compression delima?
 

Hey iv got a Gen V 454 Mag. Its in the process of getting rebuilt. I didnt have any luck finding a set of .30 over pistons to keep me at the stock 8.7:1 compression. I called Probe Industries and talked with Eric who had a set with slightly more dome that would bump compression to 9.5:1. He thought this ratio would be fine running 91 octane with my iron heads and I could use a thicker headgasket to get it around 9.3:1. This sounded good to me so i bought the pistons. But now Ive been told that a thicker gasket will actually increase risk of detonation in my application for reasons i didnt really understand. So, my questions are, can i get away with 9.5:1 on 91 octane? and what about these thicker headgaskets?

If it were my motor I would have no problem running 91 octane with the compression your talking. If you really want to be safe you could run a degree or two less overall timing to be very conservative, but I dont think that is needed. A thicker Head gasket will not cause detonation. Whoever told you that is mistaken. Your fine.

9.3:1 on 91 is fine. Thicker head gaskets will not 'cause' detonation, but it won't help either. What they are referring to is the head gaskets effect on something called quench. Tighter quench (closer tolerance between the top of the piston and head) increases turbulence, and therefore increases burn rates. Faster, more complete burns show less chance for detonation. This is one of the techniques racers and now OEMs use to run high compression w/o detonation. More info here:
http://www.kb-silvolite.com/article....n=read&A_id=39
http://www.popularhotrodding.com/eng...tio/index.html

So jumping from a .40 head gaskt to a .60 would indeed lower my compression but would not reduce the chance of detonation? So I may as well run a stock gasket and stay at 9.5:1? it makes sense to me, but Im just making sure i understand that correctly.

So long as you run 91 octane, 9.5:1 should be OK. Check for signs of detonation, and back off a couple of degrees timing if you see them.

I'm lowering my compression from 10.3 to 9.85 by using a .05 head gasket. My builder and cam designer both suggested this is how I should do it. Both know their stuff. I'd lower it, 9.5 is asking for trouble with Iron heads... 9.85 is the max I was told to run with aluminum heads on 91 gas with a little room for error if I was to get some bad gas somewhere.

Quench should be around 0.036" for what you're wanting to do. Sometimes you need to deck the block to get it that tight.

While your heads are off, check the piston-to-deck dimension. Add that to your compressed gasket thickness and that is your quench.

As said, that is what generates turbulence which interrupts detonation somewhat.

And you may not have the compression you "think" you have. Get the specs on the pistons - whether they are domed (+xx cc's) or dished (-xx cc's) and then cc your combustion chambers. You need those measurements plus the compressed headgasket thickness to determine the true static compression.

Also, bigger cams make the motor think it has less compression. Everything's related.

MC

If you install a thicker head gasket it will lower the compression, working off your head CC you can figure out what each thickness will change it and lowering compression will help with detonation, I would try to stay at 9-1 max for reg marina fuel, also the cam will help/hurt detonation as well, playing with timing will be your final dial in...

quench and comp ratio
 

Sorry to offer my 2 cents so late ... but

My understanding is that quench distance does indeed have a direct effect on detonation resistance due to the increased turbulence induced at low quench. A thicker head gasket increases quench and even though it will lower comp ratio, and may actually reduce the overall detonation resistance.

Also, you really should talk about dynamic compression ratio (DCR) not static compression ratio. 9.5:1 SCR may or may not be OK depending on the particular cam chosen, and the resulting DCR.