Ducati ST4 - The Gentleman's Express

This has been a good day; I have just been out for a ride on my rejuvenated ST4. Its my road bike, its not a race bike, but the one I choose for doing business on the normal streets. It looks a bit frumpy, but there are few bikes on the planet that can match its sheer competence when hustling through the bends on public roads and fewer still that can earn that accolade while carrying a passenger and panniers.

I ride very much by feel, if everything feels good I go quicker, if something isn't right then I stay slow.  The first time I went out in the wet on this thing it just felt great; I was catching and riding past absolutely everyone.  It is one of the most balanced and effective bikes I have ever had the pleasure of riding. Any one who ever saw me try to race in the wet will understand just how generous a compliment this is.

My ST4 got christened 'Stanley'; it just suited his 'face'.  Stanley is a Gentlemans express, easy to ride, very comfortable and very, very quick.   The first 6000 miles have been a bit unusual, Stan got run in while I was escorting my then girlfriend around as she learnt to ride on a 400cc Honda, we then rode across northern Spain, from Bilbao to Barcelona and back, again as escort for the Honda. Don't get me wrong, this was all very enjoyable it is just that Stanley got a long slow run in…………

Riding in the low throttle and low rev zones I was operating in a different way to the race settings I have become used to trying to perfect. As a result I have tried to critique some of the normal street things; I care about the handling with panniers fitted, I care about the low RPM response, and the midrange.  Stanley wasn't bad but he wasn't perfect when given such a long and thoughtful analysis. Several points really got to me from the beginning, there was a definite low rpm snatchiness; down at 2500 rpm, not an area of the rev band where I had ever thought that I would worry about, but once noticed it was quite annoying. Initially I thought it was merely a gearing thing, we tried a 14-tooth front.  Very nice for acceleration but no real change in the snatchy feeling. I would also occasionally feel a slight front tend tuck going into corners.

After the trip to Spain I decided to get serious about sorting out the niggles.  I made a richer chip, just down low and there was a real improvement.  In November I decided to do the first half of the research into this report and went to the dyno.  All these first tests were with standard pipes but two things came out of the day that were to point the development of this bike in one particular direction, the 'open pipe Ultimap' for this bike worked very well even with the standard pipes. The second point was that we had another restrictive airbox design.

I decided that we would run the bike for a few more miles until it had around 6000 miles on it and then do a Full Monty to make sure everything was alright.  At the same time I got a second airbox top so I could saw up the original one with impunity. 

Today (Mid July 2001) we finished off the bike (you will note that it has taken a few months to get this together but it took a lull in the workshop before I could get my own bike in!).  I have just got back from my first real ride and the improvement is brilliant, it really struck me that it was smooth, seriously good mid range pickup, and with the balancing work we had done to the fuel injection system no snatchiness down low.  In a real return to thinking about road handling the bike has had a few changes to the chassis set up and most important of all a nice new rear tyre.

For a bike like this the dyno is almost superfluous, its just that I cannot resist fiddling to get the last bit of performance out of a bike, be it mid range stomp or peak power for a racer.  The return to the dyno was a real treat though. We had noted while the heads were off that using the open pipe Eprom with standard pipes was causing it to run a bit rich, carbon deposits had been building up in the exhaust ports and had to be cleaned out and the alternator was starting to loosen.

We re-shimmed the heads and reset the squish to allow us to use the later thin metal head gaskets (better heat control due to more sophisticated water circulation). On the rocker front everything was OK but Stan has always, always benefited from a 90 second warm up before moving off. After the valves were cleaned and lapped to the seats the cams were re-timed to standard settings and new belts fitted. The fuel injection system was balanced and set up. In short one of our Full Monty services. Before fitting the new rear tyre we set off for the dyno………..

First run last November, with about 3000 miles on the clock, was with a standard Chip, with standard pipes and air intake system.  About 104 hp, not bad really for a standard 916 engine. By the time we had finished we had 110 bhp and up to 8 more through the midrange; all the dips and troughs in the powerband have gone and I love the response. Leo Vinci oval aluminium tailpipes were fitted as a comparison to the much heavier standard pipes.

The first confirmation was that the Ducati design department still suffers from 'Supermono airbox disease'. We took the airbox top off and promptly found a bunch of power, all the dips and bumps in the curve ironed out. Torque likewise was up, at 6000 rpm we had a near 7 ft-lb. improvement; 14% more than standard.  We don't like impractical improvements and leaving the top right off was going to make a bit too much noise and would leave the filter open to getting wet, this is a street bike after all. We decided to try and modify the intake system for the best balance of noise and power.

Taking the airbox intake snorkels out upped the noise but made no difference to the power. Two extra 25mm holes in the back of the top got us a couple of HP at the top, but we were after the mid range, progressively stripping out the back of the airbox top got us nearly everything we wanted.  5 bhp came from just that mod all the way from 4500 to 9000, a couple of hp for the last bit, the only loss to the completely open box being 2 to 3 hp from 8000 to the redline, we got all the torque improvements up to 9000 too.  If we have an ST4 race I'll worry about the last bit.

The quiet option

A couple of interesting asides came out of these runs. A standard Ducati air filter causes no measurable restriction at all; at least we couldn't measure any difference. The open pipes are worth 2 to 3 bhp all the way from 3000 to 9000 rpm they then hang on to the top end better with 110 bhp from 9000 all the way to 11000. There were very few mixture differences with the airbox modifications, if anything we were a little lean with the intact box and then saw a slightly richer mixture with the final mods to the box; go figure.

This sorts out the opposition - be it on tour or on track

If you try a 748/916/996 on the dyno you lose up to 3 hp if you take off the air inlet pipes; this bike is making the same, or slightly more, power as a 916 with a disabled airbox.  Impressive.

We also went down the road to another dynojet, just for a 'powercheck', 110 bhp with no mods whatsoever, standard pipes, airbox, the lot. So there you are, you can have all the power you want, with no modifications, you just have to find the right dyno!  

Then I went for my ride, just to add to the engine response improvement I was looking for a more positive steering bike with a few millimetres added to the rear ride height.  The new tyre clearly made a massive improvement; the original was really squared off from its touring trips. This thing is now a very serious piece of kit; the airbox mods give a nice throaty roar when the throttle is opened, certainly not too much to live with. The engine is dead smooth, the snatch down low is gone and the big shudder from too much throttle at low revs much reduced.  The 14% torque improvement in the midrange is a bit difficult to miss as well. The ride height change appears to have sorting out the tucking feeling from the front as well.

Now I am going to do a track day; Stanley is going on the racetrack, with his panniers on.

Neil Spalding