TR2/3/3A Compression testing again...

[karls59tr]

Just did this with my friends compression tester. Car was run for awhile but not up to operating temperature.We got 170 in all four cylinders? Did he read the gauge wrong? Seems a bit high seeing as how it was mentioned that 150 was normal. We did let the engine crank more than four revolutions maybe until it stopped going up?....it was four revolutions for the test was it not? Also we did not have the choke out...was it supposed to be out. I think I remember that when doing compression tests on American cars the carb throttle valve was supposed to be wide open?

 

[Geo Hahn]

Yes, you want the throttle plates wide open (to be able to suck in enough air to compress). Gauges vary quite a bit so 170 may or may not be the exact figure, but it sounds good to me.

 

[CJD]

An SU choke doesn't affect the reading, as it is not a blade type choke. Blocking the throttle open may help to get a quicker rise. You are looking for the highest attainable, regardless of cranking time...but if it takes a large number of strokes then that may say something too.

High compression would indicate:

Carbon build up on a completely stock motor.
Shaved head giving higher compression ratio.
Short duration cam or a cam advanced from stock index.

 

[karls59tr]

The motor has sat for 15 years. If there was carbon build up would there be black smoke from the tail pipe? The car does not smoke at all. I don't know the history of the motor. The compression readings are all the same so that's a good thing. Would a rebuilt motor account for the high readings?

 

[CJD]

It depends. Fresh carbon, from say a carb that started running rich will cause smoke. That "carbon" is really unburned hydrocarbons, which are light and do not affect compression. Carbon that is baked onto the head, valves, and pistons is what is left after everything that can burn is burned. It is hard as rock and does not cause any smoke. It can also glow read hot and cause pre-ignition.

The rebuild itself has no factor, but, again, if the head is shaved it will raise compression. Also if the cam is short duration or advanced from straight up.