igottafc

iTrader: (6)

im just curious. Normally 14.7 is the optimal a/f ratio right? Now when under boost on a turbo or S/C'ed car, does this # change or does it stay the same. is it better to run a little richer on turbo/sc cars, i was thinnking about it all day today. -alex

YearsOfDecay

JEZUZ...... HELL YES its good to run richer.... most guys pushing high boost will have their cars in the 11's (at least).

the Stock ECU with a stock motor and exhaust has the A/F comming out in the 12's at 6 PSI and FWO throttle (i checked my car before i started modding it just for ***** and grins..... had to try the wideband out!!!!)

The more boost you run, usually, ,the richer you get to prevent detonation. Yes, you can get more HP out of it the more towards stoich you get, but you also run hotter and risk detonating the motor.

Stoich is for maximum efficiency, but, the closest you'll ever get to that is on a perfectly flat surface with the cruise set at 55 and the motor on vaccume!!!

nik

Well, the stoichiometric ratio doesn't change. It's just the chemically correct mixture of air to fuel so that both are fully consumed in combustion. If you add more air (boost), you need more fuel, but the ratio for complete combustion remains the same.

However, as YearsOfDecay said, the A/F ratio is much different in a car engine(as opposed to an ideal chemical reaction), it depends on a lot of factors really.

igottafc

iTrader: (6)

hmmmmmm instresting. Thanks guys!

Evil Aviator

Stoich just means chemically correct, which in itself has no value for tuning the engine, although the stoich AFR is used as a base for useful AFR numbers. The best cruise economy AFR is usually slightly leaner than stoich, the best power AFR is almost always very much richer (somewhere around 12-12.5:1 for pump gas), and sometimes an even richer AFR is used to combat detonation in boosted or high-compression engines.

Stoich for unleaded gasoline is usually around 14.7:1, stoich for super unleaded is usually around 14.5. Stoich for racing fuel can be just about anything, lol. The important thing to understand is that the "14.7" number you hear about is only an approximation for pump gas, while the actual stoich AFR may be different depending on the properties of the fuel. Also note that an AFR gauge rarely actually reads AFR. It usually takes the lambda (difference from stoich) reading off the O2 sensor and then converts it to an AFR display assuming a 14.7 stoich AFR. Therefore, most AFR gauges are inaccurate if they cannot be recalibrated for fuels with something other than a 14.7 stoich AFR.

Just to answer your questions in simple form:

No, 14.7 is not necessarily the optimal a/f ratio

When under boost, the optimal AFR is the Best Power AFR + whatever extra fuel is needed to cool the charge to prevent detonation, if any. Pretty vague, eh? That's why you need a professional tuner rather than plugging some magical AFR number into your fuel computer that somebody on the internet said was good, myself included.

Yes, running a little richer on supercharged engines allows for a margin of safety for detonation. There will not be as much power, and the engine will experience more buildup and worse emissions, but running lean will fry the engine quickly, so richer is better than leaner.

NZConvertible

Just in case anyone's wordering why you need to run rich under load (NA or turbo), it's purely to control combustion temps. Once you add enough fuel to perfectly match the amount of air (the stiochiometric air/fuel ratio), adding more fuel does not make more power because there's no more oxygen to support more combustion. But under load the combustion temps associated with stiochiometric mixtures are so hot that very bad things start to happen, like detonation of things simply melting. If you add more fuel (i.e. a richer mixture), then that fuel is evaporated by the combusion energy, lowering the combustion temps to safer levels. The downside on this is that the energy used to evaporate the excess fuel is no longer available as engine "power".

This is why leaning out mixtures increases power, and richening the mixtures reduces power. It also explains why simply thowing in bigger injectors will always cost you power. One of the big advantages of fuel controllers and standalones is getting away from the conservatively rich factory mixtures, when ensure engine longevity at the expense or power and economy. When tuning an engine you need to constantly balance power output with engine safety.

HAILERS

Using two seperate widebands on two sepreater cars, with little modification,( 2 1/2 inch exaust, pre-silencer, no cat, aftermarket RP exaust on one car), I consistently see the low , low, low elevens and high tens afr under foot to the floor, uphill acceleration. That's with the sensor located everywhere from the stock 02 location, to two feet down the downpipe to finally locating one just prior to the left rear muffler.

igottafc

iTrader: (6)

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igottafc

iTrader: (6)

so hailers, your saying that the cars ecu naturally runs the engine richer underload right? Wouldnt The outside air temp also effect the best a/f ratio? I.E, the idea of a cold air intake is to cool the charge of air going into the engine which also means denser air, (i know we dont have CAI but lets take a civic for example) with the denser air, this means more fuel can be injected into the chambers right? i think this is starting to make more sense. i appreciate the elborate responses from you guys. thanks -alex

HAILERS

****so hailers, your saying that the cars ecu naturally runs the engine richer underload right?***

I'm beginning to think I have the only two turboii that run rich i.e. upper 10afr to the lower 11afr.....or I have two pre-built units from TechEdge that are not working right, or????????

NZConvertible

All petrol engines have to run rich under load. Otherwise they die.
Yes, that's why EFI systems have air temp correction to compensate for changes in outside air temp. The colder the air is the denser it is so the more oxygen there is and the more fuel you can inject. But the colder the air is the more resistant the fuel/air mix will be to detonation, so you can run leaner mixtures which make more power. It's all a delicate balancing act.
We do actually. Most people rip them off though...

Evil Aviator

Hehehe, you have been playing with your turbocharged engines so much that you forgot your basic ICE principles.

Just for the record, power does in fact increase as the AFR becomes richer from the stoich AFR. However, there is a certain point where enriching the mixture will cause a decrease in power. I will attempt to illustrate this with a quick diagram. The curve isn't very accurate, best economy is sometimes stoich or closer to stoich than shown, and the anti-detonation threashold will obviously vary greatly depending on the boost level, compression, and other factors, but this should still represent the general idea. I left the actual AFR numbers off the diagram on purpose so that the viewer uses stoich as a reference point rather than some worthless AFR numbers that somehow have gotten popular on the internet.

NZConvertible

Yeah, fair enough. NA engines bore me...

Good info, thanks for that.

igottafc

iTrader: (6)

hey thanks guys, its all making sense to me now. thanks for providing that graph Evil Aviator and to everyone else that had something to say in this thread. -alex