[youngleadfoot]

right now i am in the final stages of building a 400 and am wondering how much compression i can run on 93 octane?

the engine has a nodular iron crank, reconditioned rods,keith black hypereutetic pistons and an 041t cam. the block was square decked and bored .030. the rotating assembly was also balenced.

[youngleadfoot]

right now i am in the final stages of building a 400 and am wondering how much compression i can run on 93 octane?

the engine has a nodular iron crank, reconditioned rods,keith black hypereutetic pistons and an 041t cam. the block was square decked and bored .030. the rotating assembly was also balenced.

[goatnut4909]

my 400 pings like no other with 72cc heads about 10.1 on 91 whe the 32 degrees comes in at 2800-3000 rpm, but 93 you might get away with that octane. Worst case just mix a little race gas to raise octane i worked out a mix with 25% 100octane lo lead airplane gas and 75% 91 oct havent driven yet but i'll let you know when i do and what the results

[youngleadfoot]

thanks
i was figuring that i might have to run a little race gas anyway.
i actually have access to a set 68 cc(orignally 72cc)that have some work done to them but i figured that they would give me somewhere around 10.3

[tom s]

this question is beatup every month on every board.The question is WHY build a pump gas motor that will be on the ragged edge of hurting itself?The performance gain from a 1/2 to 3/4 point CR is so small its just not worth it.Build it right the first time and not worry about it.On hot days or a bad tank of gas you will be in the detonation range.IMO,Tom

[RPM]

Safe cr is the octane divided by 10. 93 octane = 9.3:1 cr. Have your pistons dished!

[youngleadfoot]

i was reading some of the other compression posts and saw that compression cam be efected by cam profile. is this true?
does anyone know if my 041t grind will effect it?

[70GS455]

>>cam be efected by cam profile. is this true?

Yes, in particular, it's intake valve closing event. Read up on Pat Kelly's DCR page, it will explain it.

[Thumpin455_GTO]

The cam has an effect at low rpm and will work only if you want to idle around town on pump gas. If you ever get on it you will have detonation. 72 CC heads on one of my .040 400's with 91 knocked a rod bearing out in short order. Smashed the top one flat.

If you go over actual 9.5:1 with iron heads in a 400 or 455, you are asking for trouble. the 9.0:1 455 in my GTO has been running for 9 years on everything from 87-91, it has ported 5C heads and a 280-288 Ultradyne. It knocks when it gets too hot or I put a load on it with 87 in the tank.

[70 Dream]

What happens to horsepower when you lower compression?

The premium unleaded octane here in the Bay Area is only 91.

So, I'd have to lower my compression from 10.25 to 9.1 to run on pump gas?

[70GS455]

You won't have to lower compression if you have a big cam...big enough to bleed off cylinder pressure due to the late intake valve closing event. But, if it's a reasonable cam then, yes you will need to lower it, or reduce your timing.

[Philcol]

I run 12:1 in my street 413 with the RA4 cam. I use about 103 octane and of course don't drive the car everyday. At $7.00 a gal. it is for special events only. I don't really recommend it but it's just what I like. It hits a hard lick and stomps the ground when you romp on it.

Phil

[youngleadfoot]

thanks guys for all of your expierences and suggestions.