Monster S4 – Differences From Other 916 Models

This page comes from the enquiry’s we’ve been receiving about the S4 Monster and the differences between it and the other 4V/916 models, and is my take on "what’s wrong with it".

Many people know of the new inlet cams, which seem to be causing much consternation with regard to the S4’s lower peak power output compared to a 916 or ST4. There is another main difference in the engine setup, and that is a smaller exhaust system. The effect of these two major differences we’ll have a look at with the help of some torque curves. Torque curves are the best way to compare this kind of thing, as any differences are more visually obvious than on a power curve ( as power is torque multiplied by RPM ) and torque output at various points across the rev range is what we are trying to maximize.

As a side note, my many emails and phone conversations with Doug Lofgren over the past few years have changed the way I look at and interpret torque curves, as Doug takes a rather simple, overall look at things and comes up with some pretty cool ideas accordingly. Once again, the benefit of experience. So, torque curves can be very useful in picking what you don’t like so much, and how to improve it.

Firstly, let’s look at the inlet cams. The "old" 4V Strada inlet cams have timing of 11/70 @ 1mm lift, with max lift of 9.6mm. These are fitted to every non SP/SPS/R model from 1988 on. The new inlet cams, fitted to the S4 and ST4S, have timing of 11/60 @ 1mm lift, with max lift again 9.6mm. Which means that the profile has been changed to close the inlet valve 10 degrees earlier. Some people are getting rather upset about this, but in my experience this can only be a positive thing. It is not a de-tuning, as many seem to think, but simply a better idea. With proper setting up and a little playing, I would expect this engine to produce the same, if not more, power through the mid range AND at the top end. I certainly wouldn’t bother fitting the old Strada inlet cam personally, although I could be proved wrong. I just don’t think it’s worthwhile, an opinion again based solely on my experience.

Secondly, we should look at the exhaust system. For some reason, probably a perceived "more midrange" thing, Ducati have fitted the S4 with a 40mm exhaust. This is the same size as the 900 2V engines, and the 851/888 range. Whether or not it was done to simplify the Termignoni/Ducati Performance Monster slip on muffler range I don’t know, but the S4 mufflers are a bit different to the others anyway.

I do have a little insight to effects of this, however, based on recent playing with my 851 ( which up till now I thought was quite irrelevant ). The torque curve on the 851 is a bit different to what I would expect from a 4V. The 851 has another difference too, being 1mm smaller valves than the 748/888/916/S4/ST4 engines. This will have an effect on the top end power also, but the similarity between the 851 and S4 torque curves, compared to the 916/ST4 engines, is rather obvious.

The graph below shows these differences. The green curve is 851 with std 40mm headers and some slip on mufflers. The red curve is 851 with the 45mm ST4 headers and some Staintune mufflers. The blue curve is S4 with some Termi slip-ons. The purple curve is ST4 with std mufflers ( only ST4 curve I have ). Also apparent from this graph is the S4’s 9,500 RPM rev limit, controlled by the ECU. Most of the other 4Vs go to at least 10,500 RPM, and the S4 DP ECU lets it rev to 10,000 RPM. The patchiness in the 851 curves is due to the fact that the inductive sensor used by the dyno software to measure RPM doesn’t seem to like my 851.

Both the 40mm header curves ( red, blue ) show a torque peak at around 7,000 RPM then a steady decline from then on. The drop off is steeper on the 851, due to the smaller valve sizes, but don’t worry so much about that. The 45mm header curves, on the other hand, show another torque peak up around 9,000 RPM. Not quite so pronounced on the 851, but, again, don’t worry about that. This last peak is consistent with all the other Strada cammed engines I have torque curves for. It is this peak that makes the top end power missing from the S4 power curve, which goes flat from 8,500 RPM on.

So, it is my contention that the top end power missing from the S4 is due to the smaller exhaust system, and has nothing to do with the inlet cam specs. To further back this up, we can look to the Ducati Performance catalogue. The DP catalogue for the S4 contains dyno graphs for the expected improvements that each kit will bring. The 3 offered kits are

1. Termi slip on mufflers, Air filter with open lid, remapped ECU to suit
2. Full Termi "Spaghetti" style 45/50mm exhaust, Air filter with open lid, remapped ECU to suit
3. Full Termi "Spaghetti" style 45/50mm exhaust, SPS cams, Air filter with open lid, remapped ECU to suit

The graph above is one I re-created from the three graphs in the DP catalogue. I couldn’t scan the originals as they are printed across the centre fold, and the PDF files on the Ducati site are exactly the same. If you were guessing I have too much spare time on my hands at the moment, you would be spot on.

The power figures given are a little lower than I’ve seen on the Dynojet I use, but that’s different dynos for you. The increase from std to Kit 1 is about the same as we measured, so they are certainly representative. I wasn’t so much after the numbers, but the shape of the curves. As you can see, Kit 2, with the big pipe, is supposed to make power all the way through to 10,000 RPM. It would probably hold the power further if not rev limited. The peak power improvements claimed for the kits are 5, 10 and 15 Hp respectively

As far as options for a bigger system go, you could go the Ducati Performance way, or, fit the ST4 headers and mufflers and modify the muffler mountings as required to suit ( as I have done on the 851 ). Or, have a custom system made. The advantage of the DP system is that it comes with an ECU to suit, although the Power Commander is now available for the S4, and FIM’s aftermarket ECU will be available soon, so they are other options. Whether or not a larger system will need much more fuel anyway remains to be seen, and it may be within a reasonable range for the Mathesis ( Ducati diagnostic tool ) to handle.

It all depends how much you want to spend really, and how much you need the extra power at the top end. How often do you rev yours over 9,000 RPM? For me, with the 851, I had to admit to myself that the pursuit of top end was a bit irrelevant. It rarely sees more than 8,000 RPM anyway.

The other thing to say is that the midrange torque output for the S4 is pretty flash anyway. The graph below shows the torque curves from the std S4 in red, std ST4 in green, the S4 with the Ducati Performance Termi slip on/Air filter/ECU kit in blue, and a 916 with its cams dialed and slip on mufflers in purple.

You can see the under 7,500 RPM torque advantage the std S4 has over the std ST4. With the power kit ( but no cam timing changes ) fitted to the S4, it’s torque curve is better up to 8,000 RPM than the cam dialed 916. So the S4 is certainly not short on low and mid range torque. Dialing the cams on the S4 I imagine would pick up both the bottom ( in particular ) and top ends of the curve, improving what is already there.

The final issue with S4 performance I am going to deal with is the usual Ducati standard over-gearing. For noise test or other reasons, most ’90 on Ducati models have come from the factory with very tall ( or high ) gearing. The ST range is one exception to this, although I’m not sure why. Anyway, the table below shows the comparative gearing of the S4, ST4 and 916. The primary drive ratios are different between pre ’98 and post ’98 models, being taller post ’98 to allow a higher top speed for the 996 in racing mode. The S4 has the close ratio or 748/SP gearbox, while both the ST4 and 916 have the std, wide ratio box. To interpret, the smaller the number, the taller the gearing.

As you can see, the 3 lower gears on the S4 are taller than the 916, which was considered over geared by many owners anyway. The closer ratios of the S4 box mean fitting a smaller front or larger rear sprocket to lower the gearing in the bottom gears will reduce the top speed capacity of the bike. But it will no doubt still be capable of going fast enough for the wind blast to peel you off the back. A 14T front, or 39/40/41T rear sprocket will help the performance no end, and be rather cheap to boot.