My 383 has a Compression Issue!
03-25-2007, 03:27 AM
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My 383 has a Compression Issue! okay got a call from Haddad Motorsports here in LA, it appers my flat top pistons with -6 cc reliefs and the aluminum lt1 heads I have yielded some HIGH compression numbers. With the .40 head gaskets the compression is 12.93:1 and with .60 gaskets they compression is 12.19:1! He feels the best way to solve this problem is with differnt pistons. Does this sound like the best course of action to you guys? Could a differnt set of heads be used with bigger chambers instead? I need some advice here because tearing down the engine and putting new pistons in with all assembly top and bottom and install the price is gonna be somewhere in the $4-$5,000 range......HELP! it has basically doubled in price because of this compression issue.
03-25-2007, 05:56 AM
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Originally Posted by Superbeasst
okay got a call from Haddad Motorsports here in LA, it appers my flat top pistons with -6 cc reliefs and the aluminum lt1 heads I have yielded some HIGH compression numbers. With the .40 head gaskets the compression is 12.93:1 and with .60 gaskets they compression is 12.19:1! He feels the best way to solve this problem is with differnt pistons. Does this sound like the best course of action to you guys? Could a differnt set of heads be used with bigger chambers instead? I need some advice here because tearing down the engine and putting new pistons in with all assembly top and bottom and install the price is gonna be somewhere in the $4-$5,000 range......HELP! it has basically doubled in price because of this compression issue.
54cc heads
.039 thick felpro gasket 4.125 bore
3.75 stroke
4.03 bore
6cc valve relief
.005 piston to deck=12.26 to 1 SCR
.000 piston to deck=12.43 to 1 SCR
I run 12.5 to 1 SCR, daily in my car....9 to 1 DCR.....No problems on pump gas...
Piston to deck, and actual head cc's would be nice to give exact SCR....I don't think it's as high as he's telling you....with those numbers....
03-25-2007, 08:09 AM
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even if they are right, 12.19 doesnt sound all that bad for a street car, surely it would run fine on 92 octane. if worse comes to worse, the knock sensor will just pull some timing out, and allow you to do something about it before it hurts any parts.
03-25-2007, 06:09 PM
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yeah but I live in CA with crappy 91 octane gas, so unless I'm getting octane boosters or the 100 octane for like $7.00/gal from the 96station I think bigger chamber heads is a good idea. I got 12.1 as well, unless those heads have extremely small chambers like 52cc then I don't know where 12.93:1 came from. I'm visiting the shop to see where they're getting the math for these number so I'll get back to you with what they show me.
03-25-2007, 10:46 PM
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doesn't sound right at all....
my old 355 with a -5 cc valve relief flattop piston and 53 or 54cc chambers and an .039 compressed thickness head gasket was 11.75:1..... 12.9 sounds way off....
my old 355 with a -5 cc valve relief flattop piston and 53 or 54cc chambers and an .039 compressed thickness head gasket was 11.75:1..... 12.9 sounds way off....
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03-26-2007, 03:58 AM
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the only thing I can think of is if the heads were milled to SH*T and they now have really small chambers. I used these pistons from Probe
383 - 3.750 Stroke - 6.000 Rod- 1.125 C/H - .927 x 2.500 - 118 Gram Pin
and these Rods from Eagle
327-400ci 6.000'' Connecting Rods, (+.300)Large 2.100'' Crank Journal (2.100'')
Bushed Piston Pin
590-grams
and this crank from Scat
350 Main, 3.750" Stroke, 6.000" Rod Length
383 - 3.750 Stroke - 6.000 Rod- 1.125 C/H - .927 x 2.500 - 118 Gram Pin
and these Rods from Eagle
327-400ci 6.000'' Connecting Rods, (+.300)Large 2.100'' Crank Journal (2.100'')
Bushed Piston Pin
590-grams
and this crank from Scat
350 Main, 3.750" Stroke, 6.000" Rod Length
03-27-2007, 01:25 PM
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Here are the specs we get.
I'll let you guys do the math.
Head = 52cc
gasket bore = 4.100"
gasket thickness = .040
deck = .005
piston = -4cc
bore = 4.030"
stroke = 3.750"
rod length = 6"
Please remember we have junk 91 octane over here. Unless he wants to run race gas all the time anything over 11.5 is pushin it.
Thanks
Mike Haddad
I'll let you guys do the math.
Head = 52cc
gasket bore = 4.100"
gasket thickness = .040
deck = .005
piston = -4cc
bore = 4.030"
stroke = 3.750"
rod length = 6"
Please remember we have junk 91 octane over here. Unless he wants to run race gas all the time anything over 11.5 is pushin it.
Thanks
Mike Haddad
03-27-2007, 02:57 PM
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Yep, with a head of 52cc's things can change quite quickly on compression....especially with a piston of -4 cc's..not -6 as stated...
I get the same numbers...Like 12.93....
I due run, no problem on 12.48 SCR to 1 on 92 OXYgenated gas (**** gas). So with a DCR of 9 there is no problem for me.....Friend on mine runs 12.1 SCR on his 380 cube and the DCR is 8.9 to 1. No problems....You would have to have one huge cam or severely retarted Intake ABDC to get that DCR down...There is a guy over on CZ28 that has that kind of compression running around daily but no idea what his DCR could be....Sounds like new piston with a -8 to -13 dish are what's in order...11.5-12.1 is a good number to shoot for...and leaves alot of good cam grinds on the table...
I get the same numbers...Like 12.93....
I due run, no problem on 12.48 SCR to 1 on 92 OXYgenated gas (**** gas). So with a DCR of 9 there is no problem for me.....Friend on mine runs 12.1 SCR on his 380 cube and the DCR is 8.9 to 1. No problems....You would have to have one huge cam or severely retarted Intake ABDC to get that DCR down...There is a guy over on CZ28 that has that kind of compression running around daily but no idea what his DCR could be....Sounds like new piston with a -8 to -13 dish are what's in order...11.5-12.1 is a good number to shoot for...and leaves alot of good cam grinds on the table...
03-28-2007, 02:09 AM
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yeah the 52cc chambers are the reason.... I think my ported LT1 heads were at least 56cc.....
what's the thickest head gasket you can run (compressed thickness)?????
might make a difference but i'm not sure if you'd get it down below 12:1....
what's the thickest head gasket you can run (compressed thickness)?????
might make a difference but i'm not sure if you'd get it down below 12:1....
03-28-2007, 02:44 AM
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I did not think that he could get above 11.00:1 on 91 POS Cali gas, with out any retard from the computer. but I do agree 12.0:1 and more, RACE GAS ONLY, and rember no led in your gas if you have O2 sesnors in you exaust stream, or kiss them goodbye, I saw that myself.
03-28-2007, 04:12 PM
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Originally Posted by 94 F-body
I did not think that he could get above 11.00:1 on 91 POS Cali gas, with out any retard from the computer. but I do agree 12.0:1 and more, RACE GAS ONLY, and rember no led in your gas if you have O2 sesnors in you exaust stream, or kiss them goodbye, I saw that myself.
03-28-2007, 09:05 PM
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Thanks for the input Mike! Just got off the phone with Mike at Haddad, going down to check everything out on friday, so yeah got the Trick Flows so I'm on my way to getting this thing done right.
