Flexplate or Flywheel?
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Flexplate or Flywheel?

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    Member Toyboata's Avatar
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    Default Flexplate or Flywheel?

    Looking at a 427 SBC fresh off the dyno and arrived at my door. Good numbers, 528 H/P 580 TQ. Unpacking and found they had installed an 18 to 20 lb flywheel instead of a flexplate? The old SBC had a flexplate, so I'm thinking that I might need to talk to the builder.
    But before I do that, for jet boat engine performance, I thought that the lighter weight of the flexplate compaired the a higher weight of a flywheel was the way to go. Looking for input.

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    Senior Member jockorace's Avatar
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    Quote Originally Posted by Toyboata View Post
    Looking at a 427 SBC fresh off the dyno and arrived at my door. Good numbers, 528 H/P 580 TQ. Unpacking and found they had installed an 18 to 20 lb flywheel instead of a flexplate? The old SBC had a flexplate, so I'm thinking that I might need to talk to the builder.
    But before I do that, for jet boat engine performance, I thought that the lighter weight of the flexplate compaired the a higher weight of a flywheel was the way to go. Looking for input.
    Leave it alone and run the thing....really! Jocko

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    Senior Member Chop Shop's Avatar
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    After seeing several flywheels I dont want one in my boat. In a boat there is no flexplate to help hold it together if that even helps.


    SFI flexplate for the win.

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    Quote Originally Posted by Chop Shop View Post
    After seeing several flywheels I dont want one in my boat. In a boat there is no flexplate to help hold it together if that even helps.


    SFI flexplate for the win.
    I found what feels to be a 30 lb flywheel on mine. Wouldn't that just kill the ability for a quick revving engine compared to a flexplate?

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    Quote Originally Posted by Double A View Post
    I found what feels to be a 30 lb flywheel on mine. Wouldn't that just kill the ability for a quick revving engine compared to a flexplate?
    Yep just like its easier to spin a small tire radius compared to larger ones.it takes inertia absorb to move a flywheel..sfi approved flexplate gets my vote as well but i wld talk to the builder and make sure rotating assembly wasnt balanced witj flywheel.if it was youre stuck with it.i used to race dirttrack modifieds with a claim rule for 300bucks.if i got claim i had tp pull my lil alky sbc and let the guy have it for 300bucks.i got claimed once only, the guy ran me with my own motor the next race and it lasted almost a full main before it let go he got my motor for 300 sure but my flywheel he didnt wanna buy for 1600 had to be on it to survive like it did for me never got claimed again lol

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    Senior Member Double A's Avatar
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    Quote Originally Posted by IMPATIENT 1 View Post
    Yep just like its easier to spin a small tire radius compared to larger ones.it takes inertia absorb to move a flywheel..sfi approved flexplate gets my vote as well but i wld talk to the builder and make sure rotating assembly wasnt balanced witj flywheel.if it was youre stuck with it.i used to race dirttrack modifieds with a claim rule for 300bucks.if i got claim i had tp pull my lil alky sbc and let the guy have it for 300bucks.i got claimed once only, the guy ran me with my own motor the next race and it lasted almost a full main before it let go he got my motor for 300 sure but my flywheel he didnt wanna buy for 1600 had to be on it to survive like it did for me never got claimed again lol
    Looks like I get to lighten it up with a flexplate. I never got to race cars of any kind. I bet a dirt track mod was a wild ride. I had a bud that raced stock cars for years then got into a modified on the asphalt and said it was a little hairy sometimes compared to his late model. He only ran it a couple seasons then switched back.

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    Member Toyboata's Avatar
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    Good word from the builder, neutral balance on the flywheel so if I want to shift over to a lighter weight Flexplate, can do with no problems. This does help with driveline installation, will not have any change any additional equipment, can use the same PTO adapter with a non modified 1310 drive line. Do not think on the long run this will have much effect on the total dyno number, anyone?
    By the way Jocko, the buider is right next door to you in Baldwin.

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    gn7
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    Quote Originally Posted by Toyboata View Post
    Good word from the builder, neutral balance on the flywheel so if I want to shift over to a lighter weight Flexplate, can do with no problems. This does help with driveline installation, will not have any change any additional equipment, can use the same PTO adapter with a non modified 1310 drive line.
    This isn't completely true. Flex plate PTO register of the OD of the crank flange the sticks thru the flexplate. Flywheel PTO register off the ID of the flywheel



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    GN7, your correct on the differences between the PTO's as they relate to Flexplate or Flywheel. The new engine came with a thick flywheel not the flexplate that I wanted but failed to make that clear to the builder (my bust). My concern was could I pull the flywheel and replace with a flexplate and reuse my current PTO and driveline. Having looked and measured with the new engine, I wasn't sure if I had to change all of the running gear from engine to pumpshaft.

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    BTC cardcarrying member sunkisst's Avatar
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    Default Toyboata

    Are you gonna have this thing together in time to bring it to Jacks ?

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    Flywheel mass will effect hp only during rpm transients. It takes hp to accelerate all of the rotating mass including the flywheel. At steady state operation, flywheel mass does not cost any hp. Inertial losses are more important in a race car than it is in a boat because cars must shift gears down the track and they have way more rotating mass than a boat (ie; tires and wheels). All this rotating mass must be accelerated every time the car shifts gears. In a boat, and especially in a jet boat, the engine reaches its peak running rpm (off idle to 90%) in less than a second and after that any small rpm changes will not cause any significant inertial losses (the last 10%). Most circle boats operate at mid rpm in the corners (less time in the corners) to max rpm down the straight (most of the lap time). Over this narrow rpm range the inertial losses would be small but not insignificant so if you run a competitive SS boat you would go after every last percent and a very light flywheel would help. It would also benefit all the high end drag boats since the whole idea is acceleration. I still think that the fastest drag boats reach 90% rpm before they reach the starting gates so the inertial losses have occurred before the race even begins.

    Inertial losses are not the only thing to consider when selecting the flywheel for a boat. I ran flex plates in lake boats and race boats for years and they all cracked sooner or later... In my own experience with a Comp Jet, I have found that a very light weight flywheel has a negative impact on engine life. This is caused by rough water operation (normal racing conditions) with the engine at max power and rpm. Since a jet must maintain suction with the water to keep the engine loaded it is a big problem when the boat breaks this suction at full song. This sends the rpm to the moon every time suction is broken which leads to piston to valve contact unless heroic efforts are taken for the engine to survive. Hence, I now run titanium valves, solid lifters and very high rate valve springs. I also run heaver (12 lb to 22 lb.) steel flywheels to arrest engine acceleration during these extreme transients.

    In my opinion, only the fastest drag and circle boats gain any (small) benefit from this and I'll bet that none of the fastest run flex plates but rather light weight steel or aluminum flywheels. The trade off with a flex plate is not worth the loss of reliability in my opinion, especially in a lake boat, yours may differ.
    Shirl Dickey
    CJ60

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    Quote Originally Posted by MrEracer View Post
    In my opinion, only the fastest drag and circle boats gain any (small) benefit from this and I'll bet that none of the fastest run flex plates but rather light weight steel or aluminum flywheels. The trade off with a flex plate is not worth the loss of reliability in my opinion, especially in a lake boat, yours may differ.
    Shirl Dickey
    CJ60
    shirl, don't bet. most people run sfi approved flex plates.

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    gn7
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    Quote Originally Posted by MrEracer View Post

    In my opinion, only the fastest drag and circle boats gain any (small) benefit from this and I'll bet that none of the fastest run flex plates but rather light weight steel or aluminum flywheels. The trade off with a flex plate is not worth the loss of reliability in my opinion, especially in a lake boat, yours may differ.
    Shirl Dickey
    CJ60
    If you are ever planning to run a v drive circle boat, you quickly learn that there is more to the weight of the wheel than acceleration. As much energy that it takes to spin the wheel up, its carrying when it comes to slowing it down. One trip into the turn with a 24 lb flywheel verses a lightened alum wheel will point that out to FAST. The weight the wheel will kill your time on BOTH sides of the turn.

    I agree, CHEAP SFI stamped flex plates are a disaster in any boat. There is nothing hooked to the plate to arrest the harmonics and ring like a bell in operation.
    There are only 2 flex plates I will consider running. The Maeziere billet, and the JW "The Wheel". EVERYTHING else is junk looking to crack. The stamping process puts stress the wheel forming the damn curves that are useless in boat wheel, and the hole ALL have a sharp ass edge on one side and radius on the other from the stamping. EVERY HOLE!!! They all act as stress risers if they are addressed.

    I prefer an aluminum wheel. But even the newest aluminum are failure prone because they are cut from bullet with the grain structure all wrong.
    If its cut from plate, the grain run across the wheel in one direction, and it cannot possibly right in all directions. If its cut from bar stock, the grain runs the thickness of the wheel, which is probably worse. I have seen the center tear of more than few billet aluminum wheels. That's why I run a PTO on the wheel instead of just bolting the POS to cranks. It clamps the wheel to the crank and helps support the center better.

    The VERY BEST aluminum wheels are the old forged pieces made by Wilcap, M/T, Weber etc in the days gone by. Re-enforced ribs on the backside, and vastly superior grain structure. When I see one on Ebay for the right price, I snap it up.



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    Last edited by gn7; 04-26-2014 at 09:15 AM.

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    Quote Originally Posted by gn7 View Post
    If you are ever planning to run a v drive circle boat, you quickly learn that there is more to the weight of the wheel than acceleration. As much energy that it takes to spin the wheel up, its carrying when it comes to slowing it down. One trip into the turn with a 24 lb flywheel verses a lightened alum wheel will point that out to FAST. The weight the wheel will kill your time on BOTH sides of the turn.

    I agree, CHEAP SFI stamped flex plates are a disaster in any boat. There is nothing hooked to the plate to arrest the harmonics and ring like a bell in operation.
    not disagreeing if you are talking about circle racing a v-drive. even if we're talking about circle racing a jet. but I know of several very fast jets that run flex plates. and I won't count my slowazzed 118mph jet in that bunch. even though I've been using a flex plate for over 20 years, which I did replace about 12 years ago with a new one, I wouldn't characterize it as "cheap".

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