Dwayne Williams

Gents,

Been reading through the archives for recommended A/F ratios and have seen varying recommendations. Most tend to agree that for optimal power and reliability you want to be around 12.5 to 12.7 at WOT. So, what A/F ratios do folks recommend for:
- Idle?
- Cruising?

Pump gas (93)

turbo951fan

Idle and mid range should be around 14.5 to 14.8. Cruising should be 16.5 to 17.8.

Jake951

Here's what I learned on the dyno. At WOT above 4000 rpm (15 psi on my car), 12.5 or so produces maximum power/torque. At WOT below 4000 rpm when boost is still building, a little leaner, around 13 or 13.5 produces maximum power/torque. This has been noted before by others.

If you run leaner than stoichiometric while cruising, I think you'll fail any emissions test done on a dyno, if that's a concern for you. You need to be stoichiometric all the time to pass.

MySwiss

I have found on the dyno, that a mixture of 12.0/1 to 12.5/1 is best for TQ and HP. Any leaner,will give you more hp but less tq,and any richer will have the opposite affect.

rage2

How's that so? TQ is related to HP, so as TQ increase HP also increase .

The secret is to balance EGT's. You'll make more power at 13:1 than 12:1, but EGT's will go up as you go leaner, and chance of detonation (octane dependant) increases.

Timing, the more advance, the higher the power, and higher potential of knock. The less the timing, the higher the EGT's, the lower potential of knock (and less power).

I find the best balance for timing vs a/f is around 12.2 to 12.5, at around 20 degrees of timing. I tune this at a certain boost level, if there's no knock, turn up the boost, repeat, until I find the knock limit for that particular octane fuel I'm using.

Some engines, you'll make the same power at 12.2:1 and at 11.5:1. In that case, I richen up the mixture to 11.5:1, so the fuel helps cool the charge, allowing higher boost, and more power safely for a certain octane. 944's seem to drop a lot in power as you get richer than 11.8:1. My car is tuned at 12.3:1.

As for EGT's, the general rule of thumb is 1500F limit for NA motors with standard exhaust valves, and 1650F limit for turbo motors with sodium exhaust valves.

MySwiss

Rage. I'm simply stating what I have found,with 24 dyno runs,using the SMT5,to correct my fuel values. Timming was static for these runs.

emwporsche

I agree with rage2 - his first line at least (I've left my timing alone).
I have had a lot of problems with fuel on my car, so it's been on the dyno a lot. (75-80 runs)
My car LOVES to run lean, lots and lots and lots more power and torque.
But it will start puffing out a little smoke and isn't really safe to run.
It likes about 14.#, but we set it about high 12 - low 13 for pump gas.

Duke

Rage2>> Since you have mapped your car from scratch, perhaps you can give me some ideas about A/F ratios for low load?

Turbofan>> How did you came up with around 16.5 to 17.8 for cruising? Are we talking about very low load around 10-30 %?

TurboTommy

There are tonnes of theories on air/fuel vs power and torque. I've read/heard several times that at higher RPMs the mixture should be a little leaner. This produces faster flame speeds needed for the "push down" on the pistons. Otherwise the piston would start to outrun the flame front. Obviously, there are also different factors involved like ignition advance, head design, turbulence. Where the descrepancies come in is that some that are " in the know" actually advocate richer mixtures for higher flame speeds. So, who the f#$% knows.

As far as part throttle is concerned: I didn't think it was possible to be far off stiochiometric because the O2 sensor will always bring it to around that level, within reason.

Somebody on this board who is really "in the know" should pipe in with their opinion.

4G64 944

Are you talking about race gas or pump gas? I know every engine is different, but my real world expierence is that running 12.5:1 on 93 octane in my eclipse produces a very inconsistent car. My management system has a knock indicator and depending on temps, load, how long I've been driving, 12.5:1 is too lean, I will get a good clean pull, then the next pull I'll get some knock, since I like to be safe, I tune my wideband (in the car all the time) to 11.5-11.0 for everyday use. I've been down below 12.5 with race gas at the track and it's been great.

rage2

If we lived in a perfect world, where we had killer octane, and motors were strong enough to handle 2000+F egt's, we'd make the most power at around 14.7:1... too bad if you do it within real world parameters, the motor would blow up pretty quick hehe.

I tuned for about 14:1 to 15:1 at part load just to get a half decent map going. The O2 sensor and closed loop kicks in at this point, and dithers the A/F at around the 14.5 to 16.0 range. I run closed loop all the way to about 0.75bar.

This setup gave me about 15% better fuel economy than the autothority setup .

Turn the boost down a notch. You'd be surprised, but in most instances, you can make the same power at lower boost, leaner mixture (in the low 12's), and proper timing. This will also give you a more fuel efficient (at WOT) and more reliable (less knock) motor.

4G64 944

I've tried all boost levels from 17-24, 20-21 with 11:1 ish produces the most power and best times for pump gas, I keep timing conservative until past the peak torque point then add in more as rpms rise. Of course what I can get away with in 95dg days and 10dg days varies alot

Bengt Sweden

I am not sure what you mean by the perfect world, but maximum power is made with richer mixture. With stoichiometric mixtures, not all available oxygen will have the time to find a HC partner and availability of oxygen is the bottle neck.

Combustion is also faster with richer mixture, that is why you get lower exhaust temperatures. The fuel is used for propelling the car in stead of being burnt while leaving the combustion chamber.

Bengt

TurboTommy

Bengt is correct. Richer mixtures give more power. (Bengt, you mean it's the availability of a HC is the bottleneck). I think 13 - 13.5 to 1 is probably rich enough for max power. The slight loss of flammability at 12 to 1 is over come by the benefits of better fuel cooling, however.

emwporsche

My car was running on the dyno.
We did some runs and found out it was running lean.
We tuned the pro M elec. MAF tuner to give it more fuel.
We did some more runs, and my car made less power.
The closer we got to stoichiometric, the less power it made.