Compression Test

[hemibum]

Here's a set of compression test numbers from a Fordie X Flow. Interpretations anyone?

All figures taken with throttle wide open .

Cold Engine

1 150 psi

2 150 psi

3 150 psi

4 150 psi

Engine at working temp.

1 175 psi

2 170 psi

3 175 psi

4 175 psi

Again at working temp, but with a few CC's of oil added.

1 240 psi

2 240 psi

3 220 psi

4 235 psi.

Looking forward to your comments.

[Mr2Mike]

Looks absolutely fine, all cylinders well balanced and good compression shown. Perfectly normal for cold compression to be a little lower as the ring gaps are wider.

[hemibum]

Well, thanks for that. Still doesn't explain the amount of oily fumes coming out of the breather!

Looking at the possibility of wrong dipstick allowing too high an oil level.

[stevieturbo]

Or your rings simply might not be bedded in correctly. A compression test wont always show such problems, as I learnt earlier this year.


All 8 of mine were within about 5% of each other, yet it would chuck oil out at WOT, and the breathers stunk.

Rebuild, hone + new rings, and breathers are almost spotless.

[hemibum]

Thanks Stevie, this engine has only done about 500 miles in the last 7 years, and now , although it idles and revs beautifully it chucks so much st out of the breather.

[stevieturbo]

defo rings...or given it has sat for so long. The bores could simply have rusted, and are now damaged.

[350Matt]

If a comp test goes up when you add oil the there's a problem with the bores or rings

Matt

[Daveuk9xx]

hemibum said:

Thanks Stevie, this engine has only done about 500 miles in the last 7 years, and now , although it idles and revs beautifully it chucks so much st out of the breather.

You don't say what the compression ratio or cam is so it's hard to say how good those numbers are. You might simply have insufficient breather capacity or a blockage somewhere in them.

Dave

[Crash Test Dummy]

350Matt said:

If a comp test goes up when you add oil the there's a problem with the bores or rings
Matt

The wet compression check will, in most cases, be higher than the dry compression check. Wet being where you added the oil. This is because the oil creates a better seal against the top of the piston rings and the cylinder bore.

The LOWER the difference in readings between the dry and wet checks, the better the condition of the bores and rings.

hemibum said:

few CC's

I was always taught to add a few drops. It is possible that you created a unnaturally perfect seal. Such seals are possible, but your talking perfect manufacturing with minimal tolerance.

As for the Cold temp test - these are a pretty unfair tests on the manufacturers tolerances. As the engine warms up the pistons and rings expand, creating a better seal, hence the higher compression.

Do you know what the factory figures are supposed to be? But as above they seem to be fairly uniform so they are all equally shagged, or they are fine.

[hemibum]

I had run a pipe from the breather outlet into a plastic bottle as a half assed catch tank.

Since I've done away with that it's running fine again. The bottle must have been creating a back pressure in the system somehow. Whatever, the problem is vanished, gone, end of.

[ss64ii]

Try a Leakdown test, this will show you whats going on.
If the engine leaks through the breather then this usually means the rings are shot as suggested.

[smckeown]

hemibum said:

I had run a pipe from the breather outlet into a plastic bottle as a half assed catch tank.

Since I've done away with that it's running fine again. The bottle must have been creating a back pressure in the system somehow. Whatever, the problem is vanished, gone, end of.

Sounds like the engine needs a breather outlet as well as a method of capturing/recirculating any oil

I recently designed/improved my existing system here:




Cam cover breather oulet was previously located above cam and not baffled...bafling design!


Here's Burton Power's excellent technical article on the subject that I used:
http://www.burtonpower.com/technical_1/engine_brea...

[hemibum]

smckeown said:

hemibum said:

I had run a pipe from the breather outlet into a plastic bottle as a half assed catch tank.

Since I've done away with that it's running fine again. The bottle must have been creating a back pressure in the system somehow. Whatever, the problem is vanished, gone, end of.

Sounds like the engine needs a breather outlet as well as a method of capturing/recirculating any oil

I recently designed/improved my existing system here:




Cam cover breather oulet was previously located above cam and not baffled...bafling design!


Here's Burton Power's excellent technical article on the subject that I used:
http://www.burtonpower.com/technical_1/engine_brea...

Thanks for that, and I've got the Burton catalogue which describes the breathing set ups.

I have to admit that I was being a bit sneaky in describing the engine as a crossflow.

Yes it is a Xflow block but with a Lotus twin cam head on it. I just got fed up with posting anything referring to TC's and getting so much hassle from the "Lotus" bunch.

At the end of the day it's the same engine, and apart from the head and water pump it's a X flow as punted out by old Henry.