Yesterday we dynoed a friends car, and it seems to be down on horsepower. Car runs great and makes good torque, but is down probably 50rwhp. It is a 396 that is set-up pretty good. Similiar cars have made close to 400rwhp and little over 400rwtq....this car isn't. Best it made was 350/410.
I had the a/f ratio pretty flat on all passes....except right before the car was shut down. Probably right around 5500-5800 it started to get fat. Since I don't have the dyno sheets in front of me, I'm going to guess it was dropping close to .75:1 afr.
So first pass a/f ratio was pretty steady around 13:1 and dropped off to almost 12:1 when it was shut down. We decided to fatten it up and see what happened. Car seemed to make the most area under the curve around 12.4 - 12.5:1.
So with this PE vs. RPM set up, I was probably dropping off to 11.7:1 when we shut it down.
Question is, should I have a/f ratio fattest where peak torque occurs, and then start leaning it out as horsepower comes in? For example 12.5:1 at peak torque....then carry that out and slowly lean it out to 13:1 at peak hp or maybe a little past.
I would think if you lean it out up top that the engine would want to rev higher and make more power...right?
If someone would mind sharing, what did you're PE vs. RPM look like and how was your a/f ratio......and what should it be?
Do you want a/f ratio to slowly lean out as rpm goes past peak torque?
I'm hoping that if we lean his car out up top that I can pick up some of the "missing" 50rwhp...or is this just a pipe dream?

 

Anybody?
Wouldn't you want your a/f ratio fattest at peak torque and then start to lean the car out as rpm rises in order to make peak horsepower?
Or should the a/f ratio stay at the "ideal" ratio for the whole rpm range?

 

On the LS1's the AFR is set for 14.7 under everything except WOT. Under WOT mine richens to about 12.5, I need to get it around 12.9-13.0. I don't think it needs to lean out after the peak, I think you want to keep it at the optimum ratio for the best performance throughout the RPM range.

I mean the WOT power range of RPM's, not from idle to rev limiter, all the time. On my dyno sheets, once I hit WOT, it gets richer, to about 12.8 and stays there until I let off the gas.

 

I always understood that your suppossed to lean the car out after peak torque....sounds like your right to me.

 

Hmmm....pretty dead in here. I was hoping more people would bite on this...but I guess it's not a typical "how do I" post.
Kevin V 2002 SS and 96Z...thanks for your opinions guys.
Anybody have any real experience with this though?
I'd like to throw the car back on the rollers but it would take some convincing of the owner that it would be worth it.

 

Stop being so fixated on the AFR. Just use it as a general guide to see if you're in the right area, and not getting super lean. Use HP to determine what the engine likes at each RPM.

When tuned for best power across the RPM range, my AFR varies from 12.2:1 to 12.8:1, leaning out towards the top.

 

I'm not fixated on anything.
Just wondering if it's better to lean a car out as rpm increases and if the engine needs more fuel at peak torque than it does at peak horsepower.
What I'm gathering from your last statement is that is indeed true in your case, correct?
With the car I was trying to tune, the AFR got fatter as rpm increased. Only thing is, rpm wasn't reading quite right on the dyno software, so I'm not sure exactly where it was starting to get fat.

 

I think there is some confusion here. At peak torque, the VE is at maximum, so proportionately more fuel is needed. Maybe that's where the idea that the AF ratio should be richer comes from? But the ratio of air to fuel that is optimal for a given combo will not necessarily be highest at peak torque. Most of my tuning is with forced induction. What I have seen is that they like to be a little richer as the rpm increases. The timing is another important variable. As timing changes, so will the AF ratio. So it can get a little confusing.

Anyway, the best way to do this in on a dyno with a wideband and see what works best. In general, being a bit on the rich side at WOT cost just a few hp while being lean can put holes in pistons. So err on the side of richer and be careful if you try to lean it out at high rpm.

Rich Krause

 

Thanks Rich.