Compression on Ducati 900 Supersport

[kerryg]

I would really appreciate it if anyone who has access to the right information is able to answer these questions concerning a stock 1990 Ducati Supersport (that's the 2 valve aircooled belt-drive engine):

1. what SHOULD compression be in a low mileage well-maintained example of these engines? Is 130 ~136 PSI a decent figure? (It seems a bit low to me, but I'm comparing it to a Japper high HP multi I had before with compression of around 170psi so that might be unfair)
2. Is a 6 PSI difference between cylinders anything to worry about?
3. Are compression figures useful anyway? I have heard that low compression can come from causes other than worn cylinders/rings (e.g. from mis-adjusted valves). Any commments?

Could be REALLY important information 'cos I've just bought a bike with these compression figures, with less than 4000kms on it (ex Japan) which has got some other gremlins which make me wonder if I've bought a bad one, and/or that the mileage is understated (highly possible) ....

Any authoritative information from those in the know would be gratefully received. Cheers

[Paul in NZ]

A lot of factors to consider....

I assume the engine was in good tune, hot and the throttle held open while you did the test? If so, then thats a little low IMHO.

At 9,000 km my Guzzi was 200 and at 50,000km 185

The Duke is a more modern engine and I'd expect better from it.

[SixPackBack]

I have a manual at home [ welcome to borrow it]. Will check cylinder pressure when i get home........don't think 6 psi is much to worry about!

[pete376403]

This page http://www.floridacycles.com/new_vehicle_detail.asp?veh=1243&pov=160215 suggests 9.0:1 compression ratio. Given atmospheric pressure is 14.7PSi at sea level, 135 PSI looks pretty close to 9:1. What is petrol like in Japan? If it's anything like the swill sold here, high comp motors will pink or detonate with any reasonable sort of compression

[kerryg]

A lot of factors to consider....

I assume the engine was in good tune, hot and the throttle held open while you did the test? If so, then thats a little low IMHO.

At 9,000 km my Guzzi was 200 and at 50,000km 185

The Duke is a more modern engine and I'd expect better from it.


Thanks Paul. The test wasn't done by me, and I wasn't there. It wasn't in a particularly good state of tune (poxy Weber carburettor playing up, which is why it was being looked at). 130psi compare to 185-200psi seems a hellish big difference though. So I think I must delve deeper. Thanks again

[kerryg]

I have a manual at home [ welcome to borrow it]. Will check cylinder pressure when i get home........don't think 6 psi is much to worry about!

Hi SPB. If you can be bothered checking I really love to know what the manual says about compression. Appreciate the offer to borrow it too. Cheers

[kerryg]

This page http://www.floridacycles.com/new_vehicle_detail.asp?veh=1243&pov=160215 suggests 9.0:1 compression ratio. Given atmospheric pressure is 14.7PSi at sea level, 135 PSI looks pretty close to 9:1. What is petrol like in Japan? If it's anything like the swill sold here, high comp motors will pink or detonate with any reasonable sort of compression


Ohhhh...so that's how you calculate it :weird:

Maths was never my best subject, as you can tell. If the math is right it sounds like I'm worrying about nothing much. I'll try to corroborate it from another source for my total peace of mind ...and then I'll rest easy. Cheers for that. As for Japanese fuel I would hazard a guess that it's better than our stuff but wouldn't really know for sure. Anyway, thanks for your help

[SixPackBack]

Checked the haynes manual last night, buggered if i can find the reccomended psi.....still welcome to borrow it. pm me if you want

[kerryg]

Checked the haynes manual last night, buggered if i can find the reccomended psi.....still welcome to borrow it. pm me if you want

SPB that's really good of you. If I keep the bike (actually there's a real possibility that the shop will be getting it back if several issues aren't sorted out to my satisfaction) I'll buy a manual 'cos I'll need one anyway, but it's very decent of you offer.

[geoffm]

Have a look through the workshop manuals thread - I am pretty sure there are some Duck manuals there.
IIRC, the cyls should be within 10% of each other.
Geoff

[kerryg]

Have a look through the workshop manuals thread - I am pretty sure there are some Duck manuals there.
IIRC, the cyls should be within 10% of each other.
Geoff

Thanks for the suggestion, Geoff. I'll have a look and see what I can find

[pete376403]

Ohhhh...so that's how you calculate it :weird:

Maths was never my best subject, as you can tell. If the math is right it sounds like I'm worrying about nothing much. I'll try to corroborate it from another source for my total peace of mind ...and then I'll rest easy. Cheers for that. As for Japanese fuel I would hazard a guess that it's better than our stuff but wouldn't really know for sure. Anyway, thanks for your help
Theoretical CR is swept volume (ie the cylinder plus combustion chamber volume (ie the cylinder head divided by the combustion chamber volume. This is easy when the piston crown is dead flat, gets tricky when the piston is either domed or has a bowl.

[Kickaha]

3. Are compression figures useful anyway? I have heard that low compression can come from causes other than worn cylinders/rings (e.g. from mis-adjusted valves). Any commments?



My Ducati used to drop the compression as it came due for having the valves done,once they were reshimmed to correct clearance they'd come back up but I only needed to do them every 10,000km

The mechanic told me to do a compression test about every 2500-3000km and as soon as I noticed a drop to get them done

[mjvduc]

1. what SHOULD compression be in a low mileage well-maintained example of these engines? Is 130 ~136 PSI a decent figure? (It seems a bit low to me, but I'm comparing it to a Japper high HP multi I had before with compression of around 170psi so that might be unfair)
2. Is a 6 PSI difference between cylinders anything to worry about?
3. Are compression figures useful anyway? I have heard that low compression can come from causes other than worn cylinders/rings (e.g. from mis-adjusted valves). Any commments?



Any authoritative information from those in the know would be gratefully received. Cheers

hi,

sounds good to me, early 900 engines came with 9 and a bit bars (130PSI+) nothing to worry about.'

mjv :cold: