Static Compression Ratio Seems High?
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
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From: Las Vegas Nevada
Static Compression Ratio Seems High?
So I'm rebuilding the top end of my fairly stock (1990s Chevy pickup engine) bored 350 (355) and I decided to calculate my static compression ratio
I'm getting a readout that seems much higher than I'd expect from my engine. Math is telling me I'm getting 10.68:1
I have flat top pistons with insignificant valve relief so I measured all this as follows:
Bore: 4.030
Stroke: 3.48
Gasket Thickness: 0.028 (GM Composition gasket)
Deck Height: .025
Piston top volume: 0.00cc
Combustion Chamber: 64cc (Promaxx Aluminum heads)
What did I measure wrong, or am I really getting that high of a compression?
_____________
This engine is being fitted with a Comp xe274H .49 lift 274/280 duration cam and 1.6 roller rockers alongside a Promaxx 2180 aluminum 64cc combustion/190cc intake cylinder heads
I'm getting a readout that seems much higher than I'd expect from my engine. Math is telling me I'm getting 10.68:1
I have flat top pistons with insignificant valve relief so I measured all this as follows:
Bore: 4.030
Stroke: 3.48
Gasket Thickness: 0.028 (GM Composition gasket)
Deck Height: .025
Piston top volume: 0.00cc
Combustion Chamber: 64cc (Promaxx Aluminum heads)
What did I measure wrong, or am I really getting that high of a compression?
_____________
This engine is being fitted with a Comp xe274H .49 lift 274/280 duration cam and 1.6 roller rockers alongside a Promaxx 2180 aluminum 64cc combustion/190cc intake cylinder heads
Last edited by Shdggsdv; Oct 31, 2016 at 04:38 PM.
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Typically those pistons have maybe a 5cc or so volume in the valve notches..so that has to be input as a negative number. If using the .028 gasket...otherwise you're pretty close. With a .040 gasket and 5 cc piston volume I'm getting around 9.9 or so.
JIM
JIM
Le Mans Master
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I used this one http://www.csgnetwork.com/compcalc.html
Includes things like the gasket bore which effects it too.
It matched what the machine shop come up with too when they had all the parts in hand.
Includes things like the gasket bore which effects it too.
It matched what the machine shop come up with too when they had all the parts in hand.
Le Mans Master
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From: Hermosa
Like Jim said probably 5cc pistons. You could cc them in the block to find out exactly. The heads can also be cc'd. If they are "as cast" heads, their volume may vary to some degree from advertised volume.
I get 10.0 if 5 cc pistons are considered with a 4.1 bore gasket.
I get 10.0 if 5 cc pistons are considered with a 4.1 bore gasket.
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
Likes: 15
From: Las Vegas Nevada
Still a lot higher than I expected. I was assuming 9:1 would be like the tip of what I might end up with being a fairly shitty stock engine
Thanks for the help
Last edited by Shdggsdv; Oct 31, 2016 at 06:17 PM.
Pro
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From: Vernon Hills Ill
Did you put it as +.025 deck height, I would think it should be 0 or less?
Thread Starter
Racer
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From: Las Vegas Nevada
Dr. Detroit
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From: New Braunfels Texas
Given these number using a Fel pro 1003 gasket (4.166x.041) you get this:
Which should be fine with those heads......your .025 deck number will make it even lower.......why are you using the .028 gasket....I believe it is a GM part.....but still.....
Jebby
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Melting Slicks
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From: Woodinville WA
Not acceptable as a number to input......most I have ever seen after 1985 or so come in about .015-.016......
Given these number using a Fel pro 1003 gasket (4.166x.041) you get this:
Which should be fine with those heads......your .025 deck number will make it even lower.......why are you using the .028 gasket....I believe it is a GM part.....but still.....
Jebby
Given these number using a Fel pro 1003 gasket (4.166x.041) you get this:
Which should be fine with those heads......your .025 deck number will make it even lower.......why are you using the .028 gasket....I believe it is a GM part.....but still.....
Jebby
Is 0.025" acceptable for older blocks? When I was researching my 79 L82, I also came across the 0.025" deck height #...
Adam
Melting Slicks
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From: Detroit MI
Why do you say that? Do you mean the calculator will not take it?
Last edited by 69autoXr; Nov 2, 2016 at 02:30 PM.
Dr. Detroit
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From: New Braunfels Texas
The only .025 I have ever seen is in a 69' truck motor I tore apart...in fact it was like .030 if I remember......
The last 5 or so 350's (even my 406) were at .014-.016.....
Do not take my word for it......and what does it matter for calculations sake....? Use the big number and know the worst case scenario......
I have never built an engine without zero decking the assembly....the increase in compression is gladly offset by having proper quench.
Jebby
Dr. Detroit
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Older engines are routinely .018-.025 maybe in the hole. Mid 80's 302 Fords and many Mopar small blocks used POSITIVE deck heights where pistons are above the deck. My 555" has the pistons .009" out of the hole.
Also have to look at replacement pistons. Many are made "short" to allow for block decking and end up as much as .040" down in the hole..and then folks uses a .040-.050 gasket and end up with zilch for compression.
Only way to know for sure is to measure it.
JIM
Also have to look at replacement pistons. Many are made "short" to allow for block decking and end up as much as .040" down in the hole..and then folks uses a .040-.050 gasket and end up with zilch for compression.
Only way to know for sure is to measure it.
JIM
Thread Starter
Racer
Joined: Jun 2014
Posts: 431
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From: Las Vegas Nevada
Not acceptable as a number to input......most I have ever seen after 1985 or so come in about .015-.016......
Given these number using a Fel pro 1003 gasket (4.166x.041) you get this:
Which should be fine with those heads......your .025 deck number will make it even lower.......why are you using the .028 gasket....I believe it is a GM part.....but still.....
Jebby
Given these number using a Fel pro 1003 gasket (4.166x.041) you get this:
Which should be fine with those heads......your .025 deck number will make it even lower.......why are you using the .028 gasket....I believe it is a GM part.....but still.....
Jebby
Using that, the head gasket I have currently lying around, and your deck clearance; I'm getting almost 11.75:1 which seems WAY too high
Edit: Okay I'm doing something wrong here in my calculations. Numbers have never been my forte
Edit#2: Yep. Now it looks normal again. Accidentally calculated for a 5cc dome piston. Getting 10.35 now
Last edited by Shdggsdv; Nov 2, 2016 at 02:48 PM.
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
To be honest I'm pretty new to this. I'd originally assumed that my engine wouldn't be making much compression due to it's stock nature even with 64cc heads so I chose a gasket that would raise it as much as possible without damaging the mounting surface of the heads. Also, my engine was rebuilt at one point and was bored over. Just went out and measured with a caliper. Measured 4.045
Using that, the head gasket I have currently lying around, and your deck clearance; I'm getting almost 11.75:1 which seems WAY too high
Edit: Okay I'm doing something wrong here in my calculations. Numbers have never been my forte
Using that, the head gasket I have currently lying around, and your deck clearance; I'm getting almost 11.75:1 which seems WAY too high
Edit: Okay I'm doing something wrong here in my calculations. Numbers have never been my forte
Jebby
Melting Slicks
Joined: Jun 2016
Posts: 2,979
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From: Woodinville WA
To be honest I'm pretty new to this. I'd originally assumed that my engine wouldn't be making much compression due to it's stock nature even with 64cc heads so I chose a gasket that would raise it as much as possible without damaging the mounting surface of the heads. Also, my engine was rebuilt at one point and was bored over. Just went out and measured with a caliper. Measured 4.045
Using that, the head gasket I have currently lying around, and your deck clearance; I'm getting almost 11.75:1 which seems WAY too high
Edit: Okay I'm doing something wrong here in my calculations. Numbers have never been my forte
Edit#2: Yep. Now it looks normal again. Accidentally calculated for a 5cc dome piston. Getting 10.35 now
Using that, the head gasket I have currently lying around, and your deck clearance; I'm getting almost 11.75:1 which seems WAY too high
Edit: Okay I'm doing something wrong here in my calculations. Numbers have never been my forte
Edit#2: Yep. Now it looks normal again. Accidentally calculated for a 5cc dome piston. Getting 10.35 now
(Don't forget that the bore of most of the gaskets is actually 4.1" -I believe to support 0.060" overbores.)
Adam
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Really need to measure deck height. As mentioned, many aftermarket replacement pistons will put the deck height pretty low.
JIM
JIM
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I am saying that if you do not measure it......you do not know.
The only .025 I have ever seen is in a 69' truck motor I tore apart...in fact it was like .030 if I remember......
The last 5 or so 350's (even my 406) were at .014-.016.....
Do not take my word for it......and what does it matter for calculations sake....? Use the big number and know the worst case scenario......
I have never built an engine without zero decking the assembly....the increase in compression is gladly offset by having proper quench.
Jebby
The only .025 I have ever seen is in a 69' truck motor I tore apart...in fact it was like .030 if I remember......
The last 5 or so 350's (even my 406) were at .014-.016.....
Do not take my word for it......and what does it matter for calculations sake....? Use the big number and know the worst case scenario......
I have never built an engine without zero decking the assembly....the increase in compression is gladly offset by having proper quench.
Jebby