Aluminum flywheel?

[Ross Nicol]

Bill As a matter of interest apparently Porsche have made the weak link in the GT3 the clutch and have done so on purpose, so heavy handed drivers destroy the clutch before more expensive driveline bits.Maybe Steve could enlighten us on how many drivers get caught out by this.Yes I can attest to porsche power they are bloody hard to beat.
Ross

 

[Bill Bayard]

Ross,

Sounds like a good plan to me! I've smoked one clutch in

my 40 so far(at 1500 miles!), but my gearbox has held up

just fine.

And I thought you racers didn't care about that sort of

thing because you work with unlimited budgets! /ubbthreads/images/graemlins/wink.gif

I'll pose the question to Steve, and let you know.


Bill

 

[Chuck]

I guess I'd add that the ALMS cars are really built for hp and not torque. They adjust the gearing for the track and getting the acceleration curve where the engine is most responsive.

Now I'd also bet that Steve's advise was relative to his Porsche experience on street cars too. The lightweight clubsport clutch and flywheel packages were popular on Carrera 2's, RS America's and 993s. Unfortunately they caused stalling nightmares in all but a few cars and only a few chip manufacturers were able to fix through the idle speed regulator functions. Accordingly, most Porsche mechanics I know of are wary of lightwright clutch/flywheel setups for all but outright race cars.

When was the last time most Porsche had a carb on a stock vehicle? About 1972/3. CIS was slightly adjustable. MFI requires a PhD (and some really neat old tools that are good $ now). Most of the 80's cars used electronic injection that wasn't very adjustable without a new brain. Same for the 90's cars.

Most of the GT40's have plenty of torque (3.6 ltr vs 5.7 ltr) and the reciprocating mass is already much heavier than the weight on a 911/993/996. I'd guess the weight savings would always be beneficial for any form of driving in a GT40 and until you got to super lightweight reciprocating mass throughout, a simple adle adjustment will suffice for any stalling on free deceleration.

One other thing. In the late 70's and 80's lot of the lightweight flywheel setups proved to be shortlived due to manufacturing problems and material quality.

Chuck

 

[Ross Nicol]

Bill- My budget is definitely not unlimited, in fact if I didn't have the ability to work on my own car I couldn't afford to race.Actually it's funny we're talking about Porsche drivers as the unlimited budget stereotype generally fits these people.Thats why it's so much fun to beat them when you can.
Ross

 

[Robert Logan]

Inertia (I) is equal to Mass (m) times Radius of Gyration Squared (k2).

The units of Inertia are therefore kg.m2 (not per m2).

Obviously the mass is important as reducing the mass using an aluminium flywheel reduces the Inertia. I have assumed that the mass is lower and the purchase of the aluminium flywheel is not solely to do with "bragging rights" or as us uncooth Australians call it "wank factor".

The major component of the right side of the equasion is the Radius of Gyration. This is defined as the radius of an infinitaly thin disc that could replace the complete flywheel. If all the mass of a solid flywheel were able to be concentrated into this infinately thin disc than the radius would be 0.7 of the outside radius of the complete solid disc. 0.7 squared is down to 0.49 , less than a half. The 'k' is therefore the dominent factor of the equasion.

Inertia is defined as the opposition to Acceleration. It is the force that stops / reduces something from changing its velocity either upwards or downwards.

The real trick is therefore not reducing the mass of the flywheel but in fact reducing the radius of gyration. This is the reason for the ever decreasing size of clutches and therefore the increase in the number of plates (friction surface area must remain the same or even increase) that are common place in race cars today.

I agree with Ross and others above that you must be careful with road ability and we have done a lot of work to get that balance correct. There is nothing more frustrating than stalling a car at a set of traffic lights because the flywheel is "too light".

As a little foot note, this is a little more complicated than I have explained, all the rotating assemblies (harmonic balancer, crank, pistons and rods and the clutch assembly ) must be considered.

The advantages of slipping through the revs quickly and therefore accelerating the engine and therefore the car quickly are great but stalling - yuk!!!!!!

Best wishes,

Robert