Re-doing my big block stroker motor. Strictly street. I know that camshaft selection makes a big difference, but as a rule of thumb, what static compression ratio should I be shooting for to use 89 octane mid-grade fuel with aluminum closed chamber heads? 4 speed 3.23 car.
compression ratio
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RustyRatRod
Well-Known Member
You're really not doing yourself justice excluding the camshaft specs. Without that, you cannot really come close, since what you need to be concerned with is dynamic compression ratio and cam specs is a huge part. .
Current cam is a Racer Brown SSH-25. .485 lift 238* at .050. At .050 lifter rise, the intake closes at 44* ABC. Currently have 77CC open chamber iron 906's and the cranking pressure is 150#. Sometimes rattles on premium. Ordered the MP452 version of the Eddy RPM heads today and I know that they will have to be cut, but I no longer want to have to burn premium, so I need a ratio to shoot for. The aluminum head thing is new to me.
I guess that another way to ask the question is how much cranking compression should I shoot for in order to use 89 octane pump gas with aluminum heads?
I guess that another way to ask the question is how much cranking compression should I shoot for in order to use 89 octane pump gas with aluminum heads?
RustyRatRod
Well-Known Member
With only 155 PSI pressure, you shouldn't have trouble with any rattling. The reason you do is because of that teenity cam. If you run aluminum heads, you should upgrade the cam.
RustyRatRod
Well-Known Member
I have no good guess. Don't know what pistons has or which heads he plans to use. Maybe if we could get that info. Pullin one tooth at a time.
Garys1969RR
Well-Known Member
There are so many factors come into play here. A tight piston to head clearance (flat part of piston to flat part of cyl head) will help reduce ping probs. This is the squish area, and is typically .035 to .055". I like closer to .035". Also, what is the elevation the engine will be running at? Above 2500 feet MSL you can go up in C/R some. Aluminum heads help, as well as a late closing point on the intake valve. Seems like most guys I've read about in the forum are running between 8.5 and 10 to 1 C/R. Oh yea, ignition timing advance plays a big role as you probably know. Good Luck
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Where does the intake open at? How much was milled off of the 906's to get 77cc? .050"+?
Meep-Meep
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You say you have a stroker motor with 906's so my guess you have quite a bit of compression already, especially if your pistons are flat and at zero deck. That cam is probably ground on a 108 lobe separation and is pretty healthy for upper mid range power. If you want just a street motor that runs reliably I'd change the cam to a 112-114 lobe separation grind with duration in the 270-280 range.
You must first get the static compression correct for the fuel used with full timing advance of around 38 deg, so for the 89 gas I'd take a stab at 9.0:1. Of course you can tweak the CR a bit higher by ensuring you have a clean shot for flame travel (no dome pistons) and good quench, but under no circumstances should you band aid the high compression by running less timing. Get the compression right. I'd pick a cam that gives you about the same cranking pressure you have now or even up to 160 PSI. Many things contribute to detonation such as static compression, chamber efficiency, intake charge temp, mixture, engine temp and they all need to be addressed. I'd say start with compression ratio and chamber design then cam and cylinder pressure and tune from there.
You must first get the static compression correct for the fuel used with full timing advance of around 38 deg, so for the 89 gas I'd take a stab at 9.0:1. Of course you can tweak the CR a bit higher by ensuring you have a clean shot for flame travel (no dome pistons) and good quench, but under no circumstances should you band aid the high compression by running less timing. Get the compression right. I'd pick a cam that gives you about the same cranking pressure you have now or even up to 160 PSI. Many things contribute to detonation such as static compression, chamber efficiency, intake charge temp, mixture, engine temp and they all need to be addressed. I'd say start with compression ratio and chamber design then cam and cylinder pressure and tune from there.
RustyRatRod
Well-Known Member
I went back and read my one good eyeball out and didn't see where he said that. But then again, there's a lot of things he left out.
Garys1969RR
Well-Known Member
Could you tell us your distance below deck of the piston top? Also bore and stroke, piston dome or dish in cc s? And closing point of intake valve on final cam selection, and cam specs. Also head cc s, head gasket thickness, then we can give you a better answer to your question. Thanks
The pistons are at zero deck with a 36cc dish. I was exactly at 9.9:1 with my 906's. They were milled extensively for another build. They were setup by Koffel in Michigan. The engine bore is 4.375 and stroke is 4.15. The rod length is 6.76". I am thinking that cutting the aluminum heads to 75cc should get me about 9.5-9.7:1 as the Eddy head gaskets are 4.450X.038. Trying not to go to anything with more than 245* @ .050 because I am using manifolds, not headers. If I am on the right track, maybe someone can chime in to let me know how much has to be cut from the MP452 heads to achieve 75cc? Thanks guys. I really do appreciate all advice.
Garys1969RR
Well-Known Member
Well that should be about perfect quench then at .038. Plugging those specs into the KB C/R calculator, it comes to 9.47 to 1 Static C/R. And with an intake closing point of 44* ABDC, it calculates to a Dynamic C/R of 7.9 to 1. But I'm not sure what you need there, far as Dynamic C/R goes. Maybe some one else can help out there? Ok
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Oh yea, I am using stock HP ex manifolds on my 451 now, and it runs pretty good. Also using 906 heads and a 509 cam. But my C/R is about 12.2 to 1. Have to run Sunoco 100 octane fuel!
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Oh yea, I am using stock HP ex manifolds on my 451 now, and it runs pretty good. Also using 906 heads and a 509 cam. But my C/R is about 12.2 to 1. Have to run Sunoco 100 octane fuel!
RustyRatRod
Well-Known Member
Nope. There's your problem. A 36cc dish is removing any quench you have. That's why you're rattling. Iron heads and 9.9:1 and no quench. There's your problem. You're going to have to change pistons before you have a quench motor. Do you have a picture of them?
RustyRatRod
Well-Known Member
Well, there's certainly a quench area there.....but what a frikkin huge dish. I figured it up on the united engine calculator and you'd have to be running a pretty thin head gasket to get the numbers you say....but it's certainly possible. Stupid as it sounds.......I think you're grossly undercammed.
Garys1969RR
Well-Known Member
Since C/R ratio is figured as swept volume over compressed volume, then you are probanly at around 9.5 to 1. The long stroke increases swept volume alot, so compressed volume has to also be increased. Hence the large piston dish.
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Plugging these specs into the KB C/R calculator: Bore 4.375 Stroke 4.15 Head Gsk thickness .038 Deck Height 0 Head cc 75 Piston Dish 36cc, it comes out to 9.47 to 1 Static C/R.
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Plugging these specs into the KB C/R calculator: Bore 4.375 Stroke 4.15 Head Gsk thickness .038 Deck Height 0 Head cc 75 Piston Dish 36cc, it comes out to 9.47 to 1 Static C/R.
Meep-Meep
дворянин
He didn't say it so your good eyeball is still good! I was just throwing that in as a for instance.
Garys1969RR
Well-Known Member
Then you plug in the rod length of 6.76, intake closing at .050 is 44*, then add 15 * for the dynamic C/R of 7.9 to 1. This according to the calculator instructions. Then you can plug different intake closing degree specs and you come up with dynamic C/R with various cams. Then advancing the cam 4 degrees gives a Dynamic C/R of 8.1. So advancing the cam usually increases low end torque. But will also make it more likely to ping under load at lower RPM. Hope this makes sense.
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Wow Meep Meep you must be Psychic, because his pistons are at zero deck!
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Wow Meep Meep you must be Psychic, because his pistons are at zero deck!
Meep-Meep
дворянин
I'm running a 440 with 10.1:1 CR and 915 heads. With the 750 Eddy AFB and the stock intake (iron) my car rattles like an SOB on 91 under hard acceleration, but with good gas it goes like a bat outta hell. Recently I put a factory 6 BBL on and all my rattling issues are gone. I also noticed it seems pretty rich, and judging by my trap speed it's the lowest yet.
Your pistons with the dish look just fine and the dish is a way to correct your compression ratio back to realistic levels with the 4.15" crank. You really should pour the cyl at TDC with a compressed head gasket in place and the chamber of your old head and new head. Get the real number and make adjustments if necessary. I personally would run a compression ratio that is suitable for the gas used, and for 89 R+M/2 that may be 9.0:1. Then pick a cam that has a power band from idle to about 5000 RPM and make up some cylinder pressure that way. As opposed to running too much compression for the gas used and bleeding off cyl pressure with a late intake closing cam. Sure, the big cam will make more top end power but what percentage of your seat time will you spend at 5000+ RPM?
Your pistons with the dish look just fine and the dish is a way to correct your compression ratio back to realistic levels with the 4.15" crank. You really should pour the cyl at TDC with a compressed head gasket in place and the chamber of your old head and new head. Get the real number and make adjustments if necessary. I personally would run a compression ratio that is suitable for the gas used, and for 89 R+M/2 that may be 9.0:1. Then pick a cam that has a power band from idle to about 5000 RPM and make up some cylinder pressure that way. As opposed to running too much compression for the gas used and bleeding off cyl pressure with a late intake closing cam. Sure, the big cam will make more top end power but what percentage of your seat time will you spend at 5000+ RPM?
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