Compression test

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Compression test

Post by richardvernon » Tue Nov 21, 2017 1:05 pm

Another method -If you have access to compressed air and an adapter(for spark plug hole)- is to get a bit of air into the cylinder.

A few things may be deduced about engine condition/suspect cylinders:

With the cylinder in question at TDC, and the car in gear, if you can hear air hissing predominantly through the carb/inlet manifold then that indicates inlet valve sealing probs.

Likewise noise from the exhaust tail pipe indicates exhaust valve issues whilst hissing through the filler cap would be ring/bore wear issues.And bubbling of the coolant indicates head gasket issues.

There is a leak-down tool that can give you a reading of pressure loss from a cylinder -but i have never used one myself - but I have used the above tests to check cylinder issues.
Last edited by richardvernon on Tue Nov 21, 2017 3:35 pm, edited 1 time in total.


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Compression test

Post by Old and Slow » Tue Nov 21, 2017 3:25 pm

I like the idea of putting pressurised air into the cylinders, but not having a compressor etc, I'll have to rely on the compression tester.
When I've tested 660XYB I'll post the results, will be interesting to compare with Tony's figures above.


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Compression test

Post by Oaksey » Tue Nov 21, 2017 4:01 pm

A leak down tester is well worth having. I've tested countless engines that have performed reasonably well on a compression test but failed miserably on a leak down.


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Compression test

Post by Old and Slow » Thu Nov 23, 2017 1:07 pm

Well, I think 660XYB might have failed miserably on the compression test. :(
Of course, I broke all the rules, I didn't warm up the engine, and didn't check the tappets before starting.
Raymond Mays head, Volvo pistons i.e. high compression
Here are the results, in no particular order to quote a phrase; well actually, cylinder number, dry pressure psi, wet pressure psi.
1,170,290
2,140,255
3,145,260
4,130,230
5,160,240
6,150,170
So then I checked the tappets; 4 and 6 inlet had no clearance.
I started up in clouds of smoke, it still hasn't cleared.
Next time I'll warm the engine before testing.
All comments welcome, but it looks like I need new rings sooner rather than later.


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Compression test

Post by Oaksey » Thu Nov 23, 2017 1:24 pm

Aside from 4 (and 2 and 3 kind of) the dry compression results don't sound terrible. The wet results sound very very high to me though?


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Compression test

Post by chrisgallacher » Thu Nov 23, 2017 1:51 pm

Agreed, they do. Can't recall what my PSI readings are, but your #1 for instance - 170 dry is pretty high already so increasing it all the way to 290 psi wet doesn't sound right. How wet was 'wet' exactly?


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Compression test

Post by Oaksey » Thu Nov 23, 2017 3:29 pm

I can't remember what effect the Mays head has on compression but the pistons shouldn't add much. The most I'd expect from a high compression setup is about 190 but I wouldn't think a S6 would be run with that high a compression ratio


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Compression test

Post by Roger Pennington » Thu Nov 23, 2017 3:35 pm

I'd agree that some of the wet readings do look rather high, however I think I'd be inclined to repeat the test with a properly warmed-up engine before I came to too many conclusions?


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Compression test

Post by Old and Slow » Fri Nov 24, 2017 8:40 pm

Thanks for the feedback.
I think I overfilled the recommended "teaspoon" of oil in #1 cylinder so that probably accounts for the high reading (i.e. not much room left for the air to be compressed!).
It will be some time before I have an opportunity to repeat the tests, and it is a pain getting at the threads of plug hole #6 to screw in the gauge. :evil:
but when I do I'll post the new results.
And I'll take heart from Oaksey's positive (ish) assessment, before putting out a plea on the forum asking where I can source Volvo rings (given that I've no idea what pistons have been used)!


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Compression test

Post by scimjim » Fri Nov 24, 2017 8:45 pm

Do you know what the compression ratio is? I presume “Volvo pistons” are the ones that Robin Rew was fitting 40 years ago (and I think Danny still is)?


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Compression test

Post by efi_sprintgte » Fri Nov 24, 2017 9:28 pm

P1800 iirc?? I'm sure a part number will be stamped on the piston??


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Compression test

Post by Old and Slow » Fri Nov 24, 2017 10:47 pm

Thanks for the further feedback.
Well, I guess I'd need to get the USB camera out and see if I can read something on the piston crowns (and find out whether it needs a de-coke!!)
According to the Robin Rew articles on the archive here, he said:
". I suffered frequent piston breakage on the Sprint ¬Sabre and set about looking for an alternative which I found in the Volvo P1800 piston. This needed only a slight change in size of small end bush to 22mm and the standard bore Volvo was equivalent to a .030" rebore (approx). As the crown height was identical, this was a more than satisfactory substitute which is now commonly used."
I believe Richard Prosser had the engine rebuilt by Iain Daniels, but that was late last century - he might still have records.
Regarding compression ratio, as Robin Rew says the piston crown height is the same, so it will depend on the Mays head combustion chamber. Perhaps someone out there will know? Comparing with Tony's figures, the average of 660XYB is around 10bar, i.e. not too dissimilar. Lower than the GTC that started this thread.


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Compression test

Post by GTC Ian » Tue Jan 23, 2018 8:58 am

philhoward wrote:
Mon Nov 20, 2017 9:09 am
I hope the rattle behind the drivers' seat isn't the roll bar ends...
After having a look it is the roll bar end rotted through. Luckily it is only the end plate and not the tube. When I first bought the car 14 years ago I did coat it all in hammerite and underseal, so that will need removing before I weld it.
This is now turning into a minor rebuild. All the interior is coming out and is off to a local trimmer to be recovered in leather. Did some humming and harring over what material to use (crushed velour in the same colour unobtainable) and as this is going to be a once in a lifetime for the car, leather was the final choice. So the stripdown will start now the days are getting longer. Interior out, carpets binned and doors off, then the welding can begin. Hopefully I can get all this done while the seats are away. He reckons from the end of February it will take 6 weeks to retrim. All the other jobs will follow on, as and when.
On the subject of carpets has anyone used Coverdale carpet sets?


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Compression test

Post by philhoward » Tue Jan 23, 2018 9:04 am

GTC Ian wrote:
Tue Jan 23, 2018 8:58 am
On the subject of carpets has anyone used Coverdale carpet sets?
Mixed reports, I believe... :w Geraint and AJL have certainly had recent experience.


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