I was curious to see how the torque curves of the std comp motors compared, so I made up a spreadsheet generated graph using exported data form the Dynojet dyno software to compare the old dyno to new dyno runs I did of the high comped 750. Using the factor between those two runs (average of 0.882, 0.860 - 0.912) I calculated a std comp run for the big valved, 900 cam 750, albeit with std carbs for the new dyno. I also took the dip out of the previous 750 run, just to make it look nicer.
The result is the graph below. Black is 900 with FCR39, slip on muffers and open airbox lid. Red is std comp 750 with jet kit, slip on muffers, open airbox lid, 900 cams and big valves. Blue is high comp 750 with FCR39, slip on muffers, open airbox lid, 900 cams and big valves. The yellow line is the comparison between the two std comp torque curves, as % difference of 750 from 900. The 900 engine is 20% bigger than the 750, meaning an ideal situation should show this as a straight line at 20. But with the reality of engines being engines, you don't get that. Interesting that the centre part of the curve is just under 20% (17 - 19), which is as expected and it's always nice to see some correlation to the theory. Once the 900 torque peaks just under 6,000 rpm the factor drops as the 750 both peaks later and doesn't suffer the same rate of drop off after the peak.
Thinking about it some more just now, the 750 was running the cam timing at 107 degree inlet centrelines, but the 900 was "as delivered". Meaning not checked. At a later date I did check the 900, and found both cams at 117 degree inlet centreline. They were advanced to 107 degrees with a couple of 10 degree offset keys. At 107 the 900's torque curve would drop away even quicker, possibly to less than the 750 at the end.
And there you go. I found it somewhat interesting, hope you did too.
I was going through some dyno runs yesterday looking for some 750 carb jetting info, and looking for a comparison when I came up with this one.
Looks like two runs with different jetting? Not, as it happens. The blue is a 900SS fitted with FCR39, open airbox lid, some sort of slip on muffler and no other mods. The red is my hotted up 750 motor when fitted to Minnie. Same cams as the 900, similar valve sizes, different FCR39 (you'd hope very much the same) and the exact same 152 main jets. I have a bit of a running joke going with this pair of FCR 152 main jets. I fit them to everything I do with FCR and see how they compare. So far they've never kept a permanent gig. Anyway, two things come to mind.
1/ Massive difference in fuelling where the only real difference is engine capacity, and therefore airspeed through the carbs. The 750 has lower airflow for the same time frame (same cams), so less vacuum drop across the venturi (Bernoulli's equation) and therefore sucking out less fuel.
2/ Almost identical power curves run against road speed, with the red run stopping earlier appearing to be just a "as run" thing. For the gearing to match it up this way is pretty neat, and shows that, on a short straight or in this gear at least (possibly 4th on both, 900 has two more to go) it'd be a fair fight to the next corner.
The next graph shows the power versus RPM, which shows them to be different. As you would expect, the 900 will make more power at any given RPM under the power peak as it is larger, but also points out how good the 750 is comparatively.
Comparatively is better shown in the torque curve, as below. The 900 engine is 20% larger than the 750, so it should make 20% more torque all other things being equal. In this case, the big variable is compression, as the difference in peak torque is only 8%. The 900 has a bit over 9:1, the 750 allegedly around 12:1 (I never did check). The runs I ran with the 750 as a std compression motor were done on a previous dyno which gave higher readings, so I don't have a std comp run that directly compares.
