Chris Prack

It's a lot cheaper than starting over after you build a $15k turbo motor only to have to start over after it pukes a rod through the side of the block. Of course you can spend a whole lot more than that and still only have the Porsche crank to use.

Vilhuer

Exactly. Makes one wonder how many "well build" engine rebuilds are actually shaking their internals to death because their builders thought they knew what they were doing.

m42racer

Chris, you speak wise word here. The amount of counterweight is designed as a percentage of the reciprocating and some of the rotating masses. Typically, street engines are upwards of 39%. in line 4 Cyl engines sometimes have more. The bigger the Bore and Piston weight the higher the %. Race engines lose alot of this weight with lighter internal componets. Ther are many vendors here that wish to sell something. Cross drilling a Crank Journal, Knife edging Crank throws just to name a couple. Makes good sales and looks good to the ignorant customer.

Custom Cranks will show up soon with better Oiling and with counterweights more in line with modern ways.

333pg333

So do we think it's possible to have one of these lightened or knifed stock cranks and be doing some damage to our motors, or is damage too harsh a word? What are the logical outcomes.

Douglas Sorrells

Sounds like Chris Prack is saying you can't have your crank - knife edged, and still have a reliable motor?

Comments................ Special Tool, Chris White, John?.............

special tool

I left my cranks on the heavy side just because everything else is light.
Despite this, the engine would STILL wind down too fast for MY TASTES.

That's why it was really nice when I got the big intake plenum - which allows more air at throttle closure and helps to keep the engine spinning between shifts.
It was refreshing.

Keep in mind also when you make your decisions, I don't have a real windage issue with the dry sump.

Douglas Sorrells

Thanks ST. That's what I've been wondering - if the engine feels more like a 2 stroke dirtbike with the crank lightened - quicker to spool up, but "wind's down" while decelerating in gear fast also? Downshifting the 951s while decelerating really slows the car down nicely without having to lay on the brakes. I'm wondering how that is effected as you lighten stuff up.

thanks d

333pg333

Well I had the knife crank and lite fly and I liked the response of the motor, specially heel and toeing downshifts. With the shorter final drive the motor would just zing up and down the gears.

special tool

Well the extreme case would be:
-full weight crank/fly/PP/rods/pistons
-dry sump
-large plenum.

I don't know if it would slow down 100 RPM's between shifts if you did it like this!

Conversely:
-lightened everything
-wet sump
-stock intake

Like this, the engine would all but die between shifts.

Choose a point in between to tune to your likings.

Chris Prack

Reliable? I don't know. What is important here is the factory balanced the engine with the OE pistons, pins and rods. Once you start making changes to the componets in the bottom end you are changing the balance of the engine unless the weight of the componets is very close to those being replaced. 100mm JE pistons are actually heavier (at least the ones I have used in a "stock" F or E engines) then the factory Mahle or KS pistons Porsche used. An example that I found, an engine with a two ounce imbalance static, at 2k rpm will have a 14.2lb imbalance, at 4k it will be 56.8lbs an at 8k rpm will be 227.2lbs. This is only two ounces static. What if it were worse? It is exponential, double the speed, quadruple the force. Can you check the balance of your knife edged crank at home? You are trusting the guy doing the work and unless you understand how to do the job and see the results then you are going by "blind faith". I personally hate that. I don't like dealing with things I cannot verify myself.

The imbalance in the engine in the example above at 4k rpm is 56lbs of weight being applied to the crank trying to bend it on it's axis. This leads to engine harmonics, possibley damaged main bearings or a broken crank. I don't think any of this occurs at this weight but what do I know. I have seen broken 944 cranks. I have seen broken 944 lightened/knifeedged cranks. This is why I have only used one. It is still running in a 951 engine I built 4 years ago.

What does all of this mean to me and you? I am not really sure. It is somewhat of an unknown. Maybe you get a proper knife edged crank that will work or maybe it's swinging 200+lbs of weight around it's center of gravity. I think there is less risk with the counter weights left alone. Can you lighten a crank too much? Sure, I had to have weight added to a crank in a 427 cu. in. small block Chevy that I had converted to an internal balance.

The weight of the crank in our 944's is 58lbs. stock. I want to know why. Why does it weight this much? What does Porsche know that we don't? I asked almost 10 crank people at PRI about this and noone could give me a reason that they felt confident about. They almost all agreed that it was very heavy for a 4 cylinder engine that used two balance shafts.

Like I said, I sent a stock crank to a manufacture that assured me they could make something lighter, stronger and have better oiling. Hopefully I will know something soon. I really hope they can tell me more about the crank in general.

333pg333

Very interesting points Chris. Without giving up your crank manufacturer, is it a US company?

Chris Prack

No.

333pg333

Didn't think so...

Waterguy

Link to dynamic balancing service for air-cooled Porsche engines: some good information. http://www.aircooledtechnology.com/d...-balancing.htm

I like this statement: Note the link applies to boxer engines, which are inherently balanced, and not inline 4's, which are inherently imbalanced.

Chris Prack

Horizontally opposed engines are at a balance advantage. The crankshafts require much less weight to keep them in balance due to the pistons/rods naturally counter each other running in the same plane.