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Ducati 916 SP – 955 BIG BORE KIT
On the surface a relatively easy conversion, the increase in capacity comes from the bore of 96 mm, up from 94 mm. Installing a 955 kit to a 916 SP is almost simply a matter of removing the old cylinders and pistons and fitting the new. This is due to the fact that the crankcases on the 916 SP are machined larger at the cylinder spigot ( hole ) than the normal 916. To convert a 916 Strada or Biposto to 955 the cases need to be machined to accept the larger cylinder spigots. This requires stripping the engine down to bare cases, adding more labour to the job.
This is not, however, the major drawback some would imagine. The kit we used in this instance came with pistons that weighed approximately 410 g. The standard pistons were around 490 g. Quite a difference in percentage terms. As this engine had to come apart due to a big end problem anyway, rebalancing was not a problem. Without rebalancing, the effect of the lighter weight pistons would be very noticeable. Just how bad it would be is hard to say, as I have never done a job like this without rebalancing. Our balance man thought it would be a bad combination though, and he does know what he is talking about. You could always suck it and see, but the taste might be bitter indeed.
Some of you may be surprised that larger pistons can be so much lighter. Piston construction and design varies greatly, however, and such was the case here. The lighter pistons help reduce the effective rotating mass of the engine, the effects of which were obvious in engine response.
We also spent a little time making sure that everything was set up just right. We juggled base gaskets for the desired piston deck height and set the cam timing to suit. We also went through the procedure of checking the new compression ratio. We had previously fitted a big bore kit to another model ( against our best advice ) that had far too much comp. We didn’t want a repeat of that debacle.
Fortunately, the final figure was 12.0 : 1, about as high as I would have been confident of running on a street engine. This can sometimes cause starting problems, especially when combined with the cam timing figures we run. A good battery is required, as is good fuel.
So, at this point we had a well built, high comp, cam dialed, rebalanced 955 SP. Just what we had set out to do. All that was required now was a new chip and trip to the dyno to set it up.
However, at this point it all went bad. Due to people with little idea, the bike went back to the owner with the std 916 SP/Termi chip and I went home a little insulted. Needless to say, there are no dyno runs.
I was later told that the bike went very well, and that the owner was very happy. It took me a long time to care ( this happened three years ago ). I got to ride the bike a couple of years later and was amazed at how well it went. It would certainly equal most, if not all, of the 996 SPS’s we have played with. The lightweight pistons, the compression and the cam timing combined to make it stand up on the back wheel at a rather alarming rate, even in third.
Why this bike runs so well with the 916 SP chip I have no idea. Ducati had got their mapping pretty good by this stage as a general rule. It should be lean at full throttle, given the capacity and the cam timing, although the increase in comp may have reduced its fuel requirement a little. Whether or not there is more to be had with a properly set up chip I don’t know, nor am I ever likely to know with this bike.
If anyone with a 916 SP would like to start and then finish the job properly I would be most eager to oblige. The result would be well worth the effort.
Also, as a side note, the main differences between a 916 SP and a 916/996 SPS are as follows. Capacity is 916 cc. There were some 955 SP’s built ( I have seen a 955 engine number in a photo ), but lord only knows where they went. The SPS are all 996 cc. The cams in the SP have far longer duration than the SPS cams. Nominally, they are referred to as a G inlet and A exhaust. The same cams as the 888 SP5. The lift is not a great as the SPS cams though. The SPS cams are later technology and suit the engine well. The G and A cams come from the 851/888 race days. The power peak is lower and later on a 916 SP. The exhaust has 45 mm header pipes into a 50 mm collector and mufflers. The SPS exhaust is 50 mm all the way through. The 1998 and later SPS have Titanium rods. All the pre ‘98 engines that I have had apart have had Pankl steel rods. The weight difference is approx 115 g per rod. The SP has 34/30 mm inlet/exhaust valves, the SPS 36/30 mm. Compression ratios are nominally 11.0:1 on the SP and 11.5:1 on the SPS.
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