Compression Test - What do you think - Vintage Mustang Forums
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post #1 of 9 (permalink) Old 09-14-2019, 05:19 PM Thread Starter
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Compression Test - What do you think

I have a 1966 with a 289 and I’ve been working on it for the past few months and about ready to start it up. Since I was changing the spar plugs I decided to do a compression test with the following results:

1 - 160
2 - 160
3 - 160
4 - 155
5 - 143
6 - 168
7 - 127
8 - 165

7 is concerning maybe even 5
What do you think?
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post #2 of 9 (permalink) Old 09-14-2019, 05:43 PM
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First, I have to ask, what was your test procedure that determined these CR#s.?
Yes, 5 & 7 are suspect? Could be valves, depending on additional tests.
Was this test taken on a warm engine?

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post #3 of 9 (permalink) Old 09-14-2019, 06:17 PM
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Depends on the story of the motor. If it’s been sitting a long time and a used motor, get some hours on it and then see where everything lands. If it’s a daily driver that was driving yesterday, you have an issue,. When it becomes bad enough where you can tell when driving something is wrong, you should fix it. Otherwise I’ve gotten lots o miles outta motors like yours.

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post #4 of 9 (permalink) Old 09-14-2019, 06:26 PM Thread Starter
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Quote:
Originally Posted by kenash View Post
First, I have to ask, what was your test procedure that determined these CR#s.?
Yes, 5 & 7 are suspect? Could be valves, depending on additional tests.
Was this test taken on a warm engine?
Cold motor not started.
I think it had been sitting for a while when I bought it in May. I drove it a little bit and then started working on some things and has been sitting since June.
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post #5 of 9 (permalink) Old 09-14-2019, 07:12 PM
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Needs a B cam.


Mark
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-Mark-

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post #6 of 9 (permalink) Old 09-14-2019, 07:33 PM
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Block the carb wide open too, that can skew readings
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post #7 of 9 (permalink) Old 09-14-2019, 07:59 PM
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Needs a B cam.


Mark



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post #8 of 9 (permalink) Old 09-14-2019, 10:25 PM
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5 would probably be okay. 7, not so much. Do a leakdown.
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post #9 of 9 (permalink) Old 09-16-2019, 11:56 AM
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5 OK IMO

for #7 squirt a shot of motor oil down the hole and try it again

if its bad rings that will seal it long enough to bump up the readings

IIRC you should have a 15% spread from the highest to the lowest on a warm motor

so even #7 isnt too far gone to worry about

just drive it

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