What compression should I be looking for with a Z head?

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Model T Ford Forum: Forum 2017: What compression should I be looking for with a Z head?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Daniel Kraft on Sunday, August 20, 2017 - 08:00 pm:

I put a Z head on an otherwise stock T. What compression should I have in each cylinder.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan Treace, North FL on Sunday, August 20, 2017 - 08:05 pm:

"Z" head on my '26. fresh rebuild, bored and new pistons.




Top of pagePrevious messageNext messageBottom of page Link to this message  By Stephen, South Texas on Sunday, August 20, 2017 - 11:22 pm:

If the gauge is accurate, ~70psi is REALLY good!
Was that wet or dry?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Monday, August 21, 2017 - 12:27 am:

The pressure generated on then compression stroke is related to the external air pressure (p) and the compression ratio (r). The formula for the pressure (P) is

P={r-1} x p

With a CR of 4:1 the pressure will be about 44 psi
At a CR of 5:1, the pressure 58psi
And at 6:1, pressure 73psi

This is the maximum that can be achieved.
If the results are lower, there is probably excessive leakage
If tested dry, there will be some leakage, so the pressure will be less. Adding a little oil will cure some leakage and raise the pressure close to the theoretical values. If higher pressure reading is achieved, the the effective CR must be higher, caused by either machining of the head (or block) or build up of carbon in the combustion chamber.
As a general rule, it is generally considered unwise to exceed 6:1 with splash lubrication for reliable operation.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Daniel Kraft on Monday, August 21, 2017 - 04:32 pm:

I got a reading of between 55-70 on a motor that's stock bore. I'm not sure if that's an acceptable variation in readings.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Monday, August 21, 2017 - 04:39 pm:

I also think it makes a difference if you have a Hand or electric starter.

Great post Tony!
Your gauge has a 300# scale so only reads best accuracy at 150.

We like to see the readings in a close range but yours are certainly acceptable. I'd recheck the 55.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Daniel Kraft on Thursday, August 24, 2017 - 01:03 am:

Thank you all. I have a good amount of blow by so it could be that the 55lb on is a bit loose


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Thursday, August 24, 2017 - 04:14 pm:

Oil or water leaking into a cylinder will change the reading quite a bit.

The center 3rd area of the gauge was always considered the most accurate area when I was taking Navy fire training, with the center area being the most accurate.


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