Knife-edge a race crank?

[Nader]

In building a full race 1750 engine, how important is it to have the crank knife-edged? It has already been nitrided. The engine will have 12:1 pistons, Carillos, windage tray, etc. Thanks.

[jrclem]

Santo's Italian Car Service is currently doing a complete engine rebuild on my #33 two liter... The new engine will be identical to the last version (With ALL the bells and whistles) with the only exception being that the new crank (the old crank showed cracks developing in main #3) will be knife-edged. We will dyno the new set-up and see the exact difference (if any). I suspect a difference in "on track performance" due to a significant reduction in rotating mass (up to 7 pounds!) will be significant. The dyno result will likely reflect only a nominal change if any. That's one of the details of racing that only experience (i.e. old age) brings out... the dyno only tells part of the story. I'll report here both the objective and subjective results.

[jrclem]

BTW: If you decide to "Knife Edge" your crank you will have to stress relieve (shot-peen) and re-nitride the crank. Also, if its old, (and it IS) you should magnaflux it for cracks. Then check the line-bore of your block and plastic-gauge the main bearings...
oh well, you have the books with 99% of this info.

[PSk]

Quote:

Originally Posted by jrclem The dyno result will likely reflect only a nominal change if any.

[jrclem]

Indeed the proof is in testing (and racing)... I would point out that even F1 teams that recruit the very best test drivers/engineers, and employee amazing computer assisted analysis have too many variables to make the ultimate perfect choices. In the end it's just going on your gut ...
That's what separates the winners from the losers!

[FMG_V6_btb]

Hi, modifying the crank taking some weigth from it and giving knife-edged shape to the counterweights only will give you a little power increase (maybe 0.2 HP) by the fact of a better aerodinamycs of the crank with the oil fog inside the crankcase.

The benefits are three:

1- The inertia of the rotating masses (crank) is lower, so less torque is needed to rev up the engine when accelerating, instead this torque is sent to the transmission to accelerate the car.

2- The better aerodinamics of the crank inside the crankcase improve the lubricating conditions of the oil in the engine (less foam formation).

3- If in high g cornering the oil in the sump reaches the crank (not so probable), the knife-edge shape of the counterweights will made the power loss be lower.

Paco.

[PSk]

Pictures of F1 cranks I have seen show that they do not knife edge them ... thus the only benefit from doing this I see is the reduced mass.

And the damage to your engine caused by a crank hitting oil at high revs during cornering would be surely more than a few hp lost ... more like rod out the side???
Pete

[Nader]

Well, I won't be doing it. Being after the fact, it's too much cost, too much delay, too little in benefit. Maybe I'll save it for the 2nd or 3rd rebuild, unless I really take to this vintage racing scene and become competitive. Thanks for the advice.

[Gordon Raymond]

These are available for many racing applications. Building one from scratch is costly and not necessarily, cost effective. Given a choice of a crank with less rotational mass, and one with more, for a racing application, one would choose less mass. So... if available use it. If an expensive experiment, forget it. Just my opinion. Gordon Raymond

[James1750]

would a lightened drive shaft be another option???

[scole]

I just spent $900 to have a 2 liter crank lightened and knife edged plus a little magic dust sprinkled on it...it should really work

[velocedoc]

I am waiting to hear how your crank works in your two liters, as I have an engine build waiting for my knife edged crankshaft. The crank is .10 /.10 and balanced. There is a lightened flywheel that will go on it, so the crank should spin up nicely. Thank you for posting what the benefits are to using this type of crank.

[BlpltGTV]

I don't have a knife edged crank in the Alfa, but I do have one in the track car. I lost 5lbs of rotational weight over the stock one. I did notice the car was more willing to rev, but seeing as I was able to loose over 20lbs between the crank and flywheel, thats a given. I did have the flywheel on the car before I did the bottom end (I even had some stock rods that someone had taken 140 grams each off. All I can say about those is.... SCARY! I didn't use them.) I can only imagine how an Alfa engine would rev with that much weight cut out of the rotating mass. I do have to say it takes some getting use to with a light bottom end and flywheel, when you figure it out however, its all good.

Will

[FMG_V6_btb]

Some physics of angular (rotational) movement dynamics:

A = T / I

A = angular acceleration
T = torque
I = Inertia

For a given torque, the lower the inertia (of the rotating masses) the higher the angular acceleration (and the car acceleration).

So, any modification trying to take weight from that components is wellcome:

Pistons (are attached to the rotating masses), piston pin, rods, crankshaft, pulleys, flywheel, gearbox pinions, drive shafts, brake discs, rims, and wheels

Regarding rods, somebody knows a titanium rod manufacturer for Alfa engines? (especially for the V6)

Here a crankshaft designed by me 10 years ago:

[jrclem]

Here's the one going into my car this week: