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  1. #1
    6S Moderator
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    LW Flywheels: Pros and Cons

    I am making this write up to explain the benefits, and downfalls of a lightweight flywheel... on both boosted, and n/a engines.

    Take it with a grain of salt, and if you find anything that I missed, or you feel is wrong, please correct me.


    For N/A engines:

    Pros: Engine revs quicker, allowing you to get off the line quicker, and accelerate quicker. Netting you faster times at the track.

    Cons: Engine braking is decreased due to lack of rotating mass to slow the engine. You have taken more than 1/2 the weight of the stock flywheel away, and replaced it with only 8lbs.



    For Boosted engines:

    Pros: Quicker launches, quicker spool, and quicker acceleration. The power trio right?

    Cons: When you use a lighter than stock flywheel on a boosted engine... you have to take into consideration a few things.
    • 1. How much boost are you planning to run? 5-10psi.. ok. Anymore, and I wouldn't use something like the fidanza.
      2. What condition is your stock flywheel in? Is it still within service limits? If Yes, have it re-surfaced, and lightened. If not, get a new one, and have it lightened. You can snag one at the parts store for about $65... as the salvage yard will charge you almost $50 for a used one... gay.


    Too light of a flywheel on a boosted engine can cause compressor surge. Which is when you disengage the clutch, and the engine slows down rapidly... which leaves your tubo that was at full boost without anything to support the compressed air in the intake stream... now all that air is surging back through the compressor... yes, that is bad.

    Click the images to see a larger version

    HTML Code:
    <body link=#FFFF00><body vlink=#99FF00><a href="http://www.sixthsphere.com/storage/990/62_1_b.jpg" rel="lightbox" title="This is your typical lightweight aluminum flywheel.... many people have it."><img src="http://www.sixthsphere.com/storage/990/62_1_bsmall.jpg"></</a>
    HTML Code:
    <a href="http://www.sixthsphere.com/storage/990/lightenstockfront.jpg" rel="lightbox" title="If you want to remove material from the front of the flywheel... you need a really good machinist to do it. This is typically where they would remove it, and it would be all hand work. It's expensive to do it this way."><img src="http://www.sixthsphere.com/storage/990/lightenstockfrontsmall.jpg"></a>
    HTML Code:
    <a href="http://www.sixthsphere.com/storage/990/lightenstockrear.jpg" rel="lightbox" title="This is where I would have them remove most of the weight...
    right on the back of the flywheel. 
    It's the safest route to go.
    Notice the white X's... those are where material 
    was removed for balancing. 
    You need it re-balanced 
    everytime you remove material."><img src="http://www.sixthsphere.com/storage/990/lightenstockrearsmall.jpg"></a>
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  2. #2
    Leafy's Avatar
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    Re: LW Flywheels: Pros and Cons

    shouldnt a blow off valve take care of the compressor surge?
    1994 SL2 HCE auto Head Gasket is still good, just had condensation in the crank case.
    02 Chevy Cavalier LSS 5 spd S/C
    one shitty car to another one
    fly wheel, shifter, pcm, and misc saturn parts for sale.

    WPI class of '12 ME
    WPI Engineers Paintball Team Captain

  3. #3
    6S Moderator S.Bretz's Avatar
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    Re: LW Flywheels: Pros and Cons

    A lighter flywheel HURTS launches due to less angular moment. I compensate, you need to rev higher, but it makes the lauch a little more tricky.

    I have run about 400hp on a fidanza. I have over 180K on it. I wouldn't limit them to 5-10 psi. What you do want to limit with an aluminum flywheel is the amount of clutch slippage. Aluminum expands at about twice the rate as steel. When that clutch slips the aluminum flywheel will heat up and put added stress ont he fylwheel bolts. Really bad thermal expansion can "pop" the heads of the flywheel bolts off.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

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    JRSTANGE's Avatar
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    Re: LW Flywheels: Pros and Cons

    interesting. i've been running one for the n/a days and boost on 8-10psi with no side effects. but, this is for sure good to know.

  5. #5
    Level 3 Post Op. Special Forces's Avatar
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    Re: LW Flywheels: Pros and Cons

    Quote Originally Posted by S.Bretz
    A lighter flywheel HURTS launches due to less angular moment. I compensate, you need to rev higher, but it makes the lauch a little more tricky.

    I have run about 400hp on a fidanza. I have over 180K on it. I wouldn't limit them to 5-10 psi. What you do want to limit with an aluminum flywheel is the amount of clutch slippage. Aluminum expands at about twice the rate as steel. When that clutch slips the aluminum flywheel will heat up and put added stress ont he fylwheel bolts. Really bad thermal expansion can "pop" the heads of the flywheel bolts off.
    But isn't the actual contact surface of the disc to flywheel steel on the Fidanzas? When I put the new insert in mine, it wasn't the same material as the flywheel itself and it was sold as "Steel Insert". Now granted there is heat transfer from the insert to the flywheel, but either way the heat is being generated at the steel plate. Does that still mean your talking about a heat problem in reference to the thermal expansion?
    //// Need valve bodies? Click here \\\\

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  6. #6
    6S Moderator S.Bretz's Avatar
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    Re: LW Flywheels: Pros and Cons

    Yes, I'm still talking about thermal expansion. The heat generated in the steel friction ring will transfer into the aluminum. The aluminum will then expand at about twice the rate as the steel.

    With a stock FW the heat isn't much of an issue since everything is expanding at the same rate.


    What I did to heat rid the heat from my alum FW is I applied a small amount of heat transfer compound between the steel friction ring and between the crank and fw. The idea is to try and dissipate some of the heat into the crank. The oil will cool the crank. Does it make much difference? IDK, but it doens't seem to be hurting anything.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  7. #7
    Jon2001sc2's Avatar
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    Re: LW Flywheels: Pros and Cons

    Quote Originally Posted by S.Bretz
    Yes, I'm still talking about thermal expansion. The heat generated in the steel friction ring will transfer into the aluminum. The aluminum will then expand at about twice the rate as the steel.

    With a stock FW the heat isn't much of an issue since everything is expanding at the same rate.


    What I did to heat rid the heat from my alum FW is I applied a small amount of heat transfer compound between the steel friction ring and between the crank and fw. The idea is to try and dissipate some of the heat into the crank. The oil will cool the crank. Does it make much difference? IDK, but it doens't seem to be hurting anything.
    never thought of trying to radiate the heat into the oil, I'm going to have to steal that idea for the next time the trans comes off.

    This is what I came up with granted its from a n/a road racing standpoint, I'll leave the boost heads to deal with the other side. A lot of this probably deals too much with ideal theoretical engine and transmission operation and might not be enough with street driving a lightened setup on a saturn. All my experience has been on saturn roadrace cars other than about 3000 street miles in a saturn
    The lighter the flywheel the harder it is to get going. When you release the clutch there isn't as much mass, (or enough in some cases), to accelerate the transmission and start pulling everything forward. This causes the need for increased revs on launch to make up for the loss of rotational inertia. After you get going then this problem goes away. After the car is moving then you want as light of a mass to spin up as possible. This is why you see flexplate mounted 5in and smaller diameter multi-disk clutches on race cars. Rather than dealing with that parasitic drag of a flywheel and large diameter clutch you just have to use much higher engine speeds and/or massive amounts of slipping to get going (your crew will kill you for the latter) then added to this problem are really long first gears. Think of it as trying to get rolling in second gear, or if you have more torque, third gear. These cars are only expected to start from a standstill leaving the grid and leaving from a pit stop. This works great on the track but in the real world of street driving clutch changes get expensive fast and the alternate of heavy slipping is having to ride the thin line between a stall and a huge burn out. Deputy Dan behind you is not going to accept your excuse of a light flywheel as why you left the light in a smoke show. So if its hard to get going and can lead to increased operation costs whats the benefit? Less rotational mass. What is good to launch is bad for accelerating. The reduced rotational inertia allows the engine to both accelerate and decelerate faster. The faster the motor accelerates so does the car. Now engine breaking will change but this is mainly dealing with the resistance to the crankshaft from the compression in the cylinders and not as much from weight of the flywheel. Finally the car will shift faster. Note the car, NOT that the Transmission will shift faster. The transmission has a physical limit to how fast the shafts will match speed and allow the gear change. The increase in speed is that the engine will reach the correct output rpm to match the input rpm of new gear being selected for a given vehicle speed at a faster rate. when you up shift the engine has to slowdown to match the next higher gear's speed. If these rpm's don't match, then you have to make up the difference by slipping the clutch to bring the speeds together. If the engine is allowed to do this faster, then the shift can take place faster, or with less clutch slip. For down shifting, (assuming you know how to drive properly and heal toe), you have to rev-match the engine to increase its speed to smoothly and quickly engage the next lower gear (for those of you who don't rev-match, then you will slip the clutch less). [If your noticing a trend here, clutch slipping is bad; it increases wear and produces all sorts of unneeded heat] There are some other issues such as idling that are effected with changes in the flywheel mass. But none of these issues are really going to be effected by using any currently, or formerly, available flywheel or clutch for a saturn.
    Jonathan
    Just a guy with a thing for tubeframes and motorswaps...
    Saturns, I have them, too many of them actually

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