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.

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 "




LAter