Cranks and rpm limit

A Duratec 2.3 engine can safely be revved up to 7200 rpm with the standard valve springs. With uprated springs it the limiting factor is the crank and according to Cosworth it can be revved up to 7700 rpm with the standard crank. Above that, a very expensive steel crank is needed. (source)

Balanced crank and front pulley

Among other places, a thread on Pistonheads several people writes that the standard crank may be up for more than 7700 rpm. On my engine the power curve was still climbing when I hit the rev limiter and I would probably gain some top end power if I could raise the RPM limit to say 8000.

What kills a crank is not power or torque, it is RPM. The force on the crank is exponential to the rpm. (source missing) That's why forced induction engines can have very much power and still use the standard crank, as long the rpm limit is kept down. Apparently the forces are specially high during downshifting.

I wrote an email to a Ford Focus engine builder and asked about their view on the subject.

"For us we have a race car and we redlined it at 8500 with no problems ever. But the engine was balanced and we even balanced it with the flywheel and clutch to make sure it was perfect! It can be done but just take all precautions needed..."

I did a simulation in Optimum Lap and a 8000 rev limit would do about 0.5 seconds on Mantorp. Not much and I'm doubtful it is worth the extra risk and engine wear, but as my engine is in pieces I went to a engine workshop anyway to have my crank, pulley, flywheel and clutch balanced. It doesn't cost much and it would decrease the loads on the bearings even if I increase the rev limit or not. I also hope it would reduce some vibrations at high rpm making it easier to stay focused on the race track.

Balanced clutch and flywheel

The mechanic explained they put the crank in their machine and start rotating the crank and measuring the imbalance. After some adjustments by removing metal the balance "was zero" and he put on the flywheel and balanced again. Then the clutch and finally the front pulley. That way individual components can be replaced without the need to rebalance the crank. On V engines the weight of the crank counterweights depend on the weight of the rod and piston. That is not true for inline-4 engines as there is always the same weight going the opposite direction. (as long as the piston/rod assemblies are of equal weights, which they normally are if bought as a set)

More very informative reading about cranks here.