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Compression

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TRdynamo

TRdynamo

Freshman Member
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I checked the compression of my TR6 and all the cylinders are reading around 110....my calculations with the compression ratio figure that the readings should be much higher. I understand its a 75' and it has low compression (7.75:1) but still, can anyone interpret that info?
 
Make sure tests were done full open throttle & engine warm. Your reading should be between 115 & 125 for a 7.75: 1 model year.
 
Are you having a problem that led you to do a compression check in the first place? Depending on the mileage, 110 wouldn't necessarily be a bad reading on a low compression engine. I would be more concerned with variations between cylinders, or two adjacent holes reading low.
Just curious.
Jeff
 
Between probable innaccuracy of the gauge and difficulty in calculating the 'right' number I would have to agree that this is more of a relative measurment than an absolute number.

In the absence of any symptoms of problems, if the 110 is fairly consistent across the board then probably not an area for concern.
 
Calculations of compression pressure from compression ratio are difficult as Geo says. It depends greatly on valve timing - The piston cannot begin to compress anything until the intake valve gets closed somewhat after BDC as the piston has already started back up. Altitude will also lower compression pressures about 3.4% for every thousand feet elevation increase.
D
 
I did the compression test on my 73 TR6 earlier this year. I had about 120 across all 6 cylinders. I've always been told that as long as they are close to each other you don't have a lot to worry about unless your having other symptoms. Of course I didn't have the throttle wide open. Not sure if that would raise or lower the results. Answers??? Thanks
 
My understanding (could be all wet) is that you do the test with WOT so the restriction of the butterfly (throttle disc) does not mask a leak in an intake valve. I also prop the carb pistons up out of the way.

I also remove all the plugs so a leak from cylinder to cylinder is not hidden by partial compression in an adjacent cylinder.
 
Thanks everyone, I just did the check to make sure my engine didnt have any compression issues I was unaware of. Engine does have 60,000 miles on it with the head being rebuilt in 99' by previous owner (reciepts varify rebuild). Hoping with good maintenance I shouldnt have to rebuilt the engine for another 60,000 miles.
 
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Of course I didn't have the throttle wide open. Not sure if that would raise or lower the results. Answers??? Thanks

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I have found very slight difference, 5 psi higher, with throttles open vs closed, & 5 psi higher with hot vs cold engine. If in doubt, try it both ways.
D
 
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Test until compliant? /ubbthreads/images/graemlins/grin.gif
Jeff

[/ QUOTE ]
Yup. Always, unless you wear it out testing. It's been known to happen.
D
 
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