JustinL

In terms of performance on a turbo car, what is the ideal air fuel ratio on decel overrun?

Lean- more heat... maybe more turbo spool between shifts (popping exhaust)
rich- more fuel to cool the turbo (flames out exhaust)
cut- fuel economy

I'm all ears and willing to learn

Justin

Charlie944

Right now tuned via my TEC 2 I have my 944 turbo set to cut the fuel injectors at a decelleration above 1900rpm and when the vacuum is below 48.5 kPa with a TPS sensitivity of 5V/sec. Other that if I shift between gears while accelertaing under boost (~11.75-12.02) the AFR will drop to a quick 10.6-10.8 and shoot some flames. All AFR's are via my Innovate Wideband.
I am always willing to learn more too!
Hope this helps!

faithless

wait...

It is my understanding that the popping is caused by the ignition being too late for whatever amount of fuel inside the cylinder, lean or rich, it doesnt matter. Also, why would lean cause the EGT to be hotter than rich? There is more fuel in a rich mixture... don't you mean intake temperatures? Isn't this more a question of timing rather than fuel ratio?

Is this wrong?

Chris White

Yes

Chris White

I have found that any fuel at all kills any engine braking and I find that annoying. I program the management system to cut fuel completely on decel (same as Charlie).

Leaner = hotter EGT. It has to do with using all the available oxygen. When you run richer the additional fuel is actually a coolant. The heat of vaporization helps to cool the process and since there is not any O2 to combust the extra fuel it does not add any extra heat (or power).

If you want to do something to spool the turbo you can dump unignited fuel into the exhaust. This will light off when it finds the hot turbo. Of course there is a down side- the turbo won’t last long…

Rogue_Ant

Same as the factory programming


Rogue

rberry951

It's funny this came up actually, I've had several clients remark that their cars 'engine brake' really hard now. I have to explain to them that this is the proper behavior. But I will say the fuel/timing in the race car currently in my avatar he does indeed shoot flames going into turns. I try not to make the street cars do that. It really isn't healthy for a stock setup.

Regards,
Russell

faithless

Wait, why would the gas ignite by the turbo... how could the turbo be hotter than the headers and additional piping?

Hmm, so the charge must not be completely mixed if some of it will ignite but then some will not? I suppose that makes sense if there is a low volume of air flowing through the head.

JustinL

I've also found that engine braking is pretty well a gonner once you add fuel. This isn't a problem on track, but is quite annoying on street and I'm sure that I'm using a ton of gas unneccesarily. For normal driving I'm sure fuel cut is the way to go, but I'm really interested in autocross and track conditions.

In a quick autocross, can I run a really lean mixture on overrun and let the turbo heat up or am I going to be risking detonation even with MAP around 19Kpa? Can you even detonate with so little air in the cylinder?

On the flip side, I don't want to wash cylinders by drenching them in an effort to keep the turbo cool.

JustinL

It would ignite in the header/crossover and "blast" the turbine. I can't kill my spark, so this isn't really an option... not that I want to go through turbos that quickly.

faithless

Why would the gas ignite in the exhaust system and not in the head? I thought it wasnt igniting because there is no available oxygen, so then ignition inside of the exhaust is also not going to be possible.

It really sounds to me that flames/popping is not from dumping fuel down the exhaust because of lack of oxygen, but from improper timing to burn that fuel. Dumping vaporized fuel into the exhaust would cool the exhaust, and no fuel will be able recombust in an environment that is cooler than the head and is also absenst of oxygen. The turbo is hot, but much cooler than the head, so i dont understand. The extra vapor in the exhaust should cool the pipes and turbo, and by the time you are on throttle again, the cooler pipes reduce the temperature and pressure of the expanding gas in the first few moments.

Right?

I'm going to delete some of my upper post so I dont misinform anyone...

faithless

This thread got really confusing.... to sum up my thoughts:

Lean mixtures produce hotter EGT which probably keep the exhaust pipes and turbo hotter for better response. The matter of backfiring and flame shooting however, is not necessarily controlled by a:f, but more by ignition timing due to the fact that there is nothing in the exhaust system to mix oxygen with fuel, nor ignite it under low compression. Therefore, running an excessively rich mixture that will burn only enough to use available oxygen in the cylinder, might cause cylinder/valvestem washdown, and will certainly produce lower EGT, which will carry heat out of the whole system. If there is combustion inside of the exhaust, you cannot assume anything about the a:f, but only that the ignition was too late for whatever fuel mixture inside of it, and now the valve has popped open and you are hearing combustion inside of the exhaust. You can have backfire and flame throwing at any level of fuel mixture, just so long as the charge is ignited at the latest time for it to mostly burn as it pops out of the cylinder under compression...

The question i have is...

Regardless of a:f, what timing sequence benefits you more for hottest EGT:
Letting the charge do its burning inside of the combustion chamber, in a higher compresion environment... or... letting it burn partially inside of the exhaust?
I know that the head will absorb a lot more heat into the coolant if the charge is allowed to fully combust in the head, but it will also create more energy because its is hotter and compressed more... on the flip side.... the charge that is allowed to burn itself off in the exhaust will yield more heat into the non-cooled exhaust headers and crossover... which will benefit heat transfer, but will still yield less energy from that fuel, and might leave some unburnt to travel out and be wasted.

ERAU-944

i don't believe the turbo (under hard use conditions) is much cooler than the head due to several reasons.

1. head is water cooled much more efficiently than the turbo

2. the turbo is actually doing the compressing of the intake air, thus creating heat, AND receiving heat from the exhaust entering the hot side of the turbo

3. more reasons i can't think of at the moment - any thoughts peoples more knowledgeable than myself?

ever seen a turbo glow due to high heat? i have seen several in aircraft as well as cars. i have yet to see a head glow...

-Mike

faithless

Yea you have a point... it also depends on what part of the head (the exhaust valve side is usually hotter than the intake valve side)... I'm actually referring more to the headers since they are right there at the head but not cooled and have practically no mass.

rage2

I do this too, but there are some cars that I've done for endurance racing where there wasn't enough engine cooling, and I dumped fuel in on decel overrun to help maintain a safe temperature for long periods of time. A 2 rom setup with a thermostat flipping between the 2 programs (fuel cut or fuel dump) allows safe operation as well as fuel conservation.

The Mercedes/McLaren SLR, as well as the "65" V12 AMG benz's does this too once water temps past a certain preset, but in a much more elegant way in software . If you ever get a chance to follow a SLR, or the more common 65 AMG's, watch for the big puffs of black fuel smoke out the exhaust after a good run.
You can tune around this. I do it by dumping a bit more fuel, which cools the exhaust enough so there's no chance of ignition once it exits the exhaust. Of course, expect to wash your car a lot from the fuel on the back of your car!