Crankshaft Lightening
#31
Drifting
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.
#32
Addict
Rennlist Member
Rennlist Member
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.
#33
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.
Custom Cranks will show up soon with better Oiling and with counterweights more in line with modern ways.
#34
Rennlist Member
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.
#35
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?.............
Comments................ Special Tool, Chris White, John?.............
#36
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.
#37
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
thanks d
#38
Rennlist Member
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.
#39
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
thanks d
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.
#40
Drifting
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.
#41
Rennlist Member
Very interesting points Chris. Without giving up your crank manufacturer, is it a US company?
#43
Rennlist Member
Didn't think so...
#44
Three Wheelin'
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.
I like this statement:
Some customers write to ask us if we can “Balance their crank”- that’s worthless! We have found that most imbalances come from the other parts of the engine, especially flywheels and pressure plates. IT ALL MUST BE BALANCED AND INDEXED!
#45
Drifting
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.