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Author Topic: What's my compression ratio  (Read 3166 times)

Stan Paralikis

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What's my compression ratio
« on: October 02, 2009, 05:05:41 AM »

Trying to figure out what compression ratio I'm going to end up with.
Original piston/head combo would have been 8.2:1.
4.  .039 head gaskets.
I'm guessing aprox. 10.5:1  but I'm no rocket scientist on head stuff.  Anybody have a good idea?  :-/

firedome

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What's my compression ratio
« Reply #1 on: October 02, 2009, 05:34:42 AM »

You'd need to get exact dimensions with the new pistons and gasket used
to calculate the precise cubic volume of the combustion chamber
incorporating the revised bore & stroke at TDC and the total
cylinder volume at BDC and then derive the ratio mathematically to get
a truly accurate figure, otherwise it's pure guesswork, but it will be
higher, that's for certain.  I'd go with 8:1 and stick on a
Garrett T-4! 

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Steve

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What's my compression ratio
« Reply #2 on: October 02, 2009, 05:45:28 AM »

Yep
 
Better get the rocket fuel Stan
 
That's a good guesstimate though
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Butch Houghton

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What's my compression ratio
« Reply #3 on: October 02, 2009, 06:12:28 AM »

The only way to really know is to mockup 1 piston & rod with the crank & actually see how far in the hole the piston is.    Hit TDC & with a straightedge & feeler gauge you can get real close.

Did you actually measure those heads or just use the advertised CC'a.   Should be checked,   88 cc's is closer to nominal.   

The reason I say all this is we had a 440 that someone else built here in a 70 Cuda convert that we just had to finish a couple of things on & fire the engine.    Even with the timing backed to TDC it pinged under light loads.   Supposed to be Ross 9.5's in it & after teardown sure enough they were.   Chamber cc'd right at 88 where they should be.  Compression calculated at 11.1:1?   Taled to Ross & for one thing the pistons were'nt far enough in the hole for one thing so I had to conclude the deck was heavily decked.   took it to the machine shop & he checked it for me,  sure enough it was decked .040!  

You never really know unless you check it.

Butch


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firedome

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What's my compression ratio
« Reply #4 on: October 02, 2009, 06:42:17 AM »


Nothing beats direct empirical observation and measurement.
firedome2009-10-02 11:42:56
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Stan Paralikis

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What's my compression ratio
« Reply #5 on: October 02, 2009, 08:55:45 AM »

Sheesh.  All you anal guys.  All I wanted was an aproximation.
I'm not going bracket racing, ya know....
 
I'll find out with my first tank of regular.

Steve

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What's my compression ratio
« Reply #6 on: October 02, 2009, 09:49:54 AM »

It's actually
 
What was the compression ratio of the original engine and what was the compression ratio of the head donor engine.  If the head donor was higher, then you could be in between the two.  I can't recall any 440 over 10:1 stock though.  Your original engine was around 8.2 to 1. 
 
Just looked it up.  The 73 440 was also 8.2:1.  What year is the block?  Sometimes they lowered the compression ratio with the pistons and the heads.  The piston did not reach the top of the deck.  So without knowing th year of the block, it's hard to tell.  Or even guess. 
 
POLARACO2009-10-02 14:54:03
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Stan Paralikis

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What's my compression ratio
« Reply #7 on: October 02, 2009, 10:06:52 AM »

It's my 1976 block with these pistons:
 
Piston head volume +5 cc so that's why I said my head volume was 82-83 cc.
 
Commando12009-10-02 15:08:46

Stan Paralikis

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« Reply #8 on: October 02, 2009, 10:51:33 AM »

Never mind.  i figured it out myself.
 

 
And I was right about 10.5:1....
 
 

Steve

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What's my compression ratio
« Reply #9 on: October 02, 2009, 12:26:35 PM »

well hell!  We didn't have that info.  So if that is the case, you were right
 
My crystall Ball was in getting an oil change
 

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Butch Houghton

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What's my compression ratio
« Reply #10 on: October 03, 2009, 04:39:47 AM »

Since you have the Compression height listed take your old piston & measure with a caliper it's compression height to compare as a double check.

Just measure from the center of the wrist pin to the top,  that's your compression height.

Plug the old height number into the calculator & see what it comes out with too.  The old piston should have a lesser compression height.

Just a real eyeball verification.  Irregardless they look like nice pistons!  I dubious about a real 10.5:1 compression & if it is you're gonna have issues with detonation with Iron heads.

Butch


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firedome

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What's my compression ratio
« Reply #11 on: October 03, 2009, 05:03:23 AM »

Those do look like some nice pistons!

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Stan Paralikis

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What's my compression ratio
« Reply #12 on: October 03, 2009, 07:10:55 AM »

Quote from: HemiFury
... & if it is you're gonna have issues with detonation with Iron heads.
Nonsense!  With 92 octane and the hardened seats, there will be no problem.  You know yourself there were nothing but iron heads in the sixties and even the car makers were giving us engines with 11.5:1. 
AND (!!!!!) my heads are the motorhome heads with the extra cooling passages.

firedome

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« Reply #13 on: October 03, 2009, 02:30:14 PM »

True - as I recall one of the Olds W series had 11.75 CR, and iron is far more forgiving of heat and  pressure than aluminum anyway.
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Butch Houghton

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What's my compression ratio
« Reply #14 on: October 04, 2009, 05:51:27 AM »

Yeah,  but the octane rating were much better back then too.  Everything was RON rated & now  it's it's an average between RON & MON ratings ....much lower octane.

But whatever,  go right ahead...




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