I know this has been talked about a bunch. I tried to use the key word search, but I have trouble with it, there is always some kind of internal error. I tried a Google search, but only got a couple MTFCA finds, that didn't help.
The question is, I have a friends 1918 Model T in the garage and we have been working on it to improve it. Today I tried to measure the compression. I've done this on other Model T's, but they all had a starter motor. This one does not. So I removed the plugs installed my meter and cranked as fast (fast is relative) as I could and at least 5 compression strokes. The most I could get on each cylinder was maybe 28#. Each cylinder was very similar to others. Cylinders 3 and 4 have spark plugs that look like good combustion, Cylinders 1 and 2 can oil foul quickly. Is this a good test? Can I believe the 28#, considering I hand cranked the engine? The car (touring) pulls good in 1st gear, but has a little trouble in high, especially on a grade. Thanks Mike
You could also try the test with some oil in the cylinder. If there's a big difference, you might need rings or some rebuilding.
I found 28 way too low for ignition with a hand crank
Once I did a valve job, honed the cylinders and put in new piston rings it was right up to 55 PSI and starts right up with hand crank
I have a 1920 Roadster same problem been sitting waiting time to redo Engine will pop start towing it because of low compression. but not by hand cranking
Did you have the throttle all the way open when you did this test?
(Message edited by paulmikeska on March 02, 2015)
This thread has a step-by-step compression test procedure in it:
I believe you're supposed to not only have the throttle open, but the engine warm too.
We decided to see if the car would start. Mike pulled the choke, cranked a couple of times, and Lance turned the key. We got a free start. Mike sure looked surprised, but we were all thrilled that after all that we had done on the car, that it still started like it was new.
The next step is to see what more can be done. That is why Mike asked the question.
It is good to hear of another T roaring back to life! Ask a bunch more questions!
The engine was warm not hot, I had the throttle closed. The engine starts very well, when it has clean plugs. #1 and #2 will foul after a while, then it is harder to start. The engine is loose and cranks easy, another indication of a tired engine. Mike
Please try again with the throttle open. Would be interesting to hear about the difference in compression readings?
If you add some oil in the cylinders after the first wide open test and test again, you'll learn if it has leaking rings or valves - the oil will seal the rings for the length of the test, but it won't help leaking valves.
you could have an air leak at 3 and 4 manifold port, causing you to richen up the mixture. This would cause 1 and 2 to be richer than needed which will foul out and run wet in those cylinders.
I'll try redoing the compression tomorrow. We had the manifolds off to repair a leaky core plug. The Intake has new copper ring gaskets with RTV on them. I tried the propane test on the manifold and it seems fine. Mike
We had a 85 year old club member with a very tired '14 touring who drove it everywhere. We could never get over 25 pounds of compression on any cylinder yet the car ran well, idled smoothly and started on compression more often than not. I can't say what the compression would have been with a starter spinning it but I doubt it was much more than 25 pounds.
Well.... I tried the compression test as is and got the same readings. I tried it again with the throttle wide open, no change. Added a squirt of oil and it went up a couple pounds. Well, it is what it is. At least the readings are all similar. That must indicate that there isn't a major problem like a burnt valve or one set of cylinder rings that are all broken. So, I suppose we will just improve on the external stuff in an effort to get the most out of the engine, until it is time to redo it. Thanks Mike
Good decision, hope your friend drives and enjoys the car for what it is this coming driving season!
There are a few things which affect compression. If the engine has been newly rebuilt, the rings might not be seated. If the engine is very old, the space between rings and grooves in the pistons might be excessive, and the end gap might be excessive. Burnt or sticking valves will lower the compression. If the engine has not been driven for a long time, the oil might have all drained off the rings. And the altitude can affect the compression. High altitude will lower the compression. If the compression is approximately the same on all cylinders and with a warm engine it is somewhere around 50 lbs, it is good.
Best way to check compression is to first warm up the engine and then remove the spark plugs and open the throttle while you check. If you add some oil and the compression goes up, you might need to replace the rings.