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Old 02-09-2013, 12:22 PM
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Originally Posted by S10xGN View Post
Hook up a battery and run a compression test. Cranking compression over 215 or so will be hard to get by on pump gas. You can either swap heads or run a cam with more duration or a tighter LSA. Increasing overlap will lower the dynamic compression.

All the points of this post are false.

1. the cranking compression pressure will not predict the octane requirement of a engine.

2. the only way to lower the cranking compression ( and the calc DCR) is to
make the intake valve close latter. Overlap does not effect this.

This will not make a 13:1cr engne run on pump gas.

As the cranking compression, dynamci compression and the real engine cylinder pressure that is created in the runnng engine at WOT @ rpm are not related.

You could install a bigger better cam that traps more air at RPM and the
cylinder pressure will end up higher, the engine power ends up higher and the
octane requirement actually increases.

If you want to stop the 13:1cr engine from knocking on pump gas reduce the spark advance. It is just that easy.

Or don't open the throttle.

if 36deg BTDC is normal just set the timing at 28 to 30deg.
Yes you will give up some power. There is no free lunch.
and changing the cam will not fix it. but reducing the spark timing @ WOT will allow it to run on pump gas
in a limited fashion. If you just got to do that.

If the 13:1cr engine has a big piston dome you will not lower the compression ratio enough for pump gas by changing the cylinder heads.
You best move would be to change the pistons.
You can do it right the first time or do it right after it blows up. your choice.
You want 10:1 cr not more than a measured 10.5:1 cr on pump gas.
On a 355 that means a 58-64cc head and a flat top piston.
You were dumb enough to buy this 13:1 cr motor. Now,,,Are you smart enough to fix it right? Before or after you damage it on pump gas? Or smart enough to just resolve to run it on high octane gas or Methanol.

If this 13:1cr engine has a flat top piston then you can lowe the compression enough for pump gas by
swapping cylinder heads.
One of those mini inspection cameras will allow viewing the piston top thru the spark plug hole.

Last edited by F-BIRD'88; 02-09-2013 at 12:37 PM.
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