Compression Ratio

[rgtrough]

Hey guys, I talked with the man doing my engine work today. I have a 383 ci power plant that until now has been using the stock 350 heads. I am going to install a set of cast iron World Sportsman II heads with 200 cc runners, 64 cc chambers. My engine guy tells me that these heads with my bore is going to push me into the 11:1 compression ratio range. This of course means using 100 octane fuel. My question is what size chambers were in the stock heads? What is the formula for figuring compression ratio? What is the highest ratio that 93 octane will support? Besides the need for special fuel, are there any other drawbacks to running high compression. Thanks for taking the time to read and reply. peace Bud :cool

 

[72Mako]

Bud, if this is a track only car, 11:1 would be fine with racing gas. If you intend to run this car on the street, I would consider doing whatever is necessary to lower the comp ration to a more pumpgas friendly ratio, unless you don't mind the cost of racing gas, additives etc. Your machanist should be able to tell you what is needed to end up at a more friendly comp ratio. I do know that running alum heads generally will gain you a point in compression over iron heads all things being equal. On my motor, I am using Edelbrock alumn heads and my comp is 9.5:1 ratio. But I wanted a motor that I could drive anywhere in the country and not have to worry about finding hi-test and it still makes decent power. Determining factor is how you intend to use this car. Just my .2 cents. Charles

 

[rgtrough]

Hey Charles, thanks for the reply. To say my '74 is used as a street car is like saying a AK 47 is used for plinking tin cans. Though I only put about 1500 miles a year on her alot of the time it is at wot. As much as I would like to have the extra hp that the 11:1 provides I must be resonable here. I instructed my machinist to go ahead with the piston work that will reduce the compression to a much friendlier 9.5:1. There will be times I may venture away from my back yard and the thought of limiting myself to a certain milage radius does not seem right. I would still light to know the chamber sizes of the stock heads. peace Bud

 

[wallyknoch]

Up to and including 11-1 compression, premium pump gas is OK. 93/94 octane will work with out any problems. I do not use any boosters either with around 36 degree lead.

 

[rgtrough]

Hey Wally, I had been told this is the case by other people, but my man is very firm in his beliefs. I really would like that extra power. It would be a real dragf if he is right about the need for special fuel. Thanks for the info. peace Bud
National Champions
THE OHIO STATE UNIVERSITY

 

[wallyknoch]

Your perfectly safe with pump premium. If the engine is in proper tune, don`t worry about it. The 62 above is 11.25-1 and it works perfect for street use. Not that 100 wouldn`t be better, but 93/94 is the minimum. I do however occasionally use 100 av gas in several of my pieces and some others I have to. If the plans are for strictly drag racing, use the 100,

OSU,

But I also went to MICHIGAN, University of.
I`ll raise a brimming glass to you though.

PS, Take a look at my site http://www.nitroalley.net you may enjoy

 

[mike weyman]

the original heads are probably 882 castings , these are "open" port heads and are 76cc chambers and give 8.5 to 1 compression in their stock installation.
maximum safe compression for high octane gas is 10.5 to 1 to one.
It is possible to run higher compression on 93 octane , but you have to retard the timimg to avoid detonation(gas exploding on the compression stroke from the pressure). by retarding the timing in this manner,you are actually de-tuning the engine, thus negating some of the power.
unless you have this block bored more than .030 over you should not be over 10.5 to 1 compression unless the block and heads have been decked.
Is it being bored oversize? what cam?
sounds like a pretty good build so far, mike.

 

[mike weyman]

sorry dude, me toopid. 383 cid is a 350 bored .030 over , you should be 10.5 to 1 if it's not decked.

 

[rgtrough]

Hey Wally, my brother says his '66 coupe also had 11.25:1 and he just ran the highest octane he could find, retarding the timing abit if detonation occurred. When I saw the Dearborn address in the ownership profile I thought there might be a wolverine in the woodpile. A hearty toast to you and yours. GO BIG BLUE!! Bud

 

[rgtrough]

Hey Mike, I can't bear the thought of that extra power laying on my machinists' floor. I have 94 octane available and 100+ just down the road. The idea of retarding the timing also doesn't make any sense either. This is the kind of dilemma I like. Thanks for the info on the heads. No wonder there where no compression problems running the stock heads after the original bore/stroking. peace Bud

 

[drags1998]

Sorry Mike,But a 383 is a 350 bored .030,And a .270 stroke increase!!! There are MANY combo's out there with comp. ratio's from7.5-14to1 Try to stay in the 10to1 area.36degrees of timing and good gas and you'll be a Happy hot rodder!!!! By the way you will LOVE the added torque of the 383. Good Luck.

 

[mike weyman]

wow drags i must have been asleep or something.
they call it stroker cause it has a long stroke.duh
350 block + 400 crank+.030 overbore =383 right?it's still early today so i'm fresh.lol mike

 

[rgtrough]

Hey Drags, The engine has been bored/stroked for quite a while, I am just now installing after market 64 cc heads. That is the reason for the comp. concerns. And you're right about loving the increased torque. I hope the new performance add ons do the same. Thanks. Peace Bud