Looking to get feedback on what everybody is having success with on their stroker crank. I'm running a new stock evo balancer on my 100mm OEM crank and am afraid to turn it over 8krpm on this setup. Trying to avoid the dreaded #4 journal cracking. Its got an exedy twin on the other side if that matters.

 

The Fluidampr is a really good out-of-the-box solution that is made with a silicon fluid and is basically bolt-on and go. The ATI is probably the best, but it's press-fit and made with elastics that require some tuning to cover all of the harmonics of a 100mm 4G63.

 

Fluidamper , 100 mm Manley billet crank -2.3 and run Excedy twin disk HD

 

I just run stock on my 2.15L

 

I wouldn't run any stroke over 88mm without an aftermarket damper (assuming the car is a track car).

The #4 journal cracking is due to torsional vibrations. Crankshaft harmonic dampers are made specifically to counter out these forces. Unfortunately almost all OE dampers use rubber for the isolation component of the damper. As years go by and the mileage is put on the engine this rubber will crack and degrade and its ability to counter out these harmful vibrations is greatly reduced.

Luckily companies like Fluidampr make viscous dampers that have a service life that is many times longer than a rubber damper. It's a similar concept as a rubber isolator but uses a heavy silicone based fluid that that has an inner ring which shears through the fluid as the engine spins instead of a rubber bonded ring.

The other big advantage to viscous dampers besides their longer service life is usually they are able to cancel out a larger range of torsional vibrations. All things have what's called a resonance frequency, crankshafts are no exception.

OEMs do lots and lots and lots of testing to find where the resonant frequencies are for different engines (I get to do a bit of said testing for a living ) and once that resonant point is found the damper will be optimized to work right around that range. Some OEMs will actually have the frequency range that the damper works for printed on the damper. But there is more than one harmful area of frequencies. The OEM damper doesn't do much for the other less harmful frequencies, and that's where viscous dampers are nice because they can cancel out a wider range of harmful torsional vibrations due to their construction.

The one exception I'd voice against a viscous damper is that they tend to perform about the same as a stock damper at high RPM (9000+). If you're building a car that will regularly rev above 9k then I would grab an aftermarket rubber damper like an ATI. Aftermarket rubber dampers use a multitude of rubber bands to cancel out torsional vibrations.

They work better than OEM dampers and often have a slightly better range of operation but their big advantage is they can be torn apart and optimized for whatever rev range and components the engine may be using. Keep in mind that when you change the weight of any component on the rotating assembly you're changing where the natural frequency of the engine is. It is possible to make an OEM damper nearly useless if you change the weight enough, although that would have to be a big change.

For pretty much any car that will see a significant amount of track time it's absolutely work investing in an aftermarket damper.

FWIW, I think people have grown a bit too scared of the 100mm stuff and that's why there's this large push towards the 94mm stuff. But the 100mm stuff has its advantages like you don't need to rev it out super far which bodes well for other components in the engine such as the oil pump, the valve springs, and harmonic damper getting the most benefit.

 

Lately I've been putting the ATI on all the motors I've built. The idea of the press fit hub really sells it to me as doing a better job of getting the damper to do more work. The larger diameter does overdrive the accessories, so an STM alternator pulley and EvoSpec water pump pulley (both are underdrive pulleys to slow down the accessory) are a good idea.

But due to the press fit hub, its hard to install the ATI on an engine that isn't torn down. The hub needs to be a .001"-.00125" press fit on the crank snout, and the ATI instrucitons are explicit in saying this needs to be check and the hub honed to size if the fit is too tight. All 4 that I've done so far have needed the hub honed as they were .0025"-.003" fit out of the box.

So if the motor is already together, just go with the fluidampr for simple bolt on install in the car. If the motor is apart and going to the machine shop, have them balance the rotating assembly, and get an ATI damper and take them the hub to check the fit to the crank.