Custom Stroker Crank

[1bolt]

This is one of those things that gets repeated and people sooner or later all think the 4cw 258 crank is prone to harmonics. Just like the Renix block is stronger.

So pardon me if I put emphasis on this Lee Hurley said the 4.0 8cw crank has a harmonic, he said nothing about the 4cw 258 crank

I take it from the fact that he didn't mention the 4cw specifically that he didn't test it.

[Plechtan]

I guess that i am under the assumption that at high rpm a fully counter weighted crank is better than a crank that is not fully counter weighted. in the V8 world do they run performance engines with non fully counterweighed cranks?

[1bolt]

Maybe a good question, but the I6 is a pretty different beast, more main bearings for example, and rod journals aren't shared. I've heard the opinion that an I6 crank could be run completely without counter weights only a harmonic dampner, because the paired sets of pistons are inherently balanced, don't know how truely advisable that is, but at some point an engineer decided that the 12 cw's on a older 258 crank were tits on a bull and lopped 8 of them off. They only put 8 on the 4.0 crank who knows why... I bet someone around here does

My point is only that we don't know anything concrete about the 4cw and any harmonics.

[Plechtan]

Some of the information on the introduction of the 199 AMC motor states that the first year of production they used a 4 cw and then the following year changed to a 8 cw to "smooth out" the engine and idle. A heavier crank will help idle quality, as would a bigger heavier flywheel. Again does not prove anything one way or the other. It does show that the AMC engineers thought it was worth adding cost to the motor, they must have done it for some reason.

Looking at Ford , mopar and GM, i know that GM only used 12cw cranks on the I6 produced during the 60's and 70's, I don't know about Ford or Mopor. I never heard of any harmonic problems with the chevy I6 and they were drag raced quite a bit, and commonly revved to 7,500 rpm.