Quote:
Originally Posted by ksa8907
Valid question, I don't have answers but maybe things to consider.
Static compression vs dynamic compression
operating temperature
elevation(which is going to directly affect that dynamic compression ratio)
carburetor vs fuel injection
Ambient temperature
Relative humidity
And others I'm sure.
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That is a lot to consider. Static compression should be fairly high with a stock VW engine due to the stock low RPM cam.
Operating temperatures had head temps around 350°F (verified with head temp sensor).
Elevation "affects" compression ratio. But you're also starting out with air that's warmer for the same amount of compression. So the air heats up similarly from the same amount of compression. And it's the heat, not the compression, that causes the air-fuel charge to detonate. (If you were to suddenly stretch out air at sea level to the same pressure as air at high altitude it would be considerably cooler.) A study in the 1980's revealed that when spark and AFR's are compensated for the change of altitude (which I did in the Bug), every 1,000 ft of altitude only allows for a decrease of around 0.2 R+M/2 octane. So technically at my altitude I should be using 85.5 R+M/2.
https://www.sae.org/publications/tec...ontent/872160/
Fuel injection should help improve knock resistance due to better atomization.
Ambient temps aren't that high around here, so that could be part of it.
But the extremely dry air here should also increase knock probability IIRC.