Did an archive search and did not really see anything. Is there an optimal ratio or is it based entirely on a particular engine. Thanks

 

Other experts will chime in here, but I've heard that optimal is like 12.8 or so for naturally aspirated and on nitrous I'd try to get closer to 12.3-12.5.

I could be wrong, but I'd at least try to answer ya.

 

Most n/a motors run best with a A/F ratio at peak torque of 12.7-12.9 and a peak hp at a slightly leaner 12.9-13.3...

My N2O motors I try to run around 13.0 but I've been using the stuff for a long time and am comfortable leaning it out this much...I would recommend staying around 11.9-12.1 if you're not real comfortable with it.

Forced induction motors (both s/c and turbo) generally like to be pretty fat at their power peaks, plus it keeps them a little safer. Usually 11.8-12.0...
-Jeb

 

thanks guys. So Jeb, would you say if I were to shoot for 12.9 WOT that would be pretty close to optimal for an N/A car?

 

Yup...Provided everything's up to snuff...Plus it will still be fat enough to be on the safe side should you get a $h!tty load of gas...
-Jeb

 

Ok Jeb, here's one last one for ya: What is the relationship between Fuel Pressure and AFR. By that I mean, for every # raised up or down, what is the formula (if there is one) to calculate the affect on AFR? Thanks again!

 

Jeb is exactly right. high 12s is what I shoot for, but I did venture to about 13.1-.2 once I got it dialed in.

With that said I will also say that there was VERY little difference in HP/Tq from extreme low AFR to dangerously lean AFR.

Here is a graph that further backs my finding that I got on the dyno with my car. There was one ~12hp difference from 10.5 AFR to 13.5 AFR. I have been paying particularly close attention to dyno graphs etc that people post and have several of them that show this exact scenerio as I describe above.

Here is the graph:



The bottom line is if your looking for extra power from tweaking A/F ratios, you might pick up 10-15 hp, but your not going to mustar no 30-50 hp gains.

To use the AFPR to adjust the A/F ratios, is easy. It would be very tough to say that 3psi is going to equate to .5 A/F change. I suggest you go about it in a systematic way. KNowing that increases in pressure, is going to richen the mixture. Make a 5 psi change and record the difference it made on the WB. If it moved it .5 points. Then if you need to move a full point make a 10psi change. If you overshoot your target, no harm done, just back the pressure off a little and try it again.

FYI: For the largest and most staggering HP/TQ changes, timing is where it is at. Very small changes, 2*, can yield HUGE hp/tq gains. Remember as you add timing, it will lean the mixture, so full might again need adjusted.

If it were me doing the testing I would move the A/F ratio to get ~12:1 AFR. Then systematically increase or decrease the timing 2* via the disributer to get max performance.

My car would change 40 hp/40ft-lbs with a +/- 2* change in timing! :eek:

Hope this all helps! :cheers:

 

Thanks Jesse. I would still like to make it up there but I am thinking that I may have missed the last shot with the weather!

 

Yeah the weather here today was about 8* with some flurries. We will be in hybernation for a while till the sun decides to thaw us out. :cry But on the brighter side, it gives us a change to rebuild or buildup as this years case will be. :cheers:

 

:iagree:

 

[/QUOTE]
If it were me doing the testing I would move the A/F ratio to get ~12:1 AFR. Then systematically increase or decrease the timing 2* via the disributer to get max performance.

My car would change 40 hp/40ft-lbs with a +/- 2* change in timing! :eek:

Hope this all helps! :cheers: [/QUOTE]

Wo! I will have to remember that, i may find my self in similar boat... : )

 

When setting up my race motor on the Dyno I found the optimum AFR for lean best torque to be 12:1. My exhaust gas temps were still ok but 12:1 was as lean as I felt comfortable with on a road race motor. Interesting note. I hade been running 50 psi fuel pressure. I calculated 50 psi from the standard fuel pressure/volumetric efficiency/horse power equations found in many books and magazine articles. When the dyno tech dropped the pressure to 42 psi I was skeptical but I picked up 60 HP to 402 HP which according to the equations and injector flow rates said I needed 50 psi to fuel 400 HP. Don't believe what you read. Believe the dyno.

 

Here's mine. [Click to display larger image]




[Modified by Jay 383, 10:56 AM 12/5/2003]