Wet compression test - V8 - observations and questions

**mark2** · 11th February 2008, 09:50 PM

I've recently done a compression test on my carbed V8. Results were around 155-160 Psi for six cylinders with one cylinder at 170 and one at 140.
I put some oil in the 140 cylinder to do a wet test, spun the motor a few times to spread the oil around and retested - result - 165 Psi.

This wasn't what I really wanted to see as it points to worn rings and I was hoping that it may be a worn cam lobe on this cylinder as I'm about to replace the cam/chain/lifters but dont really feel like pulling the engine down much further. I also experience a slightly rough idle, some popping on deceleration and feel that power could be a bit better than it is. Vacuum at idle is about 16 -17" Hg, depending on the idle speed - faster idle = better vacuum reading. I dont know what cam is in it but it may be slightly warmer than stock. The motor is a 4.3 (stroked 3.5).

Anyway, I decided to perform the same wet test on the best cylinder (170 dry). Same process and the gauge reads 230 psi

. Which is a bigger increase on a cylinder which should have a fairly good ring seal..........

Then I started thinking about the pistons and cylinders being at 45 degrees on a V8 and the possibility that oil may be pooling in the cylinder and increasing the compression due to a reduction in chamber volume. I may well have put too much oil in - a good squirt from a tomato sauce bottle........

Is this likely and could it mean that the rings may be OK in the worst cylinder? I've bought a dual gauge leak down tester on ebay which should be arriving soon which should hopefully should provide a better diagnosis?

Also, an interesting observation - while I did the compression check with the throttle held full open, I decided to try it with the carb air intake elbows removed and the throttle held open- this increased the result by 5 psi on all cylinders. To me, this points to the fact that my air intake system is fairly restrictive?? It also shows how sensitive compression test results are to air flow which could explain the lower reading if a cam lobe/s is worn down enough to reduce valve opening.

**Blknight.aus** · 11th February 2008, 10:32 PM

first off grab an old spark plug gut it and solder up a fitting to take regulated air from a compressor

take all the plugs out and insert that in one pot then manually crank the engine over till shes at TDC on that pot with both valves "closed"

hook up about 20 psi of air and then listen to the exhaust and the carbie.

if you can hear airflow you have a leaking valve.

repeat on all cylinders

while your at it listen to the dipstick tube and if you can hear air that way you have a ring/cylinder/piston problem.

if you get nothing on 20 psi step it up to 40 but at 40psi IF your either side of TDC theres enough force to make the pot turn the crank unless you have it locked off or in gear.

generally a problem that you cant replicate with less than 60psi of compressed air will be present when cold but will sort itself out when hot.

RonMcGr · 12th February 2008, 06:48 AM

Originally Posted by Blknight.aus

first off grab an old spark plug gut it and solder up a fitting to take regulated air from a compressor

take all the plugs out and insert that in one pot then manually crank the engine over till shes at TDC on that pot with both valves "closed"

hook up about 20 psi of air and then listen to the exhaust and the carbie.

if you can hear airflow you have a leaking valve.

repeat on all cylinders

while your at it listen to the dipstick tube and if you can hear air that way you have a ring/cylinder/piston problem.

if you get nothing on 20 psi step it up to 40 but at 40psi IF your either side of TDC theres enough force to make the pot turn the crank unless you have it locked off or in gear.

generally a problem that you cant replicate with less than 60psi of compressed air will be present when cold but will sort itself out when hot.

Wow, never heard of that one before

**DEFENDERZOOK** · 12th February 2008, 10:35 AM

Originally Posted by Blknight.aus

first off grab an old spark plug gut it and solder up a fitting to take regulated air from a compressor

some of the older compression testers had a ryco fitting that used to screw into the plug hole.....
then you just clipped the compression guage onto that.........

these were great for doing a leak down test as the airline just clips straight on to the fitting......

**mark2** · 12th February 2008, 12:18 PM

I do have an air hold fitting which I've tried to use before but without gauges it doesnt give a comparitive reading. Had lots of dramas getting it to stay at TDC when the pressure is applied, even with the handbrake on and in gear.
I like the suggestion of starting at 20PSI...........

**Blknight.aus** · 12th February 2008, 07:37 PM

this test is not a test to do comparative compression testing its a leakage test.

gasses can only get out of the chamber one of 4 ways (I forgot to mention one)

1. the inlet valve
2. the exhaust valve
3. past the rings
and
4. past the head gasket

to check the head gasktet one remove the cap from the radiator or the expansion tank and fill it to the brim if it bubbles then you have air leaking into the coolant system.

the best way Ive found for stopping the engine from rotating is with a dummied up starter shaft that has a set of holes drilled in it and a shroud with match up holes cut the end of the shaft to take a shifter/ratchet/spanner and then bolt it on inplace of your startermotor. turn this till the holes line up and pin it off then run your air up. repeat as often as you like for the length of the stroke on the pot your working on and you will have a good idea as to the effectiveness of your ring seals and if theres any damage to the bore for the lenght of it.

Time consuming but a lot less so than pulling the block down to check it.

IF your really really keen if you use a heater to keep the block warm to hot it will also give you a rough idea of how its going when warmed up as well. (you can also do this by removing the thermostat then heating the radiator and letting the coolant thermocycle through the block or by using a small motor to turn just the pump pully after pulling the fan belts)

If you want to do a comparative flow check all you need to do is place a flow gauge inline with the cylinder plug after the pressuregauge write down your numbers in various places and compare across all pots. (works for most diesels too but use the glow plug hole or injector hole).

**mark2** · 13th February 2008, 05:25 PM

Well, the dual gauge leakdown tester arrived yesterday from the USA so
last night I performed a cylinder leakdown test .
7 of the cylinders were around 10-15% leakage which from what I've read is
quite acceptable for a road vehicle. I've read that race engines target
around 2-5%. (BTW, apparently leakdown tests are compulsory when
servicing piston engined aircraft - the max allowable is 15%.)
Compression in these same 7 cyls was between 155 and 165 PSI with one at
170 PSI. It was interesting that there was no direct correlation
between compression and leakage on these cylinders. There may well be
carbon in the 170 cylinder........
One cylinder (#8) with a lower compression of 140 (90 on the first stroke)
had a leakage of 37% which was disappointing. It appears that most of
this is leaking past the rings judging by the sound coming from the
dipstick tube. Although even the cyls with minimal leakage could be
heard to be letting air past the rings. On this basis, I'd be reluctant
to use a tester without gauges as it might cause one to condem an
otherwise good engine.

I also ended up doing the test cold as things turned out - my
presumption is that ring sealing should improve when at operating temp? I
should try it again when the engine is properly hot.......

My conclusion based on these results is that this #8 cyl is at about 3/4
power compared to the rest. So, I have about 7.75 cyls out of a possible 8.
Not ideal, but certainly not bad enough to justify a rebuild in this
application I would think.

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