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Low compression test result- concerned
Craigorypeck - 9/6/11 at 10:50 PM

I have 110-120 psi across all cylinders which I think is low, should be nearer 200 with a CR of 11:1 I reckon, throttle open full on a warm engine, poured a drop of oil into the cylinders and it hits 150 psi. Engine has done bout 2000 miles since being built. Should I be concerned? Is it normal to have the extra 30 psi with a drop of oil to properly seal the rings or how long does it take for the rings to seal properly in the cylinders?
Also could an ill timed in/out cam be causing a problem?
Thanks

[Edited on 9/6/11 by Craigorypeck]

cliftyhanger - 10/6/11 at 02:15 AM

Try another gauge? yours may just be under-reading.
If so, nice easy fix.

snapper - 10/6/11 at 05:52 AM

It will take 50 to 100 miles to completely bed the rings in, 5 to 10 miles of high gear full throttle 10 seconds followed by 30 seconds light cruise do this 10 times.
The wilder the cam the less the static compression ratio.

ashg - 10/6/11 at 06:43 AM

needs to bed in as said, also check the cam timing, if its freshly built and your ring gaps are ok there is little to be worried about

MikeRJ - 10/6/11 at 06:57 AM

Were the bores correctly honed when the engine was rebuilt? Does it use any oil?

mark chandler - 10/6/11 at 08:46 AM

If you have a hot cam then the compression drops, as an extreme example the valve closes when the piston is halfway up the compressive stroke so CR appears to be halved.

With hot cams overlap increases so compression falls, you need to be worrying if you have large discrepancies between cylinders.

Also it is the combustion process that forces the rings into the bores to create a firm seal, try dripping some oil into each bore then repeat the test.

Regards Mark

Craigorypeck - 10/6/11 at 09:49 AM

I connected the air line from my compressor to the gauge and it matches the pressure supplied.

Rings should be bed in after 2000 miles?

Cams are more the high lift than extra duration type, newman phase 2 fast roads.

But I could shift the cams and see if it makes any difference.... hmmm which way for less over lap. Just too see..!

Bores honed at local engineers, so presume correct.

All the bores are within 10 psi of each other and go up approx 30psi when a teaspoon of oil is dribbled in.

AndyGT - 10/6/11 at 10:12 AM

Was the CT done with or without sparkplugs? Without sparkplugs and a wild cam can drop the compression even more. As ^^^ drive it a while cruising but I wouldn't drive it too hard in top gear. I usually drive my new engine builds light on the throttle but prefer to let the engine spin a little rather than labouring the pistons so early in buildlife.

Just my 2 peneth!!!

Craigorypeck - 10/6/11 at 11:11 AM

All spark plugs out when doing test..

I did the rev and coast thing when first driven but have pretty much driven it normally since.

I do have a reduction geared starter which turns over pretty slow maybe allowing more leak down on the compression stroke so guessing that with the long reach adaptor on the gauge might be keeping the results low across the board ...?

modee - 10/6/11 at 11:14 AM

Did you use a running-in oil or mineral oil to bed in the rings? If you used synthetic from 0 miles the rings may never bed in properly.

rusty nuts - 10/6/11 at 11:24 AM

Don't think I would be worried with that sort of compression on a fairly newly rebuilt engine,

Craigorypeck - 10/6/11 at 11:25 AM

I used 15-40 mineral oil for the first 250ish miles then onto 15-40 part synthetic since.

britishtrident - 10/6/11 at 11:29 AM

quote:
Originally posted by rusty nuts
Don't think I would be worried with that sort of compression on a fairly newly rebuilt engine,



2,000 miles makes it hardly a new build ---

britishtrident - 10/6/11 at 11:35 AM

The fact the test result for all cylinders are low and even makes me suspect vale clearances are too tight or valve timing is out.
When looking at the wet test results you have to allow for the fact that putting oil in the bores not only improves the ring sealing but increases the compression ratio.
If it were mine i would first check the valve clearances and restest, if no improvement I would check the cam timing, if nothing wrong with that do a cylinder leak down test.

Craigorypeck - 10/6/11 at 11:51 AM

quote:
Originally posted by britishtrident
The fact the test result for all cylinders are low and even makes me suspect vale clearances are too tight or valve timing is out.
When looking at the wet test results you have to allow for the fact that putting oil in the bores not only improves the ring sealing but increases the compression ratio.
If it were mine i would first check the valve clearances and restest, if no improvement I would check the cam timing, if nothing wrong with that do a cylinder leak down test.


Ok so checking valve clearances is done doing what?? Valve to piston? Have hydraulic tappets....

Craigorypeck - 17/6/11 at 08:21 PM

I connected my compressor that had the regulator set to 100 psi and with a tyre inflater connected to the spark plug hole the back pressure is 90psi.. hiss coming from crankcase breather... Healthy enough????