WhiteLightnin83

i'm nearly ready to throw some heads on my brand new 383 stroker motor that i just got together and i'm not sure what size heads i should put on it. i currently have 75cc chambered stock heads that obviously don't flow worth a sh¡t and when i get new heads i'm gonna get the edelbrock performer aluminum rpm heads but i don't know how high of compression i can run. the pistons i just bought are supposed to produce 10.9:1 compression with 64 cc heads and there are 70cc and 64cc heads available in the type that i want. with the 70cc heads i should be at about 10:1 compression and i know you aren't supposed to be able to run over 10.5:1 on iron heads but with the aluminum heads i should be fine at 10.9:1 right? well i know that pump gas may also be a problem with this high of compression but i'm curious to know if i'll be ok since its only 10.9:1 if i use premium gas and maybe a little jug of octane booster each tank or something? i'd much rather have the 64cc chambers and run 10.9:1 if thats do-able on pump gas since i'm sure it shouldn't be a problem with aluminum heads.... anyone got any info they could share on the subject i'd greatly appretiate it and i hope everything i wrote makes sense... thanks
Dustin

Damon

Don't flirt with high compression if you need to run pump gas (or mostly pump gas). You'll do just fine with the larger chambers and the lower compression unless you've got a HUGE cam in that motor. High compression isn't the be-all end-all of making power. If you choose a compression ratio that is a little too low, you lose a little power. If you choose one that is too high, you lose a LOT of power because you'll have to pull the spark advance so far back to stay out of detonation.

Mixing your gas every time you fill it up gets old real quick, even on a weekend fun car. Carrying noxious chemicals around inside the car or having to drive home to top it off with the good stuff after you fill up at the pump is no fun at all.

respectirocz

aluminum dissipates heat alot better than cast iron, so as a general rule of thumb, if you switch from cast iron to aluminum heads you can go one full point of compression and still run the same octane fuel
so if you stick with the 70cc head and get a nice 10:1 compression u can safely depend on pump gas

WhiteLightnin83

well i'm already sure about the 10:1 compression being ok, you can run that on the stock iron heads since you can go to about 10.5:1 on them. I want to know if i can use the 64cc heads and run 10.9:1 on pump gas as long as its good premium? i know they say its about 10.5 but can that be stretched just a little to 10.9?

bubba353z

I'd go for the 70's - the 2-3% HP loss due to the lower compression will be more than offset by the gains from the better flow. Besides, if the engine starts rattling you may lose more than just HP.

Bad gas is everywhere, and I can almost guarantee you'll find it someday....

If you wanted to split the difference, go for some 68cc AFR's - you'd probably come out ahead overall (except maybe in the wallet ).

DevilsAddvocate

What type of pistons are you running flat tops? domes? (if so what cc) dishes (same question) what size head gasket diameter and thickness. Block decked? What length rods? Whats teh compression height? 3.75" stroke crank correct? Answer that stuff for me and ill give ya the real compression you would be at, you cant just go off what the piston says itll be with such and such a combustion chamber size.

Black305RS

jenerally the highest you want to go is about 12 to 1 any higher and youll start getting pre-ignition and or detonation unless you run extremely high octane gas

octane boosters dont work btw

anthony714

woah 12-1 on pump gas??????????easy dog...maybe in a z06 corvette which is electronically controled but in a street car forget it......and what does rod length have to do with compresion ratio!!!!!!!!!!!!!!!!!!speedomotive has a simple equation under automath101 to figure comp ratios and campbell enterprise has a formula to figure out actual ratio based on all of the above factors such as deck height,compression height ,valve releif ccs.head gasket thickness....also if some one gives you a figure for a piston say 10-1,thats a rough estimate.it can vary,and when edelbrock says 64 cc heads again thats theoretically,it can vary but 3-4 ccs ......every engine and car is set up differently and too many factors effect how a car run with certain ratios,but as a rule of thumb and its a general,10-1 is about tops you want to run with iron and roughly 10-5 -1 on street is about it for aluminum..sure you can sneak a few tenths higher but when you hit detonation you might wish you went to the lower side,but if your a gluttin for punishment and wanna run the high side,your best defense against detonation or pre ignition w/o sacraficing too much power is keep it rich ,rich =extra fuel=extra cooling effect in chambers....

Black305RS

lol i know 12 to 1 is streching it but if you ran 100 octane gas which you can buy at a mobile by my school it is possible but seriously any more than 11 to 1 for a street car is rediculus you could aachieve more performace with lower compression and forced induction.

DevilsAddvocate

Its called dynamic compression mr smarty pants, would also need to know cam specs.

DevilsAddvocate

If you care about running on pump gas you might as well toss static compression out the window and look at dynamic compression ratios, thats what counts. Read up on it and you'll c what i mean. Heres a link.

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

89385formula

Wow, this seems to be full of misinformation. There are plenty of people running 12:1+ static compression on pump gas. Static compression and cam choice is a system. It shouldn't be stated so vague that 10.5:1 is the max on pump gas, etc, etc. Dynamic compression ratio is most important, and that is because it take in account for the vavle timing thus letting you know your effective compression ratio. What size cam do you plan on running? The bigger the cam the higher the Static compression ratio should be. You can run approx. 9:1 dynamic compression ratio on pump gas as long as you have all tweaks and tuning worked out, as well as a good cooling system, proper quench...etc, etc.

DevilsAddvocate

What I said

89385formula

I knew that you knew what was going on. i just questioned the others!

WhiteLightnin83

ok, i dunno alot of those questions you asked devilsadvocate. i bought the eagle 383 stroker rotating assembly out of summit priced at $895.99 or somethin like that with the flat top 7cc dished pistons i believe. its a 3.75" stroked crank and the block is a 4.030" bore and the rods are 5.7" I beams. i dunno how thick the head gasket is, i also bought that from summit in a "mr. gasket" gasket set for like $25 or something. my cam is from summit also, its a hydraulic cam by summit and i bevlieve its .488 and .510 lift or something like that, not sure about the degrees, they'd have to be looked up. but pretty much it looks like i won't be able to run the 64cc heads and i'm gonna have to stick with the 70cc heads to play it on the safe side right? what would the horsepower difference be between 10.9:1 and 10:1 static compression? if its nothing too substantial i won't worry about it and i'll just buy the 70cc heads and go from there.