Off the Scale Compression Pressure Values?
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
Off the Scale Compression Pressure Values?
So recently I was putting in a new set of spark plugs in my supposedly highly original/low mileage '70 LS5 (nominal CR 10.25). And, while I was at it, I also decided to perform a compression test on this engine, of which I have practically no knowledge of the mechanical state it is in... (And yes, I am aware of the fact that one can only draw limited conclusions from a compression test alone - but one can at least check for consistency between cylinders, I thought).
Before I get to the results, I have to mention one thing: The test wasn't precisely done 'by the book' in that 1) the engine was cold, and 2) no accessory belts were present (was replacing those, too; and I also took pity on the still original starting motor
).
Now, what I got was this: (cyl. # 1-2-3-4-5-6-7-8) 205-205-219-201-207-200-205-213 psi...
Now, I think I look pretty good in the 'consistency' department.
BUT, what about these absolute values??? It took me a while to find 'authoritative' numbers of what to expect - and finally found something in the '70 CSM: 160 psi... Can anyone make sense of why I'm at least 25% ABOVE that?
(And BTW, the first thing I checked was whether the pressure gauge was malfunctioning by checking it against a known pressure source (10 bar/145 psi): It was spot on...)
Before I get to the results, I have to mention one thing: The test wasn't precisely done 'by the book' in that 1) the engine was cold, and 2) no accessory belts were present (was replacing those, too; and I also took pity on the still original starting motor
).Now, what I got was this: (cyl. # 1-2-3-4-5-6-7-8) 205-205-219-201-207-200-205-213 psi...
Now, I think I look pretty good in the 'consistency' department.
BUT, what about these absolute values??? It took me a while to find 'authoritative' numbers of what to expect - and finally found something in the '70 CSM: 160 psi... Can anyone make sense of why I'm at least 25% ABOVE that?
(And BTW, the first thing I checked was whether the pressure gauge was malfunctioning by checking it against a known pressure source (10 bar/145 psi): It was spot on...)
Race Director
Joined: Jul 2004
Posts: 12,696
Likes: 329
From: Scottsdale Arizona
Here's a bump for ou as I'm interested in the answer as well.
One thought is you might have gap less rings. I had these on my stroker (before it blew up
) and the compression test readings were higher than I would have expected for the compression ratio.
One thought is you might have gap less rings. I had these on my stroker (before it blew up
) and the compression test readings were higher than I would have expected for the compression ratio.
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,579
Likes: 7,020
From: New Or-leens Loo-z-anna
Does it appear that the heads have ever been removed? I ask because my '70 350cu.in. replacement engine has different heads installed by the previous owner. Also, on my own 350SBC in my '34 truck, I installed .030" thick head gaskets after removing the .055" thick "late model" headgaskets.
Keep us updated....and be glad of good compression readings.
Keep us updated....and be glad of good compression readings.
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
That's not out of line. My '71 LS5 is modified with forged domed pistons and has a measured 10.15:1 CR. With a larger-than-factory cam it cranks 200 PSI.
Your true CR is probably less than 10.25:1 (that was the blueprint spec, seldom achieved by the factory). Couple that with a smaller cam (which increases cranking pressure) and your results are as expected.
160 PSI sounds like what I'd expect out of a later motor with 8.5:1-9:1 CR.
Your true CR is probably less than 10.25:1 (that was the blueprint spec, seldom achieved by the factory). Couple that with a smaller cam (which increases cranking pressure) and your results are as expected.
160 PSI sounds like what I'd expect out of a later motor with 8.5:1-9:1 CR.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Static compression combined with cam timing (overlap, intake valve closing point) determine compression readings. Looks like your likely at the upper limit to run pump fuel. All readings within 10% is excellent.
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
@all,
thanks for the input!
What I take away from this discussion is,
So, until further evidence surfaces, I'd like to further delude myself into thinking that I'm the lucky owner of an extremely low mileage car powered by a very low wear original engine...
thanks for the input!
What I take away from this discussion is,
- No one has pointed out an obvious (stated in or implied from my OP) deficiency in my test setup/procedure.
- While the absolute values measured seem very high, they are not impossibly so.
- While several of you pointed to possible modifications done to the engine, this remains guesswork, since I know exactly nothing of what might have been done to it in the past.
So, until further evidence surfaces, I'd like to further delude myself into thinking that I'm the lucky owner of an extremely low mileage car powered by a very low wear original engine...
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,579
Likes: 7,020
From: New Or-leens Loo-z-anna
@all,
thanks for the input!
What I take away from this discussion is,
So, until further evidence surfaces, I'd like to further delude myself into thinking that I'm the lucky owner of an extremely low mileage car powered by a very low wear original engine...
thanks for the input!
What I take away from this discussion is,
- No one has pointed out an obvious (stated in or implied from my OP) deficiency in my test setup/procedure.
- While the absolute values measured seem very high, they are not impossibly so.
- While several of you pointed to possible modifications done to the engine, this remains guesswork, since I know exactly nothing of what might have been done to it in the past.
So, until further evidence surfaces, I'd like to further delude myself into thinking that I'm the lucky owner of an extremely low mileage car powered by a very low wear original engine...
Intermediate
Joined: Jun 2013
Posts: 45
Likes: 0
much of a compression test has to do with the cam overlap. it appears that you have a low very torquey cam with some smaller chambered heads. if you can still run that on pump gas, then rock it.
im sure it makes for a very good stop light car. -as in taking off on the green
im sure it makes for a very good stop light car. -as in taking off on the green
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
)... There are still other gremlins to slay.The last one I found was an accelerator cable stud mounted in the upper/wrong (= "truck") hole on the lever - which made the secondaries open only half way. Had me driving around practically on the idle circuit for as long as I've had the car - which means two years
. Live and learn.
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
Does this suggest the cam may be suitable for only low RPM torque, robbed the high end? Wondering what I may deduce from the compression readings being as high as they are. The vacuum I get is lower than stock. No vacuum leaks, appears to be either timing related or cam.
Well, actually it doesn't (yet )... There are still other gremlins to slay.
The last one I found was an accelerator cable stud mounted in the upper/wrong (= "truck") hole on the lever - which made the secondaries open only half way. Had me driving around practically on the idle circuit for as long as I've had the car - which means two years . Live and learn.
)... There are still other gremlins to slay.The last one I found was an accelerator cable stud mounted in the upper/wrong (= "truck") hole on the lever - which made the secondaries open only half way. Had me driving around practically on the idle circuit for as long as I've had the car - which means two years
. Live and learn.
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
).
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Well, actually it doesn't (yet )... There are still other gremlins to slay.
The last one I found was an accelerator cable stud mounted in the upper/wrong (= "truck") hole on the lever - which made the secondaries open only half way. Had me driving around practically on the idle circuit for as long as I've had the car - which means two years . Live and learn.
)... There are still other gremlins to slay.The last one I found was an accelerator cable stud mounted in the upper/wrong (= "truck") hole on the lever - which made the secondaries open only half way. Had me driving around practically on the idle circuit for as long as I've had the car - which means two years
. Live and learn.http://www.popularhotrodding.com/eng...o/viewall.html
Here is a key phrase, copied from the article, that pertains to the original subject of this thread:
"If the cylinders are sealing up well, I look for 190 psi as a lower limit with preferably 200 psi as a target when using 93-octane fuel."
Scott
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
Here is an excellent article that does a pretty good job of explaining all the voodoo surrounding compression. Enjoy!
http://www.popularhotrodding.com/eng...o/viewall.html
Here is a key phrase, copied from the article, that pertains to the original subject of this thread:
"If the cylinders are sealing up well, I look for 190 psi as a lower limit with preferably 200 psi as a target when using 93-octane fuel."
Scott
http://www.popularhotrodding.com/eng...o/viewall.html
Here is a key phrase, copied from the article, that pertains to the original subject of this thread:
"If the cylinders are sealing up well, I look for 190 psi as a lower limit with preferably 200 psi as a target when using 93-octane fuel."
Scott
(But how much of this, especially the numbers given, can be transferred to a 44 year old engine in an essentially unknown condition remains an open question, unfortunately...)
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
also take into account the accuracy of your compression gauge.. try another one and you may only get 185 across all the cyls. but I agree, the fact that you are all close together like that is encouraging.
Thread Starter
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
As stated im my OP I DID check the gauge - albeit at 145 psi (highest "known" pressure available); it was spot on there, and I don't think the gauge is THAT non-linear, since the 200 psi range is still well below full scale.
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
I got readings much like yours. So did a local Corvette shop when they checked my car. The readings are probably correct.