Is It worth Knife edging a Crank?

[slow car]

Is it worth the effort Knife edging a crank on a 3.2?
Also Boat tailing the block webs?
Spinning at 7000rpm 3.4 PMO GE60 cams 10.5;1
Thanks
some photos would be nice if possible?

[HawgRyder]

Mods like that depend on your wallet ...and the need to reduce foaming of oil.
Below 6000 RPM ... probably not worth it...above 6000 RPM maybe.
The basic idea is to make the crank as smooth as possible when rotating (knife edge) to reduce drag.
The boat-tailing of the case will allow easier (faster?) oil return to the sump so that it can be returned to the tank.
On some V8 engines it was almost mandatory to knife edge...the counterweights were so big and flat that they really mixed up the oil....and created a big problem in allowing the oil to unfoam (word?) back in the pan (wet sump).
In cases like that...a windage screen was mandatory as well.
It all boils down to what you can afford/need.
Bob

[jess p]

It is written that knife edging a crankshaft will net you 10 hp on a 911.

[Steve@Rennsport]

Originally Posted by slow car

Is it worth the effort Knife edging a crank on a 3.2?
Also Boat tailing the block webs?
Spinning at 7000rpm 3.4 PMO GE60 cams 10.5;1
Thanks
some photos would be nice if possible?

Not unless you plan on operation above 7300 RPM. For race engines that will see 8K+, its well worth all the effort.

GE60 cams are a modest profile in a 3.4 so your power curves are all done by 7K, therefore I would save your money on this one for a good exhaust.

[911pcars]

I was under the impression the purpose of boat-tailing the crankcase was to produce an aero-shape to reduce windage losses when the pistons accelerate toward BDC; also related to moon-shaping the cylinder bottoms to decrease pumping losses.

In that regard, there might be some benefit in attaching a vacuum pump to the crankcase at a threshold RPM to minimize crankcase pressure. High revving larger displacement engines use them to some effect. Maybe the crankcase vent system in a high-revving 911 is adequate.

Sherwood

[Flieger]

Or just use the GT3 RMS and let the oil pump do the sucking.

[911pcars]

Originally Posted by Flieger

Or just use the GT3 RMS and let the oil pump do the sucking.

RMS?

Otherwise, sucking ....... of air? That's kind of a no-no, bearing wise.

Sherwood

[Flieger]

The GT3 rear main seal installs from the inside and acts as a check valve to let pressure out but not in, so it creates a lower pressure in the crank case.

The oil pump is a gear design so it doesn't care what it is pumping. It pumps an air-oil foam already. That is why there is an air-oil separator and baffles in the oil tank, etc.

I mean air in the scavenge section. Air in the pressure section is bad.

[911pcars]

Originally Posted by Flieger

The GT3 rear main seal installs from the inside and acts as a check valve to let pressure out but not in, so it creates a lower pressure in the crank case.

The oil pump is a gear design so it doesn't care what it is pumping. It pumps an air-oil foam already. That is why there is an air-oil separator and baffles in the oil tank, etc.

I mean air in the scavenge section. Air in the pressure section is bad.

Interesting. Any tech data links related to this?

Thanks

[BURN-BROS]

IIRC, the oil pump isn't designed to draw the Vacuum. GT3's achieve vacuum via induction.

[Flieger]

I just realized I forgot about the breather tube. You'd need a valve in there to prevent the oil being sucked back in.

I am not sure what you mean by vacuum by induction.

I believe I first read about this in Excellence.

[BURN-BROS]

I have read that too. I just can't see how that pump could generate vacuum at idle.

[Flieger]

It's not going to be a perfect vacuum of course, and it probably takes some revs to get the pressure down, but the seal should hold the vacuum until the engine shuts down. At the very least the design of the RMS (as I understand it) should allow pressure waves created by the downgoing pistons to let air out without air coming back in, reducing the pressure.

Anyway, for my engine I'd rather keep all the Magnesium in my case that I can and put up with some extra pumping losses.

[PFM]

FWIW...

To knife edge is as Steve said maybe not a worth while investment if the RPMs are kept in check. That said if light weight pistons and pins will be used having some weight removed from the crank beyond a simple knife edge can show benefits much like a light weight flywheel but even more so. A light weight crank is very sought after in all classes of race motors. Come crank shops can carefully remove weight, knife edge and balance the crank to provide usable gains even at reduced RPMs.

[911pcars]

Originally Posted by Flieger

It's not going to be a perfect vacuum of course, and it probably takes some revs to get the pressure down, but the seal should hold the vacuum until the engine shuts down. At the very least the design of the RMS (as I understand it) should allow pressure waves created by the downgoing pistons to let air out without air coming back in, reducing the pressure.

Anyway, for my engine I'd rather keep all the Magnesium in my case that I can and put up with some extra pumping losses.

Not sure if you were pulling my leg. Internet discussions claim the GT3 RS RMS leaks more oil than creating vacuum for any performance benefit.
E.g. 2007 GT3 - Page 4

Sherwood

[chris_seven]

Windage loss in a machine is the power absorbed by a fluid surrounding the rotating parts and occurs as a result of the relative motion between these rotating components and the stationary parts.

The absorbed power must be supplied by the prime mover (in this case the engine) and is not converted into useful energy, the presence of windage loss decreases the overall efficiency of the machine.

Another undesirable characteristic of windage loss, and sometimes the most important, is that the power absorbed is converted into heat which increases the temperature of the rotor and hence the surrounding fluid.

The effects of windage are quite difficult to model but the power losses produced are influenced by the following variables:

• Diameter of the Rotor (Windage increases to the 3rd Power of the diameter)
• Rotational Speed (Windage increases to the 5th power of the rotational speed)
• The shape of the rotor (‘lumpy’ and angular rotors produce more windage than
smooth shapes)
• The size and shape of the gap between the rotary and stationary parts.
• The density and viscosity of the fluid surrounding the rotor.

Producing a smooth shaped rotor and reducing the turbulence in the fluid in the crankcase must help to minimise windage.

Reducing the volume of oil mist in the crankcase must also help significantly.

The argument must be at what engine speed are the losses significant and there isn’t too much published data other than for turbines and high speed gearboxes.

[E Sully]

I'll debate the merits of boattailing if Henry doesn't make me cry again!
Can you say Windage

[Flieger]

They leak more when they sit longer, and they leak more than a normal seal because they are backwards to allow for negative pressure in the crankcase. At least that is my understanding.

[911pcars]

Originally Posted by Flieger

They leak more when they sit longer, and they leak more than a normal seal because they are backwards to allow for negative pressure in the crankcase. At least that is my understanding.

Yeah. According to a post I read, PAG's response to the RMS oil leaks is for GT3 RS owners to use theirs as a daily driver. Great car meets weak response.

Sherwood

[gestalt1]

How about the non-counterweight crank vs the counter weighted version? I would think the non cw would have less windage. However most don't use the non cw crank above 7500rpm.