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Tuning the C20LET - Breathing/Turbo
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First some pictures to help us understand what is inside the mythical "turbo"
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This is a naked turbocharger - with both housings removed. We can see the compressor wheel (left) and turbine wheel (right) |
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Here is the same turbo, with the housings fitted (the only view most people will ever see) |
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Compressor housing on its own Usually made out of aluminium: |
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Exhaust housing on its own Usually made out of cast iron. The KKK design integrates the exhaust housing with the manifold, making it difficult to use a housing with a higher A/R ratio (a major limiting factor with this engine) |
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Enough ogling, back to the LET now...
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The lip is enormous - the airpipe fits around it, so the air flowing towards the turbo faces a ring several millimetres thick. This would be nice if we were trying to prevent reversion - but here we want to minimise flow restrictions in that direction. |
The modification below is only beneficial when you use an aftermarket (silicon usually) hose pipe. The stock one (flexible as it is) has a matching 'lip' on the inside so there is no step facing the fast-moving airflow.
Tapering and smoothing out this ridiculous lip will certainly help - but there's a snag: the compressor is right after that 90degree turn, so nothing abrasive can even come close. The time to fix this design botch is when the turbo is dismantled for reconditioning. Smoothing the inside wouldn't hurt either...
Before:
After:
(pic by Dave)
Here's another one hacked a bit more If a lot more airflow is expected, every little helps.
Note that all work is done with the turbo assembly detached. Never-ever bring a drill anywhere near the sensitive turbocharger components. Just a few particles of flying metal dust are enough to damage it.
(pics by Decibelios)
This one has been brought right to the edge of thickness.
Again, any smoothing has to be done with the shaft removed, it's not worth the risk of particles straying where they shouldn't.
Even dipping the whole thing in degreaser wouldn't guarantee that it's clean enough to be reused safely.
Look closely at the face of the exhaust manifold:
There are 'gaps' at the upper holes where the exhaust studs go through (the lower studs when the manifold is fitted).
The factory 'cut' them for a reason: expansion.
If you found sheared studs during disassembly, you can prevent that happening again by 'cutting' through the remaining ones (especially the ones that failed previously) A millimetre or two should be enough, and it shouldn't affect the structural integrity of the manifold. Don't go crazy, because the turbo has no other mounting points. It's a balance between a couple of slightly weakened holes - or a couple of missing mounting points later on.
During disassembly, some of these nuts can be pretty tough. If they haven't been disturbed for years, they're probably baked in there.
Some may come out with the stud embedded forever, others may refuse to move at all.
Innovative tools may need to be used, as the clearance is not always ample
These threads have to be cleaned and dried thoroughly. Studs that have come loose may not have to be replaced with new ones, if their threads are OK.
These nuts have to be renewed, as they are copper and deform as they tighten up. If reused, different torque will have to be applied. Get them too tight and they will bake there forever. The washers are special, too - renewing them is a good idea if they are deformed.
Use less than 25Nm for the studs, there's no need for them to be very tight, let them tighten themselves (if they feel like it) as you correctly tighten the new nuts.
Care has to be taken with all these surfaces, because of the extreme temperatures involved. If they break themselves loose after a few expansion/contraction cycles there will be leaks that could damage the turbo (upsetting the pressure balance) or the cylinder head. Remember that the exhaust manifold is meant to redirect a certain amount of heat away from the head. If there is a gap, the heat won't be conducted properly, and there will be a hot spot as a result. That means expansion that is not uniform - something will crack or deform permanently.
If the nut has come out with the stud as one piece, then check the length of the 'free' stud and the condition of the threads. If they are both OK, there is no need to change anything, just clean the threads thoroughly.
This is the stock K16 turbine wheel. Looks tiny and unimpressive, doesn't it? Can this little thingie be responsible for filling up twice the displacement of a 2liter engine? Apparently yes. My Kawasaki750turbo had a turbine that was much smaller than this, looked like it was made by Toys'R'Us. Didn't stop the bike going off the clock without any changes to the turbo...
The stock K16 design is a good compromise among low-boost threshold, quick spoolup and acceptable backpressure at high revs. In that order.
This rare (as rocking horse stuff) pressure graph shows the stock KKK as well as the racing version. It is clear that the stock turbo is well within the max efficiency island (75%) with plenty of headroom up to 1.5bar (2.5 is absolute so it would be 1.5bar or 22psi on a boost gauge), as long as the airflow is increased by around 50% (up from 0.12 to 0.19m3/sec). This is important: if airflow is not increased (stock intake, intercooler, pipes, exhaust, head) then the turbo's efficiency goes downhill from around 16psi, and gets worse (efficiency below 70%) from 20psi onwards.
It's also interesting that a stock setup won't hit the surge limit until it reaches 30psi boost, so there's lots of headroom there.
In order to decrease backpressure one has to decrease the turbine's diameter (less torque on the shaft though!), alter the shape (but it won't spin as easily) or fit a larger housing around it (still won't spin as fast, as more exhaust gases will find the easy way out, avoiding the blades). For a lot more power, what we really need is both a larger wheel and a disproportionately larger housing (larger A/R ratio).
As the LET has the exhaust housing incorporated within the exhaust manifold, this is not straightforward, so 'hybrid' designs go for slightly altered wheel shapes (with cutback blades usually) and some machining of the stock exhaust housing to make it fit.
Cutback blades do reduce backpressure at high revs, but spool later. Their efficiency is seriously compromised on the exhaust side, meaning that a lot of gases go through but don't really spin the turbine. It's a botch - good designs never have cutback wheels. The major factor though is the final Area/Radius (A/R) ratio after the machining, and I suspect different 'hybrids' have different ratios (whether this is the result of inconsistent machining I don't know!). The result is that some 'hybrids' spool almost as easily as the stock KKKs, but also run out breath as quickly. Some others take another 500rpm to make boost, and keep it another few 100s of rpm up the rev range.
From what I've seen, there is no justification to junk a perfectly good KKK16 (reconditioned for £180) for a 'hybrid' costing around £800. Back-to-back tests have to take place for a definitive answer, comparing newly reconed KKK16s with various 'hybrids'.
Most people make the mistake of comparing an old, tired and smoky KKK with a fresh hybrid, and they naturally feel a big difference. Same as changing spark plugs or ignition leads - if the old ones were worn out, it's no wonder the 'splitfire performance' items feel better!
An old, worn turbo, apart from not boosting properly, will be leaking oil mist in the intake - this seriously compromises performance as the mist reduces the octane rating of the mixture. It also leads to carbon deposits around the valves, further decreasing performance. It also coats the intercooler inside with a nice, shiny protective oily layer, that compromises heat transfer.
Finally, a hybrid running on standard boost is no better than a stock turbo - in fact it's worse because it spools later and it's efficiency is lower. It only makes sense for high-boost engines, where it might perform a bit better.
Here's how a turbo is assembled internally
This a black art for most people, pros included. While I agree that it's not for the faint-hearted, neither is it incomprehensible. Here is a document I've made out of another enthusiast's scans from the American Sport Compact magazine (sister publication of Turbo and High Performance, a subscription to which I feel is worthy).
If you think an actuator is for life, then think again (or get a dog). It has to live with the heat of the engine bay (nothing is closer to the turbocharger!) and it's also regularly filled with the superhot boosted air (before the intercooler). As a result, after a few years the diaphragm can leak or crack, and the spring can go soft. Usually you'll still see the maximum boost on your gauge, so you think all is well. But all may not be well, you could be getting wastegate creep and by just looking at your boost gauge you'd be none the wiser.
It's easy to get the actuator off, and it doesn't take high-tech equipment to check it out.
True to our low-tech tradition, it can be done with a footpump and a piece of spare hose.
It helps if the pump comes with a decent gauge. We operate it by hand for more accuracy. Start pumping until the actuator rod moves: the pressure on the gauge shows the boost pressure that will 'crack' open the wastegate. Keep pumping until the rod is fully extended. Check the reading on the gauge: that's the max boost the actuator can take.
Notice how smoothly the rod extends as the pressure increases. If it's got steps then it's tired and may need changing. Also note how long the pressure stays in the actuator. If it leaks (you can feel it or hear it usually)then it's definitely shot.
This way you can also test if the new, 'uprated' actuator you just bought is any better than the old, stock unit you had. Doesn't hurt to check, does it?...
Below is a setup with an extra spring effectively parallel to the existing one. It sits just over the actuator rod, using two simple brackets. This allows us to reduce the pretension of the actuator's rod, even perhaps eliminate it altogether. This allows the full opening of the wastegate flap, which means better boost control
It's nothing special either: a door stop from the local hardware store!
An alternative is to use brake shoe springs like these ones. Cheap and do the job pretty nicely.
Amal Valve - how does this work then?
Do we really need this thing hanging from coolant hose? Easy to mess up the pipes, too
Technically it's known as the charge pressure control bypass valve.
It's the ECU's only way to directly control boost. In a setup where the ECU cannot control boost (as it used to be in the old days) the pipe behind the actuator was directly connected to the compressor outlet. That way, the more boost the turbo made, the more pressure was at the back of the actuator pushing the diaphragm to open. Nice'n'simple.
Only snag is that if the wastegate is meant to be fully open at 10psi, the diaphragm has to crack open at 5psi (or less) so that it gradually opens as boost builds up behind the actuator (creep)
The amal valve is meant to sit somewhere in the middle of this boost-connecting pipe playing traffic-cop. It can redirect boost away from the actuator during low boost pressures, and let it all through when the opening time is near. Or it can suddenly open the boost-gate (and lower boost) if the ECU feels there's a good reason for this - excessive intake temperatures, whatever.
In this case the ECU sends the opening (pulsing actually) signals via the valve's electrical connection. And the two pipes, unsurprisingly connect to the compressor and the back of the actuator. There's another pipe at the back, connecting to the engine intake. That's directing the boost pressure that has been bled off, back to the intake (it has been metered, so it shouldn't be dumped in the engine bay).
The pipe further away from the electric plug goes to the actuator - try to remember that, because it's all too easy to mix up these two pipes.In its idle state (say if it's broken or disabled) the valve is meant to let all boost pressure go to the actuator. That's a basic safeguard for the engine. During operation the valve has the option to restrict the pressure hitting the actuator until the very last minute, improving boost build-up.
That's rubbish, I want a high-tech, super-duper flashy shiny boost-controller. Much better, isn't it?
Well the amal valve IS part of the manufacturer's boost controller. The instructions are inside the ECU - so it's not like you're adding boost control to a prehistoric engine. Therefore the benefits of an aftermarket boost-controller will not be as dramatic as you might expect. It may help you keep the wastegate closed for a tad longer, before unleashing the boost floodgates at the back of the actuator. This will decrease turbo spoolup times a bit, but it will be at the expense of a shock to the transmission (which the manufacturer tried hard to avoid)
Nevertheless, some people fit the APEXi AVCR on this engine, arguable one of the best electronic boost controllers out in the market. Then they find out that they have to splice the loom and that it's not exactly a 10 minute plug'n'play affair. Here is a useful guide that will help you if that's the case. It originated from MIGWeb, probably user RDS.
Here is a simple, cheap (£17 here, if you find it cheaper please let me know) way to eliminate boost creep and help the turbo spool as fast as it can. We simply set the adjuster to the right setting and only boost over that value will get through to the actuator.
So if it's set to (say) 1 bar, then the actuator won't see any boost diverted until 1 bar boost is reached - (roughly, you need to fine-tune it a bit with the boost gauge on the road.)
Here is the controller fitted just before the actuator.
This one is all steel and brass so doesn't get affected by the surrounding temperatures
This is the alternative setup. The MBC (Manual Boost Controller) totally bypasses the amal valve and is connected between the turbo and the actuator. This is a more adventurous state of things, as ECU has no control over boost now whatsoever.
Here's another one , or even how to make one yourself (if you're so inclined...)
You'll also effectively lose the 1st gear boost limiter, which is no big deal as long as you're careful with your right foot. You're also compromising a bit of the ECU control for engine-safety purposes, so better make sure that your fuelling/ignition/intercooling are in top shape.
One of the advertised advantages of an electronic aftermarket boost-controller is that you can change the boost settings from inside the cabin. This can be useful in icy situations, but again I think control of the right foot should cover that. As for the flashy lights and the cool ambience? well...that's why you really buy these gizmos, isn't it?
Here is more on electronic boost controllers (local copy here). It's from an Audi S4 site, but the principles are the same. They're running Motronic, too.
Here is an account of installing a boost controller, testing, and the downside as well...(local copies here, here and here)One last thing to note is that fitting an uprated wastegate drastically alters boost control anyway, as the ECU cannot limit boost to the previous levels. If you're doing 10psi boost, the maximum the boost controller (or amal valve) can send to the back of the actuator is 10psi. If the actuator spring is dead-stiff up to 15psi, (local copy here)then the ECU is effectively denied control until that pressure is reached. One less reason for spending money on a boost-controller.
Here are the impressions of another enthusiast, using the similar (but more pricey) Dawes Device on his EVO (local copy here).
Right after the turbine, the LET sports the elbow. It's a tight 90degree turn right after the turbo, not the best recipe for smooth and unobstructed flow. The exhaust gases are still hot and expanding, so any improvement at this stage won't go amiss.
Below are views of both sides of the elbow, right after dismantling:
Here they are cleaned up and smoothed out a bit. Nothing fancy, they'll get dirty soon enough, it's an exhaust after all! But the shinier, the better the flow (and less chance for more deposits to stick later on):
This is a good time to clean these threads. There is a lot of heat involved here, and these 6 bolts normally should not be reused (torque at 20NM torque, no more). Do not replace them with stainless steel or other metals, because they'll expand at different rates, and this gets very-very hot. Maybe we should stress here that the manufacturer's torque values refer to clean & dry threads. If they are soaked in rust and oil, they'll come loose when you least want them to...
The same stands for the two studs/nuts (the ones with the springs) connecting the elbow to the rest of the exhaust: They have to be renewed, along with the round gasket.
For an improved elbow design, there is more in the chapter on exhaust
It deserves a special mention, because it's usually a major factor in the early demise of turbochargers. The oil flow through the 'floating' bearing is critical. If it's disrupted for a handful of seconds the shaft will be permanently scarred. If the disruption lasts for more than 10 seconds, then the turbo may need a rebuild: it can be that bad!
Contrary to what some people believe, the turbo oil return is not pressurised. Only the oil feed is under pressure. The return is meant to flow using gravity. That's why the pipe diameter is larger (same flow but lower pressure!) It is absolutely critical that the oil can flow freely down that pipe. If it's blocked in any way, then the turbo bearing will be immediately swamped and the excess oil will have to force itself through the turbine or compressor seals (depending on the load and the pressure balance at the time).
A kinked or misrouted pipe can cause such a restriction. So would excessive crankcase pressure.
Looking under the car, such symptoms would look like this (click for larger picture):
One way to check for excessive crankcase pressure and free oil flow is to substitute the stock rubber hose with a transparent one, Teed to a 'breather'
(seen at the lower left of this picture):
If there is excessive crankcase pressure, then there will be oil stacking up this 'breather'. In that case the engine should be stopped immediately and the issue investigated before restarting.
It seems obvious that such an oil leak will be fixed by just tightening up the oil return pipe, or the 22mm fitting (which should have a new copper ring fitted). But all the tightening in the world won't stop it from leaking if there is an inherent problem with the turbo or the crankcase pressure build-up.
Below is a guy's attempt to fit twin turbos on a Toyota V8. Nice effort, shame that he doesn't know any better: lack of space forced him to locate the turbos below the oilpan.
That means that the oil returns are lower than the bottom of the sump, therefore gravity will not help the oil flow away.
He admits to having lost 3 turbos already (SR25DET upgrade units, not cheap!) but sticks to this flawed design nevertheless...
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This is a modified oil return for an LET running a much bigger turbo. In order to retain the downward slope they had to re-route it to the top of the sump (instead of the stock point a few centimetres above.) What they haven't thought is that the splashing inside the oilpan might interfere with the oil return, especially at high revs. This will lead to the turbo smoking (best case scenario). Also note the sealant oozing from the sump gasket - a dangerous practice, because it can allow silicon bits to drop in the oil and be drawn by the oil pump (clogging tight passages) |
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It is used in competition and it does reduce both turbo lag and underbonnet temperatures. It also increases the heatload around the turbo bearing.
I would only experiment if the cooling system is already comfortable and the oil used is Mobil1 changed regularly (as in 3000miles max).
I don't see any harm in wrapping the downpipe, but not much benefit either. Most of the engine bay heat comes from the exhaust manifold and the turbo. I think a slightly longer heatshield would help and a thermal jacket for the turbine itself might be worth considering if 'expansion gaps' have been added to the manifold (it will retain more heat and expand more, there's no question about that).
Below is a US-sold example of more sophisticated turbo/exhaust manifold wrapping (meant for a Supra):
In any case, it's good to keep in mind the temperatures of a turbocharger at full operating conditions. This one is from a Fiesta RS, posted in MIG by Teddy.
The case is red-hot and the turbine is white-hot and can be seen through the thick metal!
Full boost: brrrrr....
K16 under stress during the design/testing stage
It's easy to see why special bolts are used at the elbow.
Now ask yourself:
do I really want this thing to get even hotter?
It's easy to slag off the stock manifold and admire trick designs like this one:
This one looks ceramic-coated as well Some people even make their own manifolds...
Nice they may look, but they don't always contribute to an efficient turbo setup. For a start, the pipes are several times longer than the LET design.
This means that a lot of heat will be lost before the gases hit the turbine. That's heat that should be turning the turbo and making boost! It also means there's a lot more heat lurking around in the engine bay. Not good.It gets worse: there's a lot more metal involved:
1. more weight at the front of the car
2. more thermal expansion, more cracks
3. this whole goliath (with the big turbo attached at the end) cannot be supported from the exhaust manifold nuts - not for long, anyway. I also suspect that some nuts will be particularly hard to tighten up (let alone use a torque wrench)If pulse separation was their main objective, I'm afraid that there is a common collector where all gases come together, so some cancelling out will still be happening. It's very important to keep the out-of phase pulses away from each other, right until the very last moment - when they are about to hit the blades, like this Garrett for example:
This one was custom-made:
It's hanging from an LET head, the collector looks nicely shaped and all welds look professional, both from the outside and inside (where it really matters)
Let's see how shiny it will look after a few red-hot full boost run cycles...
Looks like a modern sculpture from up close - an ornament for the living room.
Unfortunately good looks and generous size don't come cheap: the bulky Garrett meant to hang from the flange will create clearance issues for the radiator (easily addressed) and the power steering pump (ooops).
One solution would be to dump the stock PS pump and use an electric one. This sounds fine, until you realise that it may not be powerful enough to fill the 4x4 accumulator (hence people considering 'locked' tx boxes in these cases)
The above manifold in the bay doesn't look too shabby in the end. Fortunately in this case the power steering pump was retained (only just!) so the 4x4 remained operational.
Note above the enlarged pipe and 'top hat' all the way from the intercooler. Also the massive battery and alternator, essential for coping with the abnormally high-amperage needs at full throttle Low-impedance injectors mean that the ECU draws four times the current to drive them. Plus the nitrous, plus the water injection pump. Not forgetting the second fuel pump feeding the other side of the fuel rail. That's a lot of current demands at full boost.
This one below (made for an LET) does look a bit weird, and there's no reason why it wouldn't work (albeit inefficiently). The header pipes look slightly thick for a 2.0 engine, and the collector is on the hefty side. Nothing dramatic, just the boost threshold will be a couple hundred revs too high, because the gases will have lower velocity and have already expanded a bit before hitting the turbine wheels. The three steep 90degree turns won't help either...
The cast manifolds below from Italy look less clamorous than others, but would probably be easier to fit on the LET and avoid cracking perhaps.
So is it dead-easy to swap the stock turbo with a big one?
Some people who don't know any better would let you believe that you can just junk the miserable little K16 and fit a mother of a turbo leading you to an easy 400bhp - simple as that! I'd like to check and see the credentials of these 'experts'. They may be less mechanically-minded than you think, and take their car to a garage even for an oil-filter change. Armchair mechanics.
Alternatively, they may be experienced enough to know what's ahead, but commercial interests prevent them from telling you the whole truth. When you're stuck with a half-finished conversion, there may be even more money in it for them!
Below is the std KKK turbo (left) sitting next to a bigger, higher flowing Garrett conversion. The pic is from the MIG forum courtesy of two of the most experienced members (RDS and Mike Hayward)
The size of the replacement turbo isn't that far off really, and looking at the final product it looks like plug'n'play (click for larger pic).
Alas, because of the new positioning, nothing would fit properly anymore. They've had to ditch the mechanical PAS pump in favour of a Paxo electric one. They also had to remove the tensioner brackets, replace the down pipe, re- route the intercooler hosing, re-route oil feed and return and bypass coolant hoses, and ditch the Motronic for programmable engine management.
On top of that, because of the expectations of the extra horsepower they also had to fit a different radiator, replace manifold custom, customise the throttle body top, also upgrade transfer box, alter suspension, fuel pump, con rods and on and on
Seems to have had a knock-on effect, didn't it?
These are people who knew what lied ahead, and are experienced in fabricating/modifying parts. Still, the project was abandoned a few months later. Too much aggro. It all looks easy and neat on the photos, but don't be fooled by that. Ditching the stock setup needs a lot of planning ahead.
Here is another nice'n'shiny manifold on a LET head, originally posted on the MIG forum
This one started life as SR20DET (someone else's abandoned project no doubt!) and was carefully cut off and welded on an LET flange.
Not a bad idea in principle, but this maze of thick pipes is heavy and will radiate a lot of heat
From this angle the big compressor looks impressive (could be susceptible to excessive surge though).
Again, some cyl head exh nuts might be very tricky to tighten properly and the added weight of this combo will try to shear off the remaining nuts. Lots of metal, lots of expansion.
A couple of supporting brackets would come in handy, but they would have to end up in the block. If power steering has to be relocated, then maybe one of those mounts can be used for this purpose.
...and if you think that the engine bay is cramped, there's always worse:
Bigger turbos tend to create awkward plumbing...
This is the culprit for the congestion above:
Yet another one, awaiting to be fitted:
The poor thing moves between abandoned projects like a hot potato. Maybe a sign that one-off fitting and fabricating is not everyone's cuppa tea.
Note the 'flexi' joint, meant to make routing a bit easier. A pipe this thick under the car will create clearance problems even in the best of cases.
Intake A/R 0.60 More importantly, exh A/R 0.63 (note the markings right underneath the collector)
Not too extreme, certainly a lot less backpressure than the stock KKK variants.
However, t his one is meant to hold around 2 bar, with copious amounts of nitrous on top. So large volumes of exhaust gases will have to go through, making the owner wish that he had gone for an A/R closer to 1. Of course then boost threshold would be high (not much happening until 4Krpm) but under full boost and shedloads of nitrous it wouldn't suffer from reversion.
Choices, choices...
Finally, this is another decent T3/T4 LET manifold, with provision for an external wastegate at the collector:
Here's how to fabricate your own turbo header manifold (local copy here)
Now this is an interesting turbo configuration on a 6 cylinder...
The idea is that unequally sized turbos are used concurrently, the smaller one spools up early making boost while the large one produces the massive airflow needed at high revs.
What this fellow hasn't realised is that gases will follow the path of least resistance, and that's through the large turbine's housing, leaving the small one as a dead weight.
The massive external wastegate will force both turbos to produce the same amount of boost. I can't think why anyone would want this to happen.
It looks cool though...
Very nice, but I'm looking for more turbo-specific info
OK, here's some from the horse's mouth. Below in red font there are some installation instructions from Holset's turbocharger tech manual. The design and requirements are very similar for all turbochargers of this generation, including the C20LET.
After each instruction I add LET-specific comments:
1. Holset Service receives many turbocharger returns that are no fault found. Before assuming the turbocharger is not performing to specification always refer to the engine diagnostic system and the fault finding chart of this manual to make all the recommended health checks. Quite true, people tend to blame the turbo more often than not
2. It is important that intake and exhaust systems are fitted in accordance with the recommendations of the Equipment and Engine manufacturers. Limiting mass inertia loading is critical to turbocharger wholelife operation. Maximum engine vibration input must not exceed 10g. In other words, a totally free exhaust can kill a turbo as can running it unbalanced.
3. The air filter must remove particles greater than 5µm at an efficiency of 95% and be of sufficient capacity to match the air consumption of the engine. Recommended filters should always be used with a pressure drop indicator. Intake systems must be capable of withstanding depressions up to 6.9 kPa (1.0 lbf/in2). Fancy boulder strainers and clogged stock filters need not apply
4. Hose and clip connections of intake manifold systems must be capable of withstanding the turbocharger pressure ratio. V band clamps are preferred and must be used above 3:1 pressure ratio. Use proper clamps and hoses if you're running high boost (as if we didn't know!...)
5. Exhaust systems must be capable of operating at exhaust back pressures of up to 10 kPa (1.5 lbf/in2). This limit is increased to 13.4 kPa (2.0 lbf/in2) if a catalytic converter is fitted. Exhaust brake applications are permitted to impose 450 kPa (65.3 lbf/in2) back pressure.
6. Oil should be filtered to 10µm with efficiency of 60% TWA (Time Weighted Average) /20 µm with efficiency of 85% TWA. Efficiency assessed using ISO Standard 4572/SAE J 1858. ie, only use original oil filters and change them regularly
7. The oil quality must be as specified by the engine manufacturer and will be a minimum API SE - CD (MIL - L - 2104C) specification. Improved life can be obtained by using super high performance diesel (SHPD) oils, particularly in industrial applications which use extended oil drain periods. Only use proper synthetics for petrol turbos, we already knew that!
8. Normal oil temperature is 95+/-5°C (203+/-9° F). It should not exceed 120°C (248°F) under any operating condition. Interesting one, this. We want the coolant jacket 90-100C. go beyond 110 and the bearing gets it!
9. Any pre-lube oil must be clean and meet the minimum CD classification. ie, do NOT use cheap mineral oil during re-assembly
10. The orientation of turbine housing, bearing housing and compressor cover is fixed according to application. During installation, do not attempt to rotate these components. Inclined turbocharger installation is not recommended. If an installed angle is necessary, oil inlet centreline must be +/- 10 degrees from vertical and rotor centreline +/- 5 degrees from horizontal. In turbo conversions, avoid hooking the turbo on funny angles, because the oilflow might suffer
11. Holset permits oil return pipes to decline at an overall angle of not less than 30 degrees below horizontal. All turbocharger applications require a pipe of internal diameter greater than 19 mm which has integrated
connectors. To ensure oil returns into the engine under all operating conditions, the return connection into the engine sump must not be submerged and the outlet flange of the turbocharger must be 50 mm above the maximum oil level of the engine sump pan. Crankcase pressure should be limited ideally to 0.8 kPa (0.12 lbf/in2) but 1.4 kPa (0.20 lbf/in2) can be accepted by reference to Holset. Very interesting: crankcase pressure should be no more than a FIFTH of a psi. High-mileage LETs will have blowby creating a lot more than that!
12. Oil pressure of 150 kPa (20 lbf/in2) must show at the oil inlet within 3 - 4 seconds of engine firing to prevent damage to turbocharger bearing system. A flexible supply pipe is recommended. ie, 1.5bar of oil pressure within seconds, or else the turbo bearing gets it!
13. The minimum oil pressure when the engine is on load must be 210 kPa (30 lbf/in2). Maximum permissible operating pressure is 400 kPa (58 lbf/in2) although 600 kPa (88 lbf/in2) is permitted during cold start up. Under idling conditions pressure should not fall below 70 kPa (10 lbf/in2). Under load, we want 2-4 bar hot oil pressure. Cold start can show up to 6 bar. Idle below 1 bar could be dodgy!
14. Recommended oil flows for the turbochargers are 3 litre/min at idle and 3.5 - 4.5 litre/min above maximum torque speed.
15. Do not use liquid gasket substances or thread sealant as any excess can enter the turbocharger oil system to obstruct flow.
16. Recommended coolant flows for the turbochargers are 3 litre/min at idle and 10 - 14 litre/min above maximum torque speed.
This engine can also be supercharged, boost is boost after all.
side view of the supercharger
Electric supercharger?
Yeah, right - as if a fan flowing a few amps will ever meet the air consumption requirements of such an engine - let alone exceed them!
This is an attempt to fit one of these ebay wonders, claiming to flow 320cfm, no less...(local copy here)
Making a robust, shiny turbo shield out of kitchen utensils (local copy here)
Big turbo?
Massive A/R ratio, with or without a cat:
They don't get much bigger than this!
Major causes of turbo failure according to Garrett
Explanations for some turbospeak and some tech info on turbochargers (local copies here and here)
Another page explaining turbos and another one, Mitsubishi - biased(local copies here and here)
Flowmaps here and compressor map galore here (local copies here and here)
Here is a good link explaining the inside workings of a turbo in general (local copy here)
A mixed-bag document on superchargers from GrateApeRacing (local copy here)
Another one on turbochargers (local copy here)
Interested in afterburners, electric supercharger and other half baked ideas? Half-baked.com!
A document on electric turbo-compound technology (local copy here)
Things are so much cheaper in the States
On to Intercooling...
Surge is caused by stall, which is worsened by surge, its a vicious circle...
Imagine the way our turbocharger's compressor is driven, right back to basics: The turbine wheel is spun by exhaust gas, utilising also the energy stored in the exhaust gas as latent heat.
This force on the turbine shaft generates torque at the compressor end as they are physically connected. Remember that part as its the key element, Torque
Now lets imagine our turbocharger compressing air, forcing it into the engine, the engine is utilising it.. what happens then? Confus Ok, The engine expells it, and our turbine housing pressure goes up accordingly, the torque applied at the turbine wheel increases, increasing the spin speed and compressor efficiency at the other end. Great chain of events!
End Result - boost is climbing nicely.
The turbo begins to spin, as above, but the compressor wheel is generating more air than we can move and the torque required to spin that huge compressor is not being met? Remember that unless the air is processed by the engine, and expelled as hot, rapidly moving air into the turbine housing, our turbine shaft torque will not increase to spin the compressor faster, and it may well diminish.. our compressor is slowing, braked massively by the compressed air its trying to overcome and increase.....Result? Compressor starts to slow a little.. (beginings of a compressor stall)
The engine is still rotating and consuming air, but the turbo has stopped producing an excess, so our engine has now had chance to consume what excess was available and the inlet pressure is now diminishing, the turbine torque is now increasing again...VERY IMPORTANT:
Remember also that at same time, compressor resistance to spin has also dropped due to housing no longer being as pressurised and as a result, the torque required to spin it has dropped massively..Boom.. Suddenly the Turbine wheels torque massively exceeds the compressors resistance and the turbo spins to speed in an instant!! Sounds great, rapid boost climb!! The compressors acceerating at over 2G
BUT:
We just hit the same problem again, our huge compressor has made so much boost, so fast, and is trying so hard to push the massive volume or air, that our YB cant use it.... so we start to slow again........So we went: Fast, slow, fast, slow, or in other words: The turbo is Surging.
As you can imagine, it continues this cycle until it hits the revs your engine consumes all the air..Downsides to this surge are pretty obvious:
We can have serious bearing wear, due to the excessive loads imparted on the shaft supports.Compressors have been known to simply explode!!
Worst case scenario, the compressor being slowed rapidly with a turbine still trying to accelerate it can simply do as expected, part company.
Hope this helps to explain in as simple terms as i can put it, the phenomenon known as "Turbo Surge."
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The following excerpts are from Jay Kavanaugh, a turbocharger development engineer for Garrett Engine Boosting Systems.
N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.
Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.
A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.
If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”
"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.
As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”
"Here's a worked example (simplified) of how larger exhausts help turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
o here, the turbine contributed 19.6 psig of backpressure to the total.
Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.
