V12 compression figures for cylinders Low

I know there a load of posts on here about doing compression checks on the cylinders, So out of interest I thought I would do mine today. If nothing else just to record them in case of any future issues.
The engine was cold, probably run about three weeks ago after doing an oil change. I’ve had the car about ten years and the oil and filter replaced about every 18 months, and not many miles. The car is a uk spec 1973 e-type about 59000 miles.
So the car was cold, fuel pump fuse removed, all spark plugs removed and the throttle held fully open.
All the cylinders were very similar between 84-87 PSI. These seems very low compared to all those that I have read up on. Though the car seems to run fine? I borrowed another gauge just to make sure my gauge wasn’t reading low, but that had the same values.

Should I run it hot and repeat?

Any one else have similarly low values?

Many thanks

Steve,
Your compression tests show only part of the story. To do a proper evaluation, determining ring or valves, you need to do a leak down check. The testers aren’t expensive but require a shop compressor capable of 100 psi (or more). With a leak down tester you can pinpoint whether you have a badly seated valve (or other valve related issue(s)) or piston rings/bore issues. Recommend viewing on-line videos on how to perform a leak down test would also be helpful IMHO.

Happy Trails,

Dick Wells
CWO-4 USA Retired
RAF CSRO Instructor qualified - Mt. Batten

Cheers, I was thinking I might do that next, your right they aren’t too expensive and I have a compressor. Do you think the figures are low due to not much oil being on the cylinder walls from not being run for a while? The pressures are very consistent across the 12 cylinders. Is it likely I would have badly seated valves on all cylinders for example?

Thanks

Steve, drop a teaspoon of oil into each cylinder and re run the tests, that will at least show if you have a problem with rings, or get the engine running for a short while to get the oil flowing around and see if that alters the readings.

Steve,
First need a little more info. The motor, a 1973 build; has it ever been overhauled/rebuilt? If it’s factory original then consider it’s fourty-eight (48) years old. Time and corrosive factors play hard on valve seats and valve faces. A teaspoon of oil into each cylinder will, indeed, give some indication of ring seating. Valves are usually the culprit. With a leak-down tester you can hear valve leakage either thru carbs or exhaust. If its rings thru the crankcase vent. I’ve seen similar motor cylinders that still showed original factory honing. The cylinders usually show almost no wear. My bet would be valves!
Regardless of whether the motor has been rebuilt or not a leak-down test will give you very definitive results to work with. If you pump up a cylinder and hear air leaking, e.g. from the exhaust then you can be assured an exhaust valve (or seat or both and/or guide) are the problem.
This method takes most of the guess work out of locating the problem(s).
Let us know what you find.
Good luck and Happy Trails,
Dick

Thanks I warmed the car up earlier today and repeated the test hot with very similar results, then dropped in some oil which didn’t alter much. I haven’t got a leak down test kit yet.
I was going to take it for a run with some additive in the fuel, redex? and give it a hard drive (3000rpm to redline a few times). I will let you know where it leads.

If your numbers are still sub-100 psi, you’ve likely got worn bores and rings.

I was gonna ask about that. I know the US-spec V12 had low compression, but those are really low numbers. OTOH, I always put more stock in the differences than in the numbers themselves because it’s so easy for there to be differences in the compression gauge and the methods used. Heck, even battery and starter condition can affect the numbers.

I looked through the history, the engine had an overhaul about 5000 miles ago. The timing chain and tensioner replaced, cylinder heads were removed and decoked along with the pistons and valves, the valves were re lapped… At the weekend I gave the car a run, accelerating from 3000rpm to 6500 rpm about 6 or 7 times after getting the engine to temperature. I then took it back home removed the air filters and ran some valve cleaner through at about 1500rpm. I put the filters all back on, took it back out and was going to do a couple of extra accelerations, but the engine would now only rev to 5500rpm? Today I checked the fuel filter, was clean, it was a new pump less than a 1000 miles ago. Removed the sparkplugs and rechecked the compressions, they had all risen by about 10 psi. Put an endoscope down the plug holes not much to see. I have cleaned the distributor cap, but I overhauled this a couple of years ago, and don’t do many miles. Any thoughts on the rev limiting?