66mm non counterweight crank for a 2.5?

[Saint-Dave]

I need to know if a 66mm non counterweight crank is of any use in a performance engine. I am building a 2.5 shortstroke with 46 carbs, if this crank and associated components were all balanced as a unit would it do the job?
I dont think i would be going over 8000 revs, just for street use only.
Thanks Dave.

[304065]

Yes, racers use them all the time due to the lower mass- they beat the bearings more than counterweighted cranks, but given that race engines are torn down every season or so, it' s not an issue.

[kenikh]

Titanium rods and smaller SC journals mitigate the pounding effect somewhat, as well as Ti wristpins, as does coating the bearings. GenII GT3 rods are the ticket.

[chris_seven]

I have always worried about the fatigue life of a non-counterweighted crank.

I believe that the lack of conterweights makes the crank more susceptible to torsional vibrations caused by high order gas torques and these vibrations can eat up fatigue life.

It would be ineresting to try to look at the torsional vibrations but I guess that dyno's with this capability are quite hard to find.

I also don't understand why a 'lighter' unit with a lower polar moment of inertia will load the bearings more than a heavier unit. Surely the highest radial load will be associated with higher mass.

Using Ti rods will help to reduce the loads and will also help the inertia torques in the crank but these are lower order than the gas torques.

[Saint-Dave]

Thanks for all your replies, any idea on a price for a set of ti rods, and who supplies them?

[kenikh]

Originally Posted by chris_seven

I have always worried about the fatigue life of a non-counterweighted crank.

I believe that the lack of conterweights makes the crank more susceptible to torsional vibrations caused by high order gas torques and these vibrations can eat up fatigue life.

It would be ineresting to try to look at the torsional vibrations but I guess that dyno's with this capability are quite hard to find.

I also don't understand why a 'lighter' unit with a lower polar moment of inertia will load the bearings more than a heavier unit. Surely the highest radial load will be associated with higher mass.

Using Ti rods will help to reduce the loads and will also help the inertia torques in the crank but these are lower order than the gas torques.

It doesn't statically load the bearings more, but it does create more pounding load since it has less weight to counteract the rotational mass of the rods and pistons. This is the loading referred to.

[rvanderpyl]

I f you can afford TI rods, just get a CW crank

[Plavan]

We have used N-CW cranks on some of our race engines for almost 100 hours before rebuild. No problems. They are "zippy" motors with 7500-7700 RPM limiters