Compression and Timing

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Compression and Timing

Post  hotrod1956 on July 19th 2015, 11:44 am

I just picked up a 78 f150 trailer special 460. I have a compression test of 60 to 70 lbs on 3 cyl. But if I start to retard my timing while doing it the compression rises until it stalls. I know I should just turn the engine over when doing compression test. The engine has good power when timing is set where I get a low reading. Jumped time?

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Re: Compression and Timing

Post  rmcomprandy on July 19th 2015, 12:01 pm

hotrod1956 wrote:I just picked up a 78 f150 trailer special 460. I have a compression test of 60 to 70 lbs on 3 cyl. But if I start to retard my timing while doing it the compression rises until it stalls. I know I should just turn the engine over when doing compression test. The engine has good power when timing is set where I get a low reading. Jumped time?

You can not take a compression test with the engine running.

compression ratio, static cylinder pressure and running cylinder pressure are entirely different things and are not comparable.

The valve overlap, intake vacuum and exhaust pressure from the running cylinders are skewing those static results in the cylinder tested.

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Thanks, I will go try this again.

Post  hotrod1956 on July 19th 2015, 12:13 pm

rmcomprandy wrote:
hotrod1956 wrote:I just picked up a 78 f150 trailer special 460. I have a compression test of 60 to 70 lbs on 3 cyl. But if I start to retard my timing while doing it the compression rises until it stalls. I know I should just turn the engine over when doing compression test. The engine has good power when timing is set where I get a low reading. Jumped time?

You can not take a compression test with the engine running.

compression ratio, static cylinder pressure and running cylinder pressure are entirely different things and are not comparable.

The valve overlap, intake vacuum and exhaust pressure from the running cylinders are skewing those static results in the cylinder tested.

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hotrod1956

Post  hotrod1956 on July 19th 2015, 12:14 pm

Thanks. I will try this again.

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hotrod1956

Post  hotrod1956 on July 19th 2015, 2:46 pm

hotrod1956 wrote:
rmcomprandy wrote:
hotrod1956 wrote:I just picked up a 78 f150 trailer special 460. I have a compression test of 60 to 70 lbs on 3 cyl. But if I start to retard my timing while doing it the compression rises until it stalls. I know I should just turn the engine over when doing compression test. The engine has good power when timing is set where I get a low reading. Jumped time?

You can not take a compression test with the engine running.

compression ratio, static cylinder pressure and running cylinder pressure are entirely different things and are not comparable.

The valve overlap, intake vacuum and exhaust pressure from the running cylinders are skewing those static results in the cylinder tested.
Thanks again Randy, I did it with the starter and it worked out great. Happy you pro's are on here.

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