# knife edging and balancing the crank makes LESS power? how?



## Glegor (Mar 31, 2008)

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"


----------



## Prof315 (Jan 5, 2009)

*Re: knife edging and balancing the crank makes LESS power? how? (Glegor)*

Balancing the crank/reciprocating assembly is ALWAYS a good thing if it is done right. The engine wiil rev smoother and the bearings will last longer. Knife edging is done for weight and windage(oil control) and really only needed on full out race motors. Properly balancing and knife edging a crankshaft WILL give you a little more power (not less) but as the reciprocating assembly gets lighter the motor will get "peaky" It's easier to maintain rpms with some weight.


----------



## HAPPYnotEMO (Jul 29, 2004)

*Re: knife edging and balancing the crank makes LESS power? how? (Prof315)*

now that's a good reply http://****************.com/smile/emthup.gif


----------



## WaterWheels (Aug 14, 2005)

I believe some of the confussion, if you want to be civil, is the word which pops up all the time "RACE". As pointed out, in a race engine the benefit can be seen, but on a street engine the benefit either never shows or the car becomes not so driveable as one would wish for the oh so very slight gain. Kind of the same as using a lightened flywheel. People seem to wonder why their car likes to stall when pulling away when it didn't do it before , and all they did was change flywheels. Things like these really do not make any power by themselves, but free up what was already there by design and lost due to massproduction.
Small side not: People should try to avoid using terms like horsepower and torque as it can open a "Pandoria's Box". The discussions which can and often do follow might become very complex and technical as the terms are often either not really understood by themselves or as they relate to one another.


----------



## 4ePikanini (Aug 29, 2007)

losing weight loses torque.
IMHO torque makes a car fun to drive in town. I have driven petrol and diesel golfs with similar KW output but the diesel produces a lot more torque. For driving pleasure the diesel wins hands down every time by a large margin.
I am yet to hear of a TDi owner wishing he still had his petrol.
If you are racing at high revs then go for the lightened route.


_Modified by fourie_marius at 11:57 AM 4-7-2010_


----------



## Glegor (Mar 31, 2008)

*Re: (fourie_marius)*

lol, trying to tell me something i dont know? my first vw was a 81 diesel rabbit. when i got it, it was slow. now its turbo'd and REALLY fast. my 85 GTI wont even come close to touching it.
either way, back to main topic. it makes sense that its pretty much a race only thing. the only engines ive ever seen with a knifed crank are full out race engines. one being my dads 9000 rpm 327 small journal. its real peaky. it doesnt like low RPMs.


----------



## Prof315 (Jan 5, 2009)

*Re: (fourie_marius)*


_Quote, originally posted by *fourie_marius* »_losing weight loses torque.
_Modified by fourie_marius at 11:57 AM 4-7-2010_

I 'm sorry but I can't let this go. less weight means less INERTIA not less torque. Torque is a fuction of cylinder pressure and displacement. Diesels have lots of torque because of the high compression required to ignite the fuel. Big V8's have lots of torque because of their displacement. Turbo motors have lots of torque because of cylinder pressure (boost) and beacuse forced induction tricks an engine into thinking it is bigger than it really is (again boost). Diesel and turbo motors use heavy parts for durability and because inertia is your friend when you are heavily loaded (diesel) or need to keep the motor spooled up(turbo).


----------



## 4ePikanini (Aug 29, 2007)

I'm all for learning but explain to me why you lose torque when you lighten the flywheel and keep everything else standard?


----------



## Prof315 (Jan 5, 2009)

*Re: (fourie_marius)*


_Quote, originally posted by *fourie_marius* »_I'm all for learning but explain to me why you lose torque when you lighten the flywheel and keep everything else standard?

You don't. The torque curve simply moves to a higher rpm. You lose inertia which helps maintain torque across a wider rpm range. It takes more torque to move a heavy rotating assembly but it stays moving easier. A lighter rotating assembly spins up easier (it needs less torque to get it moving) but the lighter mass won't stay spinning as easily because of the lost inertia. Remember, torque is essentially a constant. for a given displacement and cylinder pressure and horsepower is a mathematical funtion of torque. (HP =TQ*RPM/5250)
That's why a 600cc motorcycle might only have 50 ft/lbs of torque but make 130+ hp at 14000 rpms.


----------



## JaseR (Apr 2, 2010)

*Re: (Prof315)*

Am I correct in saying that a knife edged crank would allow for more acceleration(well a motor that will go through the rpm a lot quicker, and higher)? But will drop the top end speed?


----------



## WaterWheels (Aug 14, 2005)

Yes and no. Knife Edged crankshafts are also balanced, it's a by-product you might say. That will aid in the "engine" being able to spin/accelerate quicker if allowed to (things can prevent that which are not the engines fault). Top speed, talking your car I assume, is a whole different set of calculations which the engine is only one player. Knife edging is one of those things that comes up when you are sitting around looking at your car and say to yourself "OK, I've put $10,000 into that motor, what can I do next to squeeze out another tiny bit?"


----------



## soontobe83 (Mar 31, 2009)

*Re: (WaterWheels)*

this sounds good. i want more info. something i have been wondering about but haven't got that far with a real full out build.


----------



## LT1M21Stingray (Sep 14, 2006)

*Re: knife edging and balancing the crank makes LESS power? how? (Prof315)*

Street motor. 300 bucks modification. Also has a lightened flywheel. Plenty of torque. Nothing peaky. Car is very driveable. http://****************.com/smile/emthup.gif


----------



## joedapper (Oct 6, 2009)

I really like the science of this thread. we need more discussions like this.


----------



## 88-ROCCO717 (Feb 23, 2010)

*Re: (joedapper)*

OK now its time to open the can of worms!!
I'm looking at a 1982 rabbit with the 1.7l petrol engine, it has a 4 speed trans.
I have a STOCK 88 scirocco16v, so I want the bunny for city driving.







WHAT WOULD YOU DO?








I don't need alot of crazy stuff, just a dependable, quick car.














http://****************.com/smile/emthup.gif


----------



## Glegor (Mar 31, 2008)

*Re: (88-ROCCO717)*

drop an ABA block in that bitch and boost the hell out of it. w/ the 16v head of course.


----------



## Scorp67 (Apr 30, 2009)

Am i correct in saying that taking too much off the crank can also be a bad thing if the rods and pistons are still left stock
The counter weighting effect of the crank is supposed to balence the inertia caused by the piston and rod moving up and down.
Taking lots off the crank might decrease overall inertia of the bottom end but the bearings are gonna start wearing quicker due to the extra forces imparted by the unbalenced weight of the rods and piston
Or have i got it all wrong?


----------



## secondgen (Jan 9, 2004)

*Re: knife edging and balancing the crank makes LESS power? how? (Mtl-Marc)*


_Quote, originally posted by *Mtl-Marc* »_









Motor details? Displacement etc? And what on earth is going on with that AFR!?


----------



## LT1M21Stingray (Sep 14, 2006)

*Re: knife edging and balancing the crank makes LESS power? how? (secondgen)*


_Quote, originally posted by *secondgen* »_
Motor details? Displacement etc? And what on earth is going on with that AFR!?










Problem with valve seals spraying oil into the combustion chamber above 3500 RPM.

2091cc 8V


----------



## ziddey (Apr 16, 2006)

*Re: knife edging and balancing the crank makes LESS power? how? (Mtl-Marc)*

http://forums.vwvortex.com/zerothread?id=4855427


----------

