I know that below a certain compression reading a Model T engine will not start ( I had one at 15 PSI) With good compression (55 PSI) they start quickly even hand cranking. What my question is to you experts is -- What is the compression readings on

1. A new well built engine, a very healthy engine

2. An engine needing a valve or ring job but still running perhaps more difficult to start (a weak engine)

3. An engine too low a compression to start? ( a very sick engine)

I can not find this is my manuals. Is there a written guidance for this info somewhere?
Thanks

1. A totally stock, unmodified T with a rebuilt engine and properly lapped valves and properly fit rings will test at 55 PSI.

2. They start just fine when totally worn out and compression as low as 35 pounds.

3. I don't think an engine will start if it has horribly scored cylinder walls or burned valves. It takes about 15 minutes to pull the cylinder head. If the engine is that old and tired you need to do this, and take a look.

Yes, there is written guidance. The Ford service manual, or Dykes manual.

David Kriegel: I have owned many Ts and can not imagine anyone buying a T and not tearing it totally down to check the motor and running gear. I don't care what some former owner tells you about what he did, I would still tear every thing down.
My shop is at 5,000 feet and all the new motors that I have checked are at 48 lbs. That is with out opening the carb wide open and the other three spark plugs in the head. You will find at sea level you will get a higher compression test especially if you do open the carb and remove ALL the plugs instead of one at a time

The engine I just replaced in my T only would pull 30# of compression. It started right away and very easily, even sitting over last winter this spring it started like it was just shut off an hour ago. It would only crank over a time or two and it was running. On level ground it ran pretty good. But it would not climb hills worth a darn. I had to use first gear where a fresh T engine will accelerate in high gear up the same hill.
Seemed odd to me it did not appear to burn or use oil with rings that weak.

Well David, if this is any help, here's how things played out with my '12. Was getting super hard to start (hand crank only, naturally) and even when it was running was hard to "keep up" with anyone....like almost wide open to obtain even 32 mph. Finally invested in a comp. tester and discovered two cylinders at barely 28# and the other two barely 30#. So out the engine went to Joe Bell. And now I'm "going over" the entire car, next best thing to a complete body off resto.




(Message edited by adminchris on November 05, 2014)

My T had the following compression figures when I first got her running:

1 - 26
2 - 31
3 - 32
4 - 31

So, she ran on pretty poor compression, but not smoothly. I have since replaced the rings and valves.

The proper way to test compression on any engine is with throttle wide open.
At idle position any change in the throttle setting will change the amount of air the engine can suck in. If it can't fill the cylinders you will get a low reading.
there can be great differences in compression gages too.
Most T engines will start and run well with 35 lbs. of compression with throttle wide open.
Anything lower would indicate the start of ring or valve problems.
If the compression is very uneven you can think valves. But broken or stuck ring can also cause a cylinder to be a lot different than the others.
Two dead cylinders next to each other usually is from a blown head gasket.

Aaron - Your first line reads,...."The proper way to test compression on any engine is with throttle wide open."

I agree with that but would add a couple more words to that statement. I would add,.....AND WITH ALL SPARK PLUGS TAKEN OUT.

.....not shouting,....upper case just for emphisis,...harold