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Fuelling - C20LET
For a given AFR, it becomes pretty obvious that a car striving to make 50% more power, will need 50% more fuel. More air demands more fuel, or else we're gonna run lean - no question about that!
A 400bhp LET will have to inject twice as much fuel, one way or another. Is the stock fuel delivery system up to this task? Of course not.
Fuelling is like a chain, the strength dictated by the weakest link. As the extra demands arise, more and more links appear to be holding everything back. Let's take it from the beginning.
Manufacturers always build in 'safety' margins, to cover extraordinary circumstances or certain component failures. Two examples: An exceptionally cold day would result in exceptionally dense charge, so the stock fuelling should be able to cope with prolonged full boost during that day. A partially blocked fuel filter or a half-blocked injector could cripple fuelling, so some extra margins would allow the engine to limp and survive until it gets sorted by the dealer. These are the margins that 'chip' manufacturers squeeze, allowing the car to run 50% or 100% more boost. The truth is that these margins are usually in the 15~20% region, so this is the max power increase that can be experienced, regardless of boost. You can run twice as much air through the engine, if only 20% more fuel is added, only 20% more power can be realised. This is theoretical of course, because 100% more air and 20% more fuel under boost will end up in a blown engine.
When going for more power, the first weak link is the condition of the stock fuelling system. If it's expected to perform 120%, it certainly needs to be 100% operational! There should be:
- no rubbish in the fuel tank
- clean fuel filter
- no leaks in or around the fuel pipes
- no kinks in the hoses
- no rubbish in the injectors.
This is an original fuel filter cut in half. It's less than 5K miles old, yet it was surprising to find all sorts of rubbish in there, including sand.
Treat this as a regular service item, especially if you're in the habit of letting the fuel tank level get very low. A clogged fuel filter will not let the engine work properly under boost.
Here is a guide to changing the fuel filter (local copy here)
For mildly clogged injectors, STP "Complete Fuel System Cleaner" is the daddy. Comes in a silver bottle and it contains a patented substance by Chevron (polyether amine, the only one that works!).
In my experience, all other injector cleaners range from useless to practically worthless. Complete waste of time.
For the serious power freak though, the injectors have to come out every 50K miles or so and get an ultrasonic clean.
Not everyone agrees on the merits of injector cleaning, there is a theory that some cleaning rigs can damage the injector coils. If the link is dead (it has disappeared in the past) here is a local copy kindly given to me by a MIG enthusiast. Make of this theory what you will.
There are several other fuel additives out there, similar to oil additives. They all claim to reduce your fuel consumption and improve your sex life at the same time. Sadly, under closer scrutiny they don't always measure up.
From the six injectors on the rig below, most have an incorrect spray pattern.
Can you guess which one(s) are OK?
(answer at the bottom of this page)
If you already have the injectors out and don't want to pay for ultrasonic cleaning, you could DIY it a bit by dipping each injector in degreaser:
Wipe it clean afterwards, and use some compressed air (if you have access to it) to blow off any remaining rubbish from the baskets at the back.
You can see the rubbish in there by using a torch. The mesh in the baskets is very fine, looks almost as solid metal. Do not stick the compressed air too close though, you only want to dislodge any softened-up debris, not to blow the whole thing up!
In an ideal world the injectors max out at 80~85% duty rate, no more. In practice an overstressed injector in a highly tuned car can easily reach 100% under full boost. This leads to the potential for inadequate fuelling (no safety margin left) and reduced injector life. To overcome this issue, a common remedy is to increase the fuel pressure a bit. Beware - doubling the fuel pressure will NOT double the flow rate, it doesn't work like that unfortunately. The fuel flow change will be proportionate to the square root of the fuel pressure change.
Here's a table I prepared earlier:
There is a price to pay for this though, as the fuel pump will start getting overstressed and the fuel lines may start to leak, as they were not meant to run at the increased pressure.
If we want more than that, further increasing the fuel pressure becomes a less attractive option. Going up to 4.5 bar will give around 22% more flow over the stock 3.0bar, provided that the stock fuel pump is happy (which it might not be). Note that the fuel pump's flow overall goes down as fuel pressure goes up, so the diminishing returns really start to hurt after 4.5 bar. Since a dead fuel pump may lead to a blown engine, it would be wise to play it safe and treat 4.0bar as the max permissible fuel pressure (at least with the stock fuel pump).
Here is a DIY fuel pressure gauge fitted temporarily on the fuel rail and the gauge dial facing the driver's seat. It clears the bonnet so you can go for a full boost run, verifying the fuel pressure under boost.
This particular setup is only for dry summer days, as the gauge is cable-tied to the wiper arm. Sudden rain would be very unfortunate indeed
Note that even when leaving the fuel pressure untouched, a high-boost car will run more pressure anyway, as the fuel pressure is increased proportionally to boost (which is now higher). But as this condition will only be under full boost (sustained full throttle), the extra stress is minimal. In contrast, a FPR setup for 4.5 bar at idle will always be running 50% more fuel pressure than the system was ever designed - and at 1.5 bar boost the fuel pressure will be 6 bar. Is the rest of the fuel chain up to it? Are the injectors happy with this pressure? Will their tiny return springs be able to oppose such pressure?
Now consider this: with a FPR of the stock kind, fuel pressure rises at the rate of the boost increase. So 1 bar boost will result in 1 bar extra fuel pressure. Aftermarket FPRs could well be of the rising rate variety.
This Weber regulator can be connected in series to the existing one. Obviously it can only increase fuel pressure, compared to the settings of the stock regulator.
Turning clockwise the small Allen screw at the top increases pressure, about half a bar per half turn up until 4 bar, then it becomes a lot less linear. Do not play with these without a fuel pressure gauge installed, it's too easy to get it wrong.
So an FPR with a ratio of (say) 3, will create a 3bar increase in fuel pressure, when the boost pressure is 1 bar. If the boost goes up to 2 bar, then the fuel pressure will be up by 6bar. That's a lot - enough to kill a tired stock fuel pump or make old fuel hoses leak. These regulators are normally sold for turbo conversions, and would not be appropriate in our case. So make sure you know beforehand the rate of the fancy alloy-polished FPR you're about to install in the bay!
If you want to bypass the stock FPR, then you need a blanking plate, perhaps similar to this one.
Note the two O-rings that need to be in good condition and in place, or else it will leak (not good!)
A small amount of engine grease will keep them in place as the plate is turned upside down to sit on the fuel rail. There's no need to reuse the original short torx bolts, stainless replacements are fine. Just verify that the blanking plate threads are nice and clean before you fit it. It can be quite dangerous if pressurised fuel pisses on to the hot engine.
Also note that the stock 3.0 bar figure is meant to be achieved at idle, with the FPR vacuum hose disconnected. If the hose is connected, then the pressure should be 3.0 bar minus any existing vacuum (in practice around 0.7~ 0.8 bar).
There are aftermarket 'tuners' promising 300+ bhp from the stock pump/injectors, purely by using their 4.5bar fuel pressure regulators. Looking at the table above, the increase in fuel flow can only be (up to) 22% over stock. So how can they promise 50% extra power while only using 22% extra fuel? Where does the energy come from?
Good question. Here's a list of possible answers:
1. The claims to power increase are slightly exaggerated. In any case it's only a peak figure, while the manufacturer's figure can be sustained safely. 2. At high boost, the injectors are now operating at extremely high duty cycle (not good). 3. The mixture at full power is not as rich as the manufacturer would have done it. Even running closed-loop under full throttle doesn't guarantee AFRs higher than 14:1, as that is the display limit of the stock oxygen sensor (at best). 4. All of the above. The safety margins are now quite slim, and the claimed max power figures are only sustainable for a few seconds in favourable temperature conditions. If you want to sustain full throttle on a hot summer day, (say for a top speed run) better have your road assistance renewed... Fuel Injector technical info (local copy here)
Fuel pressure goes up but fuel volume goes down? Weird isn't it?
Why would then anybody try to increase fuelling by raising the fuel pressure? It just doesn't make sense!
Ah, but it does. You see the volume of fuel that the pump can shift is typically more than the volume the injectors can handle.
So as you keep increasing fuel pressure, the pump flows less but the injectors flow more. There comes a point where the two lines cross, and from then onwards even the injector flow diminishes despite the ridiculous fuel pressure (assuming that the pump is still operating and has not gone into 'safe' mode or burnt out altogether!)
The following figures are real and apply to the Walboro 255pump @ 13.5V (typical of a very high performance unit)
If we had (say) 6 injectors running 600cc each at 3 bar (45psi), that would require the pump to flow 6*600=3600cc/min. This it can do easily, as we see from the table. But if we double the fuel pressure to 6 bar (90psi) then the injectors will be able to flow far in excess of what the pump can provide. So even if the injectors were happy running at that sort of pressure, the overall fuel into the cylinders would be 2/3 of what it was at 45psi.
In the above example it wouldn't be wise to run more than 50psi fuel pressure.
Also note the electrical current requirements going up as fuel pressure increases - and the fuel pump is stressed. Stock wiring and dodgy connections might further impede the pump from producing its maximum. Food for thought...
OK the Walboro is a good pump - what about the stock LET pump?
The standard LET fuel pump is officially designed to deliver 85lt/hour at 12V. If that sounds a lot, it's only 354cc/min for each injector (it will be a bit more at higher voltage, with decent wiring there should be over 13V at the pump's terminals at full throttle).
That's unnervingly close to the limits of the std injectors. In comparison, a Mitsubishi EVO8 stock pump flows double this (183lt/hr), and a stock Supra pump almost 3 times as much (265lt/hr).This figure doesn't leave us confident that we have a decent headroom over and above the stock levels. It is specified to achieved this flow at 3.0 - 3.5bar pressure with the FPR hose disconnected. Even it if achieves this sort of flow at the top end of the scale (3.5bar), we still have to allow for degraded fuel pump wiring and contacts, that might not allow the voltage to be over 13V. In that case, a 10% increase in fuel flow would be the maximum we could reasonable expect. This means that increasing fuel pressure beyond 3.5 bar might actually reduce the fuel flow, and harm the engine. Over 4.0 bar will definitely reduce the flow, and at 4.5 bar the flow will be less even if the wiring is new. It just shows the recklessness of tuners who try to run the stock injectors and pump at 4.5bar. It's simply asking for trouble - I don't think that they've bothered checking the specs and doing their sums right...
But it may not be all doom and gloom --- Manufacturer specs (like Opel) are usually surrounded by hefty safety margins, because they want to minimise warranty claims. So even if the stock pump is labelled for 85lt/hour, it could well be able to flow a lot more, especially in the later LET engines with the in-tank pump and better wiring.
Bigger injectors are another way to increase fuel flow. As always, bigger is not necessarily better - we actually want the smallest possible that will do the job. An injector that has double the stock flow will not run smoothly at low throttle openings, as the duty cycle will be below the optimum range. In fact, such an injector may not even allow idle at all, without first remapping everything.
Injector maximum flow rate is just one parameter. The spray pattern is another, if it's totally different then the fuel may end up in the wrong places, liquidifying inside the head instead of being inhaled as a fine mist. Beware, increased boost pressures do alter the spray pattern!
Injector impedance is another important factor - if it's too far off the stock values, the ECU will not drive it properly, if at all. Low impedance injectors (like those used in the Cossies) will not work with the Motronic. More on the two types of injector drivers.
The Bosch reds (0280150431, found in SAAB 9000 turbos) have a similar spray pattern and their impedance is close enough (1.5 Ohms difference, but the ECU should be happy with them - eventually)
ATTENTION: it might take a few running and "on/off" cycles for the ECU to fully adapt to the new injectors. In the meantime the car may suddenly run lean when it reaches operating temp. It's highly recommended to have an AFR meter when fiddling with turbos. If it starts running funny, it's then obvious if it's a lean condition causing it. Just stop, switch it off, and wait for a while. Do NOT go full throttle to 'clear' it up. After half a dozen cycles the ECU will have fully adapted and it will run like a dream.
Here they are side by side with the stock LET 'yellows'
It's a very good practice to change the O-rings, all 8 of them. Air leaks from the injector rings can later lead to fuelling problems that are hard to trace.
The injectors from the newer Astra Turbo (Z20LET) are being sold as a good fit, with an (allegedly!) improved spray pattern (hard to prove that, isn't it?). They are quite cheap (£140) too - but their maximum flow is only marginally better than the yellows.
Here is a table of various Bosch injectors (local copy here). The convention is to quote injector flow at 3 bar fuel pressure, unless stated otherwise.
Here is another good injector-flow link
We see the 'yellows' (0280 150 420) as flowing 304cc/min, good for 61hp each, as opposed to the 'reds' (0280 150 431) flowing 359cc/min, good for 72hp/min (18% up).Fitting bigger injectors means that we can achieve more flow at the same fuel pressure, or the same flow at lower fuel pressure. In practice, for a 300bhp LET the SAAB reds running 3.5bar (slightly over stock pressure) are fine, and result in a nice safety margin as well.
Another make of injectors I've tried is the Lucas #5208009. (new name: 01D030B)
Physical dimensions are almost identical to the stock LET yellows, and the impedance is 1.5 Ohms higher, so the Motronic shouldn't complain too much before it adapts.
Their flow is rated at 440cc, nearly 45% more than stock. That gives enough fuel for over 300bhp without raising the fuel pressure and without stressing the fuel pump any more. Actually if you're still running the stock turbo you may find that you need to lower the fuel pressure to avoid overfuelling at full throttle (see table above)
Compare that to the extra 22% gained by driving the yellows at 4.5 bar!
The Motronic is surprisingly flexible when it comes to adapting to such large flow variations (keep in mind that it was never designed to run different injectors, let alone such beasts). Most standalone programmable systems would need manual changes and possibly remapping to get them to work properly. Motronic eventually adjusts via the closed-loop learning functions and manages to idle, warm up and operate part-throttle very nicely indeed. It might take a few hundred miles and switch on/off cycles, but eventually it gets it right for closed-loop. During idle it now pulses the ICV open a lot more, especially when cold, and that is evident from the abnormally low vacuum readings during warmup. But all is well in the end.
The ones pictured here cost less than £120 from the States, so don't fall victim to rip-off merchants in the UK.
The more these vultures are encouraged, the greedier they get.
Another set of injectors I've found are the 0280 150 558 or 0280 150 559. They are from the same family, so externally everything fits nicely. On paper they flow more than the reds (418cc) and their impedance is identical to that of the stock yellows (rare quality indeed). If they can be found at a decent price they might be an interesting option. They appear on eBay every now and then.
Changing the injectors is not that big a job, but it does take some time and need preparation. Here is my guide on how to do it properly
If you are feeling adventurous in injector swaps, then apart from the impedance you have to be careful with the physical dimensions. The thickness of the body probably doesn't matter as much, but others have to be the same, or else they won't fit in the rail. These are the dimensions of a Lucas injector that fits the LET:
So how do we size up injector flow/pressure when we know our target bhp?
Simple: take the injector flow in cc/min and divide by 5 then multiply by the number of injectors.
Example: if our injectors flow 500cc/min each at our chosen fuel pressure, then they're good for 100bhp each, or 400bhp on a 4-cyl engine.Note: the above calculation is widely used in the trade, but in my opinion it errs on the bigger side, perhaps using the 80% duty cycle as a maximum. While this is fine if you have a fully programmable management system, it could get you into overfuelling problems if used with aftermarket 'chips' that may pulse the injectors at 100% duty rate during full throttle. In that case my choice to follow the manufacturer's thinking: So if the engine is known to do 240bhp with stock 300cc/min injectors (at std fuel pressure) then to go up to 300bhp only use 20% bigger injectors and then check the fuelling. If it's leaner than 12.5:1 then increase fuel pressure by 3-4 psi and check again. This way you're still in the right ballpark and have avoided using excessively large injectors (being high-impedance they are harder to control at idle). You have also avoided using excessively high fuel pressures, which is also good.
If you think that it's impossible to run lots of boost with a Compression Ratio of 11:1 and AFR below stoich, think again. Technology moves on...
Here is an interesting site on fuel pumps (local copy here)
Here is a table of Bosch injector applications
Here is another table with basic info about several makes of injectors
More injector info (local copy here)
As we all know, fuel loses density as it's temperature goes up. Same as the air.
If the fuel pipe is too close to the intake manifold (that gets rather warm after 5-10minutes), or even touches it, then naturally it also gets warm and it preheats the fuel before it reaches the rail. That's why there is a bracket there, to keep the fuel pipe away from the hot manifold.A useful improvement would be to take this one step further (like the intercooler pipes) and create a small heat-resistant jacket for that part of the fuel pipe, to further insulate it from the intake manifold heat:
Every little helps.
If you are interested in more advanced subjects, like octane mixes and the like, then visit the advanced fuelling page
Here's a nice link on how injection worksBosch paper on emission control technologies
Handy online calculator for air density based on altitude, temp and humidity
6 injectors - only 2 have the correct spray pattern, numbers 1 and 5
...on to "Gotchas & Tips"
