Thread: AFRs????
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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
Posts: 5,560
Hello!

Few things worth considering:

- Twin plug engines need less ignition advance to reach optimal BMEP. They "lighten the candle on both sides" which combusts mixture quicker.
- Boost is not always equal power. Point is to extract as much power as possible, not to run as much boost as possible. More boost equals more stress, more backpressure etc.
- Aircooled engines have finicky heads that aren't cooled well and need to run richer than watercooled pentroof-ones.
- Even if it doesn't knock, more boost will make heads "walk" more, eventually leading to sealing problems.


To conclude: My opinion is that aircooled 930 engines with stock internals that are expected to hold for more than track work should run not more than 1.0 bar with intercooling. AFR's should be 14.0 for steady load with no boost to conserve fuel (= driving at 55mph) and drop to between 12.8-12.4 when full boost is reached (depending on fuel quality and ambient heat). When tose AFR's are reached ignition advance should be tweaked to reach optimum torque untill you reach knock limit and backed down few degrees for error margins. If you don't feel confident with "chasing knock limit" you can dial ignition to preset value know to work well with your engine combo.
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Old 11-22-2007, 01:02 AM
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