BikeBoy.org - Ducati 916 SP – 955 BIG BORE KIT

Ducati 916 SP – 955 BIG BORE KIT - Written 12/00, Updated 12/09

Summary: 916SP fitted with 955 kit and tuned. An old report re-written somewhat with more info

On the surface a relatively easy conversion, the increase in capacity comes from the 96 mm bore, 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. Or you can use std 94mm cylinders bored to 96mm. This reduces the spigot wall to 2mm, which may or may not be too thin.

The need to strip the engine is not, however, the major drawback some would imagine. The kit we used in this instance came with pistons that weighed approximately 410g. The standard pistons were around 490g, which is 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 could be noticeable. Personally I have run engines with lighter pistons and without rebalancing to suit and I couldn’t pick the difference. But it’s always nice to do the job properly.

Some may be surprised that larger diameter 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.

The heads were sent out for some porting, and picked up a little flow – probably more on the exhaust than inlet. Thinner base gaskets (now NLA as genuine parts it seems) were fitted for the desired 1.0mm squish setting and then the cam timing was reset to 93 degree inlet centreline and 104 degree exhaust centreline. Similar to the 916SP in the other 955 report, with 3 degrees less inlet advance. We went through the procedure of checking the compression ratio, which came out at 12.0:1 even though the pistons were domed due to the big valve cut outs required by the high TDC overlap lift of the G/A cam combination. Back when we built this engine I thought this was high, now I’d happily run much closer to 13 on a road bike if possible. Although on the pre 2000-ish starter motor ratio change engines this can sometimes cause starting problems.

No tuning work was carried out at this point, with the bike running the std 056 eprom. I was later told that the bike went very well, and that the owner was very happy. I got to ride the bike a couple of years later and was amazed at how well it went with the std eprom. It would certainly equal most of the 996 SPS’s we have played with.

Some time later I got to dyno and tune the bike as required. I was surprised how much power it made compared to the other 955 motors I’d been involved with. This may have been due to the std 34/30 valves being used with some good port work. The 916SP in the other 955 report had the valves sunk quite a bit into the heads, which was rather odd and may have hurt the power on that one.

Anyway, this bike had been dynod prior to the original work, so below is 916cc in blue and 955cc with ported heads, reset squish, cam timing, and fuelling and spark map mods in red. Power then torque. As the torque curve shows, it still has the 916SP lower rpm range hole, but it’s moved down the rpm range a bit and really quite linear once you get over that.

The final graph compares this bike to the 916SP – 955 from the other 955 report in pink and a 996SPS with the cam timing reset in green. The midrange on this bike is very impressive, and I’m not really sure why it makes so much power in that area.

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 955cc SP’s built for USA AMA homologation (50 in total?), which were also known as the SPA. These were all sent to the USA. The 1996 production 916SP were known as 916 SP3. These are still 916cc and identical to the earlier 916 SP apart from the SP3 part of the badge on the top triple clamp. 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 as 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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