8 or 4 Counterweights

[aristg]

Hello People,

seems that it has never been discussed before, and may be a more philosophical question rather than practical since the cost of such an upgrade is big.

So the question is about crankshafts and specifically if someone knows about the difference in performance between a 8 and a 4 counterweight setup. All the newer engines -such as B230- have 8 while the older such as B19/B21/B23 use 4.

By asking around in my place i have heard people mentioning that going from a 4 counterweight to a 8 setup may be an upgrade of lots of torque and horsepower. And they are not talking weight-wise but more about balancing, ease of rotation etc.

So to make the case a bit easier, how do 2 crankshafts of the SAME weight compare, one of 2 weights per piston (so 8 in total) and one of 4 (1 per piston).

I sent an email to Farndon @ UK who are well known for their performance products of this type, and for a crankshaft of 8 they want 1500GBP = over $2000, VAT and postage excluded !!

So .. is it theory time ?

 

[JohnMc]

There's a very small and tiny improvement in NVH, more specifically the 'V' part of that.

Each crank throw of a 4 weight crank is statically balanced just as well as an 8, but the weight is not symmetrical, it's off to the side. And an asymetrically balanced spinning object will attempt to align it's center of rotation around the center of balance.

In other words, there's a very slight buzziness to a 4 weight crank compared to an 8 weight crank, at high rpms.

4 cylinders can't be balanced properly anyhow, so it's not like anyone will be balancing wineglasses on the valve cover either way.

 

[JohnMc]

To go a step farther, you could statically balance an I4 cylinder engine perfectly by removing all the counterweights, completely. Each crank throw is balanced perfectly by another crank throw, 180 degrees off.

This could be balanced to a very fine degree with all 4 weights removed.

But when it is spinning quickly, the weights that counterbalance each other would be off not just by a matter of an inch or two (all the weight just to one side of the crank throw) but by about 4.5 inches or so, the distance from one crank throw to another.

It would be statically balanced, but a real mess dynamically. It would not like high rpm's at all.

 

[aristg]

Wow !! This is called physics knowledge Nice !!

So all is about NVH and not performance .. Just the 8-style may can go higher rpms.

 

[JohnMc]

Partially it's merely NVH, but on a performance front, all that vibration can cause an engine to fall apart sooner than it would otherwise. So revving higher and staying together is a performance enhancement. With normally aspirated engines, revving = more HP (as a gross generalization).

 

[burstcurse]

btcc cars ran huge counter weights and very light pistons to keep the motor together at sustained high rpm's

a stock vw audi 16v crank has 8 small weights look at the pic in the middle of this page the race motor has 4 huge weights

http://www.clubgti.com/showthread.p...ngine-specs-305-bhp-from-an-N-A-valver/page36