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Discussion Starter · #1 ·
Hey guys
anyway to give a rough estimate of my comp ratio?
i just rebuilt a 1.6L w/ .50MM oversize pistons.
the block was decked .012 (warped out of spec) and the 1.6L head was shaved .010.
i have a standard Fel-pro head gasket.
the oversize pistons should raise the comp ratio, correct? pushing more volume into the same combustion chamber?
any ideas what this may have done to the stock comp ratio (8.5 stock?)

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4,798 Posts
Re: (dirtoval.com)

Oversize pistons give you more displacement, but not higher / lower compression. (Unless they have a different wrist pin mounting position which affects where they sit @ TDC relative to the top deck of the block...)
Assuming that you have not changed the wrist pin location, the only CR increase you will see is from the .012 deck and .010 shave. Give some numbers and I'll see what I can work out for you.
Distance from top of piston at TDC to top of deck:
Head cc's:
Piston dish? => diameter (top and bottom if tapered), depth of dish:

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883 Posts
If all else fails you can bore that 1.6L block up to 82mm.

I want to say that stock CR is 8.2:1, decking the head and block as little as you have will only raise the CR by points (if that). So your current CR should be around 8.4-8.5:1. One problem with decking these engines too much is that you'll really have to keep an eye on just how little of valve to piston clearance there is. My 1.6L block and deck have both been severely decked. Intake valves protrude roughly 1/16" past the deck of the head and the piston to deck clearance is 1/32". The pistons are dished and have valve reliefs cut into them. It is a complete interference motor.
With 7-16 year old piston rings the motor still saw 165-170 psi (10.8:1 CR). After replacing piston rings and rotating assembly bearings last weekend the motor feels much stronger, I'm expecting to see at the least 180+ psi.
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