Engine Compression 2005 RT

[dmshanks]

Greetings; I just purchased a 2005 R1200RT with 5400 miles on it. I performed a compression check on both cylinders after having warmed the engine. I disconnected the voltage to the secondary coils on all plugs, but left the injectors operational, and positioned the throttle to full. Will having fuel vapor in the cylinders affect the pressure reading at the gauge? My readings were 188 left, 188 right. Can anyone comment on these readings? What does BMW consider to be the acceptable range? Thanks

[dirtrider]

Afternoon dmshanks

 

Good question (it would have to effect compression as it adds volume to the cylinder), the question is how much? Probably not enough to make any difference.

 

Your 188 (I assume you are measuring in PSI?) is in the ballpark-- My BMW manual shows 125-145 psi but I have always measured more than that on a healthy 1200 engine. I have seen 175-195 psi on a good engine. It really boils down to engine cranking RPM, amount of carbon in the cylinders/on piston top, & height above sea level.

 

A leak-down test is probably more useful for the engine cylinder/piston/ring/valve condition.

[dmshanks]

Yes, PSI & thanks for the data on accepted pressure. I will perform the test again with the injectors disabled as I'm curious to compare compression with fuel vapor Vs. without. Regarding the leak down test, I've seen it performed but have forgotten the procedure & type of gauge needed. I really have no reason to think there's an issue with this engine as it pulls very well & there are no discernible anomalies.

[JayJay]

Boy, if 5400 miles is correct, I sure hope you don't have any appreciable valve or ring wear! The only thing I could think of that a leakdown test might show would be valve burning from running them too tight, but I would think you'd see that in the dry compression test as well. And your side-to-side comparison is excellent.

 

Interesting to see how the results vary with the injectors disabled. Thinking theoretically, if you presume the final air:fuel ratio is 14.7:1, the fuel would take up approximately 7% volume to the cylinder. So at most, assuming it's entirely liquid and incompressible, that would mean your readings would be ~7% or 13 psi high. However, the purpose of the atomizer in the injector is to turn that liquid fuel immediately into vapor, which is compressible. Bottom line - I doubt that it affects the compression readings much.

 

Back in "the day" we used to squirt a bit of oil into the cylinder as part of the compression test, the idea being that it would temporarily block leakage around the rings. Significant difference in the dry vs. oil condition would tell you roughly if you had a ring problem. We never bothered to calculate the difference in pressure due to volume of the oil injected.

 

JayJay

[Guest Kakugo]

According to the German workshop manual I have here, BMW only recommends checking minimum pressure. There are no references for "too high of a compression value".

 

Minimal pressure is given at 10bar, which is 145psi: every figure above it according to this expensive service manual is good, and who am I am to argue?

 

In short: don't worry: your engine is sound. Spend less time tinkering and more time riding.

[dmshanks]

I couldn't resist; I performed another compression test with the injectors disabled. I attempted to keep the test conditions the same as the first test, which had the injectors operational. Here's the results:

 

Left: 196

Right 196

 

This would suggest that the fuel vapor is less dense than air, wouldn't it? I do believe air is heavier than gasoline vapor, so this is intuitively correct. I think

 

Whoa, I stand corrected by what I just read on a Google search: gasoline vapor has a vapor density of 3-4 times that of air. Interesting. So I don't know why my readings are different the 2nd time around. Must be the conditions under which the test was conducted. Could be that the fuel vapor increases the pressure in the cylinder. The last test would suggest that. I wonder If I'd get a different reading each time the test was performed......

[dirtrider]

Morning dmshanks

 

Did you disconnect the lower coils so they didn't fire? (I presume you did)

 

IF you ran your original compression test at a FULL WOT then it probably wasn't injecting any fuel.

 

Most electronic fueling systems have a clear-flood-enable at wide open throttle cranking to allow a flooded engine to be started by holding the throttle wide open.

 

Different compression test numbers could be (probably is) due to cranking speed difference or some oil in the lower cyl head area.

 

Did you give it the same number of puffs as the first test?

 

Your numbers seem believable based on other 1200 hexheads I have tested, the perfect match on numbers side to side over 2 different test days leaves me wondering though.

 

Cranking compression is MUCH more useful as a side to side comparison than using peak numbers.

 

More in depth (& useful) is usually a leak down test.

[lkraus]

I'm thinking the fuel "vapor" is actually very tiny droplets of liquid gasoline mixed with air. The portion of the "vapor" that is liquid is not compressible, so theoretically, the pressure should be expected to go up.

 

Haven't had enough coffee today to attempt to calculate the theoretical differences, and it probably doesn't matter anyway.

 

 

[aggieengineer]

Counter-intuitive, but dmshanks is correct. Similarly, humid air is less dense than dry air.

[dmshanks]

I did have the lower sparks disabled. I wasn't counting "puffs". I let 'er crank until the pressure stopped going up. Maybe 12 puffs. Both sides seemed to take about the same "puffs", but I don't know exactly. I'll acquire a leak-down gauge eventually. I don't mind the tinker time, and we've had much rain and storminess here in Utah for a couple weeks now. Been nice, but the rain seems to come just after I get off work. So, the tinkering is a way to stay connected to my new bike while staying dry and I am genuinely curious as to the condition of the engine, as it has sat most of its 10 years. So far, all seems in order. I changed out all oils last week. It all looked good. Final drive had some black which settled to the bottom of the drain pan, but I didn't notice any metal flakes while directing a strong beam of light onto the oil surface and there was no metal settled in the bottom of the pan. The tranny oil was very clean. There was a bit of collected silver on the magnet of the speed sensor that is near the top of the final drive however. Probably normal after 5400 miles?

[dirtrider]

clip--Final drive had some black which settled to the bottom of the drain pan, but I didn't notice any metal flakes while directing a strong beam of light onto the oil surface and there was no metal settled in the bottom of the pan. The tranny oil was very clean. There was a bit of collected silver on the magnet of the speed sensor that is near the top of the final drive however. Probably normal after 5400 miles?

 

Morning dmshanks

 

Both normal-- back on the 05 (BMW claimed lifetime fill on final drive) they used a high moly content gear oil as factory fill. That stuff will settle out for a few final drive gear oil changes.

 

Did you use the updated lower fill level for the final drive fill level? (180 Ml)

[dirtrider]

I did have the lower sparks disabled. I wasn't counting "puffs". I let 'er crank until the pressure stopped going up. Maybe 12 puffs. Both sides seemed to take about the same "puffs", but I don't know exactly.

 

Morning dmshanks

 

 

Remember the engine has only (1) puff to get full compression while starting/running so using a lot of puffs to gain max compression can be a bit misleading. If it can't meet compression specs in 3-4 puffs then something wrong with gauge or engine. Personally I usually do 3 puffs on each side but am real careful to then do 3 on the ether side.

 

 

 

[dmshanks]

Yes, 180ml of Mobile 1 75W90 in FD. Well, If I allow only 3 puffs, that probably puts me at around 100psi on compression test.