The dyno results of this 1992 750SS Ducati, are good, but puzzling.
The 750 engine has the same ports as the 900SSs I've been doing. I used Bruce Meyers' 13:1 pistons (I haven't determined the exact compression yet, and I doubt that it is anywhere near 13:1, with the combustion chamber volume I'm using, as it cranks at 180 PSI) and the stock 900SS cams were set the same on both cylinders. Also, for cost considerations, the stock manifold length and taper-bored stock carbs were used. There has been some evidence in testing other 750SSs (of which, I've done very few) that they don't suffer as badly from the inlet resonance effect, as the 900s.
The results confirm that they don't suffer too badly from inlet resonance. One friends opinion, is, that the 750 engine doesn't send as strong a signal, resulting in less negative resonance. Then, above 8000 RPM the ports (which aren't overwhelmed as they might on a 900) start to recover and the power continues to rise.
Another theory has been thrown out there. Someone called me with several questions, then offered that my theory about inlet resonance was wrong. The caller (who never E-mailed me as I had requested) told me that the problem was the inlet volume was insufficient. Y'see, car guys like to have the inlet port volume equal the cylinder displacement.
I thought about that and; I agree. I disagree that the inlet resonance has no effect.
By using ports the same size as 900SS ports, on a 750, the port-volume-to-cylinder-volume relationship changes, or increases.
This might cause the result I've seen on Mike Summer's 750SS. There seems to be the same resonant frequency as that which occurs on a 900SS, except the power levels off instead of dropping off above 7200 RPM, and then rises as the revs climb above 8000 RPM
This can be seen in the following chart. This shows a stock 900SS, a ported, short manifold 944, and the 750 in question.
The similarity in torque curves, between the 750 and the stock 900SS, (both with remarkably similar cam and length specs, but the 900 without porting) is the characteristic, linear torque fall-off, which differs from that of the short manifold 944 on the same chart, and the RPM range where the peaks and troughs occur. The major difference (between the 750 and the stock 900) is the slope of the VE fall-off.
The sharp increase in torque from 5500 to 6500 looks to be a combination of resonance and inertia increase. The next project vehicle is using shortened stock manifolds with the same carbs/cams/valves and should shed some light on the resonance theory.
Here's another chart;
The second chart compares a 900SS without porting, but with very similar inlet tract/carbs/air-box configuration. The 900SS isn't ported but has the same open air-box top and tapered carbs, as well as JE pistons.
Again, the 900SS falls off hard above 7200 RPM while the 750 muddles through that zone to recover above 8000 RPM. I've often thought that the 900s should recover like that but the ports are just too overworked at that point.
Because of the good power output of Summer's 750, I'm encouraged about Reed's 750 race-bike.
I'll be adding more as soon as I can.