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selecting a flywheel

PostPosted: January 17th, 2009, 10:30 am
by grumpyvette
selecting a flywheel, can be rather difficult for newer guys as most of them want to get something like (THE RACERS USE) and that can be a BIG mistake, ESPECIALLY on a typical 3500 lb street car, and depending on the application, for a street car you'll rarely find that a steel flywheel in the 30lb-40 lb range is a bad choice, while a light weight aluminum flywheel almost always is the wrong choice on a street car.but ID sure suggest getting an SFI rated flywheel and a BLOW PROOF BELL HOUSING AND CLUTCH ASSEMBLY
keep in mind heavier BILLET STEEL flywheels absorb heat more effectively and are usually far easier to balance and less prone to warping or cracking, than either cast iron, which has a bad habit of cracking and flying apart at high stress and rpm levels or aluminum that tends to warp or have the surface mar under stress/heat loads.
you'll want to get the correct diam. (on a Chevy that's usually 168 tooth diam. but can occasionally be the smaller 153 tooth diam. and get a flywheel that's balanced (INTERNALLY OR EXTERNALLY) to match your application.
there's also one piece and two piece rear seals, and clutch bolt patterns matching 10".10.4", 11" clutches and both sbc and BBC designs, so shop carefully, and ask lots of questions.
IF YOUR THINKING ABOUT running a stock flex plate of flywheel, DON,T!, ESPECIALLY if the flex plate or flywheel weight looks poorly welded or loose... most stock components won,t hold up to racing stress levels forever, and you could easily have a clutch or flywheel self destruct at high rpms with catastrophic results.
buy a new SFI rated flywheel or flex plate, your chances of correctly welding it indexed and weighted correctly in relation to the cranks needs to balance the assembly are LOW at best, retaining your FEET is a higher priority, adding a blow proof Lakewood scatter shield or trans blanket would be a good idea, that weight could have removed your FEET if it came off at just the wrong time and angle at 6000rpm, Ive seen flywheels take out the dash, windshield and floor when they shatter on a high rpm shift ... /overview/
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be sure the flywheel mounting bolts extend fully thru the crank flange threads, after securely locking the flywheel to the crank flange, but don,t extend much further than the flange outer surface facing the block ... D=913.html ... wordSearch ... wordSearch ... wordSearch

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watch this (found on youtube) ... r_embedded

Re: selecting a flywheel

PostPosted: January 18th, 2009, 10:45 am
by grumpyvette
jack wrote:"Several guys on the Corvette Forum are running the light (16 lbs?) GM nodular iron flywheel on their big blocks. They all say they can't tell a great deal of difference between it and a 30 lb flywheel."

well thats a darn good reason to select the much more durable 30lb-36 lb steel billet flywheel, that stores a great deal more inertia allowing a smoother launch once you install slicks....keep in mind that a clutch and flywheel can only transfer engine power when the whole drive train is semi locked together, your spinning a 70-90 lb crank, damper rods clutch, plus a drive shaft axles rear gears wheels and slicks that are trying to force a 3000lb or heavier car or truck to accelerate, the few lbs saved in a light weight fly wheel sounds great reving the engine in neutral but it tends to reduce your ability to launch smoothly with slicks and aluminum fly wheels are much less durable. ... p-393.html ... toview=SKU ... Flywheels/ ... 0Flywheels ... t+flywheel

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viewtopic.php?f=71&t=584&p=757&hilit=bellhousing#p757 ... -flywheel/


if thats true, then it should be very obvious that the much stronger billet 30-36 lb flywheel with its sfi certification and far greater ability to absorb heat ,thats far less likely to fail under high stress and heat levels is the superior choice, keep in mind a flywheel is directly connected to a 10-15 lb damper and a 55lb-70lb crankshaft, another 20lbs-30lbs of pistons and rods, and a 15lb-20 lb pressure plate,and the only time the cars transferring power to the rear wheels is when the drive trains connected to the engine which adds a 2800lb-4000lb load to what the engine, is trying to move
so removing a few pounds from the flywheel may or may not prove to be a huge improvement
I look for and have found that American made billet fly wheels / clutches, in the 30-35 lb weight range with SFI certification numbers from HAYS,Centerforce , ZOOM, WEBER, all work reasonably well if you select the versions matching the application, get a good lakewood or other blow proof bell housing, you can,t replace feet, easily, but don,t over look your local clutch rebuilder, he may have deals that you can get if you ask.
All cast iron or cast steel fly wheels will eventually fail, its just a matter of the abuse,heat,time and rpm and LUCK!
Im sure a 15lb- 26 lb aluminum, fly wheel will work,and thats what a good deal of road racers and circle track guys will recommend, especially in a race car application, but Id have selected a 30lb-35 lb in a street car, it will make street driving a bit smoother, keep in mind the fly wheel inertia increases dramatically as the rpms increase so a race car engine operating at a much higher average rpm level can effectively use a lighter fly wheel, and if you spend all most all your time running the engine well above 4000rpm theres little reason for the heavier flywheel.
as always Id suggest calling a minimum of 3 vendors and talking to the tech guys to get a feel for customer support and get a few questions answered

IM always AMUSED at the guys with street cars weighting well over 3300 lbs that feel a light flywheel will allow the car to accelerate much faster, NEWS FLASH, the only time POWER transfers from the engine to the rear differential, where it effects the cars acceleration, is when the clutch is locked to the drive train, and your then applying the engines torque thru the gearing to move the whole cars weight and relieing on the tires traction, suspension, and the gearing, so a difference of lets say 20lbs in a 3300 lb car is like a 0.6% improvement at best, yes theres no doubt the engine revs faster in neutral, but what does that gain you??
the heavier flywheel tends to resist changes in rpm just a small bit better so the inertia gained is not lost as fast during gear changes, its not a huge increase , the heavier flywheel tends to reduce the tendency to bog on hard launches also, not a big gain but noticeable, but the main gain is your far less likely to scatter engine parts with rapid changes in rpm levels during shifts when you suddenly drastically change the stress and heat levels, both due to the billet flywheels strength and weight and the tendency to slow the rapid rate of those rpm changes compared to a light nodular iron flywheel.
the smooth flow of power tends to be easier on parts breakage rates
yes theres a slight advantage in the rate the engine gains rpms,with alighter flywheel, but its also a fact thats more necessary because the engine tends to loose more rpms due to lower inertia during shifts, inertia thats not lost as fast and energy thats transfered to the tires with the heavier flywheel, during shifts between gears.
practical experience,
I tried a 20 lb aluminum, a 30 lb steel, a 35 lb steel, and a 40 lb steel flywheel on my 1968 vette race car with a 13.7:1 cpr full roller big block 496 at various times (AT THE TIME I HAD A LARGE SELECTION, as most of my friends had BBC parts and we swapped and traded constantly.)
the 36, and 40 lb provided the best ET and 60ft times consistently, and the engine tended to idle smoother, so I selected the 40 lb for my best combo, ID launch at about 2000rpm the engine ran up to about 6500rpm where ID shift and the engine tended to drop rpms very reluctantly once it was in the peak power band near 6000rpm, in fact the tires tended to scream in protest on shifts
now IVE also built several STOCK CAR ENGINES and in that application a aluminum flywheel makes sense., the cars far lighter and you need to accelerate out of the corners faster without shifting gears.
heat, wear, age , how fast the stress is applied and previous service stress, the quality of the individual components used the care taken during installation, even the bolts used and the spring pressure on the clutch plate can have as much effect as, just rpm alone.
Ive seen clutches and flywheels fail at well under 6000rpm, bolts can shear, cranks can fracture,throw-out bearings can come apart, theres almost a limitless list of odd stuff that could happen,
theres little chance a billet flywheel will shatter like a cast one can, but a blow proof bell housing is designed to limit the damage that results, in any case, having the spinning mass encased in a 1/4" off steel plate vs 3/16" of cast aluminum when it comes apart suddenly makes a big difference to your feet that are inches away.
once you've seen a windshield or dash cut into chunks by fast exiting components from a clutch or flywheel explosion you get a different perspective.

one fairly common questions I get is about if swapping from a 35lb steel flywheel to a lighter 20lb aluminum flywheel, will help the acceleration in a 3500 lb muscle car used on the street,(IT WON,T), think about it, yes if you rev the engine in neutral it will spin up a bit quicker, but your NEVER in neutral when applying power to the rear wheels, and while the engines trying to accelerate 3500 lbs, the inertia of a 35 lb steel flywheel tends to retain energy, smooth shifts and store energy that's used in launching the car, while an aluminum flywheel might allow the car to accelerate marginally faster the trade off is seldom in the lighter flywheels favor once the et is compared.
now put that same flywheel swap into a 2300 lb circle track car that shifts constantly coming into and out of corners, and uses the engine compression to brake the car coming into corners on a short track,and blasts out of those corners at full throttle, and then yes the aluminum flywheel and light weight rotating assembly has a purpose.
heavy flywheels absorb, and dissipate heat better,they make the engine idle smoother they tend to make the car accelerate smoother and launch from a standing start easier, lighter flywheels allow the engine to accelerate slightly faster, or change engine rpms during gear changes a bit faster but remember the cars only getting power transferred to the rear wheels from the engine while the clutch is fully engaged.............think about that, youve got a 60-80 lb rotating assembly, including the crank, damper, rods pistons etc, spinning 1000-6000rpm and your debating reducing the effective weight 10-19 lbs with a lighter weight fly wheel , thats less durable, and its only advantage will be when its loaded against a clutch, and dragging a 3000-4000lb car along with it., on the street....the lure of light weight fly wheels comes from circle track racers who, need to use engine compression to slow the car coming into tight corners and need to blast out of those corners under wide open throttle conditions, there engines rarely fall below 4000rpm, under those conditions in a 1800lb-2500lb car it makes some sense, on a street car its a bad idea