TheSamba.com :: View topic - Boat-tail/Knife edge crankshaft counterweighs & rod thro

Thu Jun 10, 2010 10:47 am

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!

Thu Jun 10, 2010 11:05 am

Interesting. I'd definitely get that thing balanced when you are done.

Thu Jun 10, 2010 11:10 am

In Mini Cooper circles, this is called 'wedging'.

http://www.aptfast.com/Flyer_HowTo_Dir/HowTo_Folder/Crank_Preparing/Crank_prep.htm

Thu Jun 10, 2010 11:27 am

miniman82 wrote: In Mini Cooper circles, this is called 'wedging'.

http://www.aptfast.com/Flyer_HowTo_Dir/HowTo_Folder/Crank_Preparing/Crank_prep.htm

I like that they mention "effective crank counterbalancing". Theres way more weight in the rod throws than there is in the counterweighs. I didn't mention it in my first post but part of my goal is to reduce the weight of the rod throws, so that the counterweigh's mass is closer to what the rod throw weight is. Seems like that would help with crank flex.

It'll definitly make its way to a balancing shop when I get that far with the build.

Any Ideas on how much is safe to remove from the rod throws?

Mon Jun 21, 2010 8:54 pm

any updates?

Tue Jun 22, 2010 4:17 am

I would think that you would have to understand your complete rod/ring combo, pin, + a couple grams worth of oil weight first. Then be sure you crank throw weight would not be less??

I send my stuff to a local machine shop, but I use jewelers scales to match everything first. Then the shop will match that bob weight on the crank and balance everything.

If I had the money, I'd rather leave the crank alone and go w/ a dry sump if your worried about crank drag that much.

Tue Jun 22, 2010 4:57 am

blackflag wrote: I send my stuff to a local machine shop, but I use jewelers scales to match everything first. Then the shop will match that bob weight on the crank and balance everything.

Your V8 is showing...

Tue Jun 22, 2010 5:03 am

miniman82 wrote: blackflag wrote: I send my stuff to a local machine shop, but I use jewelers scales to match everything first. Then the shop will match that bob weight on the crank and balance everything.

Your V8 is showing...

hehe.. you got me. I have been playing w/ v8's for the past couple years. mostly Ls1 stuff- but built a 421 SBC stroker motor in a 68 camaro.

Found out I had more fun working on vw's, so I'm back finishing my outlaw turbo car.

So I have to get back into VW thinking. But at the same time, a lot of the "theory" applies to smaller engines.

Tue Jun 22, 2010 5:56 am

midtravelmidengine wrote: any updates?

I am waiting on a friend to loan me his scales, should get them this week.

blackflag wrote: miniman82 wrote: blackflag wrote: I send my stuff to a local machine shop, but I use jewelers scales to match everything first. Then the shop will match that bob weight on the crank and balance everything.

Your V8 is showing...

hehe.. you got me. I have been playing w/ v8's for the past couple years. mostly Ls1 stuff- but built a 421 SBC stroker motor in a 68 camaro.

Found out I had more fun working on vw's, so I'm back finishing my outlaw turbo car.

So I have to get back into VW thinking. But at the same time, a lot of the "theory" applies to smaller engines.

VW cranks don't use bob weights when balancing; the rod throws and counterweights are all 180 degree apart from each other, so the weight on one side of the crank almost exactly equals the weight on the other side. Balancing just gets the weight of the counterweights and rod throws on one side matched to the other side, the weights of the rods and pistons have no effect on the crank's balance.

Tue Jun 22, 2010 7:14 am

Makes sense. I didn't think about the 180..

So then this makes it much easier for the home builder to balance everything. So the rod throw eight is the same.

That being said, then I could see that a more efficient crank design or grind to cut through the oil may pick up a few hp as long as it doesn't damage the integrity of the crank.

Tue Jun 22, 2010 8:15 am

blackflag wrote: Makes sense. I didn't think about the 180..

So then this makes it much easier for the home builder to balance everything. So the rod throw eight is the same.

The crank/flywheel/pressure plate/crank pully still need to be sent out for balancing. There's no way you can do it at home.

I'm keeping mine closer to balanced by weighing how much I remove from each counterweight edge and then removing the same amount from the opposite corresponding counterweight. That way it isn't as much work for the balancing shop.

The rods and piston/pins/rings can be balanced at home if you have scales that are accurate and have the correct resolution.

blackflag wrote: That being said, then I could see that a more efficient crank design or grind to cut through the oil may pick up a few hp as long as it doesn't damage the integrity of the crank.

Yep, that's my goal.

Tue Jun 22, 2010 10:32 am

I had a guy send me a link to a topic on Aircooled Technology's Forums. Jake took 4 pounds off a stock non-counterweighted 66mm Type 4 crank for use in a SCCA car. The engine runs between 6800 and 8500 rpm on the track and had 8 hours of dyno time. The total estimated run time was about 25 hours. The engine was torn down and had no signs of crank flex.

I should be able to take around the same weight off, maybe more because my crank has counterweights that will get reshaped.

Tue Jun 22, 2010 11:34 am

blackflag wrote: miniman82 wrote: blackflag wrote: I send my stuff to a local machine shop, but I use jewelers scales to match everything first. Then the shop will match that bob weight on the crank and balance everything.

Your V8 is showing...

hehe.. you got me. I have been playing w/ v8's for the past couple years. mostly Ls1 stuff- but built a 421 SBC stroker motor in a 68 camaro.

Found out I had more fun working on vw's, so I'm back finishing my outlaw turbo car.

So I have to get back into VW thinking. But at the same time, a lot of the "theory" applies to smaller engines.

Uh oh. In before "you can't read a Chevy book to build a VW."

Tue Jun 22, 2010 12:03 pm

Sigurd wrote: Uh oh. In before "you can't read a Chevy book to build a VW."

I've been waiting for that too! :lol:

Tue Jun 22, 2010 2:10 pm

Funny thing is, a lot of the things that apply to any engine (inluding a small block) are going to apply directly to a VW as well- after all, they are both internal combustion engines. People just like to throw that line out there because some of the things you can easily get away with on a watercooled engine with respect to power production, you cannot safely do with an air cooled one. Just because the medium with which you remove heat from the engine has changed does not mean the rules apply any differently between the 2 types, it just means you have to consider things differently is all.

Things like cam timing and duration, exhaust and intake systems, and clutches are all pretty much the same in the way they react, so the same kind of methodology applied to a small block will generally have the same effect with a VW. How often in both small blocks and VW's do you see the age old formula big cam+compression x 4-barrel=power? Obviously, the rules are exactly the same there.

But because of differences in things like head chamber shape, port flow, plug angle, bore/stroke ratio, and the efficiency of the cooling system, you cannot compare things like the ignition system (timing), compression, or the amount of time you can apply full power before bad things begin to happen. A VW will generally have a more efficient combustion chamber than an SBC will, so less timing is typically required to reach peak power. 30-32* is about the average for VW, whereas I've seen anything from 35-44* on a small block depending on what heads it has. Contrast that with bore size, and you'll see that the average V8 is able to have huge valves and ports, where we VW people are restricted somewhat by the ability of the case to hold a cylinder (94mm being about the norm). That means our choices are not as large when it comes to valve size.

Biggest difference is obviously the cooling system, so you need to take that into account when applying your methodology. If you truly understand that air is a less efficient conductor of heat than water, and what that means to an engine, you'll have won half the battle already.

Tue Jun 22, 2010 2:21 pm

Well said mini.

Tue Jun 22, 2010 2:59 pm

SRP1 wrote: Well said mini.
X2
I have books about building engines that aren't VW specific. I talk to engine builders that have never laid a wrench on a VW. I don't discount what they have to say because so much of what works to increase power on other engines also works on our air cooled engines. But when I do listen and read, I keep in mind the quirks that VW engines have and filter out the info that I know won't work in our application.

One of the biggest parts of a successful engine build is the amount of thought put into it. You can't blindly trust that something that works well for engine "A" will automaticly work on engine "B". Do some research and find out how things work. Find out how different parts work together, and how changing one thing can affect the function of other parts. Do this for every part in the engine; understand the function of the part and its affect on all the other parts. Understand how a modification to a part affects other parts and their intended funtion. Start with a goal in mind and choose parts that work together to end up where you want to be.

Tue Jun 22, 2010 5:49 pm

theastronaut wrote: One of the biggest parts of a successful engine build is the amount of thought put into it. You can't blindly trust that something that works well for engine "A" will automaticly work on engine "B".

A lot of people on this site would do well to to listen to this sage advice.

Like mightymouse, Terry Clyod, Jake Raby, Michael Fisher1, and a few others. They all have something to learn, just as I do. All but one of them eventually will, because one of them is very smart and the others are not. :wink:

Tue Jun 22, 2010 7:14 pm

I never build the same engine twice. I KNOW that generic statements can't be globally applied to multiple engines, that's why recommendations are virtually worthless in the world of modification.

Tue Jun 22, 2010 7:37 pm

As I said, some are destined for greatness. :wink:

Others are destined to circle the toilet....