996SPS with bigger valves and more compression - Written 10/09

Summary: 996SPS fitted with 37/31 valves by CJS, 748R inlet cams and Pistal high compression pistons.

A customer sent his race bike in to have some heads fitted that Chris Steadman at CJS had ported and fitted with 1mm bigger valves on both inlet and exhaust.  He also had a collection of cams and some Pistal “Racing 2000” pistons.  These pistons have std sized valve cut outs and fairly big domes on them (for a Ducati 4V piston) and give around 14:1 compression I’m told.  I didn’t check as there wasn’t a lot of point.  The std sized valve cut outs mean you can’t run any of the big cams such as G or the 4xx series Corsa cams as they have far too much lift at TDC overlap, but as we weren’t intending to that was fine.

It went together with 748R inlet cams fitted instead of the SPS cams, as Chris thought these might work better with their 1.7mm higher lift.  The 748R inlet cam has more lift than the SPS inlet, but 7 degrees less duration and overall I’m not sure if the differences don’t cancel each other out so to speak in this larger capacity motor.  But it certainly wouldn’t hurt, and this combo is one that Bruce Meyers seemed to like.  We didn’t run the 748R exhaust cams as the owner was quite happy to have a sub 11,000 rpm rev limit and they tend to promote higher rpm power at the expense of midrange.  The cams were set to 114/110 centrelines, which gives an inlet closing about the same as a 996SPS inlet cam set to 110 degree inlet centreline.

I also fitted MBP collets to the valves along with light closing springs and the bike had previously been fitted with an Akrapovic 45 – 60mm tapering exhaust system and had the throttle bodies bored to 54mm.  I fitted my “96SPS with modified cam timing” eprom as a start off and set the fuel pressure to 3.5 bar via an adjustable regulator.

I took it to the dyno to do some mapping runs and was quite happy with the result.  Chris had done a similar bike previously which had made 145Hp with the DIN correction (which is the same as 141Hp with the SAE correction) so we were pretty much on the money, dyno to dyno comparisons aside.  The comparison below is with a std head 996SPS running 107/107 cam timing.  Over 7,000 rpm it’s a nice increase.

The first group of runs that I did I ran it into the std rev limiter around 11,500 or so as I wanted to see what the power curve looked like.  Turns out it peaked at 10,300 and fell away thereafter, so pulling the limiter back to 10,600 was good all round.  I did a few WOT ignition timing runs and even with all that comp it liked 40 degrees over 9,000 rpm and at least 37 under that.  Pulling it back to 34 degrees dropped the power from 5,800 rpm upwards.  Maybe the 748R inlets are a bit short over 9,000 rpm for this capacity and that’s why it wants more timing up there than I expected.

Compared to a 998S and 1098 it’s pretty good, which is what the owner wanted as the 996SPS is a bit slow these days with the 1098s around. Added to the graph above are a 998S with slip ons and reset cam timing in blue and a 1098 with full 70mm system and reset cam timing in yellow.  It’s certainly put some more life into the old SPS.

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