Flywheel Replacement???

[Frodo]

Thanks.

Makes sense: reduce the weight of the flywheel, you lose some rotational momentum, so you're more likely to bog down the engine when starting out. Ditto when you disengage the clutch to shift---low mass, low momentum keeping things spinning, so it's gonna lose revs quickly.

My kid (a budding shade-tree mechanic) found an Excel spread sheat program that calculates HP gains with FW changes. He says he would need to know the weight (OEM and LWF) and the diameter of each as well. I know I'm being lazy, but anyone know these off hand?

[insite]

Originally Posted by Frodo

Thanks.

Makes sense: reduce the weight of the flywheel, you lose some rotational momentum, so you're more likely to bog down the engine when starting out. Ditto when you disengage the clutch to shift---low mass, low momentum keeping things spinning, so it's gonna lose revs quickly.

My kid (a budding shade-tree mechanic) found an Excel spread sheat program that calculates HP gains with FW changes. He says he would need to know the weight (OEM and LWF) and the diameter of each as well. I know I'm being lazy, but anyone know these off hand?

you actually need to know the moment of inertia of each FW along with the weight. i ran the calculations awhile back and found that a 2nd gear sweep from 4800 revs to 6800 revs was good for a reduction of 21HP parasitic loss (~21HP gain) if i remember right.

i'll try to find the measurements. i still have an OEM FW; i'll weight it & measure it this weekend.

[Jake Raby]

Engine dynos test at steady state... even those that do a "sweep" of the RPM range are still sampling at steady state, just very quickly. Some chassis dynos can measure inertia accurately and may show some differences with lighter weight components.

This is why HP gains that are present from lightening components are very difficult, if not impossible to measure on an engine dyno, but they do show up very clearly on a stop watch in lap times.

The lighter components increase throttle response which gets a car out of the hole and off of a corner faster, increasing trap speeds..

Lighter parts will always increase acceleration and make any car feel faster, in a street car they cost drive-ability as the engine revs down just as fast as it revs up and that cost gas mileage as the driver is using the acclerator more and more up and down with the reduction in rotating mass.

In most any case a car with a lightened rotating mass will be faster than it was with stock weight parts, despite what the dyno says. This is why we run small 4" clutches with a reduces diameter, lightweight flywheels and even whittle 4 pounds off the crankshaft of our E and F production race engines.

Most lightened flywheels are also "single mass" meaning the factory harmonic dampening effect of the dual mass set up is eliminated. The factory flywheels were also balanced with the engine at the time of assembly as an indexed assembly, changing the flywheel removes this absolute balance- not good.

I have a 996/X-51 engine here at my shop now, it has a broken crankshaft and I feel VERY certain that this was caused by the lightweight flywheel that was used on the engine that could not be indexed to that engine as it was installed after the fact. Often times changing a flywheel creates imbalance and then removing the dual mass eliminates harmonic dampening. Double negatives that cost this 996 customer a 3,500 buck crankshaft- (we are replacing it with our billet unit to eliminate future occurrences)

(FYI- We also have IMS retaining bolts that feature a different design to that of the Porsche units and they also guard against oil leaks, especially when coupled to a sealant that we live by) We make all these as low profile as possible for interchange with as many different components as possible.

[insite]

there were a lot of LWFs out there that weren't balanced well in the first place (fidanza comes to mind). the aascos are supposed to be balanced very well. one could always take it to a machine shop and have it checked prior to installation.

as for the dual mass flywheel? jake is right that just swapping to a single mass removes the elastomer damper from the engine / driveline interface. most cars use a sprung centered clutch disk rather than a sprung centered flywheel. when i installed my LWF, i had a spring centered clutch made for my by Spec. this protects the driveline / motor from excessive shocks.

one wierd problem some of the 997 GT3 guys have experienced, and i can't really figure out why, is that when they go to a LWF, their crankshaft pulley unscrews and essentially falls off the car. not a problem on the boxster.