lightweight flywheel-do or don't

for a hot street car, 1962 tr4, autocross car, should i use a 10lb,20lb, or stock 20+ something pound flywheel?? 290deg cam,10:1 comp,dellorto's,headers, etc, typical lead foot. how many revs do you need to launch a 2200lb car with a 10lb flywheel and stock tr4a/tr6 clutch?
rob

For my hot street car (with similar mods) I have been told that by removing 25-33% of the flywheel weight is the appropriate amount.

If you buying from a catalog, and only have 10lb and 20lb to choose from, go with the 20lb'er. A super light flywheel will cause bad idling problems.

I've heard that lightening the flywheel can increase HP but at the expense of torque, is that true?

Flywheels are designed to keep turning the engine until the next cylinder fires. Lightening a flywheel reduces rotating weight, and allows an engine to build rpms quicker... and free's up HP wasted on spinning that heavy flywheel. It won't effect the HP or torque of the engine, but make it too light, and you have problems with idling. That's been my experience around the racing stuff. You have to modify the fuel, idle speed, and ignition at idle to keep 'em running with a super light flywheel. And they are way easier to stall on take off.

Kenny, when racing, the last thing I care about is the idle characteristics. When I hit the pedal, I want the engine to spool up NOW.
Jeff

Rob, What is your rear end ratio & are you running the stock transmission?

IMHO, your best bang for the buck is a LSD & good tyres. Also, tighten up that front end.

(or have you done that already)

For Auto-x its handling you want first..Power is secondary.

[ QUOTE ]
Kenny, when racing, the last thing I care about is the idle characteristics. When I hit the pedal, I want the engine to spool up NOW.
Jeff

[/ QUOTE ]

That li'l pump motor will DO that?!?!

Amazing. /ubbthreads/images/graemlins/jester.gif

He said "spool up"

Must be a sewing machine motor.

Of course there is a down side to lightening ... they can go from this;


to this;


This was in a lot of "spares" i purchased from a previous owner ... I think it was from shoddy installation and over-torqued bolts. I am using a lightened steel flywheel.

Mike

Yipes........

< covers eyes >

I put one of those 10 lb aluminum jobbies in the Elva.

Hate to have ta try putting it back on while driving.

...or gather up the pieces of yer legs... gah!

When looking at a flywheel weight alone does not tell you if the engine will spool up faster. The material and location (shape) of the weight is important. I don’t remember my physics that well, but there is a term called Radius of Gyration (general abbreviation WR2) which describes the inertia characteristics of a given shape, i.e. a solid disk or a hollow wheel. Each shape has a different formula for calculating WR2 but essentially W is the density of the material, and R is the radius (the differences in the formulas are essentially the volume calculation). The time required to change the speed of the rotating object is something like

time = (WR2 x Change in RPM) / (308 x torque)

for steel W = 490, for cast iron W = 450, and for aluminum W = 165

If we say that the engine torque is what it is and we want to rev the engine faster, then we need to make WR2 smaller. If you buy an aluminum flywheel with the same shape as a steel one WR2 is reduced because W is 165 instead of 490, if you cut down the outside radius on an existing steel flywheel WR2 goes down because R is smaller. Now we can’t really do that because there is the ring gear that we need to start the engine, but if we take away weight at the edge of the flywheel we will be helping. For example, on a spridget flywheel if the material forming the slight step between the pressure plate and the ring gear was removed it would have a greater impact than if an equivalent weight were removed by simply shaving the friction surface.

Sorry for the long post, I’m just trying to point out that if someone is planning on lightening a stock flywheel, the further from the rotating axis that the weight is reduced the more impact it will have on getting the engine to rev up quickly.

Hmmmmm... Even though this was just a clutch failure, I'd imagine a flywheel failure could do the same thing...


Don't they sell shield to save your legs in an event like this?

I dunno if they sell those shields for our cars, but fabricating a thicker firewall and trainy tunnel ought not be too difficult assuming you have all the tools necesary to build a racer. If I'm not mistaken, some race orginizations have a trainy tunnel thickness requirement. At least, I recall hearing about it... unfortunately I can't recall what type of racing has the requirement.... maybe drag racing?

Chris, thanks for the clarification on flywheel lightening. I currently run properly shaped aluminum flywheels, and have a steel spare that was lightened in the correct manner.
Scattershields are REQUIRED for most SCCA classes, and all other forms of road racing that I'm familiar with. The SCCA rule reads "Scattershields or explosion proof bell housings shall be required on all cars (except Showroom Stock, Touring, and Improved Touring) where the failure of the clutch or flywheel could create a hazard to the driver.
Minimum material specifications are:
.125" SAE 4130 alloy steel.
.250" mild steel plate
.250" aluminum alloy.
NHRA or SFI approved flexible shields.
I run a .375" 2024 aluminum shield in the plane of the clutch/flywheel rotation.
Oh, and Doc, the 948 wasn't a pump motor. That was the Coventry Climax, and they won't even let me think about it!
Jeff

Good thing it was on the starters side!

HeHe... 'k, Jeff. I'll try an' keep 'em straight.

Too bad it'd (the Coventry unit) be "recognisable" at a glance. You ~could~ consider an Elite for racing... ack! On second thought, noooo...

Scattershields are avilable but a scatter blanket (typically Kevlar/aramid blend) is easier to install and much lighter! Try drag racing outlets (like Summit) for products.

Modified drag racers at some point require a shield.