compression ratio ?

[Al]

Hi
I'm running the Edelbrock RPM heads (84 cc) on my 1968 383. I have close to zero deck height (.007) with the flat top pistons and the Permatorque gaskets have .039 compressed height. What is the compression ratio ?  Would it be around 11:1?

Greets
Al

[elacruze]

http://www.csgnetwork.com/compcalc.html

[Al]

What is : "Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')" 

Al

[elacruze]

What is : "Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')" 

Al

Some pistons, as in Poly 318's and most Turbo motors actually have a dish in the piston crown, opposite of a raised dome. If yours are flat, your dome volume is 0. If yours are flat with piston notches, close enough is -2 or -3cc's.
So, if you have flat pistons with notches, you would enter '-3'.
If you have dished or domed pistons, you would have to measure the actual cc's by putting a piece of plexiglas over the cylinder, moving the piston down the bore a known distance (which must get the highest point on the dome below deck height) and through a hole in the plexi use a burette to add a measured amount of liquid (I like ATF) to eliminate all the air in the cylinder. For a dish piston you could use 0 deck and measure, then find the difference between the actual measure and theoretical flat top piston measure.

Clear as mud?

[Al]

  You lost me.

So with my old 906 heads with their 88cc I had 10:1 compression and now with the Eddies and 84cc I get 9.044:1. Confusing 
Al

[elacruze]

What does the top of your pistons look like?
Is your engine overbored at all?

[Al]

Hi

The pistons are flat, sitting .007 in the hole. Engine has not been over bored.

Al

[elacruze]

Your actual measured CR is 9.29:1 with the new 84cc heads.
Your old CR was 8.95:1 with 88cc heads.

[Al]

Thank you for your reply. 

So the official 10:1 in the 68 HP engine were pretty far from the truth. 9.29 sounds a bit tame for a sound engine or am I mistaken?

Greetings
Al

[elacruze]

I'm not a historian, don't know much myself about 383's.

Listed compression for a '69 2-bbl 290hp 383 is 9.2:1, so you're right on target, at least for police motors-Optional 4v 330hp motor had 10:1 CR.

http://www.allpar.com/squads/engines.html

Sounds like you have a deuce, not HP 383.

[Al]

Somewhat strange, my 68 numbers match, 4 barrel Carter, HP exhaust manifolds ...
Well at least now I know the actual CR.

Greets
Al

[elacruze]

Somewhat strange, my 68 numbers match, 4 barrel Carter, HP exhaust manifolds ...
Well at least now I know the actual CR.

Greets
Al

Well, math don't lie...I'll be interested to hear the guys who know their 383's better to weigh in on this.

[Cooter]

You cannot treat the factory rating as the "Bible"...It was a guess at best. I've torn down "HP" 440 engines only to find EACH CYLINDER was a different measurement IN THE HOLE...And I'm NOT talking .007 here, I'm talking .020-.070 in the hole...

Just because the factory claimed it was so, don't mean it was......

[Challenger340]

I'm no Historian either, and I'm giving away my age here,
but,
Most of the '68/'69 383/335hp "Magnum" stuff we were tearing down back in the early 70's, had the Pistons very close to "0" deck, so your .007" sounds about right ?

However,
The Factory used to use a "Steel Shim" Gasket that was about .020" thick
and,
Most of the Factory Valve Depths from the Machining Target Pad(beside the Plug), dictated that the 906 Heads back then, were all close to the 82 to 84cc mark with NO valve Seat recession and high Valve Margins.

Although the Factory "Advertised" the 10 to 10.2:1 C.R. Ratio, we figured the Actuals in service were more like 9.6 or 9.7 to my recollection ?

Whats Interesting to Note here,
is that waaay back then,
ALL N.H.R.A. Approved Pistons for Competition, listed their C.R. Ratios based upon the 906 Head "legal Volume" as 79.5 cc(which we used to Mill to).

I think the C.R. confusion, becomes more of WHAT the 906 Heads were SUPPOSED TO BE from Factory, as opposed to what they were IN SERVICE ?
79.5cc is what the 906 Heads were SUPPOSED to be ? and the Factory was basing upon ?
NOT,
the 82-84cc(in service), and higher that we see today.

Geez I'm an OLD Fart !

[Al]

Interesting piece of information here.

[Challenger340]

Just a reminder here,
The Eddy Heads will not perform well, at low 9:1 C.R. Ratios due to the Aluminum Heads Heat Dissapation Characteristics.
Long story short;
You may find the output a little "anemic".

[TylerCharger69]

Numbers may match....but perhaps someone switched the 2 barrel for a 4 barrel intake and carb?.....i know i did!!

[Al]

Well 9.3:1 is less than I expected.

The car not being intended as a race car but as a car for cruising, it is not really that dramatic ... but as I said, I was expecting more. And as elacruze said: "math don't lie".  

By the way, I intend to use unleaded fuel. Is that ok?

[BSB67]

The 383 hp was never really close to the advertised 10:1.  9.5:1 if you were lucky.  IIRC, the blue print spec for the piston was 0.020 out of the hole. 0.005 to 0.010 in the hole typically found. Like someone suggested, if you rebuild one, re-con the rods, sink the valves add a thick head gasket and add a big cam, you just made a slug.

Because the 383 is relatively small (compared to the 440) and has a short stroke, its CR is much more sensitive to the details of the final compression volume, and you should not give up on getting the highest CR for your combo as possible.   It will make a noticeable difference in performance.  It might be worth it to mill the heads for an actual 10:1 CR.  I would suggest that you contact a High Performance professional to match up your specific application to the CR and camshaft.

[Sublime/Sixpack]

Well 9.3:1 is less than I expected.

The car not being intended as a race car but as a car for cruising, it is not really that dramatic ... but as I said, I was expecting more. And as elacruze said: "math don't lie".  

By the way, I intend to use unleaded fuel. Is that ok?

If cruising is indeed what you plan to do with the car I doubt that you'll be unhappy with the 9.3 to 1 comp. ratio.

[firefighter3931]

If cruising is indeed what you plan to do with the car I doubt that you'll be unhappy with the 9.3 to 1 comp. ratio.

For a mild build that is perfectly acceptable. KamKuda has 9:1 compression with Eddy heads in his 408 and it runs fantastic. It'll lay down 200ft of rubber from a dead stop. 

Al, you will be able to run that combo on unleaded fuel, no problem. In fact it should run fine with 89 octane pump gas. 


Ron

[Al]

Thank you Ron.  

I suppose that with the ignition upgrade (MSD 6), lunati 603 cam, eddy performer RPM heads, eddy performer RPM intake manifold, Holley SA 670 and in a few weeks, the TTI headers and exhaust my 383 should make decent power. 

Al