Knife edging/balancing a crank is a bad idea? why?

[Rabbit on Roids]

ok, so in the last week, i havent heard it from one person, but many different people that knife edging and balancing a crank is a bad idea? how could that be possible? how could someone claim that it gives you less power? i could understand it yielding less torque if that was the only modification, but less horsepower? come on people...

so, would someone that knows what they are talking about please inform me as to why all of the sudden knife edging makes less power? cause i know lots of race engine builders that have almost all there cranks knifed and balanced.

what gives? i just want a clear, definitive answer, not just "it gives you less power"

[arb]

i could understand it yielding less torque if that was the only modification, but less horsepower? come on people...

Well, hp is torque over time - so if you believe less torque is possible, then for HP to remain the same, you'll have to increase the speed.

1 hp = 550 foot-pounds per second is approximately equivalent to 745.7 watts.

On the why, that depends. Are you changing pistons and connecting rods ? Your current crank's counter weights are for the VW piston/rod combo. If you simply have someone wack off lots of material without making the piston/rod package much lighter, you will actually be unbalancing your engine. They must be done in concert, and to do it right, you'll want to start with making all the pistons the same weight, next the rods, and the weight of the rod bearings need to go into the balancing too...

Here's an interesting sales write up on the topic:
http://www.foxperformanceengines.com/crank.htm

[subsonic]

x2

The components should be balanced as a rotating assembly.  That should include the parts that bolt onto the crank, internal and external.  If you run a fully balanced setup there should be no problem.  The engine will probably run very well when done.  It's just time and money.

[arb]

x2

The components should be balanced as a rotating assembly.  That should include the parts that bolt onto the crank, internal and external.  If you run a fully balanced setup there should be no problem.  The engine will probably run very well when done.  It's just time and money.

If you truly want a balanced engine, first blue print it (balancing all the moving parts to be equal as mentioned) then put it on a test stand with a temporary starter setup so the fly wheel is exposed, then take it to a small airport with an active Experimental Aircraft Association chapter and pay one of the members to dynamically balance both ends of your engine while running - on an aircraft you must have the engine and prop perfectly balanced or the engine will depart the airframe in flight. They have a computerized dynamic balancer to achieve this and it can be used on any engine.

[sdwarf36]

As said-if the counter weights are matched to the piston/rod, then removing weight from the crank will throw things off. 
 http://www.autozine.org/technical_school/engine/smooth2.htm  Heres a thumbnail of the basics.
 With a straight 4, just matching weights is all you need to do.
 I build "price no object" vintage racing Ford 289s everyday--along with other race engines. The "cheap" cranks run $2500. And none of them come knife edged. If we told the customers you cound gain 1hp-and it would cost $1k-we'd be doing it in a second. They are lightened--yes-but knife edged? nope. not enuff of a gain. The latest trend is oil shedding coatings. I've seen the inside of a couple of year out of date Nascar truck motors-they didnt have it either. (although my drag bike motors from years back had it done.)
Lightweight parts in no way lessen the torque of a motor.

[Runt]

 With a straight 4, just matching weights is all you need to do.

Do you mean matching weights of pistons to pistons and rods to rods, or of piston&rod to counterweight?  I can see both arguments, although I think that a reciprocating to rotating imbalance would still try to shake itself out of the car, and probably set up some ugly stresses in the crank.  If I'm wrong, please help me understand, I'd like to be convinced that it'd balance properly without having to pay a shop to spin the assembly.

[lord_verminaard]

My understanding, when it comes to all VW I4's, is that the piston/rod are not balanced to the crank, and the crank is not balanced to anything else.  Therefore, if you statically balance the piston/rod combo to themselves, (make them all the same weight) and balance the crank to itself (or knife-edge, or lighten, or whatever, as long as it's balanced) then that's all you need to do.  You can dynamically balance the whole rotating assembly but it is usually not necessary like it is with most domestic V-engines.  The crank pulley and flywheel are also balanced to themselves, not the crank, so again it is not necessary to balance the entire rotating assembly.

Feel free to peruse the multiple 1.8T build threads on the vortex- some of the most hardcore builds are not doing any additional balancing.


So back to the original question- Knife-edging and balancing a VW crank is GOOD.  SCCH does really good work with VW cranks: http://scch-heads.com/viewpart.php?id=39

I think some people think it's a bad idea because there are some engines where you cannot do it easily.  One reason why we are lucky to own VW's. 

Brendan

[Rabbit on Roids]

so basically its just an old school approach to lighten the crank and make less surface area for oil to stick to the crank? i have not seen many engines with a knifed crank. my dads hot rod 9000 rpm 327 has a knifed crank, but it was built back in the 70's

and who do i believe about the bottom end? are they balanced together like the first posters say? or are they balanced as separate pieces like brendan said? im kinda thinking that they are all separately balanced. VW engines dont run very smooth for a 4 banger.

[sdwarf36]

The Knife edging stuff only helps in very hi rpm situations-we're talking over 10k.

 Like Lord said-it is an internal balance motor-meaning all the parts themselves balance to a  "zero balance"-the flywheel is zero-the balencer is zero-etc. So if yo uswap a flywheel, you dont need to get everything redone.
 If you match the assembly weights-IE: rod+piston+pin from cylinder to cylinder-you'll be fine. My piston were off 2 grams from each other (yawn-not worth touching but did anyhow) and the crank was right on the money.
 Odds are if you just bolted it together--or you fussed + worried and brought it to a shop to check it all out-the only difference you'll feel is a lighter wallet.

[truckinwagen]

I dont have much experience with knife edging, but I will say this:

thankfully our engines are internally balanced(to the extent that they are balanced at all)
an inline four cylinder engine is not naturally balanced(inline six and boxer engines are)

at the low speeds our motors turn,(and all VW motors from our era for that matter) precise balancing is not critical, it may help a little, but our motors are not going to fly apart due to poor balance.

with that said, most VW motors are built with the pistons/rods balanced to each other, and the crank balanced to itself, not as a complete package(the SCCH knife edging does not take piston/rod weight into account for balancing)

my understanding of knife edged cranks is to reduce windage, and oil resistance on the crank, but again at the speeds we turn, I don't think either is a real issue(a windage tray will keep oil from getting caught up in the crank anyway, and is much cheaper)

I personally would put money elsewhere.

-Owen

[Rabbit on Roids]

i had never planned to do it, just wanted a concrete answer as to what it accomplishes. i didnt just want "it adds power" or "it robs power", details are nice. i want reasons, and i got them.

[VW Smokr]

Supposedly VW originally balanced their diesel engines' internals to a 5g tolerance. Can't hurt to get it to 1/2g.

J.R.
SoCal

[NintendoKD]

I will be doing this, sending it off to castillo's in socal.  This guy is straight old school from what I hear and really knows his stuff.  I'll let you know how it turns out.  I will be using custom rods, and likely custom pistons as well.  The balancing will be done to the rest of the components after the crank is done for maximum effectiveness.  The hope is lower rotating mass, therefore better torque, BHP, and faster/higher revvs with less work.  I will also be lightening the flywheel to a rather extreme degree as well   Most here are correct though the gains are probably not worth it, but I just gotta know, and like I said, I'll be sure to report my results here.

thanks,

Kevin

[sdwarf36]

Supposedly VW originally balanced their diesel engines' internals to a 5g tolerance. Can't hurt to get it to 1/2g.

J.R.
SoCal

5g sounds believable-our race spec is 1g. VW pistons weigh almost twice as much as a good 4" bore JE or Diamond piston.

[lord_verminaard]

Supposedly VW originally balanced their diesel engines' internals to a 5g tolerance. Can't hurt to get it to 1/2g.

J.R.
SoCal

5g sounds believable-our race spec is 1g. VW pistons weigh almost twice as much as a good 4" bore JE or Diamond piston.

Ya.  And VW rods are about twice as beefy as most other import engines of similar displacement.  I don't consider that a bad thing. 

Brendan