Is knife edging the proper technique to reducing weight in a balanced fashion on a crank? I've heard the term used, and "seen" cars with knife-edged cranks, but wouldnt there be better gains in HP efficiency if one were to replicate the crank in a different metal? C'mon we are talking about a full rebuild here, if one is buying newer lighter pistons and rods, why not lightened/strengthen the crank?

I dont quite know how that would be done thou, just asking questions.

 

Adrial:

Is the perp drill hole made at BDC or TDC? I had mine cross drilled. That was before I had ever heard of perp drilling.

 

It is made such that oil will flow against the centripetal (inward) force.

In other words...at BDC it is pointing down.
And at TDC, pointing up.

I'm still going to change my rod bearings pretty often. That'll depend on how much time I have to get to the track though...

 

really? so you went with superlightweight components? how are the revvs? any RWHP gained from this? how does it sound? I am planning(HAHA) on doing a TON of stuff to the car including lightening many compnonets.
do you need to keep on the gas to keep the revvs up whils shifting? does it revv faster? any problems going up hills?

 

Will be dyno tuning Sept 4

Sounds mean as Heck

Revs fast

Hills pose no problems not quite sure what you mean by that ?

 

I heard that knife edging and lightened flywheels create a tendency for the 944 engine to stall when in neutral, the throttle is closed and the RPMs are returning to idle. For fuel economy reasons Porsche programmed the dme to shut off fuel under this condition and allow the rotational inertia keep the engine turning over. With the reduction of that mass the engine can stall if you don't keep your foot slightly on the pedal. Of course the chip programmers can compensate for this.

I have no first hand knowledge of this, it is only what I have heard.

 

The hole is on the outside of the throw at both BDC and TDC, which makes perfect sense. Thanks.

Adrial, when is your engine going to make some mechanical music?

 

Keith (cruise98),

Hopefully end of next weekend.

 

Less balance.....

 

Geo,

If the pistons/rods are lightened as well as the crank...I would think the entire assembly would still be balanced.

Am I missing something?

 

No, you got it exactly right. You have to lighten the upper assembly to be able to effectively lighten the counterweights. The counterweights are there to counter the weight of the rods and pistons.

You did it right.

But in Joe's case, not doing this to his car, knife edging is something I certainly wouldn't recommend.

 

I agree Geo, I don't think you should knifed the crank and nothing else, you should do the entire rotating assembly as I have, like I said above, no balance shafts and you would be hard pressed to tell the car was not stock. Well... other than the sound of HP and fast as heck

 

Does anyone have a ballpark price for drilling the necessary crank oil passage holes so that the oil is forced out, and balancing the crank (I was under the impression that this would be 1 or two holes) as .... no knife edging required. --Roy--

 

Most of the folks I have talked to stated that there car were using a far amount of oil before the #2 rod bearing went out. How many miles have been placed on cross drilled prep drilled cranks before a failure? How many failed? Are these car better maintained after this procedure? An interesting issue is that the 928 engine see a similar failure.

Good advice from Huntley for cars without crank mods:
" keep your oil temps down, limit your RPMs a bit (look at your HP peak and avoid revving past it), run a higher viscosity oil, avoid over filling, and keep the oil fresh."

 

#2 rod bearing fails from not enough oil being maintianed in the bearing.

Loose tolerances in the main and rod bearing means more oil leaks out of the bearing thus you need to have a greater oil supply to feed the bearing. I don't thing the problem is not enough oil from the one rod hole. I believe the problem is not enough oil in the crank itself to fill the ammount lost at the rod bearings.

Lets say there is fixed oil out volume from the pump.

That of course gets split between all the bearings. Well to get to the rods the oil must flow through the main bearings. Lets assume that 1000 CC/sec (guess number for example) volume of oil flow in the the main bearing. These are cross drilled already. Now some oil is lost at the main bearing. This is normal. I have no idea how much it is really lost, but for arqument say 400 cc/sec is lost at the main bearings under typical conditions on a fresh engine.

So this leaves what 600 cc/sec that flows in the crank to the rod bearings. This oil flows into the crank with two holes (cross drilled) and then through one crank passage to the single exit hole in the rod journal. Two holes really does not increase flow unless there is a restriction at the exit hole. If it is same daimeter all the way to the hole then there is no extra restiction at the exit. This probably the case since this passage would have been drilled out.

So two exit holes won't increase the flow any.

Now here we have this 600 cc/sec flow rate left over from the main bearings. Well Lets assume the rod loses 200 cc/sec. Therefore there is 400 cc/sec of reserve flow rate. So where does this go? Well it actually never flows out at all since the system is like garden hose spigot. There is a fixed water pressure behind the spigot and the water that comes out is the dependant on how "open" the spigot is.

So... in a normal setting a max flow rate of 1000 cc/sec is more than enough to supply bearings that leak oil at a combined 600 cc/sec.

Now what happens when you rev the motor faster? More oil is lost at each bearing and it heats-up more thus more oil is needed at the bearing surface to prevent metal to metal contact. Lets say 6000 RPM flow is a combined 650 cc/sec. Fine you still have 350 cc/sec left over.

When motors get old parts wear down and bearing clearacnes increase. These larger clearacnes leak more oil. So old main bearings may leak 500 cc/sec and old rods leak 300 cc/sec.
So that total is 800 cc/sec. Add on the high RPM use at the flow rate comes to 850 cc/sec. Well that is ok since you still have 1000 cc/sec. and have 150 cc left over.

Now the oil temp goes up alot and the oil pressure on the gauge drops a bit. So now the system may only be able to supply 950 cc/sec vs the standard 1000 cc/sec. Hmm still have 100 cc/sec reserve capacity

Add in a little foaming such that you have maybe 5% of the oil supplyied beign air and you are down to 50 cc/sec of reserve capacity. Then thow in a turn with low oil level and pick-up tube sucks some air down. Bang 10% (100cc/sec gone) less oil capacity again. Hmm now you are at 800 cc/sec when you need 850 cc/sec.

What happens here....

800 cc/sec of oil flows into the main bearing journal. 550 cc/sec is used there. that leaves 250 cc of oil to go the rod. Well then 250 cc/sec flows through the rod journal leaving it 100 cc/sec short.

Now if this happens for split second. You are probably fine. A little light wear. The problem comes in that this actually super heats the oil locally thus what is in there breaks down faster and flows away faster. This heats the surrounding metal this creating more heat. Thus you need more oil to flow out prevent damage. So to correct the probelm you then may need 400 cc/sec just to cool things down. Well if you get it you are fine. If not you are in big trouble as the bearing materil rubs on the crank spins, seizes on the crank and then the rod shoots out the side of the block.

Not fun. That extra hole did nothing. PS... the rotational forces do nothing, but help the oil flow out of the hole or any hole. Having two holes at the end does not increase flow unless that hole is a restriction. The restiction in the system is not in the hole, but in the bearing itself.