Thread: Rotating mass / Inertia moment
08-09-2010, 07:08 AM #1
Rotating mass / Inertia moment
3200 rpm to 8780 rpm in .5 seconds. Half a second! Thats what my Vipec data log tells me the rate of accelleration is on my SHO engine, while in the water, loaded with my twin prop setup.
Thats a good thing, right?
Yes and no.
Yes its a good thing we can create this much power with the SHO engine, ive never had so much fun.
But its not necessarily a good thing when it comes to applying the power to the water.
I just watched a SHO with a E1 wheel get beat in a drag race by a SHO with a B1 wheel, and the root cause was cavitation.
E1 equipped SHO slapped the limiter 10-15 times before it hooked up, and the B1 equipped SHO only slapped it 2-4 times before it hooked and was gone. That 30-50 horse advantage the E1 gave is lost in a drag race because of cavitation.. Control the rate/speed of engine accelleration and you control cavitation, and the end result would have been different.
All forms or racing have their traction control devices.
Top fuel cars have no transmissions or torque converters but do have a progressively engaging clutch pack that limits the power to the tires for the first half of the track or more.
Any bit of shock sent to the rear tires ends up an up in smoke pass. Too much power to quick to the rear tires and its over. Up in smoke again.
A drag race car spinning the tires on the starting line is the equivalent to cavitation in the jet ski/jet pump world.
We have no clutch packs, torque converters, gear ratio changes, tire size changes, or any other of the many devices used in the drag racing world to control tire spin. We are 100% direct drive with nothing to tame down the starting line blast of power, except to maybe try and go back to the good ole days and (feather the throttle) to control cavitation. Those with Motecs and Vipecs have traction control features which are great, but still doesnt cover all the bases, and are very pricey.
Now enter in the equation also, the terrific rate of de-celleration these SHO engines have also.
Has anyone noticed these motors go from 8500 to idle as fast as they go from idle to 8500?
Why is that?
Answer is... The rotating mass of this engine is so light, it allows the instant accell and decell rates to be abnormally high given the power we are making with them.
Consider your standard 350-400 horsepower V8 engine. First it has twice the crankshaft piston and rod weight, then a harmonic balancer on the front of the crank then on the rear it has either a heavy steel flywheel with a clutch and pressure plate attached to it, or it has a torque converter full of fluid, all which add a huge amount of weight to the rotating mass of the engine. This combined additional weight can be 50-75 pounds or more. The added weight slows the rates of accell and decell, and has little or no effect on peak horsepower. It just makes the engine a little slower in getting to peak rpms and horsepower, which can be a good thing.. It makes it more manageable.
Adding rotating mass/weight to the rotating assembly can be our simple form of traction control, to help prevent cavitation on holeshot.
It will slow the super fast rate of accelleration these engines have into a more manageable time frame so our props and pumps will have a chance to properly do their thing..
It will also greatly benefit the supercharger and its entire drive system. 32 thousand rpm to 88thousand rpm in half a second! And the same rates on de-cell. No wonder we are breaking sprags, shafts and gears. Slowing the rate of accelleration and de-celleration of the supercharger drive system will reduce the (inertia moment) that shocks the system and breaks all of our parts.
Did you know the Sea-Doo rotating mass is more than 10 pounds heavier than the Sho engine?
And its a 3 cylinder versus a 4 cylinder, and it also has a balance shaft that is crank driven that adds to the rotating mass quite a bit.
Pay attention, listen to, and ride them and you can feel and hear the difference in the rates of accell and decell in the 4Tecs, from the SHO engines.
I will soon be testing a new R&D CNC billet coupler with added weight to see just how it affects the cavitation issues.. In my mind it will work. Of course i will post my results when it happens.
Part is still being made as this post is written.
Wanted to give everyone time to think about this as usually we are trying to lighten our rotating mass for added accelleration, this time we need to go the opposite direction because of the tremendous amount of power these engines create compared to their very light rotating mass.
R&D has been working extensively to come up with a logical combination of parts that help put that power to the water, reduce rod stretch, extend clutch life, and increase rough water hook up by reducing prop shock....
08-09-2010, 07:39 AM #2
08-09-2010, 07:54 AM #3
Whaou!! This is really good info!
Thanks a lot!
08-09-2010, 10:08 AM #4
Ross we are just at the tip of the iceberg when it comes to ECU programing. The Vipec ECU has the capabilities of traction control based off APS, TPS and RPM. Example: it can be programed so at say WOT at 4000rpm the throttle blade only opens 40%. It will takes many hours of logging and mapping but it can be done.
08-09-2010, 11:01 AM #5
08-09-2010, 11:33 AM #6
08-09-2010, 11:39 AM #7
- Join Date
- Sep 2007
- Gainesville, VA
08-09-2010, 11:46 AM #8
Have you been hanging out with David Reher again?
08-09-2010, 05:48 PM #9
pro efi is nice also Kris but arent they really expensive ? also great write up Ross ! keep us updated
08-09-2010, 10:49 PM #10
You been talking to Bill eh? Sounds awesome. Can't wait to see what he comes up with.
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