best air fuel ratio

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best air fuel ratio

Post by 1972ho » Sat Nov 23, 2013 11:17 am

What should be the best A/F ratio for drag racing car i have a racepak with a oxygen sensor installed and would like to know what A/F ratio i should shoot for to make the best mph,I've been told to shoot for anywhere from 13.2 to 13.8 will make best power/mph. Thanks

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Re: best air fuel ratio

Post by dieselgeek » Sat Nov 23, 2013 11:33 am

fuel type?
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Re: best air fuel ratio

Post by Walter R. Malik » Sat Nov 23, 2013 1:55 pm

1972ho wrote:What should be the best A/F ratio for drag racing car i have a racepak with a oxygen sensor installed and would like to know what A/F ratio i should shoot for to make the best mph,I've been told to shoot for anywhere from 13.2 to 13.8 will make best power/mph. Thanks
Something leaner than stoic for good economy and somewhat richer than stoic for good power.
Realistically, that is about the only recommendation that is for sure.
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Re: best air fuel ratio

Post by RAS » Sat Nov 23, 2013 2:11 pm

You have to go rich to accelerate. DYNO-BSFC numbers tell the story. Every engine is different. Hundreds of variables. Nothing beats the best ET / top speed for tuning. Track times don't lie. If you hit it, you will know. Don't even think about lean. Nothing there.

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Re: best air fuel ratio

Post by bigjoe1 » Sat Nov 23, 2013 2:22 pm

RAS--- that is NOT what I have seen-


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Re: best air fuel ratio

Post by jmarkaudio » Sat Nov 23, 2013 3:16 pm

13 to 1 +- a point, extreme cases maybe more. The dynamics of any engines design as well as the fuels properties vary too much to put a number on it. Change the dynamics of the vehicle it's in and you change it again.

It all boils down to equal distribution and homogenization of the air and fuel, the better it is the leaner it will make best power.
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Re: best air fuel ratio

Post by dieselgeek » Sat Nov 23, 2013 3:35 pm

How can anyone recommend a number if he's not telling us fuel type?

Let's put it in Lambda terms then. I'd start at Lambda .85 and make a pass, first see if you can keep a consistent Lambda going down the track, keeping things safe, rich and slow to start with. If it looks like leaning it out from there won't cause you to go over, say .95 lambda at any point during the WOT pass, start leaning it out until you hit your best MPH and going leaner nets fewer if any more MPH.

Guessing you're carbureted? If so you have other things to worry about, the engine might not want a "flat curve" since you're measuring with a single (or pair of) sensor(s) and you'll have distribution issues by default. So don't always trust the sensor, do some WOT chops and look at plugs as you slowly lean it out. As always, give it what it wants, don't set a target arbitrarily.
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Re: best air fuel ratio

Post by englertracing » Sat Nov 23, 2013 3:58 pm

Which ever afr makes the best power ;)
There's no magic number.
Will be different at different throttle positions, rpm and loads.

All you care about is wfo on a drag car right?
Wouldn't it be best to run various air fuel ratios on the dyno and see for yourself where the best power is made?

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Re: best air fuel ratio

Post by MadBill » Sat Nov 23, 2013 4:44 pm

To expand on one of Dieselgeek's comments, another layer of complexity is added for any carburetted engine without a throttle for each cylinder (and even some with), namely air/fuel ratio maldistribution. Unfortunately there are still intake systems that deliver mixtures up to several AFRs different from one cylinder to another, often varying by RPM as well. A well designed EFI or IR system might deliver the same AFR plus or minus one tenth and make max power at 13.4:1 on a typical race gasoline, with no dangerously lean cylinders to worry about, but a bad single 4bbl/two plane manifold might be pig-rich on a couple of cylinders and smoking lean on some others, all while making best overall power with an average AFR of maybe 12.5:1.

The quick way to know where you stand re distribution is with WBEGO sensors for each cylinder. Slower but equally effective is with an O2 bung in each pipe and moving one or two sensors around them all to get the whole picture.
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Re: best air fuel ratio

Post by 1972ho » Sat Nov 23, 2013 5:09 pm

Just a little info on the it is 1972 ford mustang stocker with a Cleveland engine it runs on C-12 vp fuel
A couple of weeks ago I was at the Las Vegas division race and made a couple of qual runs with the A/f ratio
Running at a peak of 13.6 and a low of 13.2 or so,and the next morning I decide to jet up two in the front and
One in the rear and I picked about .5 mph and A/F ratio went to 13.2 or so peak to about 12.95 or so but the air change
a few hundred ft and barometer dropped a little more.So can I say if it liked more fuel and a little less A/F ratio
or is that not apples to apples.

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Re: best air fuel ratio

Post by jmarkaudio » Sat Nov 23, 2013 6:28 pm

A few hundred ft isn't much, if you jetted up that much and it picked up it's because it needed it.
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Re: best air fuel ratio

Post by dieselgeek » Sat Nov 23, 2013 6:40 pm

jmarkaudio wrote:A few hundred ft isn't much, if you jetted up that much and it picked up it's because it needed it.
Agree, it sounds like he's taking the right approach. My only other advice is for him to make a spreadsheet or somehow log every pass - weather conditions, changes to the tune, and the car's performance. Eventually he'll have a database of great information on the car.
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Re: best air fuel ratio

Post by raynorshine » Sat Nov 23, 2013 7:15 pm

RAS wrote:You have to go rich to accelerate. DYNO-BSFC numbers tell the story. Every engine is different. Hundreds of variables. Nothing beats the best ET / top speed for tuning. Track times don't lie. If you hit it, you will know. Don't even think about lean. Nothing there.
-this has been my experience as well

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Re: best air fuel ratio

Post by bigjoe1 » Sat Nov 23, 2013 8:06 pm

What I have seen is that the real strong, high HP per cubic inch engine can run leaner than the run of the mill engines, What might be good for a 1.50 HP/ cube is going to be overly rich for a 2.0 HP per cube. The more efficient engine will like a leaner jetting for best power. It shows up as a lower BSFC on the dyno. On serious race engines,I have seen over 14 to one air to fuel ratios.


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Re: best air fuel ratio

Post by rfoll » Sat Nov 23, 2013 8:18 pm

I had read recently about leaner mixtures making more power. I leaned my carb out to about 13:1 and the car slowed down about .2 seconds. The chart on the LM2 showed that it leaned out all the way to 14:1 and more momentarily when the trans shifted. It's possible that it would have showed more power on the dyno, but it wasn't happy in the car.
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