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Compression Test Expectation

 
Old 10-11-2018, 08:01 PM
  #31  
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Originally Posted by GregBBRD View Post
928 engines that have a cylinder or cylinders that are up to 40% lower than the best cylinder can fool people.

Therefore, I'm very conservative when reacting to compression readings. I never trust my first compression test on a 928 engine I've not worked on, before. I need to see compression test "problems" more than once, on the same cylinder.

928 engines have very low valve spring pressure and a tiny bit of carbon between the valve and the seat can result in a low cylinder or several low cylinders.

If I have an engine with a low cylinder on an engine, I'll always make sure the pieces involved in making spark and injecting fuel are perfect and then send the car back put to be driven. Sometimes I'll even do a carbon cleaning proceedure and run a carbon cleaning additive in the fuel.

I'll then retest in a couple of weeks.

A huge majority of the time, there won't be a problem. The "bit" of carbon will be pounded/blown out and the compression will return to normal.


Of course, if there is a test that results in one (or more than one) cylinder with 0 to 30 pounds of compression, that's a completely different story.
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Similar at the airport..if leakdown is bad, a sand hammer on the affected cyl exhaust rocker to slap the valve a few times...generally fixes it.

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Old 10-11-2018, 08:02 PM
  #32  
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Originally Posted by UKKid35 View Post
Thanks, I'll just pretend he said 50% rather than 40%

What I'll actually do is run the car for another month or two and test it again
There's no downside....it's not like it is a VW Type 1, where the exhaust valve might break off.
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Old 10-11-2018, 08:13 PM
  #33  
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Originally Posted by Speedtoys View Post
Similar at the airport..if leakdown is bad, a sand hammer on the affected cyl exhaust rocker to slap the valve a few times...generally fixes it.
Requires getting the cam cover off. But - along with rope, might solve the issue. Lets not forget these are direct acting valve on lobe, so no rocker.
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Old 10-12-2018, 04:35 AM
  #34  
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From 90psi cylinder
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Old 10-12-2018, 05:39 AM
  #35  
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Those things are called spark plugs!

Take the car for a spin for half an hour, do a couple of full rev blasts in first or second and when you get back home do a compression test on the cylinder that gave a low reading. If it is still reading low put the spark plug back in and forget about it unless you have some reason to believe the car is down on performance.

Unless you are a complete numb nuts there is no mistaking when the engine is only running on 7 cylinders- you can tell that without even leaving the garage. If you were seriously down on compression pressure in one cylinder the fuel would not burn correctly and the plug would be either very sooty or wetted with unburnt fuel.
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Old 10-12-2018, 06:14 AM
  #36  
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Originally Posted by FredR View Post

Unless you are a complete numb nuts there is no mistaking when the engine is only running on 7 cylinders- you can tell that without even leaving the garage. If you were seriously down on compression pressure in one cylinder the fuel would not burn correctly and the plug would be either very sooty or wetted with unburnt fuel.
I am assuming that because the plug shows no signs of oil contamination that it can't be a problem with rings causing the low compression

However, my assumptions are often wrong which is why I was looking for confirmation here
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Old 10-12-2018, 12:30 PM
  #37  
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Originally Posted by FredR View Post
The Lobe Separation Angle has nothing to do with when the valves open and/or close - it is a measure of the angle between the exhaust and inlet valve lobe peaks.
This is correct.

Increasing the LSA reduces the overlap
This is correct

and as I understand causes the values generated in a compression test to increase for a common cam profile
This is true only if the intake was advanced - opened/closed sooner, which will increase LSA.

If only the exhaust was retarded - opened/closed later, which will also increase the LSA, then there will be no change in the compression numbers.

The point is, it is important to know which lobe was changed to change the LSA. But a simple compression test will tell you that.
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Old 10-12-2018, 12:54 PM
  #38  
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Originally Posted by FredR View Post
Unless you are a complete numb nuts there is no mistaking when the engine is only running on 7 cylinders- you can tell that without even leaving the garage. If you were seriously down on compression pressure in one cylinder the fuel would not burn correctly and the plug would be either very sooty or wetted with unburnt fuel.
Years ago a 20V 5-cylinder Audi came in to our local Indy shop, idled fine, ran down the highway fine....trying to wind it out to redline it would misfire upstairs.

Compression was even across all 5 cylinders. Since the 20V is such a rare engine around these parts I loaned my identical car for testing / comparison (things like good ECU's for that Mitsubishi injection system are difficult to come by).

Nothing fixed the issue, then they decided to do a leak down test. Yup, bent valves in every cylinder.

Talking to the owner, something "clicked" in his brain and he recalled shifting into 2nd instead of 4th and way over spinning the engine resulting in valve float and very minor valve to piston contact.

I drove the car, it ran absolutely perfect unless it was under load above 5k rpm.
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Old 10-12-2018, 12:59 PM
  #39  
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Originally Posted by SwayBar View Post


This is true only if the intake was advanced - opened/closed sooner, which will increase LSA.

If only the exhaust was retarded - opened/closed later, which will also increase the LSA, then there will be no change in the compression numbers.

The point is, it is important to know which lobe was changed to change the LSA. But a simple compression test will tell you that.
The discussion pertains to comparing the GT cam versus the S3 cam- both have the same profile but differing LSA's- the point being whether we can expect to see a higher compression pressure asa Dave reported. Some of the increase will be down to static compression being higher on the S3 [correct?] but that only accounts for 10 psi and the issue is whether the change in LSA accounts for the additional 25 psi Dave reported he saw

Rgds

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Old 10-12-2018, 02:28 PM
  #40  
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Originally Posted by docmirror View Post
Requires getting the cam cover off. But - along with rope, might solve the issue. Lets not forget these are direct acting valve on lobe, so no rocker.
Ya, sorry, maybe I wouldnt be doing this on an OHV car, cant get to the valve with a plastic hammer.

Running is a choice too, but saves a heat cycle and reassembly deep in a 100hr/annual inspection.
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Old 10-12-2018, 06:21 PM
  #41  
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Originally Posted by GregBBRD View Post
Liqui Moly Ventil Saber. May be relabeled as Valve Clean in the US.

Thanks, will see what's available in Canada
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Old 10-13-2018, 04:08 AM
  #42  
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Originally Posted by Hacker-Pschorr View Post
Years ago a 20V 5-cylinder Audi came in to our local Indy shop, idled fine, ran down the highway fine....trying to wind it out to redline it would misfire upstairs.

Compression was even across all 5 cylinders. Since the 20V is such a rare engine around these parts I loaned my identical car for testing / comparison (things like good ECU's for that Mitsubishi injection system are difficult to come by).

Nothing fixed the issue, then they decided to do a leak down test. Yup, bent valves in every cylinder.

Talking to the owner, something "clicked" in his brain and he recalled shifting into 2nd instead of 4th and way over spinning the engine resulting in valve float and very minor valve to piston contact.

I drove the car, it ran absolutely perfect unless it was under load above 5k rpm.
Indeed and precisely why I suggested the OP take the thing for a drive and beat the crap out of it in the lower gears.

Personally, I always recommend folks do a leak down test to compliment the compression test otherwise it is a bit like having fish and chips without the fish- an incomplete meal as it were.

Having done 165 mph on the "Ring" [or wherever] it is a pretty safe bet that there is nothing wrong with that motor unless something let go on the way home- possible but not probable or so I would think.
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Old 10-13-2018, 12:21 PM
  #43  
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Originally Posted by docmirror View Post
PV=NRT. Not reading from Wiki so my apology. Adiabatic heating of compression will have a significant affect on the final reading. Also, if it is a mechanical gauge, there is the hysteresis of the bourdon tube works.

Continental engine works has done quite a bit of research on compression(static and dynamic) for aircraft engines, as well as tractor engines. They have found that readings as low as 75% of the 'book' value of a static comp test will still make rated power. It's not completely understood, but it is the way it works in real life.

While static comp tests are somewhat useful, compound compression tests are more revealing in finding issues with the upper cylinder. Air leaks can be heard going through the rings from the oil filler, or through the valves from the intake or exhaust. A benchmark is usually set when an engine is new and broken in for aircraft and that becomes the standard for losses along the way. If 80PSI is put in at TDC under a calibrated fixed orifice, the resulting losses should be evaluated as the engine ages.
As an old (as in geriatric) aircraft mechanic, I agree that a leakdown test will tell more. It should be done on a warm engine. You can buy a setup from Harbor Freight.
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Old 10-14-2018, 04:55 PM
  #44  
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Originally Posted by GregBBRD View Post
Liqui Moly Ventil Saber. May be relabeled as Valve Clean in the US.
Bought some

Thanks
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