I guess the point is that is not simple. Many have been successful and that is great, some have not been so fortunate. The dynamics are complicated. It really depends on many factors, amount of mass removed, PP, fly wheel, internals, etc. and I think that unless you have a good understanding of these factors and their contributions to harmonics it is best not to mess with with the crank a great deal. Just my 2 cents.

 

I pretty much agree with what Shawn has written. (My +2 was hurriedly put there before coming to work.) I can only go off my experience when my larger bore 3.1 motor apparently shook itself apart. In the end we put this down to crank whip / ugly harmonics. I had bought a crank many years ago and then had it sent to LR for medium lightening. It returned as a full knife version but I went ahead and used it anyway. My thinking was liberally dosed with naivety even more back then and I just presumed lighter = better. I had no concept of harmonics nor the damage they could produce. Same with Xdrilling...it has been presumed to be a known good mod to do but I've heard otherwise. Anyway, good luck to you and your motor. Not trying to spook for the sake of it. Just relaying my experience. I think for the OP money can be spent elsewhere more productively.

 

piston/rod weights balance eachother out (like cyls 1/2 and 3/4) because they are always opposed in motion.

the crank counterweights are to counter the weight of the other counterweights

heavier crank doesnt transmit vibrations as much and doesn't flex so easily along its length...what patrick refers to as "whip"

the crank weights don't have any relation to the piston/rod weights, but the balance shafts DO.

 

Yep
Fully lightened
KEP stage 1
930 disc
I had the motor completely balanced it idles as smooth as my Camry 1100 rpm with the Rouge Maf

 

I also have wondered if other parts of the car (if out of balance) can't reverberate back into the motor via the transmission...but doubt it.

 

Is that true? If the pistons and rods balance eachother, then why run counterweights at all and not just a bigger crank? Wouldn't you get a whopper of a bending moment in the middle of the crank when the 2 & 3 hit the apex?

Here's the formula I found for calculating the counter weight:

 

You don't build engines for a living do you?

 

Nope, but I'm looking forward to your next response.

 

^ lol love that statement

 

I have had a knifed crank, aluminum fly wheel, lighter after market rods, pistons etc. I did not get that big a drop in torque. I was basically running square at around 375 hp and 380 ft-lbs. All you are doing is decreasing your rotating mass for quicker revs. The down side is you are losing rotating mass, the extra rotating mass makes it easier to get the car going from a stop, i.e. it is harder to stall. For a street car this can add to the difficulty of daily driving. In a track car less so as you don't have as many dead starts.

 

Thanks to all of you. My car is slightly modified ( crank scaper, vitesse chip, vitesse stealth turbo, head porting). I'm planning a major rebuit due to oïl consumption and i want to do some more upgrade while i'm there. I'm litlle confused by you answers. My goal is to have a better car on the track and still drivable on the road. i was planning to knife edging and balancing the crank and lightning the stock flywheel.

 

I've not built a race engine for one of these cars but have followed the fortunes of those who have and know a few buddies who have experimented. My conclusion is that engine harmonics (vibration) is a serious issue with these high vol 4 cylinder motors and Porsche clearly put a lot of deign effort into managing this with crank design, balance shafts, front crank pulley and even the torque tube. My observation of others experiences is that meddling with any of these components leads to an increase likelihood of vibration induced failures.

I think the message from the majority of those who have responded is that for a fast road/DE car, crank lightning is not a great idea and that you would be better advised to putting $$ into ensuring the stock engine spec is up to scratch (rings, bearings, valves) and the same with suspension with judicious upgrades to suspension rubber, sways, shocks and springs to make it quicker round the track and still enjoyable and reliable on the road.

I have replaced/upgraded all the rubber bushings on my car and moved to Bilstein escort suspension with turbo cup car springs and 968CS swaybars and that has made a huge difference to the car ... so far I've resisted playing with the engine too much

 

It's a good advice, i think i will going that way, suspension upgrade was previous along with engine rebuilt.