Why a Heavy Flywheel Yields More Top Speed

JC-SuperJet

JC-SuperJet

Location
Texas
Many realize that switching to a Lightweight Flywheel will improve the PWC's Acceleration but not quite realize that a loss in Top Speed will also result.

This is important for Buoy Course Racers.

But how can this be, all things being equal except for the Flywheel Weight? :oops:

Let's see, for the exact same PWC, both Flywheel setups will reach the same Peak RPM. But the one with the Light Flywheel will reach Peak RPM sooner i.e. accelerate faster.

Acceleration and Speed are related but different Animals. :p

So if both identical PWC's with just different Flywheels are running at the same RPMs, how can one be faster than the other? o_O

That's because the Heavier Flywheel PWC has MORE HORSEPOWER! :eek: No way you say, it's the same damn Engine...:mad:

Whelp, the Heavier Flywheel has more Mass, which yields more Angular Momentum (the Rotational equivalent of Linear Momentum).

And a rotating Flywheel with more Angular Momentum has MORE TORQUE. :D

So? Remember that Horsepower = Torque x RPM :)

Thus the same Engine will have the same Peak RPM, but a Heavier Flywheel will yield more Torque, thus transferring more Horsepower to the Impeller, hence more Top Speed. ;)

P.S. The Engine itself doesn't produce more Torque. It's the net actual Torque at the Impeller...
 
kevbo

kevbo

Location
dfw
I know that a more solidly mounted engine block will make slightly more power than one that is loosely mounted. Flywheel weights effect on engine power is less conclusive. On a PWC it is possible that greater power stroke acceleration could increase impeller cavitation which would hurt top speed. It would have more to do with the pump than net power from the engine.
 
S

Storbeck

Location
Minnesota
The thing about "heavier flywheel has more torque" is nonsense, but it is correct to say it takes more torque to accelerate a heavier flywheel, both speeding it up and slowing it down.

Seems like, if you are in choppy water with the impeller loading and unloading, a heavy flywheel will store more energy before the rpm rises high enough to hit the rev limiter, or the rpm's rise out of the motor's power band, so there would be more energy stored in the flywheel when the impeller unloads then loads again. I could see that increasing top speed.

The speed in this scenario would be coming from staying in the power band of the motor so the average hp output is higher than a motor that is hitting a rev limiter or has rpm fluctuating up and down above and below peak hp rpm.

I don't know anything about making skis go fast so not agreeing or disagreeing with the idea that a heavier flywheel goes faster, just disagreeing with the murky physics.
 
JC-SuperJet

JC-SuperJet

Location
Texas
Not sure what's murky about Torque in a Rotating Disc It's High School Physics...

Maybe you'll accept the Laws of Physics from MIT and Volvo Engineers? https://energyeducation.ca/encyclopedia/Flywheel

"Another application of flywheels is to give the drive shaft a power output which is higher than the output of the engine alone. "

"kinetic energy recovery system (KERS) had a flywheel attached to each of the two drive wheels to store energy. The energy was then used to give the car a power boost of 81 hp"

Since the Flywheel is a Storage of Energy, the Impeller Driveshaft gets the Engine Torque PLUS the Flywheel Torque. :eek:

And, Pete and Repete, a heavier Flywheel stores MORE Kinetic Energy than a lighter Flywheel, thus MORE Angular Momentum, hence MORE Torque = MORE SPEED

I have actually proven that a heavier flywheel made my Kawasaki 750 SXi faster than when the Light Jetinetics Flywheel was installed, by drag racing against a friend's Ski, and the Stock Flywheel SXi was much closer at WOT to the reference Ski than the SXi was with the lightweight Flywheel to the reference Ski. Just didn't understand why at the time.

The best Real World test would be to GPS or Radar the peak speeds of the same Ski with both Flywheel set-ups. And see what the speed difference is...1/2mph? 1mph? 1.5mph?
 
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W

waxhead

wannabe backflipper
Location
gold coast
That will never work they told me. Even though it showed up in testing
www.waxracingproducts.com

Stainless Steel high Inertia Yamaha Coupler

Introducing our high inertia coupler for Yamaha 62T, 63M, and 65U engines. While it might seem like this would slow engine response, it actually smooths out power delivery, enhancing hole shots and hook-up. This design helps bridge the gap in power delivery between two-stroke and four-stroke...
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WFO Speedracer

WFO Speedracer

A lifetime ban is like a lifetime warranty !
Location
Alabama
I think just maybe you are forgetting something here , what comes to mind for me is automotive engines and all the extra components they have to turn, power steering pumps, alternators, water pumps , air conditioning all rob horsepower, this is why on drag engines they don't use alternators and they don't have power steering or air conditioners and they use electric water pumps, These losses are called parasitic losses and also apply to flywheels.

I see heavy flywheels the same way, it takes more horsepower to a heavy flywheel and less horsepower to turn a lighter flywheel, all the points about smoother running in , choppy water would apply though, a heavy flywheel does store more energy , it will rev slower and loose energy more slowly, that is a undisputed fact , I see no possibly way you are going to make more horsepower turning a heavier flywheel , energy can neither be created or destroyed.

All I can see it doing is moving the powerband around, a light flywheel revs quicker which on a two stroke engine means getting into the powerband quicker, a heavy flywheel revs slower and loses revs slower, the type of porting you have as well as where the exhaust makes it's peak torque and horsepower numbers at are going to determine what flywheel weight you need.
 
S

Storbeck

Location
Minnesota
Inertia makes no difference if there is no change in rpm. The way this flywheels work is that energy is stored in them by accelerating them, and released by decelerating them. In a situation that the rpm is steady the inertia has no affect on the torque. Force equals mass times acceleration, or for spinny bits torque equals moment of inertia times change in angular velocity over change in time. That's your high school physics. No change, no torque.
 
W

waxhead

wannabe backflipper
Location
gold coast
A flywheel is a reservoir of torque. You paid for it may as well keep it. Granted if you are trying to do a flat water back flip then its not what you are after. If your in the surf its awesome as it smooths out the power. I would be surprised if you can notice the difference in acceleration at that surf level. I don't double back flip so i cant talk for the flat water guys
 
MTRHEAD

MTRHEAD

Location
MotorCity
waxhead said:
That will never work they told me. Even though it showed up in testing
www.waxracingproducts.com

Stainless Steel high Inertia Yamaha Coupler

Introducing our high inertia coupler for Yamaha 62T, 63M, and 65U engines. While it might seem like this would slow engine response, it actually smooths out power delivery, enhancing hole shots and hook-up. This design helps bridge the gap in power delivery between two-stroke and four-stroke...
www.waxracingproducts.com
Click to expand...
Back in late 90’s early 2000’s a company called Fluidamper, still one of the biggest in automotive aftermarket, made dampers (their technology in our coupler design) out of stainless. They were used a lot of race Kaw triples and some on Yamaha’s. I had a few. Hard to find.
 
long beach local

long beach local

long beach local
Location
Az
Interesting reading about this topic i haven’t been on the X in a while but I can’t help with my opinion. I really like the benefits of a light flywheel running the 760 zeel lightened with no magnets TL . Bang for the Buck its noticeable revs up quicker buoy to buoy I like it better but it may just be a personal preference
 
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