Afr

[hauss]

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Originally Posted by chaider

Jim (1795 Jim),

Soliciting opinions about where someone else set their particular combination of parts AFR-wise to operate reliably is a waste of time. Most of those people never measured the actual AFR their car was using other than a tailpipe sniffer which is notoriously inaccurate.

Buy some dyno time with a competent tuner and let him tune your car. It is imoney well spent and parts well protected. The idea that AFR's change based on equipment used could not be farther from the truth. AFR's are exclusively based on the stoichiometric chemistry of the fuel being used and only the fuel being used.

The dyno equipped tuner will be able to optimize both your engine's ignition requirements and the engine's AFR to obtain a metric they call MBT which stands for Maximum Brake Torque timing. It is a three dimensional metric where the dimensions are, ignition advance, AFR and maximum brake torque.

Once your tuner finds MBT for your engine you will not be able to improve upon power without a power adder or a different engine build. Here is a chart from some emissions work that fuel system calibration engineers used in the early 90's when they were attempting to reduce the oxides of Nitrogen to meet the every changing EPA requirements.



Note that peak HP occurred between 12 and 12.5:1 AFR for n/a gasoline engines. This coincides with a lambda of 0.82 and 0.85 when using pure gasoline w/o any ethanol content.

When you add ethanol, the stoichiometric point for the fuel moves from 14.7 to 14.1. This change in Stoich point will result in a change in target AFR for max power. Additionally because of the introduction of ethanol into the fuel the target lambda will be enriched by a few percentage points to 0.81/0.83. That moves your target AFR to 11.4 to 11.7.

If you are serious about attempting to optimize your engine's fuel delivery don't rely on urban legend and so-in-so did whatever he did but there was full moon so your mileage may vary.

Fuel system calibration is not black magic, it is simple and basic but, and it is a big but, you need a competent tuner, with a dyno and with all those little air bleed and fuel metering gizmos used in what ever carburetor you are using. Anything less and you are basically at the far end of the room, someone has blindfolded you, spun you around three times, put a dart in your hand and told you to throw it at the wall. Wherever it lands is your AFR!

We all know better but easily get suckered into the urban legend solutions. Don't do that! Go to a competent tuner who has the tools and the metering components for your carb to do the job correctly. BTW one of those absolutely essential tools is the dyno.


Ed

First let me just say, I agree with most of what you just said ,however their are guys with small budgets and no dynos other than their butt dyno with real world experience. lets just say more than one way to skin a cat . some of us have to do it our self's not because we want to but because of budgets. Again let me say to everyone I agree with most of what
eschiader has said

[Morris]

Jim
Back in the 80’s.....the way we found beneficial to tune the engine was a EGT gauge monitoring the collectors after the tune on the Dyno....so we just set up the dual EGT gauge on the left side of the steering wheel and monitored the egt’s down the straightaway.

[Morris]

Jim
Back in the 80’s.....the way we found beneficial to tune the engine was a EGT gauge monitoring the collectors after the tune on the Dyno....so we just set up the dual EGT gauge on the left side of the steering wheel and monitored the egt’s down the straightaway.

[1795]

Thanks guys. A lot of good information. My concern is not getting the most optimum HP, but rather keeping the engine alive. At the last race it was running hot. I increased the main jets by one size with the Webers and that cooled the engine down a few degrees. I have also pulled a couple degrees of timing out too assist with the cooling. What I have noticed is that with the Dart block, even with the oil cooler, that the oil temps are higher than with the stock block. This is a cumulative effect, in that the first race of the day the oil temps run pretty much normal. However, subsequent sessions the old temps start higher than typical and climb higher than typical. From what I have learned this is a direct result of having about 80 lbs more iron to retain heat; they just do not cool down as quickly as a lighter block. You probably would not notice this on a street engine, as there is very little time at WOT. On a race engine I am at WOT for 20-60 minutes depending on the race and that creates a lot of heat in a hurry.

For those who asked about what fuel I am using, it is 110-112 Octane race fuel.

The extra fuel (12:1-12:5 AFR) tends to cool the engine down a little. Which is important when you are running longer races. As we all know, there are no perfect answers; only give and takes. You do a cost benefit analysis and decide on a solution that has more upsides than downsides. Unfortunately in western NY there are only three dynos and one of them is broken. You guys in more urban areas probably have access to more and newer equipment than I do here. In addition, the guys running the dynos primarily work with people going to the drag strip or are tuning a high performance street build. Therefore it does not readily come to them to conceptualize the needs of a road race car running for extended periods of time at WOT as opposed to a 5-10 second run at WOT. I do not blame them, you cannot specialize inn everything. Anybody that claims to be an expert in multiple aspects of a field is only fooling themselves and you.

Heading to the Glen next week and will let you know how it goes.

Thanks. I really appreciate all of the input and enjoy a good theoretical discussion.

Jim