23 replies to this topic

[Lt-SilverDragon]

I've done a compression test, middle two cylinders are lower then cylinder 1 and 4. 1 and 4 are both sitting at 140 psi, cylinder 2 is at 130 and cylinder 3 is the lowest at 120 on a warm engine.

It wasn't that long ago I changed the head studs, head gasket and did the valve clearences but the engine runs fine and its only done about 34,000 miles so not sure how I should take those results.

[carbon]

Here's a series of compression test results from my 1293:

- normal result: 197, 200, 197, 197

- result May 2017: 195, 190, 192, 197

- result August 2017: 197, 125, 120, 195

Head gasket had blown between 2 and 3, giving the 120/125 results on 2 and 3.

 

So the May 2017 result was early warning that all was not well.

 

But all of your compression test numbers appear very low. Did you do the test with all spark plugs out on a warm engine and with the throttle held wide open?

[Lt-SilverDragon]

Yep took all spark plugs out and cranked the engine over a few times while keeping my foot on the throttle for each cylinder. Never bothered putting any oil into the cylinders to see if there was any improvements in compression tho. Not sure what head gasket I used when I did the head but I've got payen bk450 head gaskets in my garage now so can whip head off and at some point and have a look. I want to figure out the spec of the head as well as I bought it off here but I can no longer find the post since it was years ago so don't know much about the head. If the reading for the compression is the same after doing that I've got two other heads I can try.

[Spider]

Respectfully - and I know carbon we have talked about this - the compression test numbers are totally meaningless with regards to compression ratio. It's only a sorta engine health check and little more. I really do respect and agree with 99% of your posts and thoughts, but on this, I really disagree mate.

 

Really, the cam needed to be checked to see what it is and how it's set. It can be done in-car but will take the better part of a day.

 

You can do a rough guess from where the power band starts and how the power band starts (comes on with a 'bang' or 'ramps up').

 

<Edited for clarification >

Edited by Moke Spider, 08 October 2017 - 07:20 PM.

[carbon]

Thanks Spider,

 

Agree that compression test results are best used as a engine health check.

 

The main point I was trying to make is that if the results from 2&3 are even 5-10 psi lower than 1&4 then it could indicate head gasket issues.

[Lt-SilverDragon]

Moke - I did have a little search on the forum and most people did say that as long as all the cylinders were around 10% of each other not to worry too much esp if the car runs alright so if they were all around 140 psi I wouldn't be too concerned, it's mostly because 2 and 3 were both lower then the two outside cylinders I thought maybe the head gasket was blowing between cylinder 2 and 3 or worse the head was slightly warped as I'm sure I torqued them all down in the right sequence and to the right torque (I want to get both my torque wrenches checked to make sure they're both torquing right but I believe they're both fine) .

 

If you don't mind, how would I go about checking the cam? I'm assuming it would be a case of removing bits to gain access to the timing cover and then being able to check the end of the cam once the cover is removed? I'm pretty confident it's the standard cam the BOC came with, I know the guy before me rebuilt the engine (at the very least he had it out the car so he maybe just painted it) but I don't think he did much research on performance bits for it so I don't think the cam has been changed.

[Spider]

I wouldn't loose too much sleep over the result's you've got. Not ideal, but there's a larger number of 'causes' for such variations without there being any 'issues' especially given that the engine was 'rebuilt' but unlikely to have been 'checked' in every way it could have and certainly not blue printed. eg, the stock combustion chambers in the head are only a cast finish and these vary a bit. Then con rod lengths, crank indexing, cam wear, piston pin heights etc etc etc......

 

I would suggest as far as a compression test health check goes, on an engine is rebuilt and run in, do a test then as a bench mark and then periodically check it - if you wanted to monitor it.

 

To check the Cam in car, you'll need to remove the radiator, remove the spark plugs & rocker cover. Fit up a Degree Wheel to the Harmonic Damper and a Dial Gauge to the Inlet Valve then the Exhaust Valve. Find TDC (doesn't really need to be spot on, but the closer, the better), then log Valve Lift at every (say) 50 first on the Inlets, then on the Exhausts. Be mindful to i) turn the engine over in the running direction ONLY and ii) you need to turn the crank through 2 full rotations for each cam lobe.

 

I'll put up a photo in a tic,,,

[Spider]

This is where you fit the degree wheel and the socket & ratchet is being used to turn the engine over. The wire is being used as a pointer.

 

 

and measure Valve Lift

 

[Lt-SilverDragon]

Thanks so much Carbon and Moke, think the best thing about owning the mini is the people.

 

I'm using the mini daily the now since the gearbox on my normal car packed in so I'll probably wait till the weather picks up next year before checking the cam or doing anything crazy to the it.