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Old 04-11-2010, 02:44 PM   #1
68Cadillac
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Member#: 95901
Join Date: Sep 2005
Chapter/Region: SWIC
Location: Las Vegas, USA
Default AFR theory: Maximizing Torque (and thereby Power)

I read Jeff Hartman's book
<i>How to Tune and Modify Engine Management Systems</i>How to Tune and Modify Engine Management Systems
. He's a chapter on Air to Fuel Ratio (AFR) theory that speaks directly to boosted engine that are knock-limited.

Lets have a discussion as to what is the best AFR for our engines when we want to make the most power without blowing up. Lets limit ourselves to one fuel: gasoline. 91/93 octane. Since that's what the vast majority of us use.

First some Ratio's:
14.68:1 (~14.7:1) Stoichiometric mix
12.8:1 - Lean best torque
12.2:1 - Mean best torque
11.76:1 - Rich best torque
11.01:1 - Fastest flame speed in cylinder

All five ratios are directly from Hartman's book.

He argues that for boosted engine that are knock-limited, "at a very minimum you are aiming for mean best torque" or 12.2 AFR, "if not rich best torque" at 11.76 AFR. 11.76 AFR has a side benefit of higher cylinder flame speed.

Torque starts dropping off the moment you start to go richer than Mean Best Torque and begins to drop off dramatically after Rich Best Torque. Counter-intuitively the Fastest Flame Speed at 11.01:1 AFR does not produce the most torque, but there is a cliff-like drop off in torque when you start to go richer than that point.

To sum up, Hartman states that 11.76 AFR is the richest one should shoot for. You may have to tune back the boost to get there but you'll get the same torque with better fuel usage than if you turned the boost up and went crazy rich (e.g. 10.8:1).
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Last edited by 68Cadillac; 04-11-2010 at 06:45 PM. Reason: Power <> Torque
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