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Hi guys, I'm starting to look for parts to build a 383 but before I purchased anything I wanted to calculate my compression ratio. Using an online calculator I found that with a bore of 4.03, a stroke of 3.75, 64cc aluminum heads,-5cc effective dome volume, .005 deck clearance, and a compressed gasket thickness of .04 my static compression ratio came out to be 11.84. This is way higher then I wanted it to be because I want to run pump gas. What can I change to get a lower compression ratio and would this compression ratio work on 93 if I get a cam with a longer duration to shave off some compression?
You'll need to add for crevice volume also. I think you'll be lower than 11.5 which could work with a decent sized cam if all else is very correct.
-5cc valve reliefs is quite small for most pistons is that what's advertised ?
Not sure where you are getting your deck clearance of 0.005 unless you plan on having the block shaved.
But even then, the piston you select will determine deck clearance. The piston has a dome of 0.120 I take it?
On average, the piston is 0.020 - 0.025 below deck.
To get your C.R. down to a reasonable number for premium gas, you have a couple options:
Thicker head gasket of course. Or check for a 67 -68 chamber. Once you get into the 70 cc, too much power will be lost
Static compression is not all that great of performance judging. It merely measures the squeeze from bottom of stroke to top.
But WAIT. There is an Intake Valve still open on the upward stroke.
This is where Dynamic C.R. comes in. See if you can find a Dynamic calculator to punch & crunch your numbers. It will calculate the open Intake Valve event.
You want around 8.0 - 8.5 for DCR to really bring the power level up and still be streetble w/o knock & ping.
-4cc dish, .007 in the hole (avg) , .041 head gaskets, 65 cc heads. It calculates to 11.1 static. With a xe282hr dynamic compression ratio is around 9:1. So the cam makes a lot of difference. With the right cam 11.84 static is doable on pump gas.
You can also go to dish piston, and still get a good quench or you can get larger cc heads.
You'll need to add for crevice volume also. I think you'll be lower than 11.5 which could work with a decent sized cam if all else is very correct.
-5cc valve reliefs is quite small for most pistons is that what's advertised ?
Bigger dish in the pistons or a bigger combustion chamber in your heads. The final CR is really a target for your desired RPM range. You then just cam it to match.
5cc's is typical for a flat top with valve reliefs on one side.
Like Stormin Normin said! It is very hard to even get 11.2 static out of a little 383. -4 cc flat tops are not a good idea because of the small valve reliefs. Even small 2.02/1.60 valves will have piston clearance problems with 6 inch rods and cam durations like 236/242 required to get the Dynamic Compresion Ratio down to run pump gas.
My 383 had - 7 cc Wisco forged pistons for 6 inch rods. Big intake valve reliefs for 2.08/1.625 valves. It runs fine on the california 91 octane with a 236/242 roller cam.
I quit building 383's when I found out that for the same price I could buy 396 rotating forged kits with 6 inch rods. 396 will kick but on any little 383 and everybody has 383's.
Don't be afraid of big cc heads, single plane intakes with roller cams. I drove around for about 4 years with 227cc race ready heads on my 383 and later used the 227 cc on my 427 and bought AFR 210's for the 383
If you are using as cast heads the combustion chamber is likely larger than the advertised 64cc's. could be as much as 68cc's. Measuring the volume is the only sure way to know your CR. Can also measure the cc's in the pistons once they are installed to get true volume also. It's very likely they are not as advertised either.
Doing some light porting or polishing on the combustion chambers can also increase the volume.
That CR vs cam duration is not incredibly accurate. I use a 219@ .050 cam with 10.6 CR with no pinging at all. LSA plays a big part as well.
The chart is a guideline. You can play with cam timing to alter the intake closing point. Keep the DCR where it needs to be. You can go further away from the chart but the combination will be somewhat compromised. Run great but not optimal.
You are smart to all of your planning on paper, before you buy anything.
Head chambers, piston domes, head gasket, block decking, cam timing #s, IVC point, cam adv/ret*, all has to work together.
Good hot-street Targets no matter what:
Quench: .040"
DCR: 8.0-8.5
Ideas to get you there:
CR: ~10-5
Cam: 230* duration at .050" is a good reasonable street cam goal / limit, Still drives reasonable, vacuum still ok, MPG starting to drop. Can still run A/C if you try hard. Runs better 2200+ rpm.
Power: run the best heads you can...they make the biggest difference.
So what are your driving goals?
How much power do you want?
Here are some ICON forged pistons that give you options in the 10-11 CR target range.
I like heads with small chambers if you have the choice.
Last edited by leigh1322; May 12, 2024 at 05:34 PM.
Compression ratio
