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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
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Originally Posted by 911st View Post
Nice chart but it doesn't tell the whole story. It should be used as approximation / hint.

First of all, it doesn't seem care about VE (volumetric efficiency). Otherwise, the smooth concave lines would be full of "hills". Second, not even effective CE adjusted for VE tells the whole story. Knock sensitivity is further influenced by combustion chamber shape, squish, ignition timing, intake air temperature, head temperature and position of spark plugs.

Thus, this fancy graph doesn't really say much.

You can boost 0.7 bar at middle revs and get it to knock despite rich AFR's and at the same time boost 1.1 bar at high revs w/o knock. As cams run out of breath, you simply cannot push that air into cylinders despite positive pressure before the valves.

That's why it's important to compare apples to apples.

A 4-valve pentroof watercooled/intercooled engine with knock sensors and small piston diameter will tolerate much higher effective C/R than badly intercooled, big-bore 2-valve engine with offset ignition plug, hot heads, no knock sensor and CIS.

What it all boils down to is:

Cylinder combustion pressure vs. octane vs. combustion chamber geometry.
Thank you for your time,
Old 04-08-2009, 03:09 AM
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