Im looking into building a street/strip/race(legal road race) camaro. Working on piecing together the engine, going 383 for now. What is the realistic maximum compression for premium gas. Or maybe premium gas with an octane booster i can add. I really want a fire breathing monster, but it has to be tame enough to drive to the track. Or at least the fuel needed cheap enough to drive to the track. .
Thanx for your time. ~G

 

I'm gonna say 10.5 - 11 on 93 octane. The lower the lsa, the more compression you can run in general. So, not knowing the cam specs it's hard to say, but 12 point is fairly high and 14 is pretty much race gas only territory.

 

im looking at 12-14:1. i could make my homebrew octane booster if needed. lets say i have 15:1(i wouldnt but just to be overly careful i guess), what octane rating would be needed. also how expensive is race gas (non alchahol, and lead free if possible)? Im not sure on the cam yet, im still worried about the crank(383 eagle forged), rods (eagle forged 6"), and pistons(??12-14:1??). The cam will be a very radical race one, im sure you assumed this lol.

 

that would just be to much for pump gas even with booster and you did say you dont want lead or alco in the mix so you would with that much comp (anything higher then 12 pts) you will have to use race gas now with a 383 you really dont need that much because your going with more rev's so you can make a beast that doesn't need that kind of comp ratio now race gas it self is about 5-7.00 a gal with no lead .now lead is a good thing it helps cool the engine but it does destroy o2 sensors . Id say stay on the 93 just dont go so high in the comp ratio

 

so how much power of a decrease am i looking at if i decrease from 14-11 points. If everything stays the same. This is a carbed no computer engine, so o2 sensors arent an issue. Any estimate would be helpful even if its just a guestimate. Im still mostly in the planning stage, so any info is good info. I CANT AFFORD RACE GAS, hmm wonder if i could make some..... j/k had enough experience making things that could end up killing me lol. Thanks a lot fastvette and atomonkey.

 

A 3 point drop in compression like that will lose at most I'd say 50 HP. Compression doesn't add that much HP unless you really need it, like if you had a cam that barely closed both valves during the compression stroke. Hell, you might even add power if you cam was mild enough, because you wouldn't have as big of pumping losses. Plus the starter you'd need for an honest 14 point engine would be super expensive too.

 

ATOmonkey is right about the compression and the starter. I believe that for every point cut in compression, you loose 3% to 4% of your power. So a 3 point cut in compression would cut your power by about 10%, but would be much much easier to live with. From what I have read and seen, about 9.5:1 is the most you can go on an iron headed street engine without detonation or reduced timing. Some might can get away with more, but that is a good rule of thumb. About 10.5:1 will work on an aluminum headed engine, because the aluminum rejects heat faster than iron. Just my .02 worth.

 

The cut in power from compression loss depends on where you start from and where you go to. That is to say, not all 3 point drops are equal... from 15:1 to 12:1 on pump gas you would probably gain power, but from 8.5:1 to 5.5:1 the engine might not even run any more. The rule of thumb is only valid for small changes near the customary values of around 9 or 10:1.

In my experience, static compression ratio gets WAAAAYYYYY too much attention from novice engine builders as a "holy grail" kind of goal when they're planning a build. There are alot of factors that end up determining how much power an engine can make. But there are a few things I can tell you for sure: an engine with broken pistons from detonation due to excessive compression will make less power than one built with slightly (or even alot) lower compression and intact pistons. It's a whole lot more fun to cruise around in a car when you can buy any gas anywhere any hour of the day or night and have it work right, than it is when you can only get it at the track that's 100 miles away on Fri night and Sat & Sun afternoon. And a motor that can have its "ideal timing" without pinging, typically 35°-38° total at full power for SBCs, will make as much or more power than one where the timing has to be backed way off, and will run cooler with the properly advanced timing to boot.

High 10s (10.7-10.8) with aluminum heads, or low 10s (10.2-10.3) are about all you're going to get away with in a street-driveable motor. I would avoid machining anything to hit some on-paper "ideal" CR, and would under no circumstances use a thin (steel shim) head gasket either. Choose your parts right in the first place, and settle for anything that calculates to be within a quarter point or so of those "ideal" values, and you'll never be able to tell the difference.

If I were building a 383, I would look for 64cc heads, and a piston with a dish that "reflects" the combustion chamber, with a dish volume of about 8 cc and no valve reliefs. In a stock deck block that will give you right near 10¼:1. Or using aluminum heads, flat tops and 4cc valve relief pistons, 64cc chambers, that would give you almost 10¾:1 CR. Those are combinations that will work without permanently altering your machine parts, which IMHO is a drastic step for hobbyists like most of us, to be undertaken as a last resort. If something doesn't work out, it's a whole lot cheaper and easier to use a .010" thicker head gasket or trade heads with somebody or something like that, than it is to throw away a block and start over, because it's decked too far.

Aluminum doesn't "reject" heat, it conducts it ("ejects"?) out of the chamber and into the coolant faster than iron. As a result it is not only possible, but actually even necessary in order to maintain the same thermodynamic efficiency, to run a higher static CR, to offset the cooling effect on the hot gas mixture that the aluminum has. People sometimes ceramic coat their heads (and pistons) to reduce this effect.

 

Thanx guys, as far as i can tell i'll be running victor 23* high port heads, probly machined down to 64cc(i think they are 71cc's to begin with). Im looking for around 5/600 horses out of my 383, and be able to handle a 100 shot of nitrous if needed to keep my mustang friends in line, not like i should have to worry about it. I guess i'll stick to 10/11 points then. Is 600 horses realistic for pump gas? Sorry for all the questions but ive spent most of my time on british sports cars, and this will be my first real venture in real power.

 

Thanks for all the help, i went back and looked over my plans. Im gonna go with victor jr 23* heads for half the price of the High port heads(thast with out the machine work). They said 5-600 horse potential so i'll go with them. They have 70cc chambers so i can get a good variety of pistons and C/R. Would machining them to 64cc be of any help? I think the forged rotating assembly outta be good for 600 horses at least. Around how much CFM would i need to move? I would like dual 4 barrels, even at the expense b/c i feel it would help even out cylinder differences( i find the to be the one main draw back of v8's). Thank you again.

 

That's a lot of ponies. You'd better bring your extra fat wallet to make that kind of N/A power.

 

Well for the block, rotating assembly, and heads it should be around 2500. But i want my short block to be tough, so it can handle whatever top end stuff i want to put on it. So im getting parts that are much stronger then i need right this second, even at a cost. I know it will cost a lot of money, but i think its worth it.

 

Well, I don't know how many build you've done, but you're going to spend a lot on the valve train and fuel system too. Plus this thing is going to be a ***** to drive on the street. It won't idle well, chew up tons of gas, run hot, and be a high revver to boot. You won't be able to see around the 4x2 tunnel ram manifold either. For a street car, this is just my opinion, I'd shoot for 450 horse, which is still a lot, and go with a single carb single plan manifold and a 280* or slightly less solid lifter cam. Then you can get away with a 4" or less cowl hood, rev to 6500 w/o a lot of problems and still have a half way decent idle and in town fuel economy in the double digits, and nearly 20 on the highway, unless you have 4.10s stuck in there. It's hard to get big HP numbers on the street natural. If you get a power adder, then you can run the 600+ Hp with decent street manners still. Once again, just my opinion. It's your car so you can do whatever you want.