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Old 08-31-2009, 10:17 PM   #9 (permalink)
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05 DMax - '05 Chevrolet 2500HD
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Originally Posted by MadisonMPG View Post
Jason, how so? (the fact that comp stays the same)
If you compress a cyl 80mm X 80mm to 10mm X 80mm or compress a 82mm X 80 mm to 82 X 80mm the compression ratio stays relatively the same. The actual (piR)2 math is slightly in favor of the smaller Dia but once you get into the slight dome shape of most heads it evens out.
With a squish band type head (typ for 2 strokes and some diesels) the cyl volume is compressed into a small dome 1/2 the size of the cyl Dia. Boring the cyl doesn't change this volume, just increases the starting volume that gets compressed into the pocket. In that event the final pressure really goes up.
Now you're thinking "But Jason, every engine I've ever had bored came back with more compression!"
That is due to the fact that the head was at least cut a bit to clear it up, usually some more to raise the compression ratio for a few more horses.

Either way, the higher compression can cause detonation issues and needs to be watched.
Dumping the EGR will help, as will a bit of ATF misted down the intake at half throttle. You may want to disconnect the cat before doing that as it throws ALOT of black carbon out the exhaust.
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