BikeBoy.org - Ducati ST3 Cam Timing Changes and tuning with Techlusion/SFI

We have a demo ST3 that’s got quite a few km on it now so I thought I’d have a play with it some more. In the previous ST3 report I did the usual muffler/airbox/fuel thing to see what was in it. One thing that I noticed from those curves – the “with mufflers and air filter kit” in particular – was that the power went flat around 8,000 RPM and held pretty much at that until 10,000 RPM. To me, that usual indicates something is restricting the package while everything else is happy to keep going.

The ST2 also does this, in its case going flat from 7,000 to 9,000 RPM. My ST2, with its ported heads, better shape valves and shorter cams with more lift, had a shape more like you’d expect (from a 2V in particular) with a smooth curve to a rounded peak, falling off from there.

From my experience with bigger exhausts on the ST2 and the 851/888 exhaust restricted doesn’t show as a flat power curve – all those bikes have had power curves that peak and fall and fitting an exhaust that makes more power simply raises the peak and the fall also.

And given the ST3 has the same airbox as the ST4 and 4S and shares its throttle bodies with the whole 748/888/916/996 range since ’93 it certainly isn’t inlet restricted.

So I tend to interpret the flat power curve as over-cammed or under-valved. Which is sort of the same thing, dependant on the specifics of the model. The cams in the ST3 are typical of late Ducati cams – being short duration (256 degrees inlet, 259 exhaust) and high lift (10.7mm inlet, 10.4mm exhaust) so it certainly isn’t over-cammed. In fact I’d say it’s fairly well cammed for the set rev limit (10,000 RPM). It’s certainly under-valved – 2 34mm inlet valves means it has a touch more valve area than a 916 (33mm inlet valves) and less than the 4V engines of the same capacity (36mm inlet valves). Exhaust wise the 40mm valve is sort of equivalent to two 28mm valves, again too small. Plus it has 40mm header pipes, which I know cost peak power on any 4V.

However, as much as I’d love to pull the heads and fit 36/42mm valves and a 45mm header set the management would be somewhat unimpressed with the capital outlaid, so I just thought I’d play with the cam timing to see what happened to the curve with some variation. The other side of the flat power curve thing is that you can usually sacrifice some of the higher RPM flatness for more roundness lower down. And so we did.

Checking the cam timing bore a similar result to the S2R I’ve checked recently. You’d think with the adjustable pullies every model in the range wears these days they’d have the ‘as delivered’ timing somewhat under control. Again, not so. The spec on the ST3 gives a calculated inlet centreline of 114 degrees. On this bike the ‘as delivered’ settings were 117 degrees on the horizontal and 123 degrees on the exhaust – 3 and 9 degrees retarded.

I took the opportunity to check the piston to valve clearance, of which there was quite a bit, so I decided to advance the cams while I was there to 107 degree inlet centreline. Just to se what would happen, because you never find out if you don’t try. This gave the following timing specs. Note: I don’t actually check the exhausts given they’re on the same cam, working only off the inlet profile, so the exhaust numbers are just calculated. All timing was checked/reset with belts at the running tension of 142Hz on the Mathesis twangometer.

Setting	Inlet timing	Exhaust timing	Centreline
Spec	14/62	59/20	114/109.5
Horizontal	11/65	56/23	117/112.5
Vertical	5/71	50/29	123/100.5
Revised	21/55	66/13	107/116.5

Once I’d done all that I fought with the fairing as you do (the person responsible for it clearly has no respect at all for those who have to work on them) and then headed off to the dyno. Actually, I didn’t – the dyno was still having a rest after Christmas so I waited a couple of weeks for them to re-open. In the meantime I fitted the SFI (Staintune Fuel Injection unit) I have as a tuning tool and became increasingly impatient to know how it compared. I did ride the bike around a little, and while it felt quite strong through the midrange it had lost the free revving top end it had previously had – from 8 to 10,000 it always felt great. But, as with everything, that may have been because it had lost top end or that the midrange was just so much better.

I did try setting the idle on the lean side as I knew a rich idle setting could impact on the high RPM WOT fuelling that would probably be rich anyway with the cam timing mods, but with the idle trimmer at 0 the idle was around 4% CO. Knocking it back to -10 gave me around 1.5% CO, but the idle went bad well before that. This is a bit unusual for the late bikes – the shower injector equipped 749 and 999 often idle quite well at low CO settings, which is quite amazing given that fuel is dribbling past throttle blades to get into the engine. I settled on a trimmer setting of -2, giving a CO of around 3.5%. Although with the result as below I’d use 4.5 - 5% as we usually do.

From experience I know that advancing cam timing increases the need for fuel up to the torque peak, and may decrease it after the torque peak, although not always. I was expecting, given I’d advanced the cams 7 degrees over spec, to need +10% or so. Assuming the ST3 was running 11 to 12 ms pulse widths on the WOT map line the 1.6ms maximum addition of the SFI should have been capable of giving me up to 15 or so %. The graph below shows the results from the dyno runs. I ran the SFI yellow and red pots at positions 1, 3, 5, 7, 9 and 11 to see what it needed where. The big and very quick change in air/fuel at 8,000 RPM quite surprised me, but sometimes you get that and what it is is what it is. I’ve shown torque and air/fuel – the clearest way to see what’s happening. The runs are a bit spotty too – usually an issue with the ignition pickup and in this instance generated by the lack of access to the coils with the fairings on.

So the air/fuel traces are to some extent two straight sections with a sharp transition. Realistically this is the sort of situation that suits the SFI well, as it has two WOT zones. Aiming for air/fuel ratio of around 12.7 to 12.9 I set the yellow to 9 and the red to 3, with the RPM switch at just under 8. This is due to the Dynojet air/fuel trace lagging a little – 500 RPM is often a good allowance for that. I didn’t do a dyno run at these settings, as I tend not to bother with best overall runs these days – I just find out what changes or mods I need to make.

Now to compare it to the std bike – all the above work was done with an otherwise std bike – std mufflers and air filter. This was the bit I was interested in. The next graph shows all std in green and two cam timing reset runs in red and blue. Red is with the SFI settings at 3, blue with the settings at 9.

Much to my relief it was quite a bit better. It’s 6 to 8Hp up for most of the midrange and has the same peak power, delivered 1,700 RPM earlier. While the power does drop over 9,000 RPM it’s again flat from 7,300 to 9,000, and there’s no point going past there now – you’d just change up and leave it at that. The bike was also used as a service loan bike, and a couple of customers – a 900SS and a ST3 owner – who’d ridden the bike before rode the bike just after I’d finished the work. Both were very impressed with the improvement. I rode another ST3 with Staintunes fitted a few days later and it was no stronger than this bike.

The torque curve shows a fairly consistant 10% improvement up to 6,500 RPM, which tapers off to the cross over point around 9,000 RPM.

If we add to the above graph a curve from the initial set of ST3 tests I did on this bike with air filter kit, slip on mufflers and the idle trimmer raised for more fuel – the pink line – you can see it only produces more power once the curve for the reset cam timing has gone flat at 7,300 RPM. I would like to test the reset cam timing with air filter kit and mufflers added but the mufflers I used the first time were some second handys we had that have since been sold so I don’t have any close at hand.

This sort of leaves me back where I started. This engine is restricted. A good 996 makes 100hp at 7,000 RPM, so the bottom end power is there with the 3V. With this cam timing the ST3’s power doesn’t start to drop until 9,000 RPM, which is where the 996 engines usually make their max power.

Now the ST4S is no more I see no reason why they wouldn’t improve the 3V engine. I don’t see giving the Triumph competition a 20 or so Hp advantage as being helpful to the sales men and women trying to sell the ST3. Now they’ve taken the obvious step and fitted a Testastretta to the Monster range in the S4RS they may reintroduce a 4V ST bike with the same engine. But I don’t understand why they persist with such a unique and small volume engine as the 3 valve without making use of its potential. It makes 15 more hp than the 1000DS, all over 7,000 RPM, whereas it could make 25 more fairly easily.

In that case, why not have an 85Hp 2V, an 110Hp 3V and a 130Hp Testastretta. Seems logical to me. Use them all in the Monster range and you’d have the S2R 1000, S3R and the S4RS. It’d certainly help lift the 3 valve engines production numbers. They could also commit a little trad heresy and jam it into the SS. I reckon it’d fit straight in given the head shape and the fact it uses the same belts as the DS, just a different airbox for the big throttles and some rad mounts. 110Hp SS ie anyone? That’d make the purists wail. Oh the horror.

In my opinion all the 3V motor needs are bigger valves and a 45mm exhaust system. If I had the time, money and an ST3 I’d do it myself.

We had the rocker covers off to check the valve clearances so we reset the cam timing to the std spec of 114 degrees inlet centreline, just to see what difference it would make. Completing the cam timing tests, as it were. On the road it felt great – nice midrange and revved right out smooth and fast. So the top end was definitely better, and the midrange a bit smoother than I remembered with the 107 inlet cam timing. On the dyno it came up pretty good too. The graph below shows as delivered (118, 123) in green, 107 in red and 114 in blue. The red run is a bit spotty due to the RPM pickup, but you get the idea.

Comparing the mixture shows that the change from 107 to 114 has richened the mixture under 6,000 RPM, something to be expected, but from 6,000 to 10,000 it hasn’t made much difference at all. It’s still quite lean from 6,000 to 8,000, and when I tested the bike with the 107 cam timing it made a bit more power when richened up. So I’d say even with std cam timing this bike is a bit lean and could use some more fuel. It should make a couple more hp in the midrange too