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Fuelling - 2JZGTE
Effect of octane on power output
Above is a composite graph show two power runs (power + torque) of a Euro/USspec supra running a small single turbo (61mm)
The first run is with 110 Octane fuel, the second with 91. Remember that AKI= (RON+MON)/2 so 91Octane is like 95RON in Europe. So this is basically pump gas vs race gas.
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)
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).
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...
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)
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
I've taken the liberty of saving locally the whole 'Octane booster' thread from the GTR forum, all 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 pages of it --- I think that it's one of the most interesting technical threads in automotive forums in the whole of the internet, and it would be a shame were it to disappear one day, ephemeral as internet forums are...
...on to "Gotchas & Tips"
the one thing i still cannot understand is why the 12-12.5 number keeps coming up when people say "richer than 12-12.5 and there will be no more o2 to burn fuel" when i am under the impression that 14.7:1 is stoich.. i have a feeling that this somehow has to do with the fact that fuel does not react with just the oxygen in the air at high combustion temps. clarification greatly welcome.
In order for a fuel molecule to burn it must be both hot enough and have oxygen molecues available at the right time. In a stoichimetric mixture its like you have exactly enough men and women at a party to pair everyone up. If you were to suddenly yell -- everyone choose a partner, by random chance some guys would be able to choose from 3 or 4 girls and some guys would not be near any girls. If there is not enough time to mix some guys and some girls will go unpaired. The closer you get to everyone finding a partner the harder it will be for the last few unattached guys to find the last few unattached girls.
A 12.5 :1 fuel air mixture is sort of like a party where the guys outnumber the girls. If your goal was to be sure every girl found a partner this would be the arraingement you want. When you shout eveyone find a partner (fire the spark plug) it would be easy for every girl to find a compatible guy. Some of the guys would go home single but all the girls would be happy.
Nearly all internal combustion engines make their max power rich at about 12.5:1. This is 12.5 lbs of air to each lb of fuel. This is a rich mixture that intentionally provides more fuel than can be burned by the available air (sort of like the party with too many guys). This ensures that "ALL" the oxygen (or as close as you can get ) is used up as it is the oxygen used in combustion that limits power. You are willing to leave some fuel unburned to ensure all your oxygen is consumed.
The 14.7:1 ratio or ( stoich ) mixture is 14.7 lbs of air to 1 lb of fuel, and provides enough air to burn "all the fuel". If time and condtions allow it.
In a stoichiometric mixture you should have nearly no fuel and no oxygen left after combustion. The problem with that, matching up all the oxygen with all the fuel takes some time. The last few couples take a while to find each other.
Mixtures that lean also burn too hot for most engines to make maximum power with. More of the burning process still continues in the exhaust manifold and as the exhaust passes the valve. This cooks the valve and causes very high EGT's.
Most engines can continue to make more power as you lean them out past 12.5:1 until you get to about 13.2:1 or so, but max power lean is very dangerous and very prone to detonation due to the high combustion temperatures. By running WI you can tune into that 13:1 max power range without melting the engine and without breaking it due to detonation.
This is highly simplified as other issues are involved, but engine temps, detonation and using all the available oxygen are the primary factors in producing maximum
