Cylinder compression ratios

[Saint aka ML]

Ok been trying to find that out on the other forum bt did not get one for my monster there was something found for 748 but that can not be the same values can it?

I have measured compression ratio and I have 160 horizontal and 150 psi vertical.

Now is that high or low for 1993 m900 with 944 kit fitted?

What are the by the book values for stock 1993 m900 (they will not be the sameas for post 1995 models but if you have them do post) and if some one has for a 944 kitted one?

Just to clarify I do not understand that 9:7:1 or whatever malarky would just like to compare compression in PSI as that is how it was measured (can be in bar).

Also some websites suggest that ratio will change a bit depending from starter motor and battery condition is that correct?

[Scotty]

http://wiki.answers.com/Q/How_do_you_convert_compression_ratio_and_psi

[Saint aka ML]

http://wiki.answers.com/Q/How_do_you_convert_compression_ratio_and_psi

Thx it almost makes sense except it does not.

1:1 is 0 psi and 2:1 is 14.7. Then he states to to multiply ratio times 14.7 which ratio 2 or 1 in any case ratio is given 9:7:1 so 3 numbers what then ....

I am lost. By a bit of searching I would think my one is 10:1 but then again do not know if it is high for what I have installed or low?

[Saint aka ML]

Ok I think I have the reply:

So stock monster 900 is ratio 9.2:1 which is 9.2*14.7 psi = 135 psi stock (in perfect conditions).

There are 3 types of 944 kits for monster that are direct bolt on:

one that gets it to 944 and ratio 9.5:1
one that gets it to 944 and ratio 12.3:1 !!!
and one I seem to have that gets it ratio to 10.8:1 with lighter pistons.

So 10.8:1 is 10.8*14.7 psi = 158 psi (in perfect condition). My was tested on cold.
I think I am spot on there

[dunlop0_1]

Ok been trying to find that out on the other forum bt did not get one for my monster there was something found for 748 but that can not be the same values can it?

I have measured compression ratio and I have 160 horizontal and 150 psi vertical.

Now is that high or low for 1993 m900 with 944 kit fitted?

What are the by the book values for stock 1993 m900 (they will not be the sameas for post 1995 models but if you have them do post) and if some one has for a 944 kitted one?

Just to clarify I do not understand that 9:7:1 or whatever malarky would just like to compare compression in PSI as that is how it was measured (can be in bar).

Also some websites suggest that ratio will change a bit depending from starter motor and battery condition is that correct?


I presume that you did the test with engine warm and the throttle bodies fully open?

[Saint aka ML]

I presume that you did the test with engine warm and the throttle bodies fully open?

Fully open throttle, engine mostly cold. It was 5h after 2h ride but in that time it was started multiple times and in warm doors. Why cold engine gives higher or lower compression?

[dunlop0_1]

Fully open throttle, engine mostly cold. It was 5h after 2h ride but in that time it was started multiple times and in warm doors. Why cold engine gives higher or lower compression?

Cold would give a higher compression. Should be run up to temp and then tested. 5hrs is way to long IMO

[Saint aka ML]

Cold would give a higher compression. Should be run up to temp and then tested. 5hrs is way to long IMO

I will test it again but I read on another forum that it should not be done when hot as cylinders expand and there is less leaking. Compression at cold should be higher but will double check.

[Thirdway]

I thought you would get a higher compression when warm ? There again formula one engines are rock solid seized until fed hot oil.

[dunlop0_1]

If you have ever tried to start an engine with low compression when it is hot then you will know what i mean. If you think about it logically you don't ride the bike with a cold engine. If its been running for 5 mins or 5 hours the engines optimum performance is designed for when it is at the optimum working temprature threrefore i think the test shold be done then. However this is only the way i have always done it and to be honest i do both, hot and cold and take an average because really their should'nt be much difference between the two.

Ducati being as it is you might find that any difference in readings could be down to incorrect valve clearances and nothing to do with worn bores or rings.

[Pedro]

For comparison, my 900 is around 155psi in each pot, that's stock barrells with new piston rings, measured with throttles wide open. My 1000 is a touch more

[Stafford]

As a side note note reading on Brads website http://www.bikeboy.org/index.html you can get quite a differnece between pots and correct it by dialing in the cams. He had an 888 with a 20psi difference, after dialing a better balance and a higher reading for both.

[Rockhopper]

No idea of the correct figures for your engine and to be honest i don't think i have ever seen them published for any engine, but, the important thing is that both cylinders are almost the same and are well above 100 which is pretty much the minimum need to make and engine run at all.