Is there a general guideline on "ideal" a/f ratios for our modified cars?

I was browsing through the archives on rennlist and funcars and the general consensus seems to be that during idle/cruising speeds our cars will generally run pretty lean (15.1-17.1), however, at WOT the so called "target values" should be between 12.5-13.1. Will running leaner than 13.1 or richer than 12.5 at WOT be cause for concern?

I know it depends on how lean or how rich, but can any tuning experts shed some light on this topic and comment on what the A/F values should be across the RPM band?

Thanks!

 

There are a lot of variables to this question and certainly different gas and different equipment and use will warrant what is the best. 13.1 does not always yield the most power. There is a very fine line to what creates reliablity and HP without causing longevity issues. When I tune a 930 with an aftermarket injection I have found that a AFR of 12.7 is normally where the power band is near its max and still safe. More times than not I will back it down a little. Normally these points don't warrant huge HP gains or losses. For instance a car that is running 12.2 verses 12,5 will more than likely lose about 7HP on average. The trick is to find a spot that is safe for the gas and environment that the car will operate in. For a street driven turbo that is used as a daily driver on pump gas the AFR should not be higher than 12.5-12.7..period. I have a box of pistons that couldn't make it to 15K miles on 13.1 or higher. FWIW Porsche runs about a 12.2. Keep in mind that a car running a little richer than another will keep the valves and guides cooler as well as the chamber temps. It is all a trade off when it comes to making HP.

 

Stephen,

Is the 12.5-12.7 ratio across the board from your level 1 to level IV programming? I would assume that the values would tend to run a bit leaner to obtain more hp. Or, does adding larger turbos from stage II to III balance out the A/F ratios to a consistent mid to high 12 range at WOT?

 

Here is a piston and cylinder what was on a motor. The stats were 3.4 Twin plug, aftermarket EFI system. The AFR was set in the low 12.-13.1 with nominal boost. The motor was only 22K miles into its new build when it did this. I didn't build it, but I did fix it for the customer and tuned it. Just thought you might get a kick out of this.

 

Piston

 

OUCH!!! So, what was the culprit that caused this? Running too lean (13.1)?

 

Sorry missed the other question. The AFRs on IV are about 12.2-12.5. this is a little on the fat side, but that is fine in my book

Yes, the piston in the picture was too lean. Was running moderate boost and moderate timing on pump gas.

I wouldn't call the hole moderate

 

Ummm yes I meant run it at 15 that is pefect.

 

I disagree with Stephen. There is alot more involved in detonation than just afr ratio's. Fuel, timing, afr's, temp/elevation. The one item that can effect detonation the most is timing. With the 996tt's this is not an issue like the earlier cars. I run 12.9 to 13.2 afr on 91 octane fuel up to a boost of 1.1bar. I use 100 octane over 1.1 bar and my afr's are the same. Above 560 rwhp on a 996tt you will need to add an additional fuel pump as your system will lean for lack of fuel.

I have found Stephens afr numbers will produce more torque at approximately 4400 to 4800 rpm's. Torque seems to like a little richer mixture. However, once the torque peaks and the HP takes over, 12.9-13.1 definitely makes more power on the 996tt motor.
Let's say from 5400 rpm's and above.

Incidently, I have changed my plugs three times in 5000 miles just to check them. There has never been any sign of detonation....period. Stephen, I believe your above pictures are air cooled cylinders. Do you have one from a 996tt cylinder liner? Have you ever had detonation problems with a 996tt motor? If so what were the circumstances?

 

You changed your plugs yourself??? I'd like to see how mine are doing but I'm told this isn't a DIY job on our cars like on normal ones.. There's a whole buncha stuff to remove just to get to 'em? if you have any tips that you can share, I'd be grateful...

 

I would disagree with the above. Has anyone actually flow tested the 996TT fuel pump???? I have and the fuel pump is adequate. The limiting factor on the 996TT is the fuel injectors, which are 420 CC. In order to make the power properly, larger fuel injectors are needed

 

And how much is that kit (larger injectors)?

I'm kinda leary of running as lean as 13.1 and above and would be happy to loose a few HP and run down at 12.5 or so. I'm just too wussy I guess and all of my crazy Turbo supra buddies are mad at me for currently being in high 12s. The temperature changes so much around here that I just don't want to be too close to the edge. I know our ECUs adapt blah blah...

 

Todd,

I am relating my experiences. Yes, I agree with you on the larger injectors or added injector. Your Stage IV GT is making 540 rwhp. You won't need an added fuel pump according to my experiences. I know you are working on your Stage V. I understand it produces 595-600 rwhp. Checking it on the dyno is one thing. Have you run it under full load on the road?

Again, all I can relate to is my experiences. At 564 rwhp we started to experience a leaning effect. This was remedied when we added the additional inline fuel pump. Protomotive experienced the same situation at approximately 560 rwhp.

 

Chad,
Oh yes we will disagree on this subject. I have supercharged and turbocharged about every car I have ever owned. From Mustangs to Porsche and everything in between. You absolutely can not always tell based on plugs. You can tell if you pull the motor down and measure the Lan. Then you will know what subsonic detonation is. If you EVER hear audible detonation in a turbo the damage is done. What you don't hear is the killer.

You may not have had any issues with yours yet, but you don't have any mileage on it either. I have 22K on mine on 93 octane. Unfourtantly yours has been set up to run 100 octane. Once again not practical for daily driving. Also to revisit the wet injector in the dry intake. That with a 13.1 afr will equal disaster. 5K is not enough to determine reliablity or longevity.

Yes, the piston was from an aicooled car, but the laws of physics still apply, and practical application to the street under daily use is not found in a book. I haven't had deontation because I don't run your AFRs.

 

BTW the piston in the picture was a twin plug set up with almost complete chamber burn and a timing of 22.