What Should The Flywheel weigh
 

Flywheels are available in several weights. Some are made from aluminum and others are made from steel. Both materials are offered to create a specific weight flywheel.

It is an assumed fact, the lite weight flywheel accelerates and decelerates more rapidly than a heavy flywheel.Therefore,one may select the lite flywheel because of throttle response.Conversely,when the prop leaves the water the engine may instantly hit the rev limiter or over rev.The lite flywheel will also have a limited amount of stored energy.Therefore,starting,idle quality and dock manners will be effected. Also of concern is that a lite flywheel may collect more heat and cause an imbalance,not only in the flywheel but in the drive plate.

Comparatively,the heavy flywheel will start and idle better and have better dock manners.However,throttle response will be less than with the lite flywheel.The heavier flywheel may also be more user friendly at cruise because, there is less throttle response.

In my opinion, the heavier flywheel offers most advantages.However,a throttle jockey that jams the sticks forward may increase the risk of detonation. Jamming the throttles forward would ask the engine to over come imbalances plus spool up the heavier flywheel.

Would anyone care to share their opinion?

I seems you have answered your own question. For most marine applications the heavy flywheel gets the job done with less risk to the rotating assembly. Way back in the early 80's the late, great Keith Hazell and I tried aluminum flywheels in a 38' Bertram Competition race boat. The revs came way too quickly and we were constantly zinging the limiters in load-unload-load situations. Dean Gellner and Tommy Hoffstetter could expand on this better than I can. I'm old and way past my prime. WTF do I know anyway? Who am I? Why am I here? Where's my Depends? Somebody answer that damn phone! I've fallen and I can't get up.

Ed,

You're sounding very vice-presidential tonight. Perot been calling again?



On the flywheels, the only real application where aluminum works well is in shorter-stroke drag race engines where you're making horsepower up high but sacrificing some of that mid-range toprque. The need to be able to build revs quickly is the primary factor there. Losing flywheel mass in any application where there's engagement/disengagement of the driveline to the driven surface hurts you both ways. Larger mass prevents RPM flares on the disengagement when your timing isn't millisecond-perfect and upon re-engagement where you might not be right on the throttles, one way or another. Lastly, aluminum lacks the long-term durability of steel. It's not going to like those back and forth shock loadings. We had it pretty much down to a science of how many runs an aluminum flywheel would make before it popped. I've hairline-cracked some steel flywheels but have never grenaded one, regardless of how old it was.

Aluminum works good on road race cars for quicker revs and shifting

I always thought aluminum flywheels were for drag racing but it seems every p-class racer looking for a edge is running them these days too,Smitty

The V12 Allison uses something like a 100# flywheel.First,so that it will idle and second,to help with RPM spikes.

We are not even remotely considering a 100# flywheel.However,we feel it needs to be heavier than the shelf item 27#,10-18 machined steel.The starter ring on the 27# flywheel is welded at three located points.It is my understanding,welds are required to prevent the starter ring from dislodging. Balance of the welded starter ring flywheel may or may not be a problem as temperature of the flywheel is increased.Therefore, the 27# option is eliminated.

The next shelf item weight is 35# w/pressed on starter ring.However,it is said, only 10% of the marine flywheels are sold at this weight.We have used the 35# unit and it seems acceptable.However,it doesn't seem very scientific not being sure 35# is ideal.

The next shelf item BBC weights are 40# and 50#.I'm not aware of other weights.However,custom weights are available.

I'm of the opinion, 10-18 may require Cryogenic treatment.Cryogenic treated Billet Steel may also be worth considering.The opinion is based on concern for balance.

Little is said about flywheel weight, it is an abstract, complicated component.Factors ahead of the flywheel include displacement,cam,crank to rod ratio,timing,timing ramp,induction system and etc. Behind the flywheel is the driveline, gear ratio,prop pitch,hull weight and etc. Application is also a major part of the equation. Not all boats are race boats. Durability becomes more of the requirement in the hi performance pleasure boat.

It is interesting,there doesn't appear to be a distinction between engines for V hull and catamaran.However,the catamaran seems to break loose when it is packing air under the tunnel. Therefore,there must be a change in the Torque/HP to weight rato at that point. Step bottom V hulls probably experience some of that change as well.

Engines are basicly offered in one size fits all. While that theory might be deemed "good enough", I'm not sure it is totaly professional.

In my opinion, a large part of the total design includes focus on the flywheel weight. It would then be said, this combination is recommened for the specific hull,hull weight and intended application.

What is your opinion?

I think p class racing these days is more about acceleration than top speed and not running in rough water since a majority of the courses are somewhat short or on inland waters.

Smitty please send me an [email protected]- I want to talk to you about your outdrive additive. Thanks

I am not sure I buy the free reving theory in a marine application. When the prop breaks loose you still have a direct connection to the drives . As the drives use 25-50 HP, wouldn't this act like a "big" flywheel (add resistance) to free reving? I can see a small benefit to a heavy flywheel around the docks, but not sure it is significant. Who has measured the impact at idle? I suspect the Mercury wheel weight is based around a stock GM flywheel because it is inexpensive and comes with many of the engine packages.
Aluminum flywheels may have acceleration benefits over steel, but they cost more and offer less corrosion resistance. The flywheel cost, corosion resistance and weight are the only easy things you can measure in a marine application.

Perhaps we will have some formulas in the near future. There might be some fun in applying calculations to a specific boat or two.

There is a disadvantage that is not often considered with a heavy flywheel;
Boat is propped for 120mph, rough water only going 80-90mph, prop breaks loose at lets say 4000rpm. It doesn't take but a split second for even a heavy flywheel engine to hit the rev limit. When reentering the water the water speed tries to slow down the drivetrain and engine back to the 4000rpm. This inertia event is huge. The less inertia in the engine the easier it is to slow down. Of course this assumes a missed throttle event and that the engine would have been limited by the rev limiter. A lighter flywheel engine would make this situation slightly less likely, but if the time is enough the lighter flywheel could actually be a benefit.

Falcon,Your statements are logical and I would agree. However,would you agree that the lighter flywheel primarily benefits components behind the flywheel?

Components ahead of the flywheel may be otherwise effected. The blower may see increased rotor slap,the valve train may see increased stress and harmonic ring may increase.

The heavier flywheel may serve as protective dampener for the engine. However, subject to drive model,the drive may pay the price.

All I can add, is the flywheel from an HP500 weighs 30 pounds. I just weighed one last week.:)

CcanDo, there are definitely other considerations. As you mention too much or too little weight can affect the torsional vibration, also tuned out by the crank damper. You were correct that I believe less engine inertia (lighter flywheel) could mainly benefit the drivetrain, the component between the engine and the water! The flexplate (or drive coupler) can also come into play. The problem is that most of this stuff is just trial and error, hard to determine the best combination.

Falcon, For the case of comparison, lets use Bravos vs. #6's.
We know the 6's are rated @ higher torque. Therefore, 6's are thought to be more forgiving when subjected to shock load,regardless of input horsepower. Generally speaking, the Bravos are thought to be adequate when powered by a 500 EFI. However,the Bravo life expectancy must be more effected by variables, such as hull type and weight and X dimension.

We are in agreement the heavier flywheel may shorten power train life. However, is the decreased life expectancy primarily because of jumping the boat in and out the water? If yes, how about spacing the X down and using a heavier flywheel ? While that may be counter productive, theoretically the prop should see less time in mid air and less potential shock load. The deeper X will improve planing and subject to the flywheel weight not destroy top speed numbers. It just takes a little longer to get there.

Flywheel weight shouldn't have anything to do with hp, so if a boat likes a certain x for speed and acceleration the flywheel shouldn't change things, except that a lighter flywheel should accelerate harder all other things being equal. I wouldn't necessarily claim that a heavier flywheel will shorten drive life, just that under certain conditions it could. Under the right conditions a lighter flywheel could be worse, such as what you first mentioned, short missed throttle events that the heavier flywheel could limit the rpm change enough to lessen the inertia impact. I know that a boat with high x's that chine walks can be very hard on drives, even 6's with enough power. Not sure what the flywheel weight would have done in that condition.
I do believe that many of the older supercat and open class engines did use aluminum flywheels, but they turned more rpm too. Inertia isn't just weight, it's the speed it is going.
Interesting things to think about.

I've heard of re-inventing the wheel, but never re-inventing the FLYWHEEL!
All that rhetoric and still we're no smarter than we were before!
Mercury can order engines with any flywheel they want, yet they still keep delivering engines with the heaviest flywheel they can get. Do you think they are on to something here?
If you are so sold on an aluminum flywheel, just go buy one and try it out and stop all the masturbation!
If putting on lightweight flywheels would give MerCruiser an edge, don't you think with Brunswick's resources for research and development that this would have happened already?

Falcon, My comment about the x,HP,speed was with respect to shock load. logarithm from the heavier flywheel should counter balance some degree of lost efficiency found with a lowered x. In other words, The heavier flywheel inertia should maintain RPM longer or from bump to bump,while ultimately increasing MPH. The deep x may or may not cause MPH to suffer. However,would the deeper x be safer for the components ?

CcanDo, I don't think that it would necessarily change any of the lost efficiency, but it is known that a deeper x is easier on components. Deeper keeps more of the blades in the water at once for smoothing out the impacts of each blade entering the water (surfacing), although the tendency toward 4-5-6 blade propellers also helps this. It also reduces the bending load and torque steer loads on the propshaft and gearcase. The current flywheels usually used are already on the heavy side, probably to reduce the chance of shift stall, but the newer computer controls also help this.

OK,Mr.Edward R. Cozzi,What is the ideal flywheel weight ? And,how much TIME will this take ?

Damn! Just use the one MerCruiser sends with the engine and take advantage of the information they already paid to figure out. If you insist on trying another combination, simply install a different flywheel and take it out for a ride and see for yourself if it works or not. If you're looking to have an engine that over-revs faster than any human can feather the throttles when the prop becomes unloaded, then go for it.

Here is an idea. In a twin application, put an 8-15 lbs alum flywheel in one engine and a 30+ lbs steel flywheel on the other. Run at maybe a steady 5000 RPM and jump a wake, chop the throttles back as the props break loose and measure the RPM peak/curve (how?) on both. My bet is you won't bother to pull and re-rig either engine based on the difference.

I'm going to fill you guys in on some flywheel history...

The flywheel was originally invented by Isidor and Marybelle Trundle, (yes, they also invented the hideaway bed), in the early 1700s. Their first design was hollow and they kept filling it up with a mixture of corn flakes and farina to experiment with different weights and consistencies. Mrs. Trundle was a lousy cook and Isidor was always looking for places to hide food rather than consume it. Isidor stumbled on a near perfect combination with some left-over meat loaf from earlier in the week. The engine revved-up smoothly and when the propeller left the water in Lake A, (where they tested their inventions), the engine reacted favorably. The rest, as they say, is history.

Ed,
I had heard that Isidor ending up spending most of his time hideing the sausage and he ended up losing focus on most of his other activitivies.

A heavier flywheel will greatly extend crank life.

P.S. Don't fuc& with Cozzi. Just listen & say thanks.

For drag racing flywheel weight woud depend on how much energy would need to bee stored for the best launch. Mostly depending on the size of the engine vs the weight of the vehicle. There are also increased mass flywheels. Due to the way they are machinedd a 30 lbs flywheel could have the inertia of a 40 lbs one.

Not shure why a boat would need the stored energy other than re-enteing the water to reduce shock load and help maintain rpm on impact. As Chris stated a heaveier flywheel increases engine life it smooths out the pules between the cylinders firing.

Actually there is some truth to that.

Mrs. Trundle was suspected of fooling around with a Great-great Grandfather of Carl Kiekhaefer...

About 15 years ago, I was helping a friend with an engine project. He was a fairly well-known pro-stock engine builder. He had a theory about some things that were going on inside an engine- basically that on launch, the tires went through extremely brief periods of slip and stick which, if isolated and slowed way down, amounted to the engine loading and unloading at a fairly high frequency. His theory was that induction systems could be tuned around this frequency for a more stable inlet charge flow. In his experiments, he discovered that flywheel weight could make it go away, or at least lessen it to the point it wasn't a factor. Problem was, it took over 70 lbs to get it there. He did discover significantly less rotating assembly fatigue with the larger flywheels. Everything from rods and rod bolts to cranks to main bearing caps benefit from flywheel mass. The downside is a drag engine with a 70 lb flywheel is a real stone. I don't know how he ever made out on his intake tuning experiment- he got sick & left the biz shortly thereafter.

There is really no ideal weight of a flywheel. Here again flywheel weight is directly related to many variables. Horsepower,weight of the boat/ vehicle,inertia, ect.... play a roll in selecting the proper weight flywheel. We have run many light weight aluminum flywheels in race boat applications were the boats are fairly light, higher HP , and we are looking to accelerate the engine. In a heavier boat such as an Apache and non-step hull design with Supercharged engines I like around a 25-28 lb. wheel. The weight of the flywheel will in rougher water help keep the mass of the boat accelerating. In this type of application we have gone from a 40 lb. wheel to this weight above and picked up 100-200 rpm do to the reduction of rotating mass. Yes, in Drag race applications weight and horsepower also relate to flywheel weight.

FINALLY, Dean! Where the heck were you when I needed you?

Hope you're having a Great Thanksgiving!

Ed

Yes, to all, Happy Thanksgiving !

The following is a recap from above:

1. Heavier flywheels
A.some delay in engine over rev
B.Improved bottom end life
C. 100-200 RPM max Rpm reduction w/40# vs. 25-28#
1.Is that enough to cause increased risk of detonation ?
D. May cause increased gear train stress w/ high x
E. May help protect gear train w/ lower x
F. Up to 70# flywheels have reduced harmonics
1. It is said, the Merlin/ Allison uses a 100# flywheel.
2.Harmonics are known to produce heat, which may effect the drive plate.

Is it possible idle quality and dock manners would be improved ?

Is it logical, while some will use full throttle continuous,most will use max. RPM occasionally ?

Is it possible, all things considered, the masses would prefer the heavier flywheel for the sake of durability ? Even if maximum effort performance was effected ?

Is it possible the valve train would benefit from a heavier flywheel ?

CCD,
Including my input, I would sum it up by saying you have numerous opinions and no measured science.

Ben:
I suspect CCD wanted to up his thread count AND impress us all with his pseudo-intellectual grasp of physics.
Ed

I want to thank EVERYONE that has particpated in this thread.

Perhaps,some reader will avoid an otherwise disappointing weak end, boating. Hopefully,I will.

Ben,I feel,the input has been very productive. Your suggestion of two simultaneous weights is very interesting.

Cozzi, you get the Gold Star. Your input and humor is priceless. As Chris suggested, THANK YOU!

My next stop will be at the Drive Plate. As any and all of you may participate, your participation will greatly appreciated!

why stop at the drive plate....what about the balancer?

How about some SpinTron testing? The valve train is what fails most offen in the BBC design. 55 mm cam bore?
What size and weight push rods? Where do we stop? What is going on with the BBC designs in the Northern Dirt Modifieds? Let's get into the real $$$ with some serious testing, we got all winter!
The real BS hasn't even started yet...

Wait a second ... just wait a second





























are we talking running in salt water or fresh water?

Flywheels......who woulda knew....:rolleyes::D

We need a brotha to fix the wheel situation!

I received an irate phone call from Fred Kiekhaefer about me outing his great, great, great grandfather messing with the late Mrs. Trundle.
Sorry, Fred, I thought you knew.