How much compression?

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Model T Ford Forum: Forum 2015: How much compression?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Lonnie Smith Baxter MN on Tuesday, October 27, 2015 - 07:24 am:

What would you say is good/average compression on a T engine and at what point is it time for a rebuild?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Tuesday, October 27, 2015 - 07:44 am:

45-55 psi is good for a std head T, when it's more than 10 psi difference between cylinders and/or if it's down to 25-30, I think it's time to check?

First check is to pour a little oil in the cylinders, if that makes a significant improvement you'll know it's the piston rings that isn't sealing good. If not much change but still bad, it's likely in the valves.

It's not sure you'll need a full rebuild just because the compression is low, a valve job may help a lot and keep you going for a few more years.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Lonnie Smith Baxter MN on Tuesday, October 27, 2015 - 08:46 pm:

One of my winter projects has 35 on all 4 cylinders and I was thinking that might be a little low. Thanks for the advice.


Top of pagePrevious messageNext messageBottom of page Link to this message  By joe bell on Tuesday, October 27, 2015 - 09:53 pm:

Lonnie, If the engine has set for many years the compression will be low, maybe you should try firing it up once and let it run then check it, you may be surprised on the difference in the compression. Just a thought for you.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Aaron Griffey, Hayward Ca. on Tuesday, October 27, 2015 - 10:01 pm:

Always check compression with the throttle wide open too.
Count the revolutions you spin the engine and do the same on each cylinder.
Usually 10 will bring it up as high as it is going to go.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Todd, ............Red Deer, Alberta on Wednesday, October 28, 2015 - 11:56 am:

Altitude will affect the compression also, it looks like roughly 3% loss in pressure every 1000'

Altitude Factor
1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860


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