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why do engines pull less vacuum on hot start?

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Old 11-08-08, 12:26 PM
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rotorhead

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why do engines pull less vacuum on hot start?

I've noticed this on both my friend's FD I've been tuning a lot recently (both have Power FC) and my own FC, both of which had engines with good compression. On a hot start I notice both cars pull about 1 to 1.5" less vacuum according to the mechanical boost gauge (14" versus about 15-15.5 on motors with significant porting). This makes them fall into a different cell in the fuel map. Once the car gets going for a few minutes the vacuum reading returns to normal.

Has anybody else noticed this? I am guessing that engines make less compression when they are heatsoaked, which explains why weak motors are prone to flooding.
Old 11-08-08, 01:34 PM
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Apex seals and rings on pistons seal a lot better when cold. That is why the manual always state " warm the car up and then perform the compression test "




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