How many of you folks have a flywheel laying around?
What are they off of and how much do they weigh?
THERE IS A QUIZ AT THE END OF THIS RATHER LONG POST
Here is the deal. A lister sent me a private post of some of the things
I should do with C/GT while it is apart. One of the suggestions was to
lighten the flywheel. I hemmed and hawed about doing that for a couple
of reasons:
1.) I'm cheap and I don't know a good machinist.
2.) The Sept. '97 of Hot Rod mag just had an article on flywheels.
They say that the average street car and, also drag racers
don't particularly need a lightened flywheel.
The flywheel stores kinetic energy. That is, once that thing is spooled
up it'll take some effort to stop it. A heavy flywheel can help launch a
drag racer when the clutch is dropped. For street cars, a heavy flywheel
allows easy clutch engagement from a stop without excessive clutch
sliping. The example was Indy cars at pit stops. They don't have much
flywheel effect so they spin the tires leaving the pits to perserve the
clutch. So, a heavier flywheel enhances driveability off the line.
OTOH -- David Vizard in "Tuning BL's A-Series Engine" (c 1986-89 2nd Ed)
ISBN 0854294147, page 356 has a lengthy side bar on why lightening the
flywheel is a good idea. He says a light flywheel will have no effect on
power output or top speed but can greatly help acceleration (without
tire or clutch slipping) in the lower gears. According to him, engine
power works against weight in acceleration and a lighter flywheel can
help substantially.
The first couple of gears has the greatest torque multiplication
factors. He had a formula where you multply the first gear ratio and the
final drive ratio. That is divided into the vehicle weight to get an
estimate of power per pound the engine will see. To that you must add
the total weight of the flywheel.
For instance, my C/GT has a 2.98 first gear and 3.90 rear gears. The
product would be 11.62. The car weighs about 2,600 pounds. Dividing the
1st into the 2nd results in the engine "seeing" 225 pounds accelerating
out of first gear. With a 2.058 second gear, that would be 2,600 /
(2.058 * 3.90) or 325 pounds in second.
But then the weight of the flywheel is added. I weighed my flywheel and
it comes in at 22 pounds (plus 13 for the clutch housing) so that is
about 35 pounds not effected by the engine gearing:
1st gear: 225 pounds + 35 = 260
2nd gear: 325 pounds + 35 = 360
What can be deduced is that if you can take 10 pounds off the flywheel,
the engine will 'see' and accelrating weight equivalent to dropping 116
pounds off the entire car (10 * 1st gear * final drive). On modern cars,
there is something over 100 pounds of just sound deadening material.
So, for road racing, where the benefits would be greater acceleration in
the lower gears (like coming out of corners) a lightened flywheel would
seem to be a good deal.
Which makes me wonder how light is light? If you remove too much, the
car will require too much clutch slipping to take off. Also, the car
will stutter at low speeds. A 4-cylinder engine only has 2 firing pulses
per revolution. These pulses want to speed up the crank. Conversely, on
the compression stroke, the resistance will want to slow down the crank.
A heavier flywheel counteracts this problem. On my 6 cylinder, the
effects are not quite so evident (by half).
So again, how light is light? Well, I think the C/GT flywheel has
already been lightened. The forward side of the flywheel has been dished
out quite a bit. Here are the measurements:
Diameter with ring gear: 13.5 inches
Width at center mounting surface: 1.15 inches
Width at ring gear (outside) surface: 0.935 inches
Depth of dish cut from outer edge: 0.335 inches
Effective flywheel thickness at clutch surface: 0.600 inches
Weight of flywheel: 22 pounds
Vizard's book shows a lightened flywheel and all the work was done on
the front side of the flywheel. You can't remove material from the back
side of the flywheel as this is where the clutch mounts. Removing
material here would move the clutch further away from the release
bearing and cause clutch disengagement problems.
NOW BACK TO THE ORIGINAL QUESTION:
1.) Do any of you folks have a loose flywheel laying around?
2.) What car it is from, how much does it weigh?
3.) What does the surface look like on the front side?
4.) Has it been machined out to give a dished surface or is it flat?
ERRATA:
The Hot Rod article says an 'aftermarket steel billet' flywheel weights
in at 26 pounds. I'm assuming if you're buying from the aftermarket that
is a light flywheel. To get lighter, to say 10 pounds, an aluminum
flywheel is used with a steel face pinned to the clutch side.
They also say that for street cars with moderate gearing, a 28 to 35
pound flywheel works out okay. Of course, these are 3,500 pounds
American cars with V8s.
They showcase a drag racing Mustang with a 20 pound flywheel. They also
say that tests with a stock four-cylinder engine, removing just a pound
was disasterous -- the car wouldn't idle.
Sorry about taking up all this bandwidth on this serious stuff.
--
Bob Allen, Kansas City, '69CGT, '75TR6, '61Elva(?)
"However, I must agree with Bob Allen" -- Larry Unger, Sept. 15, 1997
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