In other words, should A/F ratio richen with boost or should it be held constant? Thoughts please.

constant

constant

Don't understand. Isn't the generally accepted A/F ratio for NA engines 12.8 to 1? And don't forced induction engines require a richer mixture? Assuming this is true, shouldn't there be a transition in A/F ratio?

Willie

Under boost I will tune for 11.0-11.5 as a starting point.

/N.

Do they require a richer mixture, or is it just done as insurance against detonation?

Do they require a richer mixture, or is it just done as insurance against detonation?

Very good question. Not only would I like my original question addressed but how about this one too? Also, all you blown guys, what do you tune your A/F ratio to and why?

Willie

Constant. Your fuel curve should be nice and smooth whether your using a power adder or not. I like mine between 11 and 12.0 for assurance. Force fed applications usually recieve a much richer condition because if you go lean at all your screwed. My dyno sheet continues to make power until my A/F hits about 12.5-13.0 and then my power falls on its face. Boosted cars definetly like the extra fuel.

I'm tuning for 12.2:1 here on the V6

I shoot for 11.5-11.8 a/f, alot of guys shoot for 11.0-11.5 to be safer and anything in the 11.0-12.0 range seems acceptable(at least on my motor)

HTH,
Steve

For best power the AFR should be what ever ratio make that best power. (about 12.5:1 AFR) Dumping in extra fuel beyond this is sometimes used to help supress detonation by cooling the combustion chamber. As far as burning goes, this extra fuel is wasted.
Water and or methanol/water injection serves the same purpose, but more effectively.

You'll make the most power by running the correct WOT AFR using very high octane gas that does not need extra cooling to stay out of detonation. But wether the combustion chamber will stay within acceptable heat limits over time is another matter. being a little too rich may not produce absolute maximum power output but will produce maximum safe power over time. Engines are expensive to rebuild.

What was said above, basically. I shoot for a 12.1-12.2 constant A:F ratio under boost. Most NA folks go 12.7-13.1, on personal preferences. People like to run rich as insurance to running lean, so many start running force fed WOT at mid 11's, and eventually lean out as they learn the engine combination won't let them down by leaning out and burning a piston. More important I believe is your intake temps, and timing. Start conservative, and add timing gradually. Changing timing will also change your fuel requirements. I'm not sure what ECM system you've got Willie, but log everything, pull over, and look over the laptop before the next test run. Keep a little note pad where you write the changes, and eventually you'll see a trend forming (i.e. add "4", and you see the ratio richen by .2 or something), and you'll get much faster at zeroing into the perfect tune. Also, test in different gears, as the acceleration rate will change fueling slightly.

I agree with the above.I`ve always heard 11.0 11.5 for safety,12.0-12.5 for max power.Have a friend who runs NMRA Drag Radial and he shoots for low 12s.Either way it`s constant not boost related.
Timing however is what seemed to make the most difference for me last weekend.I feel that with a tank of straight C16 and intake temps below 150* I should not have to retard any timing at all but I`ll slowly feed it in and check plugs to get there.

I think A/F ratio should definatly be a function of boost. The brake specific fuel consumption for an engine goes up with respect to boost.

I couldn't see it being constant at all. Boost would change the characteristics of too many things.

The better your tune, the more constant your A/F ratio will be. You are correct variables will change, to get maximum potential you gotta tune your setup to account for those changes and keep the A/F at your desired mark. Poor tuning can cause your A/F ratio to fluctuate which can be bad news under certain conditions. Like everyone said, its a compromise between safety and power output. You wanna get as much power as possible without leaning out the motor. This is why WB02 is so important when tuning the car.

I believe that the AFR (or amount of added fuel) should change with the level of boost. The reason is that under light boost there isn't a need for 10.5:1 AFR. At 2-3 psi not much additional heat is being generated in the engine. So a moderate AFR will give good power without engine damage.

As the boost level increases the AFR needs to decrease (or added coolant: gasoline, water, methanol needs to increase). This is done to cool the engine and keep it out of detonation.

When running EFI systems in open loop it is common to vary the commanded (and resultant) AFR according to manifold pressure. As the manifold pressure increases the AFR decreases.

Al.

What was said above, basically. I shoot for a 12.1-12.2 constant A:F ratio under boost. Most NA folks go 12.7-13.1....

Exactly my point. Remember this is a hypothetical discussion. So if your running a supercharger, it will take time to produce maximum boost, whatever that value is. At the instant you throttle it, you may not be producing any positive pressure at all. It may "begin" at atmospheric pressure, then as the blower starts producing, boost increases. So during this timeframe, shouldn't the A/F ratio be 12.8 when no boost is being made and increase as boost is created? I believe this would be hypothetically the perfect state.

Now if A/F ratio is held constant, would you be running pig rich for the instant the blower isn't yet producing boost? So why hold it constant?


I'm not sure what ECM system you've got Willie, but log everything, pull over, and look over the laptop before the next test run.....

749, 2-bar, $58. I totally agree with your approach! Thanks!


As the boost level increases the AFR needs to decrease (or added coolant: gasoline, water, methanol needs to increase). This is done to cool the engine and keep it out of detonation.

Exactly my point.

Willie

I think I'm beginning to understand your question now Willie. My previous post was written in regards to tuning the car for power on full boost, I wasnt even thinking about what the A/F Ratio should be when off boost or when building boost. You guys are correct, the A/F Ratio should change between idle and full boost. What I was trying to say is that if you divide up that range (say idle, building boost, full boost) and then decide you want specific A/F ratios for each portion of that range, optimal tuning will allow you to maintain the ratios you want for each portion of the range. I think Al summed it up pretty good "When running EFI systems in open loop it is common to vary the commanded (and resultant) AFR according to manifold pressure. As the manifold pressure increases the AFR decreases"

As for the best A/F Ratio, I dont think there is a universal answer here because everyone has their own idea of what the best A/F Ratio would be under different conditions. And another thing to consider is that alot of people dont have the resources to do their tuning on a dyno. That would be the only true way to see how much power you're making in regards to the compromise between safety and power. For most of us the bottom line is you get as much power as you can from seat of the pants feel and then use your own determination of whats safe (as far as detonation) to remain as safe as possible by reading scan tool output or sensor outputs.

If you really want to get down to perfect A/F Ratios I think you need to gets down to the cylinder level, where I'm sad to admit, most of us running batch fire EFI Systems (like the 749 ECM) are at a disadvantage. Depending on the intake design you may have certain cyliners running lean while others are running rich. This can be a major problem if you're making alot of power at high boost levels.

so decrease afr till u start washing the cylinders down?????

if u want to stay out of detonation look into these areas first intercooler, compression, octane.

I dont think thats what he was trying to imply. We were on the subject of tuning so changing the engine compression or adding an intercooler were not really what we were thinking about. Higher octane gas and some form of cooling injection would be right in line with what we were thinking. But yeah you are right, those things you listed are good areas to research before building a boosted combo.

Anyone here also use EGT sensor(s) to tune their car?

I do, and I don't even have a power adder.

At WOT, you're looking at making the most, safe, horsepower possible, at whatever AFR that happens to be. Different engines respond to different AFR's, well, differently, and folks also have different risk tolerances, to how close to the edge they'll go, for that last few hp.

For part throttle, consider manifold pressure more a function of throttle %, rather than boost pressure. An engine built for 20 psi can run 10 psi with a much different air fuel ratio than someone with a stock iron headed 9.2:1 L98 that's pushing 10 psi on the edge of reliability (at WOT), so I'd consider low boost on a big boost engine to be part throttle, and not needing a super rich 12:1 air fuel ratio. If it's strictrly a race car, they only live at WOT, so who cares, once you get crisp throttle response. If it's a street car, and you're hoping to maximize fuel mileage in this age of $2.75/gallon gas (I'm sure you CA guys have it worse, but this isn't the thread for that), run it leaner for cruise mileage. Yes, HP will be down at that air fuel ratio, but BSFC will be more efficient. Since you're not at WOT, it shouldn't make a difference if you're down a few HP.

Aviation folks have been using EGT's for decades, to tune for lean cruise. Keep leaning out, and your EGT's will keep climbing, up to a certain point, and then they'll start going down again. I've found a lot of theory about EGT, exhaust, and ignitions from kit plane parts manufacturers that cross references well with race car engines, but don't have the links handy. Personally, I think with a wideband air fuel meter on the collector, using 8 individual egt's is a great way to make sure all cylinders are working equally as well. With sequential injection, or even with batch fire, you could trim individual injectors (like using the Accel VIC) so all run equally, temperature (and therefore air/fuel ratio) wise.

EGT’s are only useful WRT to tuning once you figure out what the EGT means for the specific engine and what the parts that you have will tolerate.

WOT AFR that you make the most power at is mostly up to the charge motion in the chamber and how well suspended the fuel charge stays. In most chambers you’ll have a spot or 2 where some of the fuel will come out of suspension effecting the burn efficiency, and as that happens your AFR has to go richer to make max power without detonating (the liquid fuel settled out on the chamber surface is worthless WRT to initial combustion, so you have to raise the amount of fuel in the mixture to compensate).

Traditional chamber/piston designs make best power at somewhere around 12.5:1, and best torque somewhere in the mid to high 11.x:1 range. With newer, more efficient designs you’ll make better power at higher AFR’s, but your overall safety factor before you blow something up drops (well, until you get in to stratified charges where you might end up using a lean rather then rich mixture to control detonation). On my brother’s supercharged car (TFS heads/intake on a 302 ford) it kept gaining power/going faster as we leaned it out till it got well into the 13x:1 range. We didn’t find out _exactly_ where it made best power since we chickened out before finding where it started loosing power (well, it did hit a range that it didn’t seem to make much of a difference what you did to it).

WRT to how boost effects that, it really depends on how boost effects the flow pattern through the head/chamber, it can actually go either way, it’s not really dependant on the boost but again, what the A/F mixture is actually doing.

Since HP is a function of torque and RPM, why would tuning for best torque make less HP? If you max out the torque at all spots on the RPM curve, you'd make best HP and best torque at all times possible. Old racers tale I've heard for NA is 12.5:1 at torque peak, and lean out as rpms go up to 13.1 at the horsepower peak to maximize the curve. As RPMs climb, the mixture in the chamber gets more turbulent, so less timing is needed, and I suppose less fuel as well, since the burn is more complete.

(dozens of reasons, this whole deal is greatly oversimplified, but I’m not going to write a freaking book here…) In most cases, more turbulent = less chance for the fuel to settle out and not as much need for as low an A/F ratio to compensate. Secondly cylinder pressure is highest at the torque peak, as cylinder pressure drops detonation sensitivity drops off so you do not need as much fuel to prevent detonation (also the reason why you could go nuts with boost on even a high compression engine as long as you keep that boost above the torque peak)…

BTW, yes, it is a function of torque and RPM… SO? You’re just proving that you can do more work at a less efficient state with more rpm in most cases… doesn’t really pertain to the discussion (not trying to be a *****, just making a point…)

I may be slightly off topic here, but this discussion reminded me of a previous discussion I tuned in to over on TurboMustangs.com. How exactly does AFR effect turbo spool rates?

I used to be under the impression that you would want extra fuel (lower numerical AFR), which would burn in the exhaust to help spool the turbo up. I think most of the guys on turbomustangs were thinking along the same lines.

However, one import tuner chimed in and said a rich mixture will lower EGT and hurt turbo response. He claimed he ran a leaner mixture at low manifold pressure to get the tubo speed up quicker. He also said that any fuel left over from combustion will not burn becuase there would be no oxygen left over, which makes sense.

I'm pretty happy with the response of my turbo systems, but I would like to have more knowlege in this area (and tuning in general).

Under normal conditions you don’t change the tune, the best tune will spool things the fastest, extra fuel will slow response just like it will on an NA engine. There are assorted schemes used to dump a/f mixture into the exhaust on shifts…, but that requires an actual burnable mixture getting through the engine and igniting in the exhaust before the turbo. If you’re having problems off the line, run a 250hp shot of N2O in addition to your turbo and you’ll leave fine…

Ok, that sounds good.

Most of the people worried about so called 'turbo lag' haven't driven a well set-up turbo car, and I wasn't tryng to sound like I was one of those people. I know Rick from Innovate has to detune his boost curve to get a good launch, so that says volumes about how a well tuned turbo system will respond.

I'm 11-11.5:1 at WOT,
and in the 13-14:1 range at idle.

My car is fatter at idle than stock,
stock is 14.7:1.