Compression Number

[Anonymous]

What are the compression numbers normally seen on the 4 cylinder 11 Perfo engine.

Just to check my compression but would be good to have an idea of what to expect.

Thanks
Ian

[Anonymous]

Published ratio is 6.2/1 and compression circa 7.7kg/cm2 or 111 psi with flat top pistons (domed pistons which you fitted should lift the ratio)

Before my engine was rebuilt with new pistons and liners, bearings etc. it showed just over 102 psi on a compression test, but as the crank had to come out to be reground and there was a lip on the cylinders it was decided to change the pistons and liners anyway (on a BL you really can’t do anything much without removing the engine) for the sake of another £300 considering the work involved to do it later.

Funnily enough, I haven’t done a compression test now everything is run in, maybe I should…..

[Anonymous]

So I have done the compression test working from left to right 1, 2, 3, 4

Cylinder 1 – 8.8 bar (127.6psi)
Cylinder 2 – 9.2 bar (133.4psi)
Cylinder 3 – 9.0 bar (130.5psi)
Cylinder 4 – 8.0 bar (116.0psi)

So I am a bit low on number 4 – why could this be.

I am super worried about the seal on the base of the cylinder sleeve, not that I have done it wrong, just because I have never done this before. But then that should not have an effect on the compression. Maybe the piston rings are lined up with the gaps?? Just guessing here so any advice would be appreciated.

Otherwise the figures seem OK to me.

[Anonymous]

To test cylinder 4, check the compression as before, dry, then put a few drops of oil into the bore via the plughole and test again which should confirm if it’s a piston/rings issue if the pressure increases, but also recheck the valve clearances on cylinder 4 just to make sure that they aren’t the problem.

Those domed pistons though certainly seem to improve things.

[Anonymous]

Right, I warmed the engine up and took the rocker cover off.

I unscrewed each head bolt (in order) a tiny bit and then re-torqued them a little tighter in turn. Before I did this I saw some leakage from the rear of Cylinder 4. I have heard the modern head gaskets require more torque than the original spec. So with a warm head I torqued them about 15Nm more.

I also checked the valve clearances again, with a warm engine. found two to be slightly out including valve 2 (cylinder 1) and valve 7 (Cylinder 4).

Retested the compression.

Cylinder 1 – 9.2 bar (previous 8.8)
Cylinder 2 – 9.3 bar (previous 9.2)
Cylinder 3 – 9.1 bar (previous 9.0)
Cylinder 4 – 9.0 bar (previous 8.0)

So the head may not have been fully tightened.

I also checked for water in the oil (before any adjustments) using the splatter test (yes I have been up all night study You Tube videos 😆 ) and with heating the oil in a jam jar lid, if the oil just smokes it is fine but if it splatters i.e. the water boiling off, then it has water in it. I have some water in the oil. So I will run it again and drive for another 50km or so and then replace the oil. By that time my CO2 tester will have arrived and I will be able to check again for any leakages (in this case combustable gases in the coolant).

[Anonymous]

Nice result, hopefully everything is secure and there is no coolant getting where it shouldn’t

[Author]

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