What's the difference?

[Randra]

They say if you use 69 heads which are open chamber on a 66 which are closed chamber it will lower the compression ratio. My question is what is the difference. Is it the block or the pistons. And if it's the pistons what's the difference there. And how low is it really. I've had this combination for years and it ran well. But it never seemed like it had the power it should. This is a 383 with 906 heads.

 

[Garys1969RR]

Anytime you put an open chamber head of 79cc on an engine it will result in lower C/R than a closed chamber head of say 73cc. These are the factory specs, but the actual cc is closer to 78 on the closed, and 86 on the open chamber. Or there abouts. The compression height of the piston is what is changed in these motors to get various C/R. The various blocks are not intended to be different from the factory as far as deck height is concerned. The 383 piston has a max compression height of 1.935". This would put the piston even with the deck, with a stock rod. The KB 162 piston has a C/H of 1.908" which puts it .027" below deck. So using a 1.908 C/H piston which is .027 below deck with an open chamber head of 86 cc, your C/R would be 9.08 to 1. Putting on a closed chamber head of 78 cc would get you a C/R of 9.8 to 1.

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The 1966 383 prob has pistons that are .027" below deck from the factory. And the High Performance 383 with an advertised compression ratio of 10 to 1, and closed chamber heads was prob close to that. So putting open chamber heads on there prob dropped your C/R about .8 points.

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There are prob some guys on here who know the exact factory compression height on these motors.

 

[Cranky]

I have never checked a set of open chamber heads that were still factory and checked at 79cc. They were always 88 to 90cc and close chambered heads were right around 78-80. Now all the books say different but the checking tube doesn't lie.

 

[Runner 68]

I have never checked a set of open chamber heads that were still factory and checked at 79cc. They were always 88 to 90cc and close chambered heads were right around 78-80. Now all the books say different but the checking tube doesn't lie.

Neither does Cranky.

 

[Cranky]

Neither does Cranky.

Just little white ones

 

[Garys1969RR]

I have never checked a set of open chamber heads that were still factory and checked at 79cc. They were always 88 to 90cc and close chambered heads were right around 78-80. Now all the books say different but the checking tube doesn't lie.

Yes Cranky that is true. I was using the factory specs as an illustration. Whenever I calculate C/R with a closed chamber head I always use the 78cc figure to get close. Does anyone know the factory deck clearance for a 1966 383?

 

[Cranky]

A 4 bbl engine was rated at 10-1 and a 2 bbl was 9.2-1. IIRC, the piston in these engines were pretty close to 0 deck....maybe .010-.015 below. Doing the math says around .010 below but was 10-1 a real figure? I have a 440 with closed chamber heads on it with factory style gaskets and a .050 down piston that's pushing 10.3-1 but the 440 does have a larger bore. Figure open chamber heads on that .010 below deck 383 piston and the compression ratio will be right at 9-1 instead of 10-1 with the closed chambered head....

 

[Meep-Meep]

I measured exactly the same chamber volume as Cranky. Pistons on 440's can be nearly .100" in the hole. If you take the theoretical 10.7" deck height and work backwards with the rod length, 1/2 the stroke; factor in the 440 6 BBL pin height, you get a zero deck. But that's theoretical and in reality you will likely get -.020" or so. A typical late 60's Magnum 440 (I believe) uses pistons that are right around 1.990" pin height and that comes out to -.075" in the hole. The difference in chamber volume of the closed and open chamber heads is about .4 points of compression on a 440. Then you have the gasket, which can be worth another .3 point or so. I am at 10.1:1 with a generous valve relief volume, 79 CC chamber, steel shim gasket and -.015" in the hole.

 

[383man]

The 906 heads I used on my old 440 build were cut .060 and with that much cut off they CC'd at 76. I agree most stock 906's cc'd about 88 to 90 cc's. Ron

 

[Randra]

Ok I get it. So what should i do with it to increase the compression. Engine Is stock with a mild cam. Looking too get near stock compression ratio of a 68 383 HP. I see there is different thickness gaskets too which I would assume would change the ratio also a little bit.

 

[Propwash]

Ok I get it. So what should i do with it to increase the compression. Engine Is stock with a mild cam. Looking too get near stock compression ratio of a 68 383 HP. I see there is different thickness gaskets too which I would assume would change the ratio also a little bit.

Toss on some closed chambered 75CC Eddy RPM heads, grab some quench, fry your tires off, and wonder why you ever ran old iron heads for so long.

 

[Randra]

I can't Afford those. I'm married and have six kids. Lol

 

[HT413]

I cc'd my 915's at 80cc, 906's at 89. Cc'd my 440source heads at 81.

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Wow, I have 3 sets of heads. I'm starting to sound like a gear head.

 

[66chargerpat]

@ Randra, Some of the 68hp 383 had the pistons out the hole some and they came with the 906 iirc. As for the 66,67 2bbl 383s the piston are about .100" down in the hole. Pat