acceptable GT6 cylinder compression

[Adrian]

Hi all

Now I know this isn't right but I've cranked the engine by hand (engine is out) just to check compression consistency (I've only just got a tester). So I know I can't achieve proper compression (my arm only works so fast) but noted that I got around 70 in 6, 5 and 3. 4 was 50 and 1 and 2 was 20-30. Even by hand that seems to be a big variation. The engine ticked over quite nicely before I took it out with no oil burning, etc.

I haven't check the valve clearances as yet. The car history seems to be a replacement engine was put in around 1990 at the same time the speedo was zeroed so basically the car has done 23K in that period. I know it tends to be a house of cards when dealing with this sort of thing and have enjoyed (ish) learning but I really don't want to be messing with piston rings - particularly as the sump has been on twice already, thanks to a stripped thread! 

If I read the manual right, compression should be 140ish? So I assume muscle power has only achieved 50% and so 6, 5 and 3 are OK and 1, 2 and 4 are lacking.  I'm hoping it may be valve settings.

Any thoughts?

Many thanks and looking forward to the 17th.

[Colin Lindsay]

To be honest I don't think you'll get anyway near the proper compression using only muscle power!

If the engine is running ok before being taken out I'd try a compression test again once it's refitted; there's no point in initiating repairs based on an incorrect assessment unless you just want to be sure and refurbish it anyway, which going by your post, I don't think you want.

If nothing else, can you rig up a starter connected directly to a battery to get it up to a proper speed for testing? It might give a more realistic reading and allay your fears somewhat.

[Pete Lewis]

yes think i echo colins thoughts,  

and  were the throttles open , any restriction will reduce the readings , especially hand cranking, if the front carb was shut  then you get low reading

if the carbs are off ,  well more shreaded wheat may help !!!

 

Pete

[Adrian]

That's good to know - I can carry on as planned! No carbs or manifold on. I knew I wouldn't get anywhere near - just wondered about the observed variation.

[JohnD]

It's impossible to say, this is an acceptable pressure, this is not, as the gushes are uncalibrated.     They are invaluable to compare the bores in a block, or else if you have a known ( believed?) good engine to compare with that.    Don't omit to do the oil test:  teaspoonful down the p!ug hole after doing the test 'dry'.   This will improve even poor ring sealing, but have no effect on valve seal, so diagnostic.

 

John

[Pete Lewis]

was dreaming about making a clear   U tubed  water filled manometer to compare the  swept volume  condition but reckon would need one as high as the house    Hmmm   !!!!

back to compressions ,  one good stroke from btc to tdc and compare , will be low figures but only takes half a turn of the crank 

which is an easier effort  if by hand  and 3 shredded wheats  rather than  try to turn over and over enough to get the best readings 

just a ramble 

Pete

 

[JohnD]

Swept volume is a dimensional calculation, if you have the head off, but if you wanted a water manometer type device, the height will depend on the diameter of the tubing.     But a better design would be, fill the bore with a liquid at BDC, stick a hose in the plug hole and turn the crank to TDC, catching the liquid INA measuring jug.  Don't forget to empty the hose into the jug!

John