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Old 07-04-2011, 07:56 PM   #1
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Default If you have a low comp built motor please come in

What kind of hot compression results are you guys seeing with lower comp built motors once broken in? Say 8.5:1-9:1 compression.

Thanks.
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Old 07-04-2011, 08:47 PM   #2
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I will test tomorrow and letter you know.
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Old 07-04-2011, 08:54 PM   #3
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Dope, thanks.
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Old 07-04-2011, 09:05 PM   #4
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What other people get shouldn't be what your worried about. Just do a comp test and look for the consistency.

If something is wrong either 1 cylinder will stand out or two cylinders next to eachother will stand out

I had a shitty gauge once and had 90 across the board, I wasnt worried because it was consistent. 3 weeks later I tried it with a different tool and it was 160 across, this was on a nissan motor though.
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Old 07-04-2011, 09:22 PM   #5
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I know. I just want to have an idea of what others are seeing for compression on low comp motors. Mine is 170-170-165-165. It's within 5PSI of the last comp test, but that was with a different tester.
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Old 07-04-2011, 09:53 PM   #6
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I had all high 180's on my 9.1 IIRC

But da chop is right: it doesn't matter what we have, consistency is key
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Old 07-04-2011, 10:26 PM   #7
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So within %5 is fine I assume? The car runs great and makes good power.
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Old 07-04-2011, 10:48 PM   #8
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If something was really wrong with your motor you would see 170 170 170 45

Something along those lines, your numbers are very good.
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Old 07-04-2011, 10:51 PM   #9
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Quote:
Originally Posted by falcon View Post
So within %5 is fine I assume? The car runs great and makes good power.
Your engine is fine. Stop worrying
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Old 07-04-2011, 11:18 PM   #10
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It was the first compression test since the initial run in. . I worry... all the time. But since it's still basically where it was before 4 autox's, 5 8kRPM runs down the 1/4, a hill climb and a track day,.. all shifting from 7500-8000, I think it's safe to say the motor is pretty decent thus far.

New injectors are on the way, gotPSI intake manifold and smaller pulley for the rotrex are waiting to go one. Time for the dyno!
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Old 07-12-2011, 01:45 AM   #11
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Borrow a cylinder leakdown tester and see how "much air" each cylinder holds at TDC compression stroke. Test pressure is typically 80 psi into the cylinder; ideally you would have all cylinders reading in the mid 70s. If a cylinder is leaking a lot air (more than 15-20%, or lower than 65/80) you can easily determine if the leakage is rings (listen for air leaking into the crankcase), intake valve/guide (air leakage into the intake manifold) or exhaust valve/guide (air leakage into the exhaust pipe).

A leakdown test takes a little longer than a cranking compression test but it can be a more reliable indication of compression stroke sealing; especially if you have higher or lower compression than stock.
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Old 07-12-2011, 02:17 AM   #12
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Running an oil analysis helped me get over "racing" a significant portion of my income.
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Old 07-12-2011, 02:29 AM   #13
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Quote:
Originally Posted by sn95 View Post
Borrow a cylinder leakdown tester and see how "much air" each cylinder holds at TDC compression stroke. Test pressure is typically 80 psi into the cylinder; ideally you would have all cylinders reading in the mid 70s. If a cylinder is leaking a lot air (more than 15-20%, or lower than 65/80) you can easily determine if the leakage is rings (listen for air leaking into the crankcase), intake valve/guide (air leakage into the intake manifold) or exhaust valve/guide (air leakage into the exhaust pipe).

A leakdown test takes a little longer than a cranking compression test but it can be a more reliable indication of compression stroke sealing; especially if you have higher or lower compression than stock.
Yeah I have a tester actually. Just haven't gotten around to doing one yet.
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Old 07-12-2011, 02:30 AM   #14
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I have a blackstone kit on the way. I've been running Rotella T6 synth since after break in.
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Old 07-12-2011, 02:46 AM   #15
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l-e-a-k-d-o-w-n
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Old 07-12-2011, 06:09 AM   #16
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Has anyone yet calculated the compression ratio you get with a '99 head and a 94-97 bottom end?
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Old 07-12-2011, 07:24 AM   #17
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Quote:
Originally Posted by 18psi View Post
I had all high 180's on my 9.1 IIRC

But da chop is right: it doesn't matter what we have, consistency is key
My low comp built motor was 160-160-160-160 8.5:1 compression IIRC

Leakdowns are more telling to specific issues, compression testing is the quick n dirty of engine health.
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