383...diff. year question...

-
65Val

65Val

Average Length Member
Joined
Oct 14, 2005
Messages
5,362
Reaction score
1,315
Location
Victoria, BC, Canada, EH!
How did Mopar keep the same comp. ratio in a 383 when changing from early closed chamber heads to later open chamber heads? Early heads are ~73.5 cc's, and later heads are ~79.5 cc's. What changed to maintain 10 to 1 compression?...rod length, piston pin height???
 
65Val said:
How did Mopar keep the same comp. ratio in a 383 when changing from early closed chamber heads to later open chamber heads? Early heads are ~73.5 cc's, and later heads are ~79.5 cc's. What changed to maintain 10 to 1 compression?...rod length, piston pin height???
Click to expand...

Nothing. The standard 383 just dropped compression. Only the 383 HP remained at the higher mark. And to answer the question, the HP pistons used a longer compression height, bringing the piston closer to the deck.
 
383 "Stock Piston Depth"

68-69 383hp had positive deck height. Actually I thought the 70 and 71 383 hp engines did also but maybe not. 68-69 hp used the 906 heads. I don't remember if the 70 and 71 did or not.
 
Last edited:
RustyRatRod said:
Nothing. The standard 383 just dropped compression. Only the 383 HP remained at the higher mark. And to answer the question, the HP pistons used a longer compression height, bringing the piston closer to the deck.
Click to expand...
I just checked Sealed Power pistons for '67-down (closed chamber heads) and '68 up (open chamber heads) and they have the same pin height.
 
65Val said:
I just checked Sealed Power pistons for '67-down (closed chamber heads) and '68 up (open chamber heads) and they have the same pin height.
Click to expand...

That's because the aftermarket picked "ONE" compression height and ran with it.
 
65Val said:
Ok, thanks. How did they do that? Did the '68-up block have lower deck heights from the factory?
Click to expand...

No. The deck heights were the same. They used two compression heights.
 
RustyRatRod said:
No. The deck heights were the same. They used two compression heights.
Click to expand...
Thanks Rob. I see 1.84 for the Sealed Power cast pistons, and 1.92 for KB forged.I assume the taller factory cast ones were 1.92 as well then, to gain back the lost compression?
 
65Val said:
Thanks Rob. I see 1.84 for the Sealed Power cast pistons, and 1.92 for KB forged.I assume the taller factory cast ones were 1.92 as well then, to gain back the lost compression?
Click to expand...

Perhaps. You never know regarding aftermarket. The best thing would be to find a factory piston and compare....but that might be difficult. I would guess the KB would be it, but that's a guess.
 
Jadaharabi said:
383 "Stock Piston Depth"

68-69 383hp had positive deck height. Actually I thought the 70 and 71 383 hp engines did also but maybe not. 68-69 hp used the 906 heads. I don't remember if the 70 and 71 did or not.
Click to expand...
I'm pretty sure 68 was the only year 383 Magnum was zero or positive deck.
I pulled apart several 68 383's and the pistons varied from slightly under zero to .005 out of the hole.
 
fishmens67 said:
I'm pretty sure 68 was the only year 383 Magnum was zero or positive deck.
I pulled apart several 68 383's and the pistons varied from slightly under zero to .005 out of the hole.
Click to expand...

My first Road Runner was a 69, 383hp, 4 speed with positive deck height.
 
If you look on the Summit site, you'll see the stock cast pistons have like a 1.8xx compression distance. That seems to be the shortest. That's probably the stock "non HP" compression distance.
 
I stand corrected 68 and 69 were listed as positive deck. lol listed LOL
 
fishmens67 said:
I stand corrected 68 and 69 were listed as positive deck. lol listed LOL
Click to expand...

Yeah you said it. "Listed". LOL I wonder if you measured 100 68 and 69 factory assembles engines "how close" they'd come? LMAO
 
Last edited:
RustyRatRod said:
Yeah you said it. "Listed". LOL I wonder if you measured 100 68 and 69 factory assembles engines "how close" they'd come? LMAO
Click to expand...



Just one with positive deck height...........from the magazine test car that Ronnie Sox drove.
 
All of the original 383 4bbl engines that I'm directly or indirectly aware of had the piston at or slightly below the deck. Factory tolerances vary, but actual positive deck seems to be more the exception.
 
Last edited:
RustyRatRod said:
Yeah you said it. "Listed". LOL I wonder if you measured 100 68 and 69 factory assembles engines "how close" they'd come? LMAO
Click to expand...
Just measured my ‘68 block with 1.84 pin height Sealed Power 366NP pistons... about .051 in the hole... confirms 69 Cuda 440’s numbers.
 
65Val said:
Just measured my ‘68 block with 1.84 pin height Sealed Power 366NP pistons... about .051 in the hole... confirms 69 Cuda 440’s numbers
Click to expand...

When I say my 69 Roadrunner 383 HP had positive deck height I was talking about the factory pistons not aftermarkets.
 
65Val said:
Just measured my ‘68 block with 1.84 pin height Sealed Power 366NP pistons... about .051 in the hole... confirms 69 Cuda 440’s numbers
Click to expand...

Yup. That sounds just right for a non HP.
 
You must log in or register to reply here.
-

Back
Top