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 ***

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

from 1975ish to 1978ish, the big pontiacs ran stock at 7.6:1, so i'm not that far away, but man that seems low.

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.

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.

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

Here's one I simulated with a 191/196 cam and 6.9:1 compression. HP peaks at 3000 rpms

Attached Thumbnails

 

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.