blade7

Read a thread on here that said the S2 crank is balanced harmonically by the front pulley and the 968 crank by it's DM flywheel. So what happens if the 968 crank is used with a 944t flywheel/clutch ?. Also do 2.7 balancer shafts cause any vibration issues when used with a 3.0 crank.

badcoupe

Not sure all engines crank pulley is harmonic dampner of sorts. Some engines used offset weight in the flywheel and balancer others are "neutral" in being that they are balanced, no offset weight. I know that you can buy a solid flywheel for a 968.

95ONE

But, Chris White has suggested.. strongly, that you just use an S2 balancer on the front. I also heard that the 968 crank is lighter than the S2. I have no facts on that though.

Thom

I posted that some years ago but I was wrong. Both weigh the same 25.0kg.

Dubai944

My 968 crank was actually a few hundred grams lighter than my S2 crank when I was deciding which to use in my last rebuild. Both stock as far as I could tell.

Karl at RE warned me to run the S2 balancer when I first put the 968 motor in the race car, but I didn't do it at first. I broke three alternator belt tensioners in a row and and cracked the oil pick up flange the first season I ran the 968 motor with just the standard crank pulley. Then I swapped to the S2 harmonic balancer pulley which is a lot heavier and never had another problem like that in 3 years of racing.

Thom

Thanks for your input - do you remember the precise difference? It wasn't that much, by the sound of it?

fortysixandtwo

I bet the difference in weights that people have measured are just manufacturing variations.

The S2 uses a harmonic balancer
The 968 uses a dual mass flywheel

Both devices are designed to reduce vibration within a specific range. Adding a harmonic balancer to the front of any engine engine is a good idea. It's on my to do list, for the 3.0

Four cycle, four cylinder, inline engines have zero balance crankshafts. They do not require any additional counter balance weights. When balancing, just the bare crankshaft is spun and balanced, with no need to account for other reciprocating and rotating masses.

blade7

Thanks for the replies, S2 front pulley it is then. How about the balancer shafts are 2.7 and 3.0 the same ?.

fortysixandtwo

The PET is showing the same balance shaft part numbers for the 2.5 and 3.0 engines.
There is an early and late balance shaft, where it looks like the windage inserts were added to the late shafts. Considering the shafts are the same for the different cylinder sizes, I doubt there would be a mass difference between the early / late shafts.

blade7

Good to know but balance shaft windage inserts , not heard of that ,is it 2.5 without 2.7 & 3.0 with ?.

fortysixandtwo

Early 944 does not have them, all the turbo cars will have the windage inserts. The insert is the black plastic piece that is screwed on opposite of the offset weights. Its function is to make the an uninterrupted surface, to reduce the mount of air being moved by the spinning shaft.

blade7

I guess it cuts drag but how much difference would it make I wonder.

eman930

So what if you just send your bottom end out and have it all zero balanced, including flywheel and crank pully? I had this done to my 924S and it was way way smoother, Would you still need the balance shafts if you had it all zero balanced?

fortysixandtwo

Depends.

The engine will be a lot smoother with all of the components balanced down to the last gram, but the vibration caused by component imbalance is not the vibration balance shafts counteract.

Balance shafts are designed to cancel out second order harmonics. In particular, the ones caused by acceleration / deceleration of the pistons and their location in the bore vs crank throw location, in an inline four cylinder, four cycle engine. It doesn't matter what you do to this engine architecture, the vibration cannot be removed. To smooth it out, balance shafts (really, they're more like vibration shafts) are spun to produce a vibration at the same frequency, but 180deg out of phase of the vibration being created by the reciprocating mass. The goal is to have the two, cancel each other out.

If you lighten the reciprocating mass, the frequency will stay the same, but the amplitude will be reduced. With reduced amplitude, the need for the balance shafts is reduced. Theoretically, if you lighten the parts enough, the balance shaft will become the source of the vibrations, at which point their throw masses should be reduced. I haven't done the math on this or know the balance shaft amplitude, so I don't know where that line is.

reno808

I know from the V8 world when you blueprint an engine you also change the harmonic balancer. therefore if you were to balance the entire engine my guess you would have to the balance shafts on a 2.5 build. if you are using a harmonic balancer on the bigger engines then you have to do something to the shafts and or harmonic balancer. Then again i could be wrong