...it is what it is." (Alberto Puig)
So what was it? I'd sorted the squish problem with the 851, made a few other improvements (shorter intake trumpets, reduced rotating mass, optimised squish etc), but what had it actually achieved?
I sourced the laptop needed to run the Axis dyno software...interestingly they prescribe a gaming type system to cope with the fast data acquisistion required. The final piece of the puzzle was a cheap lil' weather station, so I can plug air temp/pressure/humidity into the software to standardise results:
It's kinda important to know that the 2hp you gained/lost is via your muggling around in it's innards vs simply the difference between a cold wet day and a hot clear one, otherwise you just end up chasing your tail.
Interestingly, looking back over some previous professional dyno testing there was often no air conditions entered. All well and good if you are only after a one off tune, but it's not ideal when you are hoping to evaluate incremental changes over time.
Mr Google was good enough to inform me I stood at 272m above sea level, and with that I think that we're good to go.....
....oops, back up the bus! The O2 bung on the 45mm ST4 exhaust (and many other Duc's from that era) is only 1/8" BSP :
...while the wide band Lambda probe is 18mm. Hmmmm...
Luckily, amongst all the dyno cables, electronics and various other bits and bobs was a plastic kit-box containing a very nice surprise:
The dyno seller had been very generous in including a complete "SAFR" kit: basically a means to assess the AFR of your bike as you ride around, no dyno required. It includes a standalone AFR gauge, and the means to insert a tiny nozzle into your exhaust, even without an 02 bung, which feeds a small alloy sample chamber in which the O2 probe is fitted. One of those concurrently simple/clever ideas.
So the nozzle/chamber setup was the perfect solution, I just drilled/tapped a couple of 1/8" BSP brass bungs to suit the nozzles (the Axis dyno supports 2 x 02 probes concurrently, and 2 chambers were supplied with the kit) and now I'm good to go.
Alrighty, enough procrastinating, time to hit the (rolling) frog and toad! Here's the base run, with the original aftermarket FIM "916 + full exhaust" chip I'd been running all along, with the standard redline:
(Yes, smoothing is set a tad too high, but it doesn't affect the overall shape or numbers)
I'm not about to suggest it's making 10hp more than previously, it's a completely different dyno, with genuine ram-air, so there is absolutely no way to compare the two. But it's a solid baseline: pretty good AFR, nice flat torque curve with no real holes, and certainly didn't run out of puff approaching the redline.
Time to fit the custom P8 916 eprom that Carmo Electronics burned with an extended redline that allowed my single injector/TB bike to rev like a twin injector/TB SP. (all the single injector P8 bikes are limited to 10krpm)
I could pump out a heap of curves here, showing the progression, but bottom line it was so nice to be able to run the thing with no crushing time or financial pressure.
And this is where we landed a few days later (note the AFR/hp scales are slightly different to above):
There are some things to like but also some things to wrinkle your brow. Peak power is pretty good at 131hp (8.4hp gain), but the fact the peak is under 10krpm is a mixed blessing: it's great for preserving the 'cases/reliability, but is ultimately limiting the power it will make (748R peak power is normally up close to 11krpm).
Why? I have a couple of suspicions rattling around in my scone:
- the cam timing is an obvious one, retarded when sorting the squish
- the Carmo chip is just a stock 916 Strada P8 ECU eprom, which they have simply modified to extend the redline vs the racier SP's (and 748R) which probably have quite different ignition tables.
- The 45mm exhaust is perfect for an 851/888/916, outperforming a 50mm system in comprehensive testing carried out by Brad Black:
...but they all have old-school long duration low lift cams.
The 748R heads on my bike are almost a kind of bridge between Desmoquattro and Testastretta, having later Testa'-style short duration high lift cams in Desmo' heads. Think about the difference in exhaust pulses, given the same volume is being exhausted: Desmo looooong low pulses vs Testa short sharp pulses.
With an inherently higher exhaust gas velocity Testa certainly seem to like bigger exhausts than Desmo with a 996RS (Desmo') having a 57mm exhaust, where say a 999RS (Testa') had 63.5mm, albeit with very different bore/stroke. FYI, a 45/50mm half system is the pick for a stock-ish 748R, where a 57mm is the ducks nuts for a 749R.
I have no intention of pulling RS-type revs but I have a not-so-sneaky suspicion a larger (at least 50mm) exhaust would be a step in the right direction for a 926 with these short duration high lift cams.....but that won't be happening any time soon with the financial cupboard now well and truly bare.
