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1993-97 Gen II Small Block V8

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Old 09-06-2011, 11:55 AM   #1
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Default Good Compression Ratio for 383?

I am building a forged 383 Stroker LT1 and I was curious what compression ratio I should shoot for...

Basically this will be a weekend driver that I will take on road trips and cruises and run at the track. I will be running a direct port 150 to 200 shot on it but I don't plan on boosting it anytime soon.

After talking to AI about it, we came to the conclusion that if I used the -5cc pistons with the heads I chose, gasket thickness and correct deck height, I will be around 11.4 to 11.6...Does this sound about right or where should I be? If I ever boost it I will change the pistons to lower the CR
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Old 09-06-2011, 11:59 AM   #2
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thats perfect
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Old 09-06-2011, 12:13 PM   #3
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Sounds good. If you wanted to you COULD go even higher (mine is around 12.5:1) but for a more street-than-strip 11.4-11.6 is ideal.
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Old 09-06-2011, 12:28 PM   #4
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Agree with RamAir...high 11s should be good. You can go higher if you want, just depends on what your end goals are with the car. Be careful with going too high though since pump gas might start to factor out of the equation. I think the rule of thumb is that as long as you are below 9.30:1 DCR, you should have a good chance at running pump gas. Eventually it comes down to a good tune though.

I will be running 12.7:1 (9.22:1 DCR) with the build I am doing now and the car will see more street than strip...
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Old 09-06-2011, 12:45 PM   #5
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Ok, also how do I properly calculate the CR and DCR myself? AI calculated my CR but I have no idea how myself. I know the chamber size, pistons, deck height, gasket thickness, bore, etc all factor in but how do I take the hard numbers (62cc chamber, .030 bore, -5cc pistons, .021 thickness gasket) and get the CR and DCR? I wanna learn! Lol
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Old 09-06-2011, 12:49 PM   #6
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Compression Ratio Formula - How to Calculate
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Old 09-06-2011, 12:51 PM   #7
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Quote:
Originally Posted by Z28Camaro30Ann View Post
Ok, also how do I properly calculate the CR and DCR myself? AI calculated my CR but I have no idea how myself. I know the chamber size, pistons, deck height, gasket thickness, bore, etc all factor in but how do I take the hard numbers (62cc chamber, .030 bore, -5cc pistons, .021 thickness gasket) and get the CR and DCR? I wanna learn! Lol
http://www.kb-silvolite.com/calc.php?action=comp


static compression as you know is the volume of the cylinder and chamber at BDC vs the volume of the cylinder and chamber at TDC.........

Dynamic compression takes into account the valve event timing based off the cam.........problem is this calculation is done in a static (aka not moving) state.........seems like a good idea if the motor never spins

dont get too hung up on DCR its a internet fad........
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Old 09-06-2011, 12:59 PM   #8
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If you're doing standard 383 stuff:

Bore = 4.030"
Stroke = 3.75"
Rod Length = 6"
Deck Height = 0 (most motors are 0 decked, unless you are going to have some kind of clearancing issue - I am going to assume 0 decked since you are going with a standard -5cc piston)

Add in all your other factors and you should come out to (from what I calculated from your previously provided specs), a static compression ratio of 11.98:1

Hope this helps
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Old 09-06-2011, 01:01 PM   #9
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keep in mind standard deck height is like 9.026 on these motors.....the pistons are usually .020-.024 down the hole at TDC stock.......so unless you deck the block down to 9.000 its not 0........i cut mine down to 9.006.......with a .026 gasket i have a .032 quench.......DO NOT go under .030 quench.....
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Old 09-06-2011, 01:07 PM   #10
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Quote:
Originally Posted by quik95lt1 View Post
keep in mind standard deck height is like 9.026 on these motors.....the pistons are usually .020-.024 down the hole at TDC stock.......so unless you deck the block down to 9.000 its not 0........i cut mine down to 9.006.......with a .026 gasket i have a .032 quench.......DO NOT go under .030 quench.....
I stand corrected...I do remember my builder telling me that my pistons were set to sit at .006" in the hole. Sorry about that...
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Old 09-06-2011, 01:15 PM   #11
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I stand corrected...I do remember my builder telling me that my pistons were set to sit at .006" in the hole. Sorry about that...
odd u went .006...........i did it for PTV/quench reasons.....i would've love to go closer but on a street driven race motor the little extra quench volume will help just incase i got carbon build up......
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Old 09-06-2011, 01:32 PM   #12
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AI told me what height to deck my block to and it wasn't zero but I couldn't hear him very well so I am waiting on an email with all the specs.
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Old 09-07-2011, 07:35 AM   #13
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I went with .039 quench area. The math worked out to be 12.3 CR. Pump gas seems to get better times than 100+ octane race fuel. Have not done any work to the car in some time. Cam and tune + converter change should get me better times. I really like the 383.

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Old 09-07-2011, 10:26 AM   #14
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I am running right around 12:1 with no problems on 93 octane. I went with a Eagle forged rotating assembly with SRP pistons and it ran me right around $1400 for the kit with rings bearings etc. I also really like having the extra cubes.
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Old 09-07-2011, 11:03 AM   #15
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I am not using Eagle...after doing research they are too hit and miss with balancing issues...I will be using Callies. Also AI just sent me the specs, and if I use a .029 gasket he wants me between 9.005 and 9.010 deck height. I will probably stay on the upper end of that because I wanna use a .026 gasket...
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Old 09-07-2011, 02:19 PM   #16
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Im running 11.3:1. Mine runs on 91 no problems. I know i coulda went higher but it stills runs damn good
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Old 09-07-2011, 08:35 PM   #17
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Quote:
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dont get too hung up on DCR its a internet fad........
Lol.......
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Old 09-07-2011, 08:35 PM
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