Jump to content

Compression test result on 2.5 with 89k miles


Recommended Posts

Hi guys here is my conpression test for my gt6 mk3, the head has new valves too. I know numbers don t matter, but there is a big variation...


C1 dry 85 Wet 95

C2 dry 90 Wet 95

C3 dry 75 Wet 70

C4dry 73 Wet 70

C5 dry 100 Wet 100

C6 dry 105 Wet 125


Is it possible to have a lower result wet ??

But overall what do you guys think ? Thx

Link to comment
Share on other sites

The numbers do matter, Sean , but only in comparison across the block.


That they are all virtually the same, wet or dry, is down to your new valves, but to have some that are 25 psi less than the best indicates poor ring sealing.

Can't explain lower when wet, except unless technique was faulty.

So double check.   Did you have:

all the plugs out?

Throttle wired wide open.  Tie it with a piece of wire!   If necessary, disconnect the throttle cable

Fully charged battery - if possible do the test with the car powered by jump leads from a running modern car, so that battery voltage will not creep down.    This is an abusive test to a battery!

Enough turns of the engine to stabilise the gauge reading each time.



Link to comment
Share on other sites

Thx a lot john for your message! So !


All plugs were out - throttle full open (carbs and inlet is out)

I used my MGB battery that was fully charged. And yeah i have rotate on average 6 times but i waited to get the highest result. I doubt it s a battery problem, because i did c6 at the end and got higher results.

But what do you think, is it still ok to run with this for a bit, even though it s not perfect ? Thx

Link to comment
Share on other sites

Poorly sealing rings are due to wear, of the rings or the bore.

Going on running it will do no more harm except you should change your oil more frequently.


I don't understand what Pete means about valve seats - your wet test shows well sealing valves.


The "wrong head" for dome top pistons?

Dome tops were fitted to Mk.3s from car KC 10,001, so that the new 2.5L cylinder head could be fitted without modification and a CR of 9.0 (it had been 9.5 before) could be maintained.

If your engine had domes and the older head with smaller chambers suitable for an ordinary 2L , the compression ratio would be about 13, or if you had the 2.5 head and non-domed pistons then it would be about 7.


Unless you have been plagued with pinking, then I think you can discount the first.

You may be able to see the domes if you peer down the plug hole with a very bright light, with the piston at TDC.

Or else probe with a small screwdriver - you may be able to feel the dome.

But I fear he holds out a failing hope.  That your readings are much lower than others have posted is just in the nature of uncalibrated measuring tools, and when those others brag of 180psi, take salt from the same cellar.



Link to comment
Share on other sites

John different school ,I guess ,

to me it wet and dry are similar its not rings its valves

If wet improves significantly then suspect rings

too much oil will tend to raise the readings


Gauge calibration is a valid point , so yes just look for the differences rather than actuals.


whilst domed should give higher CR with mis matched head but if there is a low CR then you get low results



Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...