Hi All,

So I have done quite a bit of flywheel and torsional dampener design work, and thought that some of what I have learned may be worth sharing on here for those of you looking at a lightweight flywheel and/or crank damper replacement.

Most of this will be focused around the stock crank damper. The stock evo damper (at least on my IX) is a basic rubber element damper. Basically this is a pulley attached to the crankshaft which has a rubber ring pressed or bonded to it, and a heavy inertia ring pressed / bonded onto the outside of that. So why would Mitsubishi stick a heavy chunk of metal on the crankshaft on purpose? It is to protect the crankshaft from torsional vibrations. The evo cranks are cast instead of forged, and as such relatively soft, and thus will tend to have torsional peaks in lower RPMs than a forged crank would. The rubber element and inertia ring on the damper are designed and tuned in such a way to target the worst RPM for torsional vibrations. A rubber element damper can typically only dampen out vibrations over a fairly narrow RPM range, and as such the tuning of it is quite precisely matched to the engine to protect it against the worst vibrations.

So what happens when you go to a lightweight flywheel? The torsional vibrations in the rotating assembly always view the flywheel as "ground," i.e. the flywheel is not torsionally vibrating or at least not enough to have any effect on the system as a whole. When you move to a lightweight flywheel, this assumption may not be true anymore, and the RPM of the torsional vibration peaks may shift, thus making your stock damper essentially useless if they shift enough.

What can you do to correct this? The best option would probably be to switch from the stock rubber damper to a viscous damper such as the fluidamper one. The reason for this is that the viscous dampers dampen over a far broader range of frequencies / RPMs and have better damping across the entire RPM range typically. Many guys and threads on here will say you don't need one, no performance gain, etc., which as of performance is true. As of needing one, if you have 1) a lightweight flywheel, or 2) non-stock internals and are running decently high hp I would strongly recommend one, especially if you have a lot of $$ in the engine, this is cheap protection for the crankshaft.

The guy I learned most of this from designed a ton of the dampers for GM and Ford for a long time, and was always banging his head on the table about these stupid kids putting on ultralight flywheels and what it did to the crankshaft . That being said I am about to order a ACT prolite flywheel and probably won't get around to adding a fluidamper until next year, but I am also at relatively low hp and on a pretty tight budget.

 

thanks for the info, however isn't the stock crank forged?

any chance you can get some example plots of torsional vibrations with/without a crank damper?

 

Hi Mrfred,

I am pretty sure the stock evo crank is cast. I haven't seen one in person yet (will this winter) but every 4G crank I have ever seen is cast (have seen a ton at work out of other 4G variants). Take a look at the pictures at this site of the OEM crank:

http://www.streettunedmotorsports.co...crankshaft.htm

From the surface finish and profiles of the counterweights I would put solid money on cast. The fact that it is only $879 leads me to believe cast as well . If anyone has seen different though I'd be very interested to see.

I will see if I can dig up some torsional plots that I can share with and without a damper.

 

I, personally, wouldn't recommend a lightweight flywheel at all. It chatters a lot, especially on a daily driver. Friend has one on his car because previous owner thought it was good to go that route. First drive, it didn't feel like it gained much of any performance at all. Only thing it did was rev faster without being in gear.

Is a forged crank supposed to be more? Because the Manley forged crank is like $600.
http://www.streettunedmotorsports.co...rged_crank.htm

And I thought it was confirmed that a long long time ago, the stock crank was actually forged.

 

Correct, the crank and rods are forged steel and the pistons are cast.

I was lucky and aquired an ACT streetlite flywheel for $50 off of an asian friend of mine during my build. Complimented by the Fluidampr HB that STM hooked me up with and a well-balanced rotating assembly from my machinist, my engine runs incredibly smooth with no chattering or any non-sense like that.

I could imagine though, that throwing a streetlite flywheel on a stock engine could create some issues as described in the first post, and many posts before.

 

Curious as where you guys got the info that the stock crank is forged. From all the pictures I have seen it is fairly clear that it is cast. Cast and forged cranks are pretty easy to tell apart as the cast cranks have a very distinct cast finish on the counter weights, as well as a parting line down the center. The profile of the cast counterweight is also much more complex and "curvy" which is inherently easier to cast. All of these features are visible on all evo cranks I have seen.

I could be wrong here but unless someone can show data or a picture of an evo OEM crank that does not show the cast features, I am sticking to my guns on the cast argument . Nothing wrong with a cast iron crank, just "sounds" worse.

As of the chatter in the drivetrain, do you know what was chattering? My car will be pretty much 100% track next year after the rebuild so I want the lightweight wheel for the quick rev matching, but would like to eliminate the chatter if possible as well.

As of the cost forged is typically more expensive is all. Didn't know there was a forged crank out there for $600 as I usually hear like $2k and up for a nice forged crank.

EDIT: Just found this that seems to confirm that the crank is in fact forged. Oddest looking forged crank I have seen but looks like I was wrong! https://www.evolutionm.net/features/...0&pagenumber=4

 

2k would be a billet crank. manley forged is in the 600 range.

 

+1 on that reference. I was trying to post that last night but my phones lame. There's a lot of good information and references from different sources in the forums if you search enough.

 

And as I said earlier, on a balanced 2.3 rotating assembly with a fluidampr, lightweight flywheel, and CC 6 puck my car is surprisingly such a smooth ride; before, during, and after strong pulls.

 

Interesting to hear that the balanced assy with the fluidamper is very smooth, that may change my mind on pulling the fluidamper into this year's build.

Attached is a quick write up I threw together with a few simulated plots for an inline 4 engine with and without a damper. Even though these are simulated the real thing is pretty darn close. If you want more info on this and how it was done PM me.

Also as of the forged vs cast crank, I was confusing forged and billet. My bad on that, thanks for setting me straight there .

 

I would really like to see a simulation for the 4g63 EVO engine done at 2L stock with and without balance shafts. Then have the data with fluidampr and lighter flywheel added for those.

I think that would put to rest the argument of the balance shafts being eliminated or not. It would also give support to seeing if the fluidampr does really work on a broader range than the stock and can support a light weight flywheel addition as well.

Just curious..was this done in an Excel type sheet? or a modeling program or FEA ?

Any possibility to laying hands on?

thanks for an excellent post

 

Regarding the billet vs forged crank debate:

Billet is essentially one grain extending down the length of the billet. Then the crank is cut from that. This process leads to weak spots where the main bearing surfaces join to the throw surfaces...essentially everywhere the crank looks like a U as the grain doesnt follow the turn. Think of wood where the grain runs the length of the piece. Difficult to break by bending. However if you apply force in a lateral movement on the end of the wood, it splits apart more easily. That is the issue on billet cranks as you follow the crank down the crank throw and turn into the journals whether that be the rods or mains.

Forging starts with a billet then stamps it in several steps into the rough approximation of a crank before grinding. In the process , the grain is bent along the throws so you dont run into the open grain issue.

To put it simply they are stronger and have less weak spots.

This is why high end cranks are forged. Ferrari Porsche etc They have been the choice for more than 60-70 years

 

The balance shafts are meant to address the rotational vibration harmonics associated with the rotating assembly as a whole while the crankshaft damper is meant to damp out twisting harmonics of the crankshaft. Its two completely different things, and I wouldn't expect the crankshaft damper to have any effect on the rotational harmonics.

 

I feel like there is one problem here with saying that going to a lightweight flywheel essentially eliminates the "ground" for the crank with regards to torsional vibration.

When the torsional twisting will be greatest (full load, high RPM), the clutch will be disengaged and the entire mass of the car is essentially resisting torsional acceleration through the drivetrain. Also, the accessories add effective mass and damping to the nose of the crankshaft.

I’m not saying a good damper won't help keep bearings alive though, I’d have a Fluidampr if they would make the aluminum case ones again. Just saying that eliminating the damper all together and using a super light flywheel probably doesn't closely mimic a completely bare crankshaft.

 

Agreed. The balance shafts really are addressing a different problem than the damper. While these second order vibrations are present in the crank, the balance shafts are more counter balancing the second order imbalance of the engine as a whole due to the engine geometry. The damper is only for crankshaft torsional vibrations, and as I noted in the PDF can't really do anything about the engine inherent imbalance.

As of what program was used, it is a proprietary design software, so unfortunately I cannot share that. Maybe someday I will throw the measurement equipment on my car, but probably not anytime soon.