Fully Counterweighted Cranks, Needed or not?

[unahanaretsu]

I've been reading around and a lot of people say that the half counterweighted crank for the ka destroys the harmonics above 6k. The thing is, people also say that there is no reason to weld on weights to make it fully counterweighted or buy an aftermarket crank because there really is no need. This is due to the fact that peak power output is around 5-6k anyways so you really dont have to rev higher. (the gt3 guys run a half counterweighted crank with just the crank snout motified) Can we have more discussion on this topic? I really would like to understand it more.

[Ajax]

There hasn't been much discussion because most people on here just say turbo it and if it breaks, buy another- they're cheap.
The real issue is that it's expensive to develop a proper fitting fully counter-weighted crank (someone here did a lot of legwork going through a chinese manufacturer- search archives?- but I think it was still pretty expensive). No one wants to spend the R&D money on the KA, because other engines are more profitable due to common swaps. Few would deny that the engine would rev out better if it was fully counterweighted, but due to other factors in the engine, the power is not immediately found from just the crank. Cams, manifolds, stroke, and tuning all play a factor in the powerband. I'll be in development of a custom header come winter in the hopes of increasing top end performance (my stroked, fully-counterweighted KA still drops power above 6k) without losing too much low end. Even with that, I'll likely need a better tune than my piggy-backed JWT ECU can deliver.

[corn322]

Needed? No.
Wanted? By some.

I'm sure it would help upper end horsepower, but if I wanted a high reving inline 4 I wouldn't start with a KA. I'd probably pick a Honda engine.

[Bigvinnie]

Depends on what the demand on torque and power you are asking for.
Half weighted cranks fall short on the torque band in the midrange power, usually after 6000RPM range there is a gradual drop in torque for half weighted cranks. Fully counter weighted cranks are more consistent in achieving linear torque bands up to 9000RPM before there is a significant drop in torque. This is also something that is acquired by how finely tuned the engine is built.
You can look at dyno graphs between rb25det neo's and rb26dett's. rb25's were fully counter weighted, and rb26's were half weighted cranks. In some cases most people not only purchase rb25's for the price but because the torque band is much more superior to the rb26dett for its displacement size.
As far as harmonics are concerned fully counterweighted cranks are more efficient for high rev tuning. Fully counter weighted cranks may weigh slightly more than half weighted, but it has more control an distribution of each piston from torsional stress than what half weighted cranks go through, this allows for finer tuning of ignition timing, and valve overlap at high rpm.
If you havent noticed KA's go through alot of bent valves when there is high rev in the 7000+ rpm range because the halfweighted crank throws upward piston momentum at different times in crank rotation. A fully counterweighted crank has evenly distributed mass to each piston to allow fine tuning of ignition timing, allowing less opportunity of piston timing to be thrown off in upward piston momentum. Fully counterweighted cranks have more accuracy at higher RPMS.
Fully counterweighted is just preferred over halfweigheted, if you want a race engine.
If you want a fully counterweighted crank brian crower sells them custom.