11.5 to 1 on pump gas

The answer to that question is in part based on the available octane rating of the some what questionable term "Pump Gas"
Here where I live, Premium "Pump Gas" is 91 octane, and with 91 octane 11:1 compression is wishful thinking.

You can run 11:1 on pump gas. Will you enjoy it? No. It would take much more work and money to get it to run good then what you will get out of it. A good friend of mine put together a 440 6 pack engine with 11:1 in the early 80's and after a while he had to confess that it was a mistake and that he should have gone with 10:1 instead for high test gas at that time.

Now if you plan on E85 gas as pump gas then you should be able to easily run even higher, however you could not put anything else in instead, like 91 octane.

One of the better things that I did on my 69 AMX is to use a 360 engine from a Wagoneer which will run on 87 and give NO problems at all. I also loved my MSD rev limiter and how that worked with a manual transmission and being driven hard. I loved having a 3000 chip for when someone else might drive the car.

Static compression isn't the issue, dynamic compression is. Chamber and piston design play a huge factor, as does how you set up the quench.   Tight quench and a good quench area on the head and piston are more detonation resistant due to mixture motion reducing dead spots in the chamber that are detonation prone.

Dynamic compression is basically computed from the volume remaining in the cylinder when the intake valve hits the seat on the compression stroke.   The wilder the cam, the less volume there is in the cylinder since the intake valve closes later. This becomes very important when selecting a cam. If you don't have enough static compression, the dynamic won't be high enough to work with the camshaft.   You end up not creating peak cylinder pressures where the cam works best, and you end up with a pig of an engine. Too high a static compression with a small cam, can create too high a cylinder pressure for the fuel and you end up with detonation. It's all about peak cylinder pressure in the RPM range you want the engine to work best at.

You need to figure out what you want the engine to do first, before you decide on a static compression ratio since there are way too many factors to just say "use this compression ratio and you won't have problems".   

A 9:1 engine for the street can be made to run fantastic, and even in hot weather you will probably be able to get away with mid-grade.

You'd be surprised what 8.25:1 can do with the right cam and some minor head work.



Edited by Traveller - Apr/03/2015 at 10:36am