383 comp ratio
#1130608
12/09/11 06:01 PM
12/09/11 06:01 PM
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Joined: Dec 2011
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Wadena MN USA
dustdevil
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Have a 1969 383 HO. Suppose to have 10:1 comp. Our numbers come up to 8.95:1 Flat top 1.920 PD along with 87cc heads. -.006 deck height with .040 thick head gasket. Did Ma Mopar use a thinner gasket to achieve 10:1 or is that a warmed up number?
Last edited by Bruce Johnson; 12/09/11 06:03 PM.
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Re: 383 comp ratio
[Re: dustdevil]
#1130609
12/09/11 06:27 PM
12/09/11 06:27 PM
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Joined: Apr 2005
Posts: 15,134
Kelowna, B.C. Canada
DPelletier
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I think it's fair to say that Mopar was "optimistic" with it's factory compression ratios and that actual stock compression is generally less. I've always heard that 9 - 9.5 was more realistic for the engines advertised at 10:1.  Hopefully some of the guys that have "done the math" on their own 383hp motors will chime in. Dave
1970 Super Bee 440 Six Pack
1974 'Cuda
2008 Ram 3500 Diesel
2006 Ram 3500 Diesel
2004.5 Ram 2500 Diesel
2003 Ram 3500 Diesel
2006 Durango Limited
[url] http://1970superbee.piczo.com [/url]
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Re: 383 comp ratio
[Re: dustdevil]
#1130611
12/09/11 07:12 PM
12/09/11 07:12 PM
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Joined: Jan 2003
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dogdays
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Been posting on this for years. Mopar's compression ratios were for the racers who, they knew, would blueprint their engines. Then they built the engines at between 1/2 and one point lower so their new car dealers wouldn't be bothered by a lot of customer complaints about knock.
So the typical 906 head ccs at 88cc, but blueprint value is something like 79.5cc.
I've just run the calcs out on another thread and with a piston at zero deck with 4-5 cc valve reliefs, and an 84cc 516 head, compression ratio is barely 9.0:1.
See how that works?
R.
BTW, all B/RB engines were built using the cheapo steel shim head gaskets, around 0.020" thick.
Last edited by dogdays; 12/09/11 07:16 PM.
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Re: 383 comp ratio
[Re: BSB67]
#1130613
12/09/11 11:46 PM
12/09/11 11:46 PM
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Posts: 843
Suffolk,VA
ireland383
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Quote:
Your compression distance is about what they were originally in 68 and 69. The shim gasket is .018 and about the thinkness as original. That would put you at 9.4 ish, about what they were actually back then. Blueprinted, they end up well over 10:1.
I agree NHRA stock blue print was 10 to 1. In reality it was around a point less.
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Re: 383 comp ratio
[Re: dustdevil]
#1130615
12/10/11 02:11 AM
12/10/11 02:11 AM
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JohnRR
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Quote:
Have a 1969 383 HO. Suppose to have 10:1 comp. Our numbers come up to 8.95:1 Flat top 1.920 PD along with 87cc heads. -.006 deck height with .040 thick head gasket. Did Ma Mopar use a thinner gasket to achieve 10:1 or is that a warmed up number?
What's a 383 HO ???
It's a warmed up number, the real factory compression ratio was 9.2 at best. The factory used a .021 steel shim head gasket but it won't bring your number up that much.
The pistons you are using are shorter than stock, your heads have been cut because the 906 was more like 90-92 cc. If you want 10.0 you'll need to cut the heads to under 80cc, but if you are using the stock open chamber head you don't want to run that high with the piss that is sold at the pump.
To clear up a few things, that have been posted a hundred times at least, the stock 68-69 383 HP piston has a CH of 1.932, on a factory blueprint deck of 9.980 it sits .0025-.003 in the hole. If you build a 69 383HP to NHRA BLUEPRINT you get close to 11.0 compression, the NHRA spec has the piston .021 ABOVE the deck and it uses a 79.5 CC head chamber volume, NEITHER is factory actual build spec. I have an untouched 56K mile 69 383HP in my 69 GTS Dart and this is how it , and another 69 383HP I have on an engine stand, are built as were others I have disassembled with original pistons and uncut decks.
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Re: 383 comp ratio
[Re: JohnRR]
#1130617
12/10/11 10:40 AM
12/10/11 10:40 AM
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Joined: Jan 2005
Posts: 472
Downey, IA
Gt383
mopar
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mopar
Joined: Jan 2005
Posts: 472
Downey, IA
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Quote:
Quote:
Have a 1969 383 HO. Suppose to have 10:1 comp. Our numbers come up to 8.95:1 Flat top 1.920 PD along with 87cc heads. -.006 deck height with .040 thick head gasket. Did Ma Mopar use a thinner gasket to achieve 10:1 or is that a warmed up number?
What's a 383 HO ???
It's a warmed up number, the real factory compression ratio was 9.2 at best. The factory used a .021 steel shim head gasket but it won't bring your number up that much.
The pistons you are using are shorter than stock, your heads have been cut because the 906 was more like 90-92 cc. If you want 10.0 you'll need to cut the heads to under 80cc, but if you are using the stock open chamber head you don't want to run that high with the piss that is sold at the pump.
To clear up a few things, that have been posted a hundred times at least, the stock 68-69 383 HP piston has a CH of 1.932, on a factory blueprint deck of 9.980 it sits .0025-.003 in the hole. If you build a 69 383HP to NHRA BLUEPRINT you get close to 11.0 compression, the NHRA spec has the piston .021 ABOVE the deck and it uses a 79.5 CC head chamber volume, NEITHER is factory actual build spec. I have an untouched 56K mile 69 383HP in my 69 GTS Dart and this is how it , and another 69 383HP I have on an engine stand, are built as were others I have disassembled with original pistons and uncut decks.
Just wanted to say I loved your question on here...not alot of "mopar correct" language being used this day and age...  
If I hafta come in and getchya.......I'm gonna getchya!
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Re: 383 comp ratio
[Re: AeroMonte]
#1130618
12/10/11 11:23 AM
12/10/11 11:23 AM
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JohnRR
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Quote:
I've got a 66 2bbl 383 waiting for a project to put it in. Will it be comparable spec wise to a 68-69 383?
No , that engine used a 516 closed chamber head , with a piston lower in the hole.
Question is what heads are you going to use , what is the intended build.
The info on 383 and compression can be used for all the years , it's hard to build a high compression 383.
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Re: 383 comp ratio
[Re: JohnRR]
#1130619
12/10/11 05:42 PM
12/10/11 05:42 PM
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Posts: 2,275
Desert Tracker
HYPER8oSoNic
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Quote:
Quote:
I've got a 66 2bbl 383 waiting for a project to put it in. Will it be comparable spec wise to a 68-69 383?
No , that engine used a 516 closed chamber head , with a piston lower in the hole.
Question is what heads are you going to use , what is the intended build.
The info on 383 and compression can be used for all the years , it's hard to build a high compression 383.
 with John.  The "true effective"
compression ratio numbers differ with the head used, milling specs, either or both head/block, chamber ccs', piston top design and gasket bore size/thickness and importantly cam intake closing degrees. Typical 383's (simple Magnum rebuild) can range from 7.5 to 1, up to nearly 11 to 1, "effective" CR (modified "strip-only" 383's)
which is WAY different from ADVERTISED comp ratios. It is a little easier to build compression in a longer stroke engine than a shorter stroke engine. 
"Stupidity is Ignorance on Steroids"
"Yeah, it's hopped to over 160" (quote by Kowalski in the movie Vanishing Point 1970 - Cupid Productions)
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Re: 383 comp ratio
[Re: dustdevil]
#1130620
12/22/11 10:38 PM
12/22/11 10:38 PM
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Joined: Jul 2008
Posts: 259
Khemi, Stygia
Mebsuta
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Khemi, Stygia
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I believe the compression height for pistons in 68-69 383 HP was 1.934 - 1.935.
I want to say that 20+ years ago, replacement flat-top pistons with this compression height were still available, because that's what we used in my 383 Roadrunner. Also use original profile camshaft and open chamber iron heads.
With 89 octane motor sounds like castanets and chugs when you shut it off. It runs fine on 93 octane. I used to mix 89 leaded and 93 unleaded 50/50 and it ran fine on that.
68 Roadrunner. 383 4-spd. Beat up.
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Re: 383 comp ratio
[Re: Mebsuta]
#1130621
12/23/11 03:56 AM
12/23/11 03:56 AM
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JohnRR
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Quote:
I believe the compression height for pistons in 68-69 383 HP was 1.934 - 1.935.
1.932, I have a number of factory stock pistons and that is what they measure.
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Re: 383 comp ratio
[Re: JohnRR]
#1130622
12/23/11 07:42 AM
12/23/11 07:42 AM
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Joined: Jul 2008
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Khemi, Stygia
Mebsuta
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Khemi, Stygia
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Very well, 1.932 it is. You even said that earlier but I missed it. I have a couple lying around but no means to accurately measure. Thanks.
68 Roadrunner. 383 4-spd. Beat up.
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