How much compression can you run on 93 octane? At what point do you need a higher octane mixture? TIA, Shawn

 

Hi,
What I have been told is that the most comp you can run on 91 is 9.3. but that is with a carb so I dont know about with Fuel Injection. Hope this helps.

 

Well right now I'm running about 11:1 compression and using 93 no problems....I have a friend that is going to be running about 11:5 is what he figured it out to....But all in all I do believe that it comes down to how well you get your computer tuned in....so the better it's tuned and the less detonation you have...the better off you are...

 

11.2-1 here

 

11.1:1 in AZ on 91 (get some slight knock durring the summer at WOT)

 

Depends a lot on where you live, not temp wise, but altitude also has an effect and the material of the block and heads. You can run more on aluminum block and heads simply because they deal with heat better than iorn. This is something that I would discuss with the person thats gona be building your motor

 

So far 11:25 to 1 for me with my 427 ironblock and 93 amoco ultimate without problems.

 

most ive ever heard of is fireball running 11.8:1 but he doesnt race on plain 93 he splashes some race gas in for safety but he drives around on 93.

 

Its not just the compression ratio that needs ot be taken into account. I know of street motors driven for miles at 12.4-12.7:1

 

i have a mag with a guy that has 14:5.1 with a 406cid and he ran 93 octane just cruising though, not jumping on the gas..which is still hard to beleive. 11-11:5.1 is about the highest i would go, and thats with a good tuning.

 

The LT1 guys get away with 12.5:1 compression but they are reversed cooled.

 

My old setup was 11.75:1 and on 93 octane I had no knock unless it was very hot. At the track I ran 123-124mph pretty easy(1000'da) on 93 octane short shifting. I flycut the pistons so my CR should be slightly lower on my new setup. Anything between 11.0 - 11.2 should be fine for 91 octane but 11.2-11.5 you better have 93 all the time.
Phillip

BTW on hot days I usually splash 1-2 gallons of 111 Sunoco.

 

11.9:1 here. Runs strong as hell on 93 in Texas heat...

 

How are you guys setting your timing on those plants. I think thats cool. Any cooling system modifications?

 

11.4 on Sunoco 93/94
I have Evan's coolant also with around 29 degrees of timing

 

I took a set of 6.0 liter heads and milled the .063 so i fugure i was runnin 12.5 too 1 atleast and i ran 87 for a while through it no knocks on auto tap whatsoever. for somereason my car runs better on 87 than 93 weird huh

 

10.7 was pushing it for me on 91 octane depending on what brand I put in, it would ping sometimes. I guess I'm not in the know of why a lot of people want their CR to be so high, I mean it's only like 1 HP for every .1 bump up in CR, I can see if you spray big shots of N20, but for the guy who drives their car mostly on the street...

Dan

 

I think that 11.5:1 up here in Chicago, I did run 12:1 on 93 pump but that would not fly in Texas with the high heat. I ran 26-27 degrees of timing but I also had a lot of stroke.

 

What matters much more than static is your dynamic compression ratio - this is the effective cylinder pressure you see after VE restrictions and cam timing.

You could have a huge static compression, but if you didn't close the intake valve until very late, and coupled that with a restrictive high rpm ve you could probably get away with 16:1 on pump gas (static) - but you would only be seeing the same cylinder pressures as a lower compression/differently setup car that also ran on pump gas.

That's one reason why nascar runs such insanely high compression - with their restrictor plates @ high rpm they aren't nearly filling the cylinders because of all the manifold vacuum. This is also why they are more likely to pop a headgasket at lower rpm/coming out of the pits, etc.

 

Man i'm about to get my motor together and it was going to be 11.5:1 and I was worried but not anymore. Now my only other worry is p to v clearance.