Compression test

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Compression test

Post by GTC Ian » Sat Nov 18, 2017 3:48 pm

Just done a compression test on the GTC. I have a range of
1) 13,0 bar
2) 10,5
3) 12,0
4) 11,5
5) 11,6
6) 11,8
Has anyone an idea what the readings should be?
I know I have a problem with valve stem seals, but dont want to do them just to find the rings/bores are worn.
Condition of the engine will determine how I proceed with the plans I have for the car this winter. The SS1 will have to take a back seat for a while.
All input will be gratefully received


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Compression test

Post by philhoward » Sat Nov 18, 2017 4:06 pm

Rule of thumb says a 10% difference across cylinders - yours are almost within that..

Valve clearances OK?


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Compression test

Post by efi_sprintgte » Sat Nov 18, 2017 4:39 pm

Yes do valve clearances and then do it again..

Could do wet and dry compression tests which will indicate if your rings are OK.


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Compression test

Post by scimjim » Sat Nov 18, 2017 6:52 pm

I would question number one? I’d expect them to be somewhere in the range of 150-170 psi and you have 190+


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Compression test

Post by GTC Ian » Mon Nov 20, 2017 10:04 am

Re done the test yesterday and no 1 pot that was high first time, round was 11.8. Did a wet test on the low one and hardly made any difference.
Ive got 3 days off this week so the next plan is to try Forte engine flush that has come recommended (it was free). See if it lives up to the hype.
Whatever happens the heads are coming off to do valve stem seals, along with the long list of jobs. This is turning into a bit of a project.
After the engine the gearbox needs new filter and gasket .
New headlights and new bowls need fitting.
Bumpers are coming off to be smartened up. A couple of body repairs to be done while they are off. My local bodyshop is due a visit.
Poly bushes for the radius arms
Hinge plates in the doors have rusted away so Ive made some stainless ones to fit
While Im doing the doors need a new window motor
New electric aerial to go in
And find that annoying rattle behind the drivers seat. Hoping its the roof frame and nothing more sinister
Oh and now the passenger seat diaphram and foam has collapsed and the drivers seat covering has given up the ghost its a trip to thr local trimmers to get a quote to re do the interior.
Like Ive said Barbaras SS1 has been cosseted the last couple of years, now it is my turn to lavish some TLC on the GTC. All the bit are in a big box in the garage. Been collecting them for months now.
I will post piccies when I suss out how to do it.


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Compression test

Post by philhoward » Mon Nov 20, 2017 10:09 am

I hope the rattle behind the drivers' seat isn't the roll bar ends...

No.2 may be a tappet not quite right or a valve not sealing 100% - not too bad but if the heads are coming off anyway, I guess all will be revealed. It shouldn't be anything "bad" as it's only just out of spec compared to the rest. If the wet test didn't change the reading, then the bores/pistons sound OK.


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Compression test

Post by GTC Ian » Mon Nov 20, 2017 10:16 am

Forgot to add that I bought a Powerspark distributor kit at the NEC. The wire from the dizzy to the module on my car has got hot at some time and is now very brittle.
One thing not decided on is whether it is worth fitting a mildly uprated cam.


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Compression test

Post by GTC Ian » Mon Nov 20, 2017 10:18 am

I hope the rattle behind the drivers' seat isn't the roll bar ends...
That is my worry . If it is then do them now before I do the interior.


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Compression test

Post by scimjim » Mon Nov 20, 2017 11:25 am

Have you had poly bushed radius arms before Ian? I thought it could make the ride a bit harsh at the rear?


Jim King

Current: SE5 (8Ball), TI SS1 (snotty), 1600 SS1 (G97), 1600 SS1 (C686CCR), 2.5TD SE5a (diesel 5a), 6 x random other SS1s.
Previous: SE5, 3 x SE5a, 2 x SE6a, 3 x SE6b, GTC, 2.9i GTC, 3 x 1600 SS1, 1300 SS1, Mk1 Ti Sabre, Mk1.5 CVH Sabre
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1300 SS1 (Number One) & Sarah's coupe.
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Compression test

Post by GTC Ian » Mon Nov 20, 2017 11:44 am

No I havent Jim. But these are the 2nd rubber set I have put on and they dont seem to last. Noticed another one has split.


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Compression test

Post by Old and Slow » Mon Nov 20, 2017 10:45 pm

Can anyone confirm/enlighten me on something I'd heard long ago about compression testing a worn engine, as follows:
When you spin the engine dry you usually need 3 to 5 compression strokes to pump up the gauge (i.e. 6-10 turns of the engine).
If you then put oil in the cylinder and spin the engine, if the compression comes up quicker and to a higher level, then you have "proved" that the low compression problem is caused by worn rings;
If instead, it still takes 3 or 4 or more compression strokes to get up to pressure, then it is a valve seat problem.
Some time soon I must do a compression test on the Sabre Six, but do I really want to know! :shock: :w


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Compression test

Post by philhoward » Tue Nov 21, 2017 6:34 am

From a random site:

Interpreting the readings.
Normal: Compression builds up quickly and evenly to the specified compression for each cylinder.
Piston Rings Leaking: Compression is low on the first stroke. Compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
Valves Leaking: Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you ad oil.
If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be the head gasket leaking between the cylinders.


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Compression test

Post by heathsabre » Tue Nov 21, 2017 10:04 am

We've now had our "new" Sabre Six for just over 4 months. Prior to our ownership it had only done about 250 miles in 8 years. As far as I can tell it's the original engine and has done about 60,000 miles from new. Generally it runs OK but smokes from the crankcase and rocker cover breathers. When we first got the car I measured the following cylinder pressures:-

1- 9.7 bar
2- 8.6 bar
3- 8.6 bar
4- 9.7 bar
5- 9.3 bar
6- 7.6 bar

I repeated the readings yesterday, 300 miles further on:-

1- 9.7 bar
2- 9.3 bar
3- 9.3 bar
4- 10.3 bar
5- 10.3 bar
6- 9.3 bar

I then carried our "wet" tests on 2 and 6 and got 10.0 and 10.3 bar respectively.

Do these readings look acceptable or do they suggest worn rings?


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Compression test

Post by efi_sprintgte » Tue Nov 21, 2017 10:13 am

It would suggest that maybe the rings are slightly worn, especially as the crankcase is pressurising.


When I got my first GTE it had been laid up for 16 years, once running it smoked ALOT. 1000 miles of use and two oil changes and it was like the rings bedded back in again, was spot on after that.

I would suggest using it some more and monitoring the situation.


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Compression test

Post by philhoward » Tue Nov 21, 2017 11:54 am

I’d totally agree - some running on decent petrol like V-Power should improve things even more - it certainly seems to be improving based on the readings.

Might be worth seeing inside the bores with a small USB camera and see if there’s any scoring?


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