black_89_vette

I remember a while back reading about the best air / fuel ratio. It differed between reading for hp and reading for torque.

I vaguely remember reading that 13.2 was best for torque and 12.8 was best for hp.

In an attempt to alter my air / fuel ratio which ranged from 12.3 at 3000 to 12.8 at 3500 and then managed to settle at 12.5 from 4500 on, how much adjusting of my AFPR would it take to lean it out by half a point to about the 13.3-13.0 range?

If it's set at 45 psi now, should I drop it to like 41 psi in order to gain that half point leaner?

Has anyone co-related the two, psi and a/f ratio?

mseven

I always though at 12.8 more torque and 13+ more HP.

vader86

kopbet89c4

Last time I dynoed my car, A/F was 12.3-12.8 from 2500-6000 rpms.

tjwong

Thats true, but the gains of Hp by leaning it out to 13+ is negliable. I prefer a safer and more conservative 12.5 for most NA cars, 11.5 for FI cars.

black_89_vette

Ok, I guess I had it reversed, lol.

So, I should leave it the way it is instead of tinkering with my afpr TJ?

tjwong

From the looks of what your AFR is in your first post, I would say its good. It could be made a bit better but only to even it out to 12.5 across the board, but as I mentioned it looks good.

Dominic Sorresso

black,

Messing with your VAFPR without tuning will cause the part throttle to be either too rich or too lean.

Aardwolf

The computer corrects for this.

69427

Can someone explain the physics behind this for me? Horsepower is a function of torque, and both are just functions of cylinder pressure. If you maximize (increase) cylinder pressure by employing the correct A/F ratio you increase torque, which automatically increases horsepower. What am I missing here?

Aaron's 87

I dont' know if anyone can explain it because it is just one of those internet perpetuated myths. Different cars like different AFRs at different RPMs. The only way to know which is best for your car is to experiment. Don't forget timing plays a factor in all of this too.

bjankuski

The Max torque (RPM) and Max hp (RPM) occur at two different RPM points. If you want maximium torque you will run a slighlty richer AFR at the torque peak then you would at the HP peak. The torque peak may be at 4000 RPM and the max torque AFR may be 12.5 to one. The same engine may make max HP at 5500 RPM and the AFR may be 13 to one to create max HP. So you would have the low RPM AFR richer then the high RPM AFR to create max torque and max HP.

sami85L98

Timing plays a major role, learned the hard way, i could not believe i drove my car for months with timing wayy advanced with pinging everyday, finally got that sucker. Finally got a good spot.

I do have AFPR, tried adjusting low/increase but did not feel much diff, probably i have to watch very closely.

Set Fuel pressure at 36 psi with vaccum hooked and 40 with unhooked.

According to LPE when i called for my combo (383/219 cam/dart 200 heads/SR ....) they say A/F should be at 14.7 at idle and 12.2 at WOT condition.

sami

Alvin

Anywhere from 12.5-13.5 will be say typically a 7 hp span with somewher in the middle max and either end lows.

Each car is different. I prefer to leave them on the rich side just to keep them running best at all loads.

kopbet89c4

Even though my tune is on the rich side, it still runs like a champ. That's good you had that insight in mind. Plus you got it right on target. 12.3-12.9 AF ratio on WOT is right on target. Just have to remind you that this tune is actually more aggressive than the tune you burned me for my old SR intake. And I like it!

Alvin

Awesome.. Glad it worked out. I really like to encourage people to put thier cars on the dyno that way we can double check them and see if theres anything left on the table.

loiq

google: stoichiometric fuel ratio

69427

You haven't answered my question. I don't wish to appear rude, but at the risk of being blunt, you just repeated the same claim regarding A/F ratios as a previous poster.
First point: Increasing torque always increases horsepower (HP= torque * RPM/5252). Increasing torque at any RPM will increase the horsepower at that RPM. (Note that I do understand the concept of peak torque and HP readings at different RPMs. That is irrelevant to the original question.)
Second point: I asked for the physics involved with the original claim. As I said before, more cylinder pressure will increase torque, which will increase the HP at that RPM. What is the physics involved that would explain why at some RPMs we want one A/F ratio to maximize cylinder pressure, and a different A/F ratio at another RPM.
I understand that with a carb you are stuck with whatever the fuel curve compromise ends up being over the RPM range, but with an EFI system you can get whatever A/F curve you want. If somebody actually programs the curve to give these ratios, again, what is the physics that would explain this? I'm not disputing that this claim may actually be true, I'm merely asking for an explanation for it, rather than relying on rumor or hearsay.

Aaron's 87

As far as stoich - and please correct me if I am wrong - stoich is complete combustion and occurs at roughly 14.7 to 1 in gas motors. It is all about emissions and has little to do with the original topic.

As far as different AFRs for different RPMs - different RPMs (and loads) will command different airflows. Your heads, intake, and exhaust will all conspire to determine how the air flows into the combustion chamber at different RPMs and load. So, the fuel injected into that air will be utilized differently in the combustion chamber depending on the above factors. That's my guess anyway. Sounded good in my head.

69427

Aaron,

We may be on a similar track here. I was thinking that at different RPMs there will be different amounts of swirl in the chambers due to the differences in piston speed. This will affect the mixing of the fuel and air in the cylinder. With 100% mixing (resulting in a homogeneous mixture) we should (theoretically) be able to run a 14.7 A/F ratio, due to every fuel molecule having a nearby oxygen molecule to combine with. Poorer mixing would require extra fuel to ensure that every oxygen molecule gets used. (Complicating this "theory" is the limitation of the pistons to deal with the continued heat of combustion. Richening of the mixture to lower the cylinder temperature might be required for a safety measure.)
Just my thoughts, subject to correction if needed.