924Board.org

[RC]

Is anyone here running a lightened or at least a knife edged crank shaft?

Not after internet opinions here but would appreciate hearing from someone who has first hand experience? How much was removed from the counterweights? Balance, vibration issues? Performance gain?
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World`s quickest 924 2L slushbox

[gegge]

I am running a knife edge crank. The reason for this was because the rods I am using are so much lighter and I was aiming to get the internal balance right. Idle and revving is silk smooth. Bare in mind together with a wild cam and an aluminuim pressure plate. The weight of the pressure plate was way more important for performance than knife edging. Even with k-jet the throttleresponce is great. I think I got a short start-up movie somewhere...

I did remove some 1,5-2kg from the crank in a lathe and "polished" it with grinder afterwards. Finally balancing the crank.

I do have some sort of resonance at ~2000 rpm but the vibrations are not severe, rather close to stock. The reason could be that I didn´t balance the crank together with the flywheel and pressure plate.

This was two years ago.

_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[gegge]

A lot lighter and with a crank scraper:

An old but polished and balanced stock crank (17,5kg):

_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[RC]

Thank you gegge, appreciate your input.

I`ve searched for some more details of your build and see you did a lot of other mods at the same time and understand it`s difficult to attribute various aspects of the end result to specific items. Your reciprocating component`s weights are also very useful to me and a good guideline.

Like you, I also have a desire to attain an optimal balance of the crank to offset the reduced reciprocating weight. Have much lighter pistons & pins and although still using stock rods they are lightened, balanced, resized, shotpened & using the ARP bolts. Most of the weight removed was from the big end cap counterweight, ~50gms to individually balance the caps alone. Then end to end on a pivot jig.

Have the crank chucked up in my lathe but ATM am reluctant to spin it. Sure that OE manufacturer`s use both sophisticated modeling software and a heap of cast iron to arrive at their optimum reduction of harmonics and lower NVH levels take precedence over acceleration performance. After receiving conflicting advice from different crank "specialists", feel there`s no definitive formula really, except for experience, trial & error, and a degree of luck.

Will keep thinking this trough for a while longer, as IMHO the safest option ATM is to do nothing. I am always willing and happy to listen to the advice and opinions of a man who has actually been there and done it however. So if you can add anything more, am all ears. Cheers.
_________________
World`s quickest 924 2L slushbox

[gegge]

From a dynamic point of view I would like to have as light reciprocal weights/loads as possible - read rods/pistons. The crank could actually be relative heavy keeping the intertial moment not getting upset by combustions or other vibations. The big ends of the rods are a minor problem the way I see it. And for performance and a revhappy engine, a small and light flywheel. A good balance is all about keeping the center of the crank in center. Thats way minimizing all other weights are beneficial.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs