assuming compression ratio stays the same, does having the piston furthur in the hole make a big difference in power? blown 383: two examples, what would you do:

1:
gasket 4.200 & .051"
deck height diff .030"
piston dish -15cc
compression ratio: 9.0

2:
gasket 4.166 & .039
deck height diff .005"
piston dish -24cc
compression ratio 9.0

example 1 would give me more piston to valve clearance and is cheaper cause of non custom pistons. race shop says example 2 is more power, and might need notching in the pistons, which is more $$$.. worth it? your thoughts..

thanks,

Mike


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Since the compression ratio is the same in both examples the engines should produce the same amount of power.

The block should be decked anyway to make sure it's square. You don't want to blow a head gasket because you cut a corner. Setting it to zero deck means installing the crank, a con rod and piston to measure how much needs to be milled off. A simple decking pass just shaves off enough to make it flat.

Piston to valve clearance has nothing to do with cam lift since the valves are open when the piston is down from TDC. LSA, valve timing, overlap, and intake centerline will all determine how much valve to piston clearance there is. Having that piston down in the cylinder when at TDC will help the clearance if the cam choice isn't perfect.

Advancing a cam decreases intake clearance. Retarding the cam increases the intake clearance. Most Comp cams are already ground with 4 degrees of advance.

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The race shop is right - example two should make more power AND be more resistant to detonation...

The reason being QUENCH.
Add up the deck height, and the gasket thickness - that's your piston-head clearance at TDC, and the quench height.
1 has a huge .081 clearance
2 has .044 clearance

What happens is when the piston comes up to TDC, the flat part of the piston (that's why they make dished pistons with a D shape) comes speeding right up and almost smacks right into the underside of the cylinder head. This near-collision causes a shockwave across the open part of the combustion chamber, which adds alot of turbulence to combustion.
This turbulence makes for a more even burn, and helps to minimize hot spots which might cause detonation.
Most engine builders will tell you to aim for .04, with .05 being abouth the limit before you loose the quench effect. Closer than .04, and you'd better be sure everything is put together right, or else you'll start killing parts.
What I've heard, is even if you have identical pistons, head volume, etc and only vary the quench distance, that the lower quench will make more power, and be more resistant to detonation (even though it has a slightly higher CR)

As for notching the pistons - I thought most piston manufacturers made then with pretty good-sized valve reliefs. Ask the maker - they'll be able to tell you approximately what is the biggest cam you can run without having to notch anything. Of course, you should still check everything yourself to be absolutely sure.