Boat-tail/knife edge counter weights & rod throws Q's

[theastronaut]

I'm building a 2276cc and I want to reshape the counterweighs and rod throw arms for better aerodynamics and also to reduce the rotating mass. I'm going to be doing it myself and would like some ideas on how much material can be removed from the rod throw arms. It seems to me that there is alot of extra metal not used, probably since one forging is used to produce more than one size stroke. I'd like to remove this extra metal and then reshape the throws so that they have less aero drag when the engine is running. It's an 82mm CB Super Race Crank.







The last pic shows what I have in mind, the black shaded areas being completely removed and the silver shaded areas laid back to round off the overall shape.

To keep the crank somewhat balanced (atleast closer than doing it blindly) I'll weigh it (in grams), reshape one counterweight and then re-weigh it to see how much metal was removed. Then I'll remove the same amount off the opposite side counterweight. I'm also going to make templates to keep the same shape on all the counterweights as close as possible.

I'd really appreciate any thoughts on this before I bust out the grinder!

[sideshow]

Well Jake took 4lbs off a stock crankshaft for a rules limited race motor;
http://forums.aircooledtechnology.com/s ... php?t=1350

[Turbo_Manx_Maniac]

My friend has an 82mm Okrassa knife edged crank from his drag motor on the bench, I'll see if I can snap a pic of it. It looks like a piece of artwork.

[theastronaut]

Well Jake took 4lbs off a stock crankshaft for a rules limited race motor;
http://forums.aircooledtechnology.com/s ... php?t=1350

Thanks, I wasn't sure how much could be taken off and I'm sure for an engine like that Jake was pushing the limits. Since stock cranks aren't counterweighted and my crank has alot of extra material, I may be able to remove more weight than that from mine.

My friend has an 82mm Okrassa knife edged crank from his drag motor on the bench, I'll see if I can snap a pic of it. It looks like a piece of artwork.

Awesome, I'd definitly be interested in seeing those pics!

[theastronaut]

The scales came in today. They measure down to a tenth of an ounce. I heavily taped the journals and established a baseline- 21 pounds, 11.3 oz. Should have time to start the reshaping tomorrow night.

[Tony Z]

I think you are wasting your time... knife edging is a V8 trick... their crank is just above the oil level and the crank hits the oil occasionally, which is why the knife edge the crank, to allow it to slice through the oil without setting up a bad vibration. This doesnt happen in a VW

[Clatter]

-SchwEEt!!-

I'm really into this kind of stuff.
FREE power!

As long as your time doing this is fun.
Otherwise, if it's work, time is money.

What we gotta do is build another identical motor;
Then we can dyno one with the knife-edging, and one without, side by side.

One with the case mods, one without, etc.
That's the only way to know for sure.

There's a lot of oil flinging off the rod and main journals.
It gets whipped into a froth that the crank and rods have to cut through.
Like trying to run through deep snow..

Plus, removing material adjacent to the rod journal makes the counterweights more effective.
Just this alone could be beneficial, as the counterweights could more effectively offset the mass of the rod's big end,
And put less thrust loading on the main bearings.

Ain't no such thing as wasting time with making your motor smoother and more powerful...
You could be saving the entire thing, because, as we all know;
Stress cracks start at surface imperfections!

You GO, brother!

[theastronaut]

Thanks for the comments Clatter. I also posted this on thesamba and was met with much resistance to the idea. Just because its a street engine doesn't mean it can't benefit from such modifications.

Does anybody know of a shop that I can send the crank to get heat treated?

[Bugfuel]

we did this to 9000-10000rpm 1600dp race engines.
Everything was lightened to the max to get instant throttle response and reduced drag. Had to get the rotating parts moving quick. Cars were very light. The best powerband was high and narrow, trans was geared close, and the tallest gear would be something like the stock 3rd. They'd only occasionally see speeds over 60mph, but they get there quick
Careful balancing afterwards.
Try to work SLOW, to keep heat buildup to a minimum and let it warm up slowly, not rapidly in one small area.
I dont' know if it makes sense to pre-heat the crank in the oven, never thought about it until now.

[Clatter]

There are heat-treating outfits in most areas.
Working at a machine shop, we used to send parts to the local heat-treat place all the time.

After all of the work, and esp. heat-treat,
I'd be looking _very_ closely at the straightness of the crank.

Might be a good idea to have it re-ground the next size under,
Depending upon how straight it stayed, or not...

Back in the day, drag racing Harley Davidsons,
Everybody would polish their cases on the inside,
and the crank (flywheels) and rods.

This was because of what they found on the dyno;
More Power!

Those rough corners on that crank are a big Hydraulic E-Brake!

[Bruce2]

Does anybody know of a shop that I can send the crank to get heat treated?

What makes you think you need to re-heat treat it? If all you're doing is grinding the corners off, you won't get it even microscopically hot enough to affect the original heat treatment.

[theastronaut]

What makes you think you need to re-heat treat it? If all you're doing is grinding the corners off, you won't get it even microscopically hot enough to affect the original heat treatment.

I'll be taking the corners off of the counterweights but also reshaping the rod throws.

Not sure if this is correct but as I understand it, the purpose of heat-treating the crank is to make the surface harder so that the journals don't wear out as easily. I guess I don't understand how this makes the crank stronger, other than just making the outer "skin" harder.

I'm also thinking about using a carbide burr to remove the metal, as it seems this would keep the metal much cooler than grinding.

[Bruce2]

You don't need to worry about heat treating with what you're doing.

[theastronaut]

I finally got around to starting on the crank. Earlier this week I purchased a 20,000 rpm straight-shaft die grinder with the collet extended 3 inches. The extended nose makes a HUGE difference in how controllable the grinder is and how precisely I can shape the metal.

Since this is a leading edge, I tapered the sides down to a point and then rounded the point a little. This seems to be the most popular way of thinking when shaping the leading edge as its thought to more gently push the air/oil mist away from the counterweight. The trailing edges will be more steeply shaped into a point.

I used an oval shaped carbide burr to make the initial cut to get the rough shape, then a sanding drum with an 80 grit sleeve, then finished with a flap wheel. I'll see what yall think before I go ahead and do the rest of the counterweight edges. The crank also stayed way cooler than I had thought it would, almost cool enough to comfortably touch the area that was reshaped.

BTW, this cut took off .4 ounces

[Bugfuel]

looking good, I like it and the choice of tools for heat control.