Brad The Bike Boy: Part 2 of Dyno run comparison of things not the same - dunno, couldn't think of what to call it, it's a bit odd

Saturday, August 27, 2016

Part 2 of Dyno run comparison of things not the same - dunno, couldn't think of what to call it, it's a bit odd

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.

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The 851

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