Folks,
Just finiahed rebuilding my W11 engine with new rings, bearings, new exhaust valves, valve seals, resurfaced head and new head gasket. Before I threw the engine back in the car, thought I'd do a compression test (I will do a leakdown test also tmw). To do so I hooked up the flywheel, starter motor, and got it going with a battery (see video link below). I got these results:

Cyl 1: 115
Cyl 2: 106
Cyl 3: 88
Cyl 4: 89

Obviously not great numbers, so I pinged some of my trusted people and they say the following:

1) Battery looks like its dying (started at 11.9V, ended at 11.7V). If I don't crank fast enough I won't build good compression

2) Doesn't make sense to do compression test until the engine has been broken-in (i.e. rings had a chance to sit in properly).

Curious if anyone has attempted to do a similar test and seen similar results. I'm basically trying to get a sense on whether this engine is suitable to get installed back in the car or am I already showing some issue with the pistons or valves.

Thanks for any insight. YouTube video at https://youtu.be/y5ZzYfSswFw

 

Never did a compression test on a rebuilt engine before or after installing it into a vehicle. My SOP was been to strive to do the rebuild correctly so I have no need to try to confirm I did a good job.

Have to say a new engine's compression may not look all that good because as you already touched upon the rings are not seated. Also, differences in how much assembly oil was used can also affect compression. (Before putting the piston/rod assembly in the engine I'd dunk the piston/rings and small rod end into a bucket of clean engine oil to ensure the rings, piston, and work the piston on the rod end bearing to get oil into this so the small rod end bearing was well oiled.)

Also the first cylinder or 2 may have low compression while the rest may show higher compression. This arises as the engine is spun and the last cylinders get some oil from the piston oil jets or from the crank. Plus the hydraulic lifters may pump up and deliver better valve timing and opening numbers.

Unless you have reason to suspect the correctness of the engine build put the engine in the vehicle then start it and run it to give the rings time to seat -- they should seat quickly though of course they will require a thousand or more miles to get to the point they are seated well enough to risk pushing the engine hard. My SOP is to after the initial 15 to 20 minute run in at fast idle to shut off the engine and drain the oil -- it will be HOT!!! -- and change the filter (HOT!!!!) -- and with fresh oil in the engine proceed to drive the vehicle while adhering to the factory break in guidelines.

 

You can always do a wet compression test to help verify. Just squirt a bit of oil in each spark plug hole prior to the compression test to see any change. Compression should come up. I have to say, though, that the compression readings you have are really, really low. You should be in the 140-150 range with no more than 10% variance between cylinders. Sounds almost as though you're not getting a good seal with your tester.

 

As mentioned, if you just honed the cylinders and replaced piston rings with new, then they aren't sealed yet. It takes a bit of run time to get the rings to mate with the new cylinder walls.

Check this out:
https://www.enginebuildermag.com/201...the-right-way/

 

Great article, njaremka.

 

Thanks all for the great input.

Yesterday I did a retest with a freshly charged battery and results were 128 psi. on all cylinders except #3, at 118 psi. I'm as amateur as they come, so the test is mostly to get a sense that I haven't f**** up in some major way 🤣. These numbers make better sense to me.

I will go ahead and break-in the engine as described in the article and recheck compression numbers after the break-in.

Thanks again for all the input!

 

Hey folks,

Fast forward from last reply, car cranked and managed to stay on, no leaks and no weird noises from the head ... success!. It has a rough idle and some other issues which I'll post about later, but thought I'd point out that after the first 30 mins of the engine being on, I'm reading 150 psi on all cylinders except one hitting 162. So I'd say that should be better, right?

 

I'd say that's MUCH better. Sounds like the rings are getting seated as expected.