Ducati MONSTER S4

The long awaited 916 monster, although it shares more with the ST4 than with the original 900M. The frame and much of the running gear is taken directly from or modified from ST4 parts. Putting the frame side to side with an older 900M frame shows the increase in frame tubing size. This frame is apparently also being used for the new 620ie Monster, and will no doubt find its way into the whole Monster range soon.

Anyway, the bit most people are interested in is the motor. Given it is pretty much straight from the ST4, which is basically a 916 motor with heads redesigned for front wheel clearance in the ST4 frame, it makes the sort of power 916 motors have been making since 1994.

The differences between the ST4 and S4 engines are inlet cams, airbox, ECU and exhaust. The inlet cams have the same lift and opening point, but close the valves 10 degrees earlier, giving 11/60 timing instead of 11/70. The airbox has one air inlet snorkel about the same size as one from the older style airbox lid, although the older (SS, Monster, ST2/4/4S) airbox lid has two. It really amazes me an engine that is probably sucking in close to 75 litres of air a second at 10,000 RPM can breathe through such a tiny hole.

The ECU is the new 5.9M, a very small unit with no replaceable chip. The exhaust is the same size as the 900 2V header, or the 851/888. This is 40mm OD, as opposed to the 45mm OD pipe used for the ST4. This makes a difference to power over 8,500 RPM. The style of header pipe/stamped cross over is the same.

Given the changes to the cam timing and the slightly smaller exhaust, I would expect maybe a little more bottom end from the S4. The tiny airbox inlet would work against it here though, so the net result is pretty much the same. The first graph shows the S4 in red and the ST4 in green. The extra top end of the ST4 is probably due to the variations in cam timing and maybe the header pipe diameter. Don’t really know without a good play.

Comparing them in an on the road sense is pretty hard, as the S4 is a heap lighter and feels totally different to me. In standard form, the S4 feels like a very smooth 750M with a strong top end. It’s not midrange punchy like a 900 is, but the quick rev nature means it accelerates as fast as you can feed it gears. I found it much tamer than expected, but, after riding my own 900M, most things feel a little unresponsive. This may have a fair bit to do with the characteristics of the carbed 900 2V engine and it’s long manifolds anyway, so as a comparison is a bit irrelevant I guess. I certainly didn’t find it the wheelie king many people are claiming it is.

Anyway, getting back to the story. I got the chance to dyno one of these bikes when it came in for some STAINTUNE mufflers. Given I wasn’t riding at this stage (another broken leg fixing operation), I drove to the dyno in the shop ute with the mufflers in the back. This let me do a direct before/after test. I added to that a test with the STAINTUNES fitted and the airbox lid removed. What this does to the mixture I’m really not sure, although I would have to assume it would put it into the lean area. Can’t say I would recommend riding it like that. The dyno curves are shown below. Removing the airbox lid certainly makes a difference, as the red curve shows. Staintune only is blue, standard is green. How much better it would be with a suitable map in the ECU will be for the future to tell. The shape of the curve is consistent through all three though, which is most likely attributable to the cam timing.

As a side note, if you want to buy TERMIGNOMI mufflers for an S4 from DUCATI PERFORMANCE, they come with an airbox lid with the top cut out, an aftermarket air filter and an ECU to suit. Should certainly be a good bolt on package. As for other remapping options, Duane at FIM is currently working on getting into the new software. Hopefully, this won’t take too long.

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