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Discussion Starter · #1 · (Edited)
i have a pontiac 400 motor that i'm trying to get running again. had a cracked exhaust manifold, led to broken exhaust bolts, needs an EDM machine & hundreds of $$. alternatively, i can put on a set of heads that i have that came off of another motor (455). the 455 heads have huge chambers and would reduce compression to about 7:1, maybe even just under 7. how bad would this be? would i need a hotter plug? change timing (which way?).

thanks.

*** edited to add: i ASSume less advance, but figgured i'd ask ***
 

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This is something I have also wondered. I am pretty sure some of the first cars had around 5:1 compression. I realize that from a performance and efficiency standpoint, this would not be the greatest, but are there any other draw backs? It would be good to know for an N/A motor, but I have also wondered for a forced induction motor. The less compression you run, the more boost you can run and stay on pump gas. What is the practical lower limit? What if you ran 6:1 compression and 40 psi boost?

If you look at this chart (page 24), you can see the pattern
http://www.procharger.com/pdf/Chevy.Prod.bb.pdf

Adam
 

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my 1951 235 Chevy I6 has 6.7:1 compression and it has no trouble starting or running, and it runs on 6v. considering that, I'd say your fine with 7:1, just don't expect much performance. some people do build blower motors to 6:1 so they can boost up to 18 PSI or more, on pump gas.
 

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NA with ultra low compression it's best to reduce camshaft overlap to trap more of the charge to increase cylinder pressure, the results is a very low RPM engine.

As for ignition timing normally larger chambers need more advance. You will need a hot plug with this combination.
 

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Those older engines were also optimized for that lower compression... timing curves, fuel curves, everything. A more modern engine will need some customization to run right at that low compression... possibly a custom cam in the 190-195* intake duration, a self-made ignition curve (or one that's locked in at full advance). You could run normal stock parts on it, but once you drop below 7:1 its a whole new ballgame.

Expect sub-150 hp numbers :(
 

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Discussion Starter · #8 ·
thanks for the simulation Curtis.

i'm gonna fish around for a set of used heads that would work better. i know there are piles of 70s era stuff that are in the boat anchor pile. the trick is just to find the pile, then find something useable.

even with a 2bbl top end on this thing it had some bark. it just wouldn't be fair to entirely neuter it with compression nums like that. unless i put a $turbo$ on.
 
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