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Old 03-19-2008, 07:31 PM
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thoughts on 383 SBC combo

Ok, got most of the setup i'm going with on my 383 sbc combo. will be dropped into a 79 nova. heres what I got: (modified it a bit double_v23 =) )

- 383 sbc 4 bolt mains
- eagle forged crank, 3.75 stroke
- SRP forged flat top pistons
- comp cams XR292r mechanical roller, .582 - .588
- AFR 195cc heads, 75cc combustion chamber
- speed demon/barry grant 750 cfm carb
- Super T10 4 speed
- 3.73 gears (maybe 4.11, haven't decided)

what should I run for rockers? same for valve springs? this should give me around 10.4 - 10.5 c/r. thoughts?

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Old 03-19-2008, 07:46 PM
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Valve spring requirements should start with the cam manufacturer's recommendations.
For rockers, I like CC's Pro Magnums, although there are other quality ones too. But I've got nearly 35K street and race miles on mine with no issues whatsoever. This is in a 388 Chevy factory roller motor. Adjustment has never varied when checked and they still show no discernible wear. They're also rebuildable whenever the need finally arises.
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Old 03-19-2008, 07:50 PM
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Run the springs Comp Cams recommends. If they recommend dual springs take out the inner spring or use weaker springs to break in the cam. Be sure to shim the springs to proper installed height during the final assembly. I would use full roller rockers for the horsepower gain. A 1.6 ratio rocker arm will give you a little more lift and duration, but if those duration numbers are at .050 I don't think you'll need anymore for the street.
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Old 03-19-2008, 08:11 PM
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I'm curious, how did you get your static compression ratio of 10.4-10.5 with flat-top pistons and a 75cc combustion chamber? I've run several combinations through the calculator and the only way I can get close to that number using the stats you provided is by using zero deck and a .020" shim gasket, and even then I'm only getting about 10.3 to one, and your quench would be dangerously tight. Using other more common combinations like a zero deck and .039 gasket, or stock deck and a shim .020 gasket, I get CRs between 9.5 and 9.8 to one.

Can you provide the rest of the relevant numbers like gasket thickness and bore, deck height, and maybe valve relief size?

Maybe I've overlooked something, but you'll need 10.4-5 compression for that cam and I want to make sure you have it. In fact, Comp recommends 11:1 CR for that cam.
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Old 03-19-2008, 08:20 PM
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Quote:
Originally Posted by Arcuden
I'm curious, how did you get your static compression ratio of 10.4-10.5 with flat-top pistons and a 75cc combustion chamber? I've run several combinations through the calculator and the only way I can get close to that number using the stats you provided is by using zero deck and a .020" shim gasket, and even then I'm only getting about 10.3 to one, and your quench would be dangerously tight. Using other more common combinations like a zero deck and .039 gasket, or stock deck and a shim .020 gasket, I get CRs between 9.5 and 9.8 to one.

Can you provide the rest of the relevant numbers like gasket thickness and bore, deck height, and maybe valve relief size?

Maybe I've overlooked something, but you'll need 10.4-5 compression for that cam and I want to make sure you have it. In fact, Comp recommends 11:1 CR for that cam.
yes, it is zero decked, .040 gasket (compressed), 5cc valve reliefs. bore size is 4.030.
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Old 03-19-2008, 08:38 PM
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i come up with 9.7.i have 64cc heads,flat top,piston in the hole 005 and it put me at 10.9

Last edited by corenele; 03-19-2008 at 08:44 PM.
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Old 03-19-2008, 09:03 PM
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Quote:
Originally Posted by corenele
i come up with 9.7.i have 64cc heads,flat top,piston in the hole 005 and it put me at 10.9
whaaat, am I going crazy? I swear this has calculated out to around 10.5 before...

http://www.linderothstryckeri.se/ltr...taticcalc.html

the bore is 4.030 (inches)
the stroke is 3.75 (inches)
gasket thickness .040 (inches)
deck height 0 (inches)
valve reliefs 5 (cubic centimeters)
I could be getting this wrong, but the combustion chamber volume in CC's is 121.6025..thats the volume of the stroke/cylinder, which would be (3.1416 x 2.015^2) x 3.75 x 2.54(inches to centimeters) .. am I missing something here? lol, i'm getting 6.8..
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Old 03-19-2008, 09:20 PM
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http://www.kb-silvolite.com/calc.php?action=comp
try this one are you entering your piston .005 or just 5 cc
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Old 03-19-2008, 09:30 PM
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Quote:
Originally Posted by corenele
http://www.kb-silvolite.com/calc.php?action=comp
try this one are you entering your piston .005 or just 5 cc
ahh, I was entering the wrong value for the combustion chamber. it's the volume when the piston is at TDC, between the heads and piston, using volume of gasket .040 x (3.1416 x 2.015^2) x 16.387 + head volume (75cc) ~ 83.361 CC's. That gave me about 9.1:1 compression...odd, I must have calculated it wrong before. If I go with 64cc heads and a .030 gasket, I get about 10.4:1 c/r. Can you run a .030 gasket (compressed)? Possibly a .020?
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Old 03-19-2008, 09:32 PM
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Some calculators require you to enter negative values for valve reliefs and others require you use positive numbers. That could affect your values.

I get 9.9:1 on this calculator http://www.bgsoflex.com/cr.html
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Old 03-19-2008, 10:08 PM
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Quote:
Originally Posted by curtis73
Some calculators require you to enter negative values for valve reliefs and others require you use positive numbers. That could affect your values.

I get 9.9:1 on this calculator http://www.bgsoflex.com/cr.html
Using that, could I go with a .010 gasket? It seems kind of small, but it's going to be zero-decked.

http://www.bgsoflex.com/cgi-bin/cr?c....00&gskt=0.010
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Old 03-19-2008, 10:15 PM
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No need to remove the inner spring to break in a solid roller cam. No need to break in a solid roller cam.

tom
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Old 03-19-2008, 11:14 PM
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Quote:
Originally Posted by mrdreex
Using that, could I go with a .010 gasket? It seems kind of small, but it's going to be zero-decked.

http://www.bgsoflex.com/cgi-bin/cr?c....00&gskt=0.010
No. Not enough quench clearance. At higher RPMs the pistons will hit the head. The simple answer is to use a smaller chamber head. Going to a 64cc head will put you at 11.2 with a typical .039" gasket.

If you haven't already built the shortblock, now is the time to consider a different piston. I would lean toward the dished piston with 64cc head instead of a domed piston with a 75cc head. Domes slow the flame front killing efficiency and requiring more ignition lead.
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Old 03-19-2008, 11:55 PM
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If you stick with the AFR heads, you can just drop down to their 65cc version. You'll retain your good quench and allow you to keep your current pistons if you already bought them. The only possible issue would be your new compression ratio of about 11:1 even. Perfect for your cam, but probably risky if you're planning on using pump gas. Then again, are you sure you want to use that cam if you won't be running the recommended static compression?
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Old 03-20-2008, 04:47 AM
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I should stop answering questions late at night after a long day, I seem to lose the ability to read....LOL. I was thinking flat tappet cam, not roller.
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