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Tuning the C20LET - Breathing/Exhaust
There are two schools of thought, both fiercely battling for world domination:
One says that exhaust backpressure is an 'evil thing' and it has to be eliminated here and now. The other one says that some backpressure is needed by the engine, to run smoothly and efficiently
The 'evil' dudes counterattack with the argument that 'how can it be good for breathing to have flow resistance', for that's what backpressure is. The 'moderates' reply that if you dump the whole exhaust it won't run much better, it will be lumpy and erratic.
Who's right and who's wrong? Can they both be right (or wrong?)
Of course they can - they are generalising, and it's unavoidable for both schools to be right in some cases and wrong in others.
Let's start from the beginning...
There is no single figure for exhaust backpressure. It varies dramatically depending mainly on engine speed and less on engine load. Typically it's almost zero at idle, and a fraction of a psi at midrange. As we approach high revs it shoots up quickly and at full revs it can be quite a few psi. When we refer to 'exhaust backpressure' that would imply full revs and full throttle.
On a n/a engine the intake also experiences some 'backpressure', which follows a similar pattern but is overall of much lower amplitude. "Backpressure" by the way is a catch-all term technically incorrect, but I'll use it nonetheless.
A (mechanically) supercharged engine will have an exhaust backpressure pattern similar to the n/a version, but all figures will be a bit higher, as more exhaust gases are trying to flow from the same-old exhaust.
A turbocharged engine will probably have a huge mother of a restrictor before the exhaust even starts. It's called a turbine, and it squeezes and upsets the outgoing gasflow like you'd never believe. Stock, OEM turbo designs have exhaust housings that are VERY restrictive, squeezing the air through a tiny passage, trying to make it give away all it's energy to spin the turbine. (low A/R ratios, but let's keep it simple here). If you want boost at low-mid revs, then the pre-turbine chamber must be squeezed like a garden hose trying to get the neighbours wet. That restricts flow big time, several psi worth of drop and we're still at midrange revs. At full revs this restriction is much higher, and that's even before the exhaust pipes contribute their own share of backpressure.
Race-preped turbos run high A/R housings, which means that the turbine wheel might be bigger, but the housing around it is FAR bigger, you can stick your finger in there. That improves flow immensely, especially at high revs. In fact, half of the air molecules may get through the turbo without even touching the turbine. This leads to low backpressure all right, but if they haven't tried to spin the turbine, who's gonna do it? Hence the 'race' turbos don't make boost until 5K rpm
Then we have the 'hybrids' and the modified turbos, somewhere in the middle (much closer to OEM, really)
OK, enough popular mechanics, why should we care what the backpressure is?
One issue is the interaction between the cam timing and the exhaust.
As we saw back in the "cams" section, during overlap both exhaust and inlet valves are open for a short while. This means that stuff could flow either way. We don't want it move the wrong way. We either want everything to stay in place, or move a bit towards the exhaust. That would encourage fresh (cool) mixture to wash out the remaining crap from the combustion chambers, push the old rubbish away and cool the valves a bit (at the expense of higher fuel consumption and emissions)
If it goes the 'wrong' way then some crap will remain in the chambers and some will go back to the intake port. This will preheat the ports, the intake valves and the whole chamber in general. It will also displace fresh mixture, contaminating what we're trying to burn. Preheating the area it also decreases the density of the mixture that DID make it into the chambers. This happens a lot actually, that's one of the reasons why VE is so much below 100%.
Race engines of yesteryears used to run loads of overlap. At some revs this exhaust gas reversion would be really nasty, because exhaust pressure waves would stuff the burnt gases back into the carbs. But at other revs (max torque revs) these same pressure waves would suck out the burnt gases, creating a vacuum below atmospheric, pulling in the fresh mixture in. VolumetricEfficiency at those revs was 110-120%. The ridiculous overlap wouldn't let them idle properly, and at low revs they were hopeless, but race engines are meant to be full-throttle all day long, so that wasn't an issue. Fuel consumption and emissions weren't a problem either, but they are now, and OEMs go for minimal overlaps nowadays.
Back to turbos: There's this notion that overlap lets the boost get away straight to the exhaust. This is perpetuated by 'tuners' and 'experts', so ordinary folk treat it as gospel. Some books on turbocharging also fuel this fire.
At last, let the TRUTH be told!
Here is a list of fallacies on the subject:
- On n/a engines the intake is sucked in and the exhaust gases are pushed out by the explosions. That's why intake valves are always bigger.
- Turbos leak boost during overlap
- A totally free-flowing exhaust can only help a turbo spin faster. There cannot be any downside to this.
There are more, but you get the picture. The reason this stuff prevails is because it 'sounds right'. Hey, it's conventional wisdom, it MUST be right. That's a good excuse for Joe Public, but specialist tuners should know better than that. How many of them have actually ever measured exhaust backpressure? Ask them to show you how they did it. There are 'gotchas' when you first try it, but once the adaptor is made, it can be used time and again. If they haven't ever measured intake and/or exhaust backpressure they're just repeating the age-old crap I keep reading in books and articles allover. But because the 'tuner' said so, it gains even more credibility. He then sells you some Slick50 to eliminate any friction between you two.
Let's look at the above gospels more closely:
1. First of all there are no explosions in the combustion chambers. If there are, then we have a problem and the engine won't last long. What we strive for is controlled burn of the mixture. We want everything to happen in an orderly manner, with no hanky-panky before the spark plug fires and no rush for cover as the flame propagates. We don't want the high chamber pressures (after the burn) to be used to push the exhaust gases out of the way. We're not in the fireworks business, we want the energy to be used to push the piston downwards!
Everything has to be timed so that the last drop of energy is squeezed onto the piston. There's only one power stroke in a 4-stroke engine, let's get value from it! Neither do we want the piston to push the exhaust gases out during the exhaust stroke, because that would be energy lost from the crank. We've got enough losses as it is, everything tries to drain energy from the crank, but this is not SSEnterprise.
Therefore an optimised engine strikes a balance on when and how easily the exhaust gases will be gone. The result is that in practice the intake/exhaust pressures are fairly similar.
2. How can someone know whether a turbo leaks boost during overlap? Do they stick their hand beside the valves and feel the breeze? Sweeping statement that.
Again, it's the inlet/exhaust pressure ratio that dictates where the flow will tend to be. Measure these, and you know. If an engine sees 20psi at the intake and 30psi backpressure at the exhaust, is it gonna leak boost during overlap? I don't think so. It will leak boost when the intake shows 5psi and the exhaust 2psi. It's still the same engine, you know, just different rev/load combinations.
Below are backpressure measurements taken by username: c20let on the MIG board
Inlet manifold / Exhaust manifold
- 0,2 / 0,25
0,0 / 0,45
0,1 / 0,55
0,3 / 0,75
0,5 / 0,90
0,7 / 1,30
0,9 / 1,80
1,1 / 2,00
1,3 / 2,20
1,5 / 2,45At high boost pressures there is almost 1 bar more backpressure than boost. That's a lot of reversion! This refers to a standard KKK and exhaust manifold, but it wouldn't be much better with a straight-through exhaust, or even a hybrid. For serious power, a larger A/R ratio is needed here.
Supercharged engines tend to have intake pressure generally higher than exhaust backpressure.
In that case you know that increasing overlap will shove boost straight through the exhaust. Some of this may be beneficial actually, cooling the valves a bit.
Turbocharged engines are totally different beasts. Exhaust backpressure rises rapidly right after the max torque revs, while boost pressure doesn't. The result: reversion. But before max torque revs, intake manifold pressure is higher than exhaust backpressure - boost leak territory.
Change the exhaust and ditch the cat, and the whole balance may change - raising the rev point where boost leak stops and reversion rears it's ugly head.
That's why generalisations and sweeping 'expert' statements can be embarrassing later on.
3. OK now, how on earth can a free-flow exhaust fail to help the turbo spin faster? Surely there's no downside to this one. Take a stock turbo car, fit a bleed valve, fit a 4" downpipe with no backbox, and you're King of the Hill.
...Boost spikes anyone? The main way to control boost is through the 'integrated' wastegate.
Yes, they are proudly advertised as a bonus, when in reality they're a miserable compromise of low-cost and low-flow. The wastegate typically sits next to the turbine and as it opens up exhaust gases are diverted from the turbine's way. For the same opening of the wastegate valve, flow is controlled by the backpressure after the turbine. The 'freer' the turbine spins, the less of an incentive for the gases to go around the wastegate. A 4" straight-through pipe will seriously diminish the wastegate's effectiveness. If the car is running high boost as well, then the stock crappy wastegate is under even more pressure.
The result is boost spikes, that can allow the turbine shaft to spin momentarily far faster than designed. Doesn't help reliability.
But I've got a boost gauge, I hear you cry. I don't see no spikes.
I've got news for you: Your boost gauge is heavily dampened. If it were not, it would be unreadable, the needle jumping up and down continuously. The spikes are evened out in the gauge's damping fluid, what you see is an average value. Oops.
Can a free-flow exhaust reduce efficiency?
Actually, it can. The efficiency of a turbocharged engine relies heavily on the cylinder head operating at the right temperatures - more precisely the gases between the exhaust valves and the silencer (or cat). The speed and temperature of these gases dictate the force that will drive the turbine. If the gases are too slow or too cold then the turbine isn't driven as hard as it could be, resulting in increased backpressure and a slower compressor (less boost).
An exhaust that is *too* free-flowing can result in the engine feeling 'gutless' at the bottom of the rev range. This is not always placebo, the gases leaving too early result in lower exhaust gas temps, therefore lower torque produced at those engine revs. This is only the case at low revs, because the very same exhaust design also results in higher flow at high revs, and lower EGTs there too. The only difference is that the lower EGTs are now welcome, because they are pushed below the maximum (safe point), while at low revs they were below the minimum (efficiency point).
Such an engine will produce improved max bhp figures, but looking closely it will be apparent that it's at the expense of low-down power. Once it's recognised however, it can be fixed - exhaust wrapping could help bring EGTs back up again, while the free-flowing exhaust can retain the max flow potential. Best of both worlds.
Here is a handy little calculator for gasflow in a pipe
Besides noise, resonances etc, it's the (in)effectiveness of the integrated wastegate that dictates how 'free' the exhaust can be. Cracks on the wastegate 'face' don't help, but they do appear due to the constant expansion/contraction, and the penny-valve banging on it all the time as the actuator pulses it (via the amal valve).
Tiny hair-cracks are not very disruptive, but once they grow to these proportions then the whole core has to be junked
The wastegate won't shut anymore, so the turbo will take ages to spool
There are mods where the wastegate is slightly enlarged to allow it to flow a bit more. I don't think it's worth the aggro. For serious performance an external wastegate is a must.
Another big advantage of an external wastegate is that it can be set to dump the excess gases out in the atmosphere, without them interfering with the 'normal' exhaust flow. This interference creates even more backpressure when you least want it: at full boost. If it has to merge with the rest of the exhaust, it can be set to merge at a very shallow angle and a few feet away from the turbine wheels. That would minimise the interference with the turbo's efficiency, that always robs power in 'integrated' setups.
How stiff does the wastegate have to be? Surely there are no vast forces pushing it open!
The exhaust gases are quite forceful at that stage. Just sticking your hand at the back of the exhaust tailpipe doesn't give you the right idea. Don't forget it's these gases that spin the turbine to 100K+ rpm!
Under low boost the exh/inlet pressure ratio is just over 1, quickly climbing to 2:1
At 1 bar boost it's around 2.5:1 and from then onwards it grows disproportionately.Of course it depends on the turbine diameter, the exhaust A/R ratio and the backpressure of the rest of the exhaust. The pressure ratio will be lower if the turbine is bigger, the A/R ratio larger and the exhaust see-through.
Very roughly, for a stock(ish) turbo and a cat-less exhaust, the exh backpressure is about 2.5 times the boost pressure. So if we're running 1 bar boost, we've got 2.5bar pushing at the wastegate (against the actuator spring)
Then we calculate the area of the flap that the gases can see. Say it's 1 inch sq.
We've got (2.5)*14.7= 35.8psi (that's pounds per square inch!)
...so the gas force pushing the flap is 35.8 pounds. Simple.That's why actuators that can hold high boost pressures need to have much stronger springs.
Uprated Actuators - there may be a catch!
An uprated actuator should be one with a stiffer spring. But beware, any actuator may appear as 'uprated' just by looking at it, and it can even appear to perform as an 'uprated' item if it's wound up long enough. But that's not the way to do it. Here's why:
Suppose that the rod has to extend 20mm in order for the wastegate to open fully. If you have an actuator with a weak spring, then you'd have to wind it up by (say) 15mm, so that it can hold 20psi (or whatever). But what you're effectively doing is simply pre-loading the spring. That's NOT the same as having a heavier (stiffer) spring.
You have a soft spring, but preloaded. Yes, it will start pushing the rod at 20psi, but it will only extend by another 5mm (the other 15mm you've already used up while preloading it!)
5mm travel are nowhere near enough to open the wastegate flap properly, and you'll end up with runaway boost.Your 'tuner' may blame the ECU, the turbo, the weather, the 'small integrated wastegate', whatever, but it really is their fault in preloading the actuator spring too much.
A halfway house would be to change the 'elbow' with one of a better design. Due to space limitations the stock one is a very restrictive 90 degree bend, so any improvements there are welcome.
This is how an alternative flange would look.
It would allow the separation of 'normal' exhaust gases from the wastegate gases - quite important for performance at higher boost pressures.
Thanks to Chris Reay for the pic
Note the gentler turns - essential to reduce backpressure, as it's so close to the turbine. This is a KKK by the way:
Note how the wastegate gases are separated during those critical first inches, so that they can't wreck havoc with the 'normal' gases. This is felt as better boost control during gearchanges under boost.
Below is the racing version of the LET. It clearly follows the same design principles:
Here is the truth behind the evil and sneaky Wastegate Creep
Another interesting idea is to have a stock (or at least not offensively wide) exhaust, Teed off at an early stage, with the sneaky extra branch coming to play only under high boost.
Smart and simple.
The silencer is too far away from the turbo to have a big impact on back pressure - but this doesn't mean the engine has to be strangulated or the car has to be noisy. For maximum flow straight-through silencers are the best, but for noise reduction AND good flow a design is needed similar to this one (see Ray Hall in the 'links' page)
Here's why we need to keep an eye on Exhaust Gas Temperatures This is meant for turbo diesels, but the principles are the same (local copy here)
EGT monitoring tips from Hahn the turbo guru (local copy here)
Here is more on what monitoring EGTs can and cannot do (local copy here)
We recently mentioned superchargers: why not have both a turbo and a supercharger on the same engine? Why not indeed...Lancia tried it and found the complexity was not worth the aggro. (local copy here)
And here's a good link on how exhausts work
On to Cooling...
The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on http://www.impreza.net regarding exhaust design and exhaust theory:
“Howdy,
This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an 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
So 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. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?
