Pump Gas Friendly Compression

[Dan White]

What is the maximum safe compression ratio I can run with iron heads for my 327, running on 93 octane? I am planing on going with 9.78:1 (67 cc chambers, 4.100" gasket bore , 0.019" thick), but have read that 9.50:1 is the maximum. The rest of the motor is stock, except for a mild crane rv cam, edelbrock intake, and carb. Thanks in advance, Dan

[Ed Jennings]

Re: Pump Gas Friendly Compression

I'm running 10+:1 (domed pistons and "fuelie" heads) on my 327 with no problems using straight pump gas.

[Duke W.]

Re: Pump Gas Friendly Compression

I depends a lot on the cam. The more overlap, the more CR you can run. There are also many other variables like the igntion timing map, your altitude and ambient temperature. There are no magic forumulas and no magic limits.

Duke

[Jim T.]

Re: Pump Gas Friendly Compression

My 68 L79 327/350 CR is 11:1 and it uses pump 93 octane.

[Tom Merkel]

Re: Pump Gas Friendly Compression

I use pump gas on my stock '67 L-79 with 11:1 CR. Either 93 octane or 94 (when available.) Runs fine; no knocks.

Tom

[Chas Kingston]

[Jack H.]

EXCELLENT!

Duke has nailed it!!!! It's not a matter of octane. It's a complex issue related to state of engine tune, ambient conditions, driver 'aggression', Etc., Etc., Etc.,

[Dale Pearman]

Ditto

It's the DYNAMIC compression ratio rather than static that determines octane requirements. Volumetric efficiency at a given rpm and load will be all over the place. If the cylinder fills to only 50% capacity, the effective compression ratio will be very low.

The primary determinant is the cam however timing will also influence cylinder pressure.

Best bet is to experiment and se what you need. (No knock or ping at WOT).

Dale.

[Mark Ring]

Re: Ditto

Here is a downloadable dynamic compression calculator. Just a little magic formula for those that believe in such things I was in the process of working on one myself when I found this one. Problem solved!

http://cochise.uia.net/pkelley2/DynamicCR.html

-Mark.

http://cochise.uia.net/pkelley2/DynamicCR.html

[Dale Pearman]

I'll go along with about 80 % Of What's Said

The rest I just don't believe is accurate.

When I want to get my head slammed back against the top compartment when leaving a stop light, I don't go to my computer. I go to the good-ole-boys that are building engines that work!

Just a fundamental understanding of the dynamics of conventional internal combustion engines is enough to get the job done. It IS supprising however that not many people have a clue about volumetic efficiency and dynamic compression ratio.

Dale.