Compression - BMW Luxury Touring Community
 
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Old Dec 10th, 2009, 12:39 pm Thread Starter
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Compression

Does anyone know what the compression of an LT cylinder is ?. I am not talking about the 10,8 :1 ratio but the one a manometer would show if screwed into the spark plug hole.
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Old Dec 10th, 2009, 1:28 pm
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Re: Compression

The end number is not the critical factor but rather the spread between cylinders. I have not checked mine to see what nominal is but I'll wager David Shealy has.

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Old Dec 10th, 2009, 2:21 pm
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Re: Compression

Pg 11.5 of the service manual.

Compression test pressure
good bar (psi)-----above 10.0 (142)
normal bar (psi)---8.5…10.0 (121…142)
poor bar (psi)------below 8.5 (121)

Dave
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Old Dec 10th, 2009, 2:25 pm Thread Starter
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Re: Compression

Thanks Dave
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Old Dec 10th, 2009, 7:34 pm
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Re: Compression

This is what a good cylinder on mine reads at 10 revolutions. The red line on the tale tel is the number.
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Dave Selvig
2004 Black LT
2000 Canon Red LT



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Old Dec 10th, 2009, 10:06 pm
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Re: Compression

A compression test is one of the first things to do if a problem is suspected, because it is easy to do. If the compression test does show too much variation between cylinders or too low readings, then the best next step is to perform a "leak down test". This is harder to do, and requires a leak down tester, which is a manifold with two pressure gauges, with a valve to control the flow into the manifold from an air compressor, a 0.040" orifice between the gauges, and a hose with adapter to screw into a spark plug hole. This is attached to a cylinder, the engine blocked at top dead center on the compression stroke for that cylinder, then the engineheld so it cannot turn. (I positioned my cylinders just past TDC then in 5th gear strapped the rear brake down).

Then the valve on the manifold is slowly opened until 100 PSI registers on the inlet gauge. The reading of the cylinder side then shows "percentage" of air held, the difference being the leakage. If the output gauge reads 85 PSI, then you have a 15% leakage (of course this is a relative number, but very useful). The greatest advantage to this type of test is that you can tell where the leakage is by listening/feeling. Air out the exhaust is signs of exhaust valve leakage, out of the intake is signs of an intake valve leaking, air out of the oil filler shows piston ring leakage.

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IBA SS, BB, BBG, 10/10ths.
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Old Dec 11th, 2009, 9:21 am Thread Starter
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Re: Compression

David, very interesting, thanks.

I note that the numbers given in the manual refer to the 98 hp engine. Are they the same for the "newer" 116 hp engine or, as the compression ratio 10,8 / 11,5 : 1, is higher, these values should also be increased ?.
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Old Dec 11th, 2009, 9:33 am
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Re: Compression

Quote:
Originally Posted by simoncharles
David, very interesting, thanks.

I note that the numbers given in the manual refer to the 98 hp engine. Are they the same for the "newer" 116 hp engine or, as the compression ratio 10,8 / 11,5 : 1, is higher, these values should also be increased ?.
When compression ratio increases, so do the pressure readings obtained. The percentage difference in the compression ratio is 6.5%, so I would expect the readings on a compression test to be similar.

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David Shealey
Dandridge, TN
EX: '01 Black LT, BAT BYKE (Totaled at 110,000 miles)
IBA SS, BB, BBG, 10/10ths.
No bike now, but maybe in the future.
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Old Dec 11th, 2009, 10:11 am Thread Starter
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Re: Compression

Yes, that is what I figured, but needed it confirmed by somebody who actually understands what they are talking about !.
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Old Dec 11th, 2009, 5:33 pm
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Re: Compression

Compression ratio & camshaft profile both have a effect on the number. On my LT all the numbers at 10 revolutions are within 3psi of each other. At 15 revolutions 1,2 & 4 cylinders pick up another 13 to 15 psi while # 3 cylinder stays the same. I have 2 to 4 % leakdown on 1,2 & 4 cylinder and 30 % on # 3. On # 3 cylinder it is leaking past the rings.

I haven't checked it in about 10,000 miles but I'll be spending 5 days in two weeks going over the whole bike.

Dave Selvig
2004 Black LT
2000 Canon Red LT



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