Brad The Bike Boy: Ducati 900 Carb models with and without airbox lids

Friday, August 7, 2015

Ducati 900 Carb models with and without airbox lids

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There's a thread going on ducati ms in the Supersport forum about airbox lids and noise and the usual resonance design stuff that comes up, so I thought I'd put up some graphs of lids on and off and snorkles in and out.

For a start, from this post: http://bradthebikeboy.blogspot.com.au/2012/07/carby-model-ss-and-monsters-airbox-lids.html , an M900, std airbox lid and jetting, some sort of slip on muffler. Blue is snorkles in, red snorkles out. How it looks is how it felt.

M900 with jet kit (Factory Pro from memory), initially it had 145 (maybe it was 140, I forget now) main jets to start with, then it got 150 and finally 155 I think. Anyway, green is 145 (or 140) mains and open lid, red 145 (or 140) mains and lid on with snorkles, blue 150 mains and open lid. No air fuel ratio back then.

Minnie the M600 with standard and open lids, no other changes. Was running 900 cams, std jetting and Megacycle mufflers. Blue is std lid, red open lid. Pretty obvious. This result is consistant for any of the carb models when running std jetting.

750SS with jet kit tuned for open lid and slip on mufflers. This bike was running 140 main, which were possibly on the too big side anyway. Blue is open lid, red std lid.

900SS runs (two different bikes) showing air/fuel ratio only, one with std lid and snorkles in red and std lid and no snorkles in blue. I haven't put up a power comparison as they are from different dynos as well, so there isn't a correlation between them.

To the above graph I'll add two curves form the second graph at the top for the M900. All std jetting, with and without snorkles. The Euro spec 900SS and Monster use the same 140 main jets, Y-2 needle jets and 5C19 needles. So theoretically they're the same, but it doesn't work out that way. Light blue is 900SS with snorkels, dark blue without, red M900 with, green M900 without. The Monster is leaner in both cases.

For an SS main jet size comparison: a 900SL with some Staintunes, open airbox lid and a Dynojet kit. I revised the kit settings a bit and ran it with both 140 and 145 main jets. As you can see, the 145 mains hurt the power a touch. From memory (probably 17 years ago), it didn't feel any different on the road. Remember, the SS needs smaller mains than the Monster. Possibly due to the SS fairing blocking airflow? Dunno.

To summarise, my experience with this topic leads me to the following conclusions:

1/ Any of the carb model bikes will make more power with an open airbox and jetting to suit.

2/ Jetting to suit is in bold because that's the important part with the std Mikuni carbs. My belief is that CV carbs are upset by the change in pressure differential across them (between inlet and outlet) more than the added airflow an open lid gives. It's not just a too lean thing, like you get with an injected bike when doing the same mod, as the dynamic behaviour of the CV carb is completely different to an ecu just putting in the same amount of fuel. But that could be wrong.

3/ Without jetting to suit, with an open airbox they won't rev out under load.

4/ Without jetting to suit, removing the snorkles will produce a pretty much nil result at best, with the power loss due to the leaner mixture offsetting the power increase due to the extra airflow.

5/ With jetting to suit, the difference between std airbox lid without snorkles and open airbox lid is mainly noise. Being less of it with std airbox lid without snorkles. It doesn't seem to affect the performance that much. I don't have any back to back runs to prove (or disprove) this though. Something to try at some point.

6/ The noise of the open lid really pisses some people off. Each to their own. I used to ride Minnie with std mufflers and open lid, so that way she was quiet when cruising and made some noise when on it.

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