Compression test interpretation

Talk about the E-Type Series 3

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doffo25
Posts: 31
Joined: Mon Apr 18, 2016 10:18 am
Location: Suffolk UK
Great Britain

#1 Compression test interpretation

Post by doffo25 » Sat Sep 01, 2018 12:58 pm

Hi All.....Im trying to decide whether I need to haul my engine out so Ive just done a dry compression test on my US low compression LB suffix v12 (1972 56kmiles) and achieved these results:

Bank A 1-6 100-105-100-100-95-100psi
Bank B 1-6 115-120-118-115-110-115psi

Firstly am I correct that this engine is 7.8:1 compression ratio and therefore these results arent too far off expected ie 7.8x14.7psi(atmospheric) =115psi or is it more complicated than that?

Secondly, what factors could make BankA be consistantly lower than BankB ?

Any possible reasons would be very much appreciated.

David
David Offord
1972 Series3 ots

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JJC
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Joined: Fri Apr 21, 2017 3:06 pm
United States of America

#2 Re: Compression test interpretation

Post by JJC » Sat Sep 01, 2018 10:05 pm

Yup......7.8 to 1 compression. Figures look ok. Car running ok ? Good news for guys like you and me with 7.8 engines is they will never be overworked, and they run on anything !! Wheeee! Hated it when I was a kid....love it now !

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lowact
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#3 Re: Compression test interpretation

Post by lowact » Sun Sep 02, 2018 2:53 am

Theory is, (v12) consistency between the cylinders on the same bank is more significant than between banks.
Better indication of engine condition is manifold vacuum, u want steady 20 ish inches of Hg vacuum at idle, steady being the equally important bit. There is lots of info about how to use vacuum test guage to diagnose an engine.
Regards,
ColinL
'72 OTS manual V12

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Topic author
doffo25
Posts: 31
Joined: Mon Apr 18, 2016 10:18 am
Location: Suffolk UK
Great Britain

#4 Re: Compression test interpretation

Post by doffo25 » Sun Sep 02, 2018 2:03 pm

Thanks guys very much for your replies. Yes the engine sounds good. I am relieved as I thought I was going to be told that the figures were suspiciously low!

best

David
David Offord
1972 Series3 ots

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