924Board.org

[gegge]

I would like som input regarding the 924 crank. It is a beutiful piece; forged and counterbalanced with dimension to handle serious power.

I am about to lighten the flywheel and pressureplate. But what about the crank? If you remove the counterweights, you destroy the balance? I will use the original heavy rods and VW pistons, maybe work some on the rods. But there is not much weight to remove, or is it?

No daily driver, I want as fast response as possible and maintain durability.
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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

[ideola]

There are definitely examples of knife edging on the 2.0L crank, a good craftsman should have no trouble taking material off and balancing the crank in the process. The theory that is always tossed about is that by removing mass from the lower end, you will affect bottom end torque, but it should definitely help with revving, both due to lighter weight and less drag. As long as you are OK with the low end compromise, I say go for it. The only reason I haven't done this to my two stroker cranks is because my applications will both be street driven, so maintaining or improving the low end feel of the car is important to me.
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erstwhile owner of just about every 924 variant ever made

[gegge]

Idle and getting away in first gear will be suffering in a street driven car - yes, ..and you have to learn shifting again, because it is a totaly new process. Same thing as with a light flywheel. A choise you have to make.

My concern is vibrations and durability. Look at one cylinder isolated. The weight of one piston and rod should be equal to the counterweights on the crank. If you remove parts of the counterweights, the stroke is out of balance.

BUT, does it make a difference? I guess that the crank will hold since it is forged and built to fit a tank. And the combustion itself will cause vibrations and inbalance anyway.
_________________
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

[ideola]

The stock rods are incredibly beefy for the size of motor, and I believe there is significant room for removal of material, particular on the bottom of the end cap, for both balancing and lightening the rods. Comparison of custom Crower vs. stock:


As for vibration, if the entire rotating assembly is internally balanced (i.e. each individual component), then vibration should not be any worse than on stock...if anything, it should be better. Since our cars don't have a vibration dampener to begin with, I seriously doubt that the stock crank does anything to help in this regard.

IMO, any knife edging should be done as part of the balancing process anyway. If you're concerned about the counterweight issue, it should be a simple matter of explaining your goals to the shop that will do your balancing, and having them accommodate your needs accordingly.
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erstwhile owner of just about every 924 variant ever made

[Cedric]

No, you dont loose low end torque by lightening. Its like gegge said, only when starting that the rotating mass helps.

the balance weigth on a straigth for should balance out the weigth of the large end tap (how do you call it the part where the big end of the rod is connected ?) and aproximate 2/3 of the rod wich is counted as rotating mass. The weigth that is moving up and down is out balanced by another cylinder. So a four cyl straigth four is totaly in balance (except for the second order mass forces which is why there are balance shafts on newer engines). So lightening should not be a problem as ideola said.

Take your parts to a good engine builder and i think he will know what to do. Dont forget that the rods must be balanced to..
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1980 924 Turbo
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