How Do Flywheels Affect Engine Performance

[Joe C.]

There is a thread below about BB 10.5" vs 11" clutches. These used 12.5" vs 14" flywheels. Different bellhousings and starters were required for each configuration.
My '65 327/365 had an incorrect 14" flywheel, which I changed to a proper 12.5" (?), with a 10.5" clutch/plate. Pirkle provided the starter, and Carlisle provided the flywheel and bell. Borg Warner provided the new clutch.
How does this affect engine performance? Is there a large difference in the rotating mass of these 2 flywheels, and how should that affect engine operation. What are some reasons that Chevy went to the 14" flywheel in 1966 (?)

Joe

[William C.]

Re: How Do Flywheels Affect Engine Performance

You have asked at least two or three questions, some of which only tnagentially involve the weight of the flywheel, but here goes. Firstly, the change to the larger flywheel in '66 was for the big-block engines onlt and corresponded with an increase in clutch disc diameter from 10.5 to eleven inches. This change was in response to the greater torque of the BB engine and was done primarily to increase durability, not for any great performance purpose. The small flywheel will not accomidate the bolt pattern of the clutch cover used with the 11 inch disc. That said, flywheel diameter, all other factors being constant, will affect clutch/Flywheel weight, and to a greater extent the rotating inertia of the flywheel. The general effect of this is that a higher rotating mass stores more energy to assist in moving a vehicle from a standing start. Especially useful with a heavy car or a high (low numerical) rear end ratio. The tradeoff is that the engine has to provide this energy and the tradeoff is that once moving, the accelleration of the car is reduced by the energy wasted in spinning the flywheel/clutch combination. A heavy flywheel car is easier to drive away from a standing start, but will accelerate at a slower rate (somewhat) once moving. Drag Racers spend a great deal of effort optimizing this tradeoff, but for street driving, it is not much of a concern once the driver is used to the combination in the car.

[Joe R.]

Re: How Do Flywheels Affect Engine Performance

Joe,

You have done the deed so now tell us how it effected the performance of your Vette when you changed the flywheel/clutch combination.

Your input could be valuable to other NCRS members thinking about doing the same thing.

Thanks,

JR

[Joe L.]

Re: How Do Flywheels Affect Engine Performance

Joe-----

In addition to what Bill described, although 14" flywheels were first used for C2s with the 1966 big block application, all C1s used a 14" flywheel (with a 10" clutch). Beginning in 1969 and continuing through 1981, virtually all Corvettes used a 14" flywheel. The only exceptions were 1969 L-88/ZL-1 and 1970-72 ZR-1.

Then, for 1984, Corvettes went back to a 12-3/4" flywheel and this continued through 1996, including the 1989-96 dual mass flywheel. For 1997, it was back to 14", again and it remains that way to this day.

[Clem Z.]

tall rear gears and flywheel weights

when GM went to tall rear gears for milage and emissions per mile they needed more mass in the flywheel to get the corvette moving from a dead stop. light flywheels and tall rear gears are hard on the clutch and with light flywheels come smaller clutches so it compounds thee problem.

[Jon #40768]

Re: smooth idle

I had a 60 big brake car years ago that had an aluminum flywheel. I dont know if that flywheel was factory (probably not), but that aluminum wheel had a very lopy idle with the dontov 30-30 cam. Nothing I have ever heard in a street car accelerated so quickly with the clutch in. Wonderful when tring to impress someone at a stoplight (I was only 17). A heavier wheel will smooth out the idle and the clutch engagement from a stop or between gears.

[Joe C.]

Re: How Do Flywheels Affect Engine Performance

JR:

I simply put the right parts back on an engine that was incorrectly dressed. Frankly, the engine was running so poorly when I first got it, because of carburetor, distributor, and valve spring problems(with the wrong starter/bellhousing/clutch/flywheel), that it is practically impossible to draw a comparison. After the engine rebuild, and fitting it with the right flywheel, clutch, bellhousing, and starter, the engine runs great, and loves to rev.

Bill Clupper's reply above, explains it perfectly. Makes sense that a wheel with larger rotational inertia would enable a better "holeshot", but would slightly impede high RPM "spool up". My car, with 365 SHP, close ratio, and 3.70 gear, would have benefitted from the heavier, but "wrong" wheel out of the hole. But, like I said, she loves to rev. Anyway, that's the way it was originally built.

[Joe C.]

Makes Sense Bill......Thanks *NM*

[Joe C.]

Re: How Do Flywheels Affect Engine Performance

Joe:

If you will indulge me here, and this is speculation. Can I infer from what Bill said above, and from what you say, that 265/283 C1's were weak in low end torque, and so benefitted from the heavier wheel. Since the 327 inherently develops more torque than the 283, engineering decided to use a lower mass wheel to enhance top end performance (horsepower).Legendary "super" ratmotors like the L88 and ZL1 could develop horsepower faster with the lower mass flywheel. ZR1 (LS6) Corvettes liked the lower mass wheel, because by this time, engines were starting to be eviscerated by emission controls, and the engineers used the low mass flywheel to assist the last solid lifter BB in revving freely.

[Joe C.]

P.S.

Thanks Joe. I didn't realize that so many design changes were made to flywheel configuration.

BTW: The speculation above, assumes "all other things being equal". It goes without saying that overall operating characteristics depends on, among other things: axle ratio, transmission gearing, camshaft profile, etc., etc.

[Mike McKown]

Maybe, but the Pass car 283's and 4/ 6 bangers

went to the small configuration flywheel in the same time frame as the 327's.

[Joe C.]

Re: Maybe, but the Pass car 283's and 4/ 6 bangers

Interesting. Well, that shoots my argument. It's fun to play engineer, but part of the game is watching out for the corporate bean counters.

[Joe L.]

Re: Maybe, but the Pass car 283's and 4/ 6 bangers

Joe-----

One other thing: the 70-72 ZR-1 performance package did not include a 454/LS-6. The ZR-1 package was based on the LT-1 small block. The ZR-2 package, available for 1971-only, included the LS-6 engine. All LS-6 engines with manual transmission were equipped with a 14" flywheel and dual disc clutch.

[Jon #40768]

Re: How Can Flywheels Affect Engine Performance

A heavy flywheel can smooth out an idle, help launch a car, and produce smoother shifts. But the amount that it helps the acceleration of a car is probably minimul. Lets think about it. The engine is not just turning the flywheel, it is turning transmission and rear end gears, which absorbs horsepower. AND IT IS ALSO PUSHING A 3,000 POUND CAR. How much differance can the flywheel weight make considering the losses in the tramsmission, rear end, and the effort required in accelerating a 3,000 pound car? Not much I would think. That is my 2 cents worth.

[bruce11495]

Re: How Can Flywheels Affect Engine Performance

Jon....I'll take your 2 cents on a bet anytime. But instead of a "drag race" lets go run a circle track like Charlotte. The car with the lighter flywheel will beat the heavy flywheel car, ALL things, including driver, being the same,almost every time. Now you ask, "How can that be?"
With the lighter wheel...the car accelerates out of the corner quicker and can go a lot deeper in to the corner before braking than the heavy wheel car. The engine will "spool" up, but will also "spool" down a lot quicker with less inertia pushing the car into the corner.
Most race cars,ie Winston Cup, Indy, Cart, are using small, 5.5-7.5" clutches and flywheels......
Want to go double or nothin'?????????