Today I spent an hour on the phone with 7K Dyno Bill Davis (fully
instrumented engine dyno @ 7000′ altitude in Marysvale Utah).
Bill and his partner Donavan are now preparing to create a sled
chassis-holding fixture like mine that will allow testing without
removing the engine from the chassis. This will be a fantastically good
thing for high altitude riders who seek optimum performance without
having to jerk the engine/ ignition/ fuel system from the chassis’.
They’ve dyno tested many hundreds of sled engines, 1000’s of dyno tests
on their engine
fixtures. But now the complexity of the new breed of EFI sleds with
hoses and wires and computers mounted everywhere means it’s time for
Bill and Donavan to begin dyno testing with engines in their chassis’.
The gist of our conversation today was how to create a long
enough dyno drive shaft to allow for motor mount flex as well as
dealing with the most evil torsional vibrations emanating from long
stroke (and even longer stroke “stroker”) big bore twins that are
in vogue today.
Tim Bender and Sean Ray have been dyno tuning their Polaris/ Ieam
Industries 800 twin snowcross engines here. Without getting into
specifics about engine output one of these 100 and something lb/ft evil
vibrating twin engines recently snapped a [prototype very stiff] dyno
driveshaft weld, and twisted out of phase a press-fit joint that
required 15 tons (measured on my hydraulic press) to assemble.
Aproximately 500 lb/ft of torque was aplied to try to reindex the shaft
to no avail. How could this be? Huge torque spikes are created by some
of these monster twins when they reach their resonant frequencies (that
engine speed where the PTO and mag ends are “ringing”, going back and
forth in different directions!). The monstrous torque spikes created by
these vibrations can be as destuctive as 1000 lb/ft impact guns,
twisting themselves out of phase and/ or turning steel dyno driveshaft
components into red dust. That heavy mag flywheel winds up, then
unloads at just the wrong time, acting like a sledgehammer on the
crankshaft itself and anything attached to the crank PTO taper. Those
torsional vibrations are the cause of Big Twins prematurely wearing out
drive clutch moving parts. Since we do mostly “sweep” testing from low
to high RPM, it is very likely that we will encounter, somewhere, the
critical speed(s) of any crankshaft. Even if that encounter is brief,
torsional vibes can wreak havoc on dyno shafts and the engines
themselves.
How can we protect our $100K dyno systems, and equally importantly, our
customers’ valuable engines from these torsional vibrations and the
resultant violent torque spikes? If we use a solid driveshaft capable
of handling the torque of. say, a 6-71 Detroit Diesel, driveshat and
dyno longevity will be excellent. But those impact-gun torque
spikes from the Big Twins will have nowhere to go, and can easily turn
on themselves (literally) and turn the crankshaft out of phase. I’ve
done that here in the past, protecting my stuff with solid shafts at the
expense of the potentially offending engines. No one wins or learns anything from
that sort of standoff.
So how do we deal with this? Simply, lots of rubber dampening media
between the engine and dyno absorption unit. The more the merrier, and
those destructive torque spikes are converted into hot rubber that
cools when the torque spikes subside. There is some engineering that
goes into determining the proper stiffeness of the rubber couplings to
absorb the torque spikes. In my case it has been 16 years of farmer
engineering which determined that sweet spot of torsional dampening
media–if it’s too supple for the peak torsionals the rubber will wind
up like black licorice chewing gum on the ceiling and shaft parts will
fly. If the rubber’s too stiff, engine components may suffer.
Today, after 1000’s of variously evil engines, we’ve come close to
learning how to deal properly with each situation. There is lots of
rubber dampening media in our shaft system, we pay attention to heat
levels in the rubber couplers between runs. With the experience gleaned
here from the holes in our ceiling, Bill and Donavan can hit the ground
running and high altitude “in chassis” dyno tuners there can be
comfortable knowing that their expensive engines will be pampered while
valuable information is learned.
A dynomometer with a properly
dampened dyno driveshaft is absolutley easier on the engine than a
drive clutch, since there is zero side load on the crankshaft PTO!
Every mod engine deserves to be dyno tuned. Bill and Donavan have found
that what works here close to sea level is totally different at 7000ft.
I need to find a way to include them more ofter in DTR, so they can
share their valuable (for mountain riders) technical info. More
valuable High Altitude stuff to come after they get their chassis
fixture/ driveshaft system complete.