aluminum flywheel worth it ?

[roverman]

Jensen Healy Hell Fire, 2,400 lbs/loaded. Turbo Supra 5spd. Theory says it should grab rpm's quicker ? Approx. 15 lbs with steel insert. I will probably build it, don't find one new. Thoughts? roverman.

[Dan B]

What engine?

[roverman]

Dan and clan, Mopar Magnum, 5.2L., Jensen Healey post. roverman.

[DC Townsend]

Can't speak to your specific combination but can say that comparing the stock steel flywheel to a Fridenza aluminum wheel on my 340HP Ford 302 there was a pronounced difference in how crisp the throttle response was. With the alloy flywheel it revved both up and down much more quickly than with the stock wheel. I know most of the flywheel suppliers are quick to tout the horsepower gains but I'm not so sure that's true based on my 'seat of the pants' measuring. While the motor revved quicker, I can't say I noticed any difference in hard acceleration although time between shifts was reduced.

[roverman]

David and clan, were this a pure drag car, where-in I could make pedal-to-metal shifts, a lighter flywheel, should make for "slower" times,(less stored energy/more rpm drop between shifts). With conventional syncro's, and more towards road-course work, slaloms/track days/etc., the lighter flywheel, should have merit. Cool the disk better ? Light car/light flywheel ? roverman.

[DC Townsend]

I think the gain in throttle response would be well worth it on a road course. Bottom line is it simply takes less effort to spin the alloy wheel. I could behind the "light/light flywheel" idea as well. FWIW, I bought my off a guy on e-bay who decided to go AOD with his car. Nice mid-line Fridenza for $150, NIB. With a stock steel 302 flywheel going for around $100-$125, didn't seem to be much a decision to go aluminum.

[Dan Jones]

A while back I ran the numbers for switching from an iron to aluminum
flywheel for my Triumph TR8.  The effect was surprisingly large.  There are
a couple of approaches to doing the math.  The more rigorous approach
is to calculate the polar moment of inertia for the two different flywheels,
adjust for the square of the overall gearing (transmission, final drive
and tires) and convert to an equivalent linear inertia.  The second
method (the one I chose) is to start with a known linear to rotational
equivalent and ratio from there.  The known relationship I used is a
solid disk rolling on its edge.  It has an effective inertia exactly
1.5 times what it would be if it wasn't rotating.  That means the
rotational component is 50% of the linear component.  Adjust for the
square in gearing and you have the answer.  I wrote a little Fortran
program to do the calculations.  I assumed a 12" diameter flywheel
which is the Buick/Rover diameter, less the ring gear.  The circumference
of a circle is the diameter multiplied by pi.  So if you roll the
flywheel along the ground it will move 37.7 linear inches per
revolution (= pi * 12).  A 205/50/15 has a diameter of approximately
23.1 inches.  My TR8's final drive ratio is 3.45:1 and first gear is
3.32:1 so one revolution of the flywheel results in the Triumph TR8
moving around 6.3 inches.  Ratio the squares and take half
((37.7/6.3)**2)/2 = 17.9.  So each pound removed from the flywheel
(equally across the face) is the same as about 18 pounds of weight
removed from the car when in first gear.  So if you remove ten pounds
from the flywheel (equally across the face), the result is equivalent
to removing 180 pounds of vehicle weight in first gear.  The effect
goes down for each higher gear, of course.   Removing weight farther
from the rotational axis has a more pronounced effect.  If the weight
is removed from the outside of the flywheel only, the effect is about
2.78 times as strong since a solid disk has a radius of gyration of
0.6 times the radius (1.0/0.6)**2 is 2.78).  2.78 * 180 is 500 lbs
equivalent weight reduction.  A non-trivial effect, particularly
in a lightweight car.  I ran the numbers a couple of ways to
illustrate.  For my TR8, assuming a 3.45:1 final drive ratio,
205/50/15 tires and LT77 gear ratios of:

 1st 3.32:1  
 2nd 2.09:1  
 3rd 1.40:1  
 4th 1.00:1  
 5th 0.83:1

along with flywheel weights of:

 stock flywheel - 32 lbs
 lightened steel - 22 lbs
 aluminum - 11 lbs

The engine in the TR8 is essentially a Buick 215 aluminum V8 from the
early 1960's.  The stock flywheels in those had a big ring around the
perimeter.  Lightening the flywheel by milling off the ring is similar
to removing the mass from the perimeter (from 32 to 22 lbs).  In the
numbers below, I didn't do it that way but a more accurate approach for
the aluminum flywheel would be to assume a reduction of 22 to 11 lbs
equally across the face and add that to the difference of the 32 to
22 lbs across the perimeter.  In any event, a lighter flywheel looks
like a good thing to do for performance.  Here are the numbers:

 32 to 22 lbs (across face assumption):
 1st 177.5 lbs
 2nd  70.3 lbs
 3rd  31.6 lbs
 4th  16.1 lbs
 5th  11.1 lbs

 32 to 22 lbs (perimeter reduction assumption):
 1st 493.4 lbs
 2nd 195.5 lbs
 3rd  87.7 lbs
 4th  44.8 lbs
 5th  30.8 lbs

 32 to 11 lbs (across face assumption):
 1st  372.7 lbs
 2nd  147.7 lbs
 3rd   66.3 lbs
 4th   33.8 lbs
 5th   23.3 lbs

 32 to 11 lbs (perimeter reduction assumption):
 1st  1036.1 lbs
 2nd   410.6 lbs
 3rd   184.2 lbs
 4th    94.0 lbs
 5th    64.8 lbs

Rotational inertia is mass multiplied by the distance from the
rotational axis (integrated over the surface).  The effect is
stronger farther away from the hub.  The best is from the
perimeter.  Equally across the face is less effective and near
the hub is the least effective.  In my example, dropping 21 lbs
from the perimeter is equivalent to over 1000 lbs reduction in
weight in first gear.  Dropping the same mass the face is equivalent
to 372.7 lbs.

Reducing the flywheel inertia does reduce the stored energy for
start from a stop.  Torque follows displacement.  Little engine
in big car with tall gearing needs more stored inertia at start.
Big engine in little car with short gearing can get away with
much less stored inertia.  On the street with a lighter flywheel,
you may need to use more RPM and clutch slip.  On the strip, you
may bog if you don't have enough excess torque at the rear tires
(more traction than engine/gearing).  Remember that HP is the
measure of how much potential torque you can have at the rear
tires via gearing.  If you have enough power to overcome your
traction, then a heavy flywheel is a loser.

Dan Jones

[roverman]

Dan and clan, seems like a more complete picture of M.xV.will include clutch cover and disk ? Partially explains why many pure race cars use small dia., multiple disk. I suspect shift times and syncronizing loads are reduced, as well. Greatly reduced, "explosion" risk. Kennedy Engineering makes alum. covers. Perhaps there are others. Thanks for science, roverman.

[DC Townsend]

If you're going with Ford power, Gordon Levy at Levy Racing has some great flywheel/clutch/bellhousing combinations that are race car specific. I think his bread and butter trade is the Cobra and Mustang racing scene but seems to be a wonderful source for go-fast parts for the Blue Oval crowd.

[roverman]

Update, If your putting the flywheel on a diet, don't forget a "light" ring gear, hung on that alum. flywheel. For instance, a 130 C, will weigh approx. double of a 130 T, only because the id. is 1" smaller, preventing a larger clutch bolt pattern to clear it, ie. a 10.5" clutch package. This set-up will likely go on a "stroker" motor, needing this, "Center Force" gripper. Slip tires not clutch, roverman.

[roverman]

Wow ! $380-$700, for alum. flywheel. Ring gear $23. New insert $44,(with screws). Satisfaction from making it-priceless.  roverman.

[roverman]

Update, 10.5" steel insert with screws/nuts = 520 grams+/-  . roverman

[MGB-FV8]

I have used an aluminum flywheel in three different cars and it seemed to have different results in each car; however, each one behaved the same at the start of each gear shift.  The surge after the shift seemed softer for a split second but the quicker engine revving attribute (from the aluminum flywheel) seemed to close the time gap in between.  The pros and cons are different with each car depending on engine built, vehicle weight, gearing, camshaft and a lot more.  My idea of horsepower gain is through combustion efficiency and savvy rotating mass application which will always yield better MPG.

On this 331 stroker built, since I'm using a light weight rotating mass, I feel that my aluminum flywheel may not cause as much "take off" lost on start because of the extra torque generated by the stroker kit.  I wish that someone would make an aluminum flywheel that would have the ability to have (removable) calibrating weights added in progression, something as simple as playing around with tire seizes.  As it stands, I have no way of telling if it will benefit the car since this is a new power-train with no history to compare it against itself.  I feel that the minimal advantages I could gain from it would be fuel mileage, engine braking deceleration, better clutch heat dissipation and I could easily replace the friction insert instead of losing material through the re-facing process.

I'm not that smart at doing complex calculations, therefore I'll take my chances with basic rules of thumb; sometimes it's beneficial and sometimes I wish that I could get my money back, most of us have been there.  Forums like this one are the heart beat for us gear-heads as we all look for proven engineering.  If I'm going to be the "guinea pig" for a major modification, I feel a camaraderie's obligation to share my findings with others.  It may open debates on my findings, but it's all for the best.

[roverman]

Jacques, Well put. I consider, this one, the best I know of. We are fortunate to have a skilled,"Science and Engineering Department" , by the name of Dan Jones. This alone, alows me to be less "pig" and more savy , Thanks Dan. Regarding "tuning weights", just so happens, one I'm building has that potential. Screws and nuts that secure the insert,(9.45" bc.). Nuts could be substitited for nut plates(weights), nested in a counterbore, on engine side. How much weight ?  roverman.