'True' Factory Compression Ratio's

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69 Cuda 440

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'True' Factory Compression Ratio's

Much closer to the correct Compression ratio when these Mopar Engines
left the factory.

1968 and 1969 ..... 383/335 HP

Factory Listed
* Compression Ratio ............ 10.0 - 1
* Piston Deck Height ........... {+.021" above deck}
* Head Gasket {Steel Shim} .. .020" Thickness

Cylinder Heads .... #2843906 {Open Chamber}
Combustion Chamber CC's .... 79.5 {Minimum} ..... Factory Level = 87.0 CC's

Typically, the Flat-Top Pistons sat below the deck, and down in the
Cylinder anywhere from {-.010"} to {-.020"}.

'True' Compression Ratio as it left the Factory ......... 9.25 - 1
 
1968 and 1969 .......... 383/290 HP {2-Barrel}

Factory Listed
* Compression Ratio ........ 9.2 to 1
* Piston Deck Height......... {+.021" above deck}
* Piston Dish ................... -.120" {Volume 13.76 CC's}
* Head Gasket ................ .020"

Cylinder Heads ... #2843906 {Open Chamber}
Combustion Chamber ....... 79.5 {Minimum} ........ 86.5 {Factory}

Typically, the Dished Pistons sat below the deck, and down in the
Cylinder anywhere from {-.020"} to {-.035"}.

'True' Compression Ratio as it left the Factory ... 8.2 - 1
 
69 Cuda 440 said:
1968 and 1969 .......... 383/290 HP {2-Barrel}

Factory Listed
* Compression Ratio ........ 9.2 to 1
* Piston Deck Height......... {+.021" above deck}
* Piston Dish ................... -.120" {Volume 13.76 CC's}
* Head Gasket ................ .020"

Cylinder Heads ... #2843906 {Open Chamber}
Combustion Chamber ....... 79.5 {Minimum} ........ 86.5 {Factory}

Typically, the Dished Pistons sat below the deck, and down in the
Cylinder anywhere from {-.020"} to {-.035"}.

'True' Compression Ratio as it left the Factory ... 8.2 - 1
Click to expand...


This is contradictory...

The fist piston spec says +.021" above the deck, then you say that they are below the deck from -.020" - -.035"...
 
krazykuda said:
This is contradictory...

The fist piston spec says +.021" above the deck, then you say that they are below the deck from -.020" - -.035"...
Click to expand...

The Factory lists that the Piston 'should' be at {+.021"} above the deck
with 'true' specifications.

But, that is 'not' the height that they left the Machining Division.

Most of the {1968 and 1969} 383/290 {2-Barrel} Engines were at {-.020"} below the deck when they left the Machining Division. Some Pistons were even further down in the Cylinder.
 
That makes more sense.

If you do the compression calculations from the raw specs in the service manual, they don't add up... Machining tolerances....

The only to know for sure is to check it yourself like my write up in the how to section...
 
krazykuda said:
That makes more sense.

If you do the compression calculations from the raw specs in the service manual, they don't add up... Machining tolerances....

The only to know for sure is to check it yourself like my write up in the how to section...
Click to expand...

Correct,

The Factory 'advertised' Compression-Ratio is way off the charts when compared
to 'actual' measurements.

Back-in-the-Day,

That is why Mopar recommended milling the Cylinder Heads {#2843906} for the
1968 and 1969 '383' {.030"}.

It really got you back to around {10.0 to 1} Compression Ratio, without touching
{decking} the Cylinder Block.

And you were fine, with Valve-to-Piston clearance, with the recommended Street Performance {484/284} 'Hydraulic' Camshaft.
 
It's not that the pistons were "down in the cylinders" the cylinders were above the pistons. That's Chrysler's expert block decking for ya. lol
 
i have been told that the stock 335 hp 383 pistons were below the decks as much as .060 and know some who have cc'd the heads at as much as 93cc.
I also understood the factory rated them so you could build a 10.5 : 1 engine and be NHRA legal.
Actual calculation on some early 10.5 : 1 340's was around 9.5:1 and those pistons were above the decks by .022
...this is a very grey area.
 
ir3333 said:
i have been told that the stock 335 hp 383 pistons were below the decks as much as .060 and know some who have cc'd the heads at as much as 93cc.
I also understood the factory rated them so you could build a 10.5 : 1 engine and be NHRA legal.
Actual calculation on some early 10.5 : 1 340's was around 9.5:1 and those pistons were above the decks by .022
...this is a very grey area.
Click to expand...

I've pulled apart my share of 1968 and 1969 ~ '383' HP 4-Barrels.

Never found one with a 'Dead-Nuts Zero' - Piston Deck height or even close
to (+.021} above deck.

Most Piston Tops were below the deck a good amount.
 
The 383 in my old 68 RR had the left bank .000,-.001,-.003,-.004
Now the right side difference -.006, -.009,-.011,-.014
 
69 Cuda 440 said:
Correct,

The Factory 'advertised' Compression-Ratio is way off the charts when compared
to 'actual' measurements.

Back-in-the-Day,

That is why Mopar recommended milling the Cylinder Heads {#2843906} for the
1968 and 1969 '383' {.030"}.

It really got you back to around {10.0 to 1} Compression Ratio, without touching
{decking} the Cylinder Block.

And you were fine, with Valve-to-Piston clearance, with the recommended Street Performance {484/284} 'Hydraulic' Camshaft.
Click to expand...



We used to run our heads on the thicker side so if they got any knicks or scratches, we had enough material to mill off and get a good surface without going under spec.

So the factory heads tend to be to the larger side on first time through heads versus "rerun" heads...
 
So, at what point if any did they get more correct in the advertized ratio?

92- 03 Magnum?

85-91 LA roller?
 
1964 and 1965 ~ '426-S' 'Street-Wedge' 365 Horsepower

Factory Listed
* Compression Ratio ........ 10.1 - 1
* Piston Deck Height ........ {-.042"}
* Piston Type ................... Flat Top
* Head Gasket ................. .020" Thickness

Cylinder Head .... #2406516 ~ 'Closed Chamber'
Combustion Chamber ........ 73.5 CC's {Minimum} ...... 79.0 CC's {Factory}

Notorious for having a tall deck, with the Pistons located 'deep' in the Cylinder,
usually around {-.075"}

'True' Compression Ratio as it left the Factory .... 9.0 - 1
 
1968 and 1969 ~ 318/230 HP ~ 2-Barrel

Factory Listed
* Compression Ratio ........ 9.0 - 1
* Piston Deck Height ....... {-.029"}
* Piston Type ................. Flat
* Gasket ....................... .020" Thickness

Cylinder Head ... #2843657 ~ 'Open Chamber'
Combustion Chamber ....... 60.6 CC's {Minimum} .... 67.0 {Factory}

Typically, the Piston was found deep in the Cylinder Bore, at approximately
{-.060"} to {-.070"}.

'True' Compression Ratio as it left the Factory ......... 8.1 - 1
 
I've only owned one 383 commando in a '68 Fury. I took off the 906 heads to have hardened seats/new valves installed, and my pistons stuck out of the block at TDC at least .010, maybe more. I did not measure them, but it was visible by eye.
 
318willrun said:
I've only owned one 383 commando in a '68 Fury. I took off the 906 heads to have hardened seats/new valves installed, and my pistons stuck out of the block at TDC at least .010, maybe more. I did not measure them, but it was visible by eye.
Click to expand...

That is very interesting.

Because the Machining Broach Units at the factory machining division were
set up to allow for a minimum of {.025"} difference between the Factory advertised
measurements and {true height} 'as they left' the machining plant.
 
318willrun said:
I've only owned one 383 commando in a '68 Fury. I took off the 906 heads to have hardened seats/new valves installed, and my pistons stuck out of the block at TDC at least .010, maybe more. I did not measure them, but it was visible by eye.
Click to expand...

we found the same with a 68 Fury Vip, maybe not .010 , but above deck.
 
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