Tom M'Guinn

My 968 crank was polished about a year ago in preparation for the then-pending assembly of my 3L turbo. As I gear up for another run at assembling this thing, I am thinking about getting the crank knife edged.

Interested in hearing pros and cons from those who've done it.

Also, who does the best knife edging work these days? I've seen some great and some terrible attempts, and would prefer to avoid the later. Thanks.

fast951

Hi Tom, For a street engine, it's not necessary. Use a lightwieght flywheel & clutch assembly.

RKD in OKC

I have the a lightweight 2.5 liter crank in my car that's been lightened and knife edged. It was on the car when I got it. I've been talking to LR about my 2.8L conversion and they were telling me that is where I can save about $900 on my rebuild. By NOT having a 3 liter crank knife edged and lightened.

Rlin

RKD, I'm thinking of knifedge the 3.0 Crank in my 2.8 litre track car. Are you saying it ain't worth the effort? Please clearify. Thanks

Kell

I got my knife edgeing done by Flatlander Racing for 600 total shipped both ways. Im not that impressed by it overall and am not sure if it was worth it or not (see sig) but in conjunction with the LR 5.75 lb flywheel and a balance shaft delete it does make for a very free reving motor. that was one thing i missed from my previous car(328is). Moving to the 951 with a 8.0:1 comp ratio left a little to desire in the rev departent.

If i could go back i probably would save the money for somthing else bit i guess its too late now.

sweanders

I have been trying to find info on how much a 3 litre crank weighs? Or a 2.5 litre if possible.

Sorry to ask this OT question but does anyone know how much a crank weighs?

333pg333

Is it true that a lightened flywheel and crank allow the engine to rev more freely but at the expense of torque? What sort of crank mods would be advisable for an over 400hp car that sees a fair bit of the track?

Ski

When I had a spare 2.5 crank shipped in, the UPS tag said 54 lbs.

Tom, make sure as you turn the new crank, that it does not hit any casting marks or imperfections during a full revolution. Here is what one person recently found after hearing a bit of small tick, which they heard with the car beside a building, then inspected further. 2.8 stroker



LR has a person who knife edges and balances, not sure on Gomes, and JME does not do it or recommend it to the best of my knowledge.

tedesco

I run a knife edged crank in my 2.5L as well and I would advise not to do it for a street car. Allthough I run lighter rods and all is balanced well there are more vibrations from the engine. It is true that when lighter the engine revs up better but all below the threshold where you can realy feel it. You notice it when no gear is engaged and you just step on the throttle but that is all about it. The main limiting factor that prevents the engine to rev up faster when driving is THE CAR`S wheight. The higher the engine power the smaller the difference. Better save your money for something else.

whakiewes

In detailed conversations with a reputable BMW engine builder of some 30 years(now retired), I questioned him about knife edging. In short, his answer was 10lbs of rotating mass removed from the engine was worth about 1% gain in power, so on a 250hp engine it would be worth 2.5hp. The cost to get it done right doesn't justify a 1% gain until you are searching for every pence of power possible. He put it in the catagory of engine mods such as titanium rods/retainers/valve springs/valves, solid lifter valve trains, etc... For someone that needs to see every possible HP it would be worth it, but as John and others stated, not really worth the cost.

Kell and I discussed in large the advantages and disadvatages prior to sending his crank to Flatlander. Although I am no expert, I would agree with him, even comming from my 32lb flywheel on my 968, that the 951 is a bit slow to rev. The otherside of the story is finding someone to turn and micropolish the crank was going to cost nearly the cost of what Flatlander charged. On top of that the only VERY GOOD machine shop in the area pretty much told us there was going to be atleast a 6-month wait.

Wes

fast951

While on this subject. Come January, I will have two 2.5L shaved/knife edged/cross drilled/polished/std cranks, intended for projects that never came to life due to time constraints. I will be selling each for $650, trying to recoup some of the $ back. Each crank is about 5# lighter than original which is ok for street or race use.
I also have a race crank (2.5L), it's 16-17# lighter than stock, not for street use. Over $1200 in machine work alone. Might be selling it as well, waiting on customer to make final decision.

Dr. Dynamics

So in basics, were better off spending the $$$ on a lightweight flywheel and cluth assbly...

Is the LR balance shaft delete worth it?

Tom M'Guinn

My 968 crank weighs in at about 64lbs on my bathroom scale. I'd be happy to conclude my scale weighs heavy though.

This crank is going in an 89 block (104mm bores). I assume the 3L crank fits with no clearance issue, unlike the rods on a stroker, but I guess its worth checking. Using Carrillo rods anyway, but interesting to see a counter weight rubbing. What was it hitting?

Tom M'Guinn

I had previously shied away from the lightweight flywheel idea because I did not want the clutch to be difficult to modulate. Anyone have any experience to share on that front?

Ski

It was hitting a small casting imperfection. The casting part was just removed from the upper section of the block, with a long deburring tool. Crank counter weight was just smoothed off. A bit of machinist dye showed no contact on rotation so it's going back together tomorrow.