When I bought the SF901 dyno 20 years ago, stock and race sled engines
were less than half the torque and HP of what we’re seeing today. The
Superflow 901 absorber is capable of controlling 1000 HP engines, but
the 1986 sales literature described how the absorber could be used for
engines as small as 12 HP. After Terry Paine, machinest extraordinaire
Skip Saupe and I built the hydraulic chassis lift, framework and
driveshaft system the first sled we had to test the dyno was a
then-new 1987 Yamaha Exciter with those notoriously troublesome
original butterfly carbs.
With dyno running directly off the crankshaft at engine speed,
pull-starting the Exciter was brutal. The bronze absorber impeller
acted like a 20 lb flywheel added to the mass of the engine’s rotating
mass. Once running, the Exciter 570 was unable to break idle and
increase revs at all. The engine would sit there idling, and increasing
opening the throttle would not cause RPM to rise at all–the engine
would remain at maybe 1000 rpm going “whaaaaa” from added intake noise
with no change in engine speed. This would only create excessive heat
in the rubber Lord torsional vibration dampeners we had engineered into
the driveshaft to protect the engine from the granite-like load of the
dyno on fragile engine components.
Working with SuperFlow engineering people it was decided that I should
disassemble the dyno absorber and machine the rotor (the internal
spinning part of the torque convertor-like absorber) down to a smaller
outside diameter. This would reduce low RPM drag on the engines, and
reduce torque capacity to maybe half.
After reassembling the dyno with the cut rotor, pull starting the
Exciter was a bit easier. But that awful 570 just sat there idling, and
appying light throttle would only make more intake noise and Lord
coupler heat, not more revs! A day later I got a call from local Yamaha
dealer Sunnyside Cycle’s tech Don Frasier who knew of my frustration
with the Exciter on my new dyno. Don had been fiddling with the Exciter
carbs and discovered that by popping out the softplugs and enlarging
one fuel passage, a light throttle lean condition was improved.
I popped out those soft plugs, drilled the second metering hole as Don
suggested, and JB Welded the plugs back in place. Unbelievably, now the
Exciter ripped off of idle and ran up to 5000 RPM where the automatic
control of the dyno took over and finally created the load necessary to
hold the engine there at full throttle. HP and torque were displayed
there, steady state. Pushing the “test” button allowed the engine to accelerate to the HP peak and beyond, and data was finally recorded. Now let’s figure out what that data meant.
Since the SuperFlow dyno did not come with a manual that expained
things like A/F ratio and BSFC etc I was totally clueless about info
that the SuperFlow airflow and fuel flow meters would eventually
provide us. All I wanted to see was torque and HP and go from there. But eventually, fuel flow numbers would be as improtant to us as HP numbers. This was a fun learningcurve.
Eventually, I would tire of pulling ropes with that 20 lb dyno flywheel adding to the load. So I acquired a huge 350 HP Cummins 12v starter, had rotation reversed, and bought a $500 german sprag clutch to alllow the starter to drive the engine to start , then idle down to zero RPM, as dyno data is recorded.
After obtaining dyno info on the Exciter, including those hard to understand
airflow and fuel flow numbers, other sleds would follow. Small 340 and
440 race engines would still be difficult to test even with the cut down absorber rotor.
We even resorted to blowing some N2O into the carbs to lift the race
engines from idle to whatever RPM they came “on the pipes”.
Reducing the dyno absorber speed with a toothed belt drive was the last
modification that would allow easy testing of those low torque high RPM
race engines. The belt drive would surely convert some of the engines’
HP to heat before being measured by the computerized dyno. To determine
that percentage I dyno tested an engine at 1/1 direct drive. Then I
installed the toothed belt reduction, and dyno tested the same engine
again. That lost HP was then programmed into the dyno computer, with
engine speed considered since friction HP increases as the square of
RPM.
I’ve dyno tested thousands of engines in 20 years, and up until last
year my reduced speed, cut down rotor SF901 has been up to the task.
But now along comes new monster turbocharged RX1’s and Apex’ with
over 300hp at reduced output shaft speed, and N2O injected sleds like
Glenn Hall’s D&D 1200 asphalt racer and my cut-rotor dyno absorber
is now on the ragged edge of not being able to control the engine RPM.
The last straw was the Honda 450 four stroke quad owned by DeNoyer
Chevrolet in Albany, overbored and modified to 50 HP plus brought here
to tune the Lectron-like Edelbrock carb and N2O system to max HP. On
motorcycles and quads, I use driveshaft adaptors to attach the dyno
drive to the transmission output shaft (never call that a
“countershaft”). The primary drive ratio is multiplied by the
transmission gear ratio to get us engine RPM. In this case, the 50 plus
HP engine was only creating 2300 output shaft RPM at 7000 RPM crank
speed and the dyno could not properly control the engine to obtain
meaningfull low RPM data.
So I recently called SuperFlow and ordered a new $1200 full size relacement
bronze dyno absorber rotor, and all new bearings and seals for another
$400. It required 20 hours of disassembling the dyno, pressing off old
bearings, cleaning parts, reassembling and pressing new components
together with my new full size rotor to be back to 1987 square one.
Now I can control 1000 HP again, so I should be in good shape to handle
all the full mod sleds have to offer for the next 20 years. But if you
have a vintage 340cc race sled to tune, you must go elsewhere.