BMW Luxury Touring Community banner

1 - 10 of 10 Posts

·
Registered
Joined
·
1,985 Posts
Discussion Starter #1
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.
 

·
Wrencher Extraordinaire
2005 K1200LT
Joined
·
14,249 Posts
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.
 

·
Registered
Joined
·
717 Posts
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)
 

·
Registered
Joined
·
12,165 Posts
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.
 

·
Registered
Joined
·
1,985 Posts
Discussion Starter #7
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 ?.
 

·
Registered
Joined
·
12,165 Posts
simoncharles said:
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.
 

·
Registered
Joined
·
1,985 Posts
Discussion Starter #9
Yes, that is what I figured, but needed it confirmed by somebody who actually understands what they are talking about !.
 

·
Enjoy The Ride
Joined
·
3,989 Posts
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.
 
1 - 10 of 10 Posts
Top