Afr

[eschaider]

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 maybe a tailpipe sniffer which is notoriously inaccurate.

Buy some dyno time with a competent tuner and let him tune your car. It is money 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 to create E-10, 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 for max power to somewhere between 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. Although it should go without saying, the tuner not only needs to have all the toys, he needs to know how to use them. 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, that is your AFR!

We all know better but sometimes we can easily get sucked into the urban legend solutions. Don't do that to yourself — or your engine! Go to a competent tuner who has the tools, the metering components for your carb and most importantly the knowledge to do the job correctly. BTW one of those absolutely essential tools is the dyno.


Ed