Compression testing...

[JimEB]

Hi all,

Got myself a compression gauge so it's time to see how healthy my PI's engine is    ::)

What should a healthy Big Six engine compression reading be? I guessing 10-15% difference across the cylinders is tolerable.

Do you do your testing with the engine cold or at normal operating temperature?

Also, what's the easiest way to disable the fuel pump - disconnect the wires at the pump in the boot or pull a fuse?

I'll be testing the compression dry and then wet (bit of oil squirted into the cylinder).

Thanks   :)

[Nick Jones]

Test should be done at operating temperature with the throttle wide open.  All plugs out & LT coil wire disconnected.

You can use the inertia switch (grey cylinder with button on the top next to the washer bottle on the bulkhead) to disable the pump.

Something in the range of 160 - 180 psi is about right but comparative readings probably more useful.  Should be within 10 Psi or better ideally.

I normally do 5 compression strokes for each cylinder.  You should see a good % of the final pressure arrive in the first couple.

Ought to do mine now the rebuilt engine is run in.  Mind you, the old engine had a pretty even 190psi - the working theory being that there was so much oil getting up around the rings that every test was a effectively a wet one!

Cheers

Nick

[JimEB]

Cheers Nick - I really should've thought of the inertia switch!  ;D

[JimEB]

Well, dry tested cylinders 1, 2, 3 and 6 yesterday, all showing around 190 psi, so that's encouraging! It's not burning oil, so I reckon the rings are fine.

Couldn't get my eBay-tastic Gunsons tester into cylinders 4 and 5 due to it fouling against the dizzy, so has anyone used an extension successfully or could you recommend a different tester?

Thanks  :)

[GT6 M]

take dizzy off, simples.

M

[Richard B]

The tester I have screws into the spark plug port and has a short (6") flexible rubber hose between the gauge and the connector.

This gets around the dizzy.

[JimEB]

Richard_B wrote:

The tester I have screws into the spark plug port and has a short (6") flexible rubber hose between the gauge and the connector.

This gets around the dizzy.

Oh! Are you going to the North Hampshire meeting on Tuesday?  ;)

[Richard B]

;D  If not, I'll see if someone can take it there, James, Tim or Cookie.   8)

[Tim Bancroft]

I will be going and have a similar gauge to Richard's, will bring along if the memory does'nt fail!!!

[TedTaylor]

Richard_B wrote:

The tester I have screws into the spark plug port and has a short (6") flexible rubber hose between the gauge and the connector.

This gets around the dizzy.

Same as the one that I prefer to use from my collection.  Gives a positive fixing in the spark plug 'ole and leaves your hands free to turn the engine over etc.

MUT

[piman]

Hello Ted,

that reminds me that when I built my car I fitted a 'jog' button under the bonnet to enable me to turn the engine over for maintenance. Simple to do and very useful.

Alec