Re: Calculating compression ratio... ugh...
[Re: PAINT IT BLACK]
#451009
08/26/09 05:55 PM
08/26/09 05:55 PM
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Posts: 12,675
Columbia, CT
moper
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No, that's right. It's because of the deck on the block being extra tall, and the compresion height of the piston being very short, and the chambers being much bigger than factory spec. It's a big issue with 383s and why building one should be a careful exercise.
Well, art is art, isn't it? Still, on the other hand, water is water! And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know.
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Re: Calculating compression ratio... ugh...
[Re: moper]
#451011
08/26/09 06:03 PM
08/26/09 06:03 PM
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Joined: Nov 2004
Posts: 7,647
Houston Texas
PAINT IT BLACK
OP
Got a CHIP on my shoulder
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OP
Got a CHIP on my shoulder
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Houston Texas
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Quote:
and the chambers being much bigger than factory spec. It's a big issue with 383s and why building one should be a careful exercise.
What do you mean "bigger than factory spec"?
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Re: Calculating compression ratio... ugh...
[Re: Neil]
#451012
08/26/09 06:05 PM
08/26/09 06:05 PM
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Joined: Nov 2004
Posts: 7,647
Houston Texas
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OP
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Quote:
Maybe you could shave the heads and bump it up closer to 9.0?
I'm not actually even going to use this engine. It's the original and I am just trying to assess what I have before sticking it in a corner. I'll be running aluminum heads on a 440 based stroker and be running much higher compression then...
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Re: Calculating compression ratio... ugh...
[Re: PAINT IT BLACK]
#451014
08/26/09 07:21 PM
08/26/09 07:21 PM
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Joined: Mar 2005
Posts: 1,373
Rancho Cucamonga, CA
D_C
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Rancho Cucamonga, CA
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Here is a very good Mopar Muscle Magazine article, all about Mopar Factory Big-Block cylinder heads: Mopar Muscle Magazine BB Cyl Heads According to the article, the "typical" factory cast combustion chamber of '906 cylinder heads are supposed to measure 88cc's. In the old-days, there were thin Steel-Shim head-gaskets that would help static Compression-ratio. Thin copper-gaskets ($$$$) can help. Milling the heads and intake-manifold mating surface, dome-pistons, or smaller cc cylinder heads are other methods.
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Re: Calculating compression ratio... ugh...
[Re: D_C]
#451015
08/26/09 07:27 PM
08/26/09 07:27 PM
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Joined: Nov 2004
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Houston Texas
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Quote:
Here is a very good Mopar Muscle Magazine article, all about Mopar Factory Big-Block cylinder heads:
Mopar Muscle Magazine BB Cyl Heads
Thanks. I found that in a search earlier, and it says that 906 heads have an 88 cc chamber. Which would give me a 9.5 CR.... but like I said, I had one of my heads cc'd and it came out to 93.4ccs. I found others on other forums that have said they found their heads to be 92-93 ccs as well. I am just wondering how it is possible that the factory heads from the same castings wound up giving these cars compression ratios anywhere from 8.5:1 to 9.5:1, all else being the same... shouldn't the performance of these cars have been very inconsistent? That's a pretty big spread.
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Re: Calculating compression ratio... ugh...
[Re: patrick]
#451018
08/27/09 09:08 AM
08/27/09 09:08 AM
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Joined: Jan 2003
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U.S.S.A.
JohnRR
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and the chambers being much bigger than factory spec. It's a big issue with 383s and why building one should be a careful exercise.
What do you mean "bigger than factory spec"?
factory blueprint spec for 906 chambers is something like 78cc IIRC. also, stock motors from the factory used a steel shim head gasket that's more like .020" thick. they were optimistically rated for compression by the factory.
No, NHRA blueprint is 79.5 , factory spec i have seen quoted as 86 and 88 , all the heads I have had checked were 90-92 cc . I thinking CC'ing depends on the person doing it , I spent a good 8-9 hours ccing heads the other night and found out I needed to PRIME the burret BEFORE starting as the first chamber I did was about 3 cc more than the rest and when I rechecked it was in line with the others I was checking .
As far as the engine in question here , I DOUBT that piston is at zero deck , there was NEVER a zero deck 383 , closest was the 68-69 at .003 in the hole , this engine is a 70 and the piston is shorter , is the engine still std bore as that does not look like a factory piston in the other thread, and the TALLEST flattop 383 piston available is at best .015 in the hole on blueprint spec and the few 383's I have dealt with have been very close to factory blueprint deck at 9.98 .
Steve advertised CR for 383's is WELL above ACTUAL , Chrsyler FIBBED big time on the 383. The available aftermarket pistons are a COMPROMISE at best because, they are made to fit both open and closed chamber engines with them leaning better suited for closed chamber duty with a steel shim gasket and you still only get 9.5 if you are lucky .
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Re: Calculating compression ratio... ugh...
[Re: PAINT IT BLACK]
#451019
08/27/09 09:13 AM
08/27/09 09:13 AM
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U.S.S.A.
JohnRR
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Quote:
The sheet that came with my car (typed by the guy who rebuilt the engine) says that my pretty much stock 383 has a 10.5 CR.
I doubted that, so I had one of the 906 heads CC'd and the chamber was 93.4CC's.
Stock 3.375 stroke.
.030 over - so that's 4.28 bore size.
Head gasket bore diameter is 4.5
The compressed gasket thickness is .039
Flat top pistons
My pistons look to be at zero deck, but my measurements are slightly questionable by probably 0.00 to 0.02 ???
The compression calculates to about 8.5:1 on these online calculators - is that right?!? Is that what a 1970 HP 383 ratio should be? How in the heck did these guys figure 10.5:1???
If the chamber cc'd at 88cc's, it would calculate to about 9.5:1 which would make more sense to me. What gives?
I just went back and read this , you are right , 8.5 if you are LUCKY , I can't remember what the ADVERTISED ratio of a 70 383 is but I think it's LOWER than 68-69 since the NHRA spec is different than 68-69 . 68-69 is .020 ABOVE the deck , 70 is ... if I remember right ... .004 IN THE HOLE . you have to build a 383 to NHRA spec for 68-69 to get 10.0 , piston .020 above the deck , 79.5 cc head and a .021 steel shim gasket .
Steve you seem to be wasting a huge amount of time on this for whatever reason, fog it and hide it away , once you have the stoker in the car you'll never go back and install the 383 unless you want to prove something at the PURE STOCK Drags ... 
Last edited by Johnahah; 08/27/09 02:52 PM.
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Re: Calculating compression ratio... ugh...
[Re: JohnRR]
#451020
08/27/09 02:46 PM
08/27/09 02:46 PM
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Joined: Nov 2004
Posts: 7,647
Houston Texas
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Got a CHIP on my shoulder
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Houston Texas
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Quote:
Steve you are seem to be wasting a huge amount of time on this for whatever reason
Part of the joy of ownership I guess. I want to learn about the car, the history, the mechanics of it all. It doesn't matter that I'm going to stick the 383 in a corner and never use it. I want to know what is sitting in my corner.
I will never have anything to prove to me or anyone else, which is why I figure racing doesn't interest me much. No offense to anyone out there. But after sitting behind the wheel of a 500 hp small block, you are dang right I'll never stick that stock 383 back in this car. 
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Re: Calculating compression ratio... ugh...
[Re: JohnRR]
#451021
08/27/09 02:55 PM
08/27/09 02:55 PM
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Joined: Nov 2004
Posts: 7,647
Houston Texas
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Got a CHIP on my shoulder
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Quote:
As far as the engine in question here , I DOUBT that piston is at zero deck
You are very likely right. I just don't have an accurate device. But regardless, it won't make a whole lot of difference on the CR with all else being the same considering the small discrepancy.
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is the engine still std bore as that does not look like a factory piston in the other thread
I think they are TRW pistons. They are .030 over.
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Steve advertised CR for 383's is WELL above ACTUAL , Chrsyler FIBBED big time on the 383. The available aftermarket pistons are a COMPROMISE at best because, they are made to fit both open and closed chamber engines with them leaning better suited for closed chamber duty with a steel shim gasket and you still only get 9.5 if you are lucky .
I don't get why they would fib like that... how much of a difference could it have made in sales?
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Re: Calculating compression ratio... ugh...
[Re: PAINT IT BLACK]
#451024
08/28/09 08:25 AM
08/28/09 08:25 AM
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Joined: Jan 2003
Posts: 75,070
U.S.S.A.
JohnRR
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Quote:
Quote:
As far as the engine in question here , I DOUBT that piston is at zero deck
You are very likely right. I just don't have an accurate device. But regardless, it won't make a whole lot of difference on the CR with all else being the same considering the small discrepancy.
Quote:
is the engine still std bore as that does not look like a factory piston in the other thread
I think they are TRW pistons. They are .030 over.
Quote:
Steve advertised CR for 383's is WELL above ACTUAL , Chrysler FIBBED big time on the 383. The available aftermarket pistons are a COMPROMISE at best because they are made to fit both open and closed chamber engines with them leaning better suited for closed chamber duty with a steel shim gasket and you still only get 9.5 if you are lucky .
I don't get why they would fib like that... how much of a difference could it have made in sales?
You're right it's won't make that big a difference in the grand scheme other than it makes it lower , at least you aren't one of those people that think that 383 were POSITIVE deck height engines from the factory  ...
why the fibbed , MARKETING ....
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