S5 Flywheel/counterweight on S4
 

does the S5 flywheel weigh less, i was told it does. if so how hard would it be to put on an S4 and what are the bad things if any of doing this?? i heard it wears the rear main bearing rather quickly. all info is appreciated.

The stock flywheels have the counter weights cast into them. If you switch the S4 and S5 flywheel, the rotating assembly won't be balanced properly.

Yes you will weare the rear bearing.. have a out of balance condition that will most likely wear all the bearings as the assy gets further out of tolerance.. with the really unpleasant problem of rotor contact with the rotor housing in short order depending on how hard you rev it.
Also remember as the eccentric moves in a non-standard form it will be causing the transmission input shaft (That plugs into it via the pilot bearing) to wear its input bearings fairly quickly as well.
You will have in short order shifting problems as well, as the transmission bearings on the input shaft loosen up.

Basically.. not a good idea.

what about the lightweight 17lb steel flywheel from racing beat, is this the same problem??

Re: S5 Flywheel/counterweight on S4
 

You have to use the S5 rotors as well or the entire crank assembly will be unbalanced and will wobble around. It will make the engine/car vibrate quite a bit and will wear the gear bearings out prematurely.

B

If you use an after market flywheel, you can select the correct counterweight to go with it from the vendor selling the flywheel. After market flywheels don't have the counterweight cast into them like the Mazda flywheels did. So if you have a 86-88 RX-7, thats the counterweight you want, regardless of whether the new flyhweel is steel or aluminum.