Compression query…

[petegardner_901]

Hi all

Any comments on this … checked the compression on the engine (done properly, engine warmed, throttle open etc) and got 120 psi on each cylinder - virtually identical. (It’s a 998cc A series)

I poured a little oil in one cylinder and then got 140 psi.

Now, does this mean there is a lot of bore wear - or an acceptable amount?

I’m not too worried if the engine has to come out for a rebore but obviously if it’s not necessary then fine.

Thanks!

 

[Clive]

Sounds normal to me. You would expect t to increase a bit with oil, and if all even that is good.

 

[Chilliman]

Having a nice even reading across the pots is a good start & your wet test is giving you just over a 16% lift in pressure which is indicative of some wear in the bores which is to be expected but if she's running well I wouldn't be contemplating a re-bore yet based on that data - it's just a used engine but certainly not a knackered one.

[johny]

yes I thought bore wear would show up more on one or two cylinders than the rest so being even is good. How does it drive, any smoke?  

[dougbgt6]

At one point the emission test on GT6s was adjusted from "Does it smoke excessively", to "Does it smoke" OUTRAGE! 

Doug

[petegardner_901]

It’ll be, “Does it vape?” soon.

[Badwolf]

Maybe "Does the car smoke more than the driver?"...... Hey, pass the joint..man! (Can you say that these days?...In my defence I quote from the 60s or was it the 70s.. I don't know, I was out of my skulls for most of the time.

[petegardner_901]

“Skulls”…? As in, more than one?

wow - cool. Two heads!!!

[andybeau]

2 hours ago, petegardner_901 said:

“Skulls”…? As in, more than one?

wow - cool. Two heads!!!

Zaphod Beeblebrox

[Chilliman]

 

Quote

Zaphod Beeblebrox

Excellent.....👍

[Badwolf]

Well spotted chaps, I knew that I could count on someone picking that up.

[68vitesse]

When the cylinder head gasket blew on my 2L Vitesse early last year thought of having the engine rebuilt as number five was always low compared to the rest. The engine starts and runs fine burning very little oil, about a thousand miles to the pint, but decided to just replace head gasket when I costed out the rebuild, seemed a lot of money for little gain.

Drive the Vitesse about three thousand miles a year.

Regards

Paul.

[johny]

Paul, was the lower compression confirmed to be bore wear and did you lap the valves while the head was off?

[JohnD]

21 hours ago, petegardner_901 said:

Hi all

Any comments on this … checked the compression on the engine (done properly, engine warmed, throttle open etc) and got 120 psi on each cylinder - virtually identical. (It’s a 998cc A series)

I poured a little oil in one cylinder and then got 140 psi.

Now, does this mean there is a lot of bore wear - or an acceptable amount?

I’m not too worried if the engine has to come out for a rebore but obviously if it’s not necessary then fine.

Thanks!

 

Pete, the 'oil test' only discriminates between bore wear and valve leaks.    Oil in the bore will always raise the CR, as it temporarily improves the ring seal, but will not raise a low pressure due to a leaking valve.

If I had obtained such consistent readings across the block, I would not have bothered with the oil, as they mean a good ring seal  in all bores, and the oil would have told me nothing.

You got 120psi.   Fine, but more important is that it was the same across the block!  The actual number is meaningless, as the gauges are not precision instruments, uncalibrated, and cannot be compared one to another.   Other variables include atmospheric pressure and the details of the procedure, so the only way to compare different engines is with the same gauge, on the same day, by the same person!

[68vitesse]

4 minutes ago, johny said:

Paul, was the lower compression confirmed to be bore wear and did you lap the valves while the head was off?

Did nothing to the valves as all held a pool of petrol, just did a thorough clean of head and block before putting it back together just over two thousand miles ago with a NOS head gasket and BMC Mini flanged head nuts without washers.

2L Mk1 engine.

Regards

Paul.

[dougbgt6]

9 minutes ago, JohnD said:

The actual number is meaningless, as the gauges are not precision instruments, uncalibrated, and cannot be compared one to another.

As John says!  On my 6 cylinder my old meter say 145 +/-3.  My new meter is much prettier  but says  128 +/-3. 

Doug

 

[johny]

Thats why all that business of doing the test hot and with the throttles open always makes me sceptical....

[NonMember]

The "hot and throttles open" defines a consistent way to test. It also minimises the common inefficiency factors, meaning that a "quick" reading is more likely to be useful.

[johny]

yes but when youre really only comparing readings between cylinders and the exact value isnt critical all that matters is that you use the same method for all of them.... 

[NonMember]

But you may also be comparing readings before and after a change, so being consistent from one day to the next is useful

[johny]

well yes by all means use the same method but it doesnt matter what that is....