Tuesday, August 23, 2016

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

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.  

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