FSTPRSH

Reading through "Excellence Was Expected" I got for Christmas and it said that when they developed the 16V head for the 944S, since they kept "the piston weight unchanged, the engineers avoided having to change the masses of the existing 944 balancer shafts." Later on, when the 2.7L and 3.0L engines were designed..."Married with the 104mm bore, this brought capacity to 2,990cc. Here too was an attractive solution. As in the eight-valve engine, the 4mm bore increase from the 2.5-liter to the 2.7-liter had already been dealt with in the use of special pistons and heavier balancer-shaft masses. The same techniques could be applied to the 16-valve engine."

Could anyone explain how the mass of each are related?

-edit-

There was also talk of a 3.2L development for the S2 that would require "the four balancer-shaft masses [to be] increased to match its slightly heavier pistons." They were considering a 105mm bore and a 92.2mm stroke.

Danno

Well, it's not really that simple since the balance-shafts work to reduce the 2nd-order dynamic imbalances, not the primary static balance you can do with weighing pistons & rods. Ideally, you would have each crank-journal fully balanced statically and dynamically; that is, the weight of the piston+rod is perfectly balanced by the counterweight on the other side of the journal. That way, if you were to chop a crank into four pieces and put it onto V-blocks to balance each main-journal, each one would be perfectly balanced at rest and in motion. Even without ideal balance on a per-journal basis with the crank-counterweights, such as if you were to get ligher rods or heavier pistons or lighten the crank, they would still be balanced on a per-pair basis. One piston+rod combo goes up while the one on the next crank-journal over goes in the opposite direction. However, this is still primary static & dynamic balance.

However, the balance-shafts have nothing to do with this. The balance-shafts actually counter-act the sideways motion of the rods' big-ends since those don't have a matching opposing motion with the same moment-arm lever. That is, the angle that the rods swing through on the bottom of the crank swing (in relation to the crank's center), is different than the angle it swings through at the top. The polar moments of inertia of the rod's big-ends is different. The rod's motion also adds and subtracts from the piston's motion in a way that doesn't cancel as well. in a 4-cyl. engine. I drew up a very quick summary of how the balance-shafts work with basic math on this thread: Balance Shafts

For a fuller understanding, I recommend getting the book I referenced at the end of the thread.

EDIT: BTW, a perfectly laid-out engine in 1st & 2nd order harmonics and perfectly even firing-order is a 60-degree V6.. or a V12. A V8 comes close but has an uneven firing-order.

Jay W

Well said danno!

SoCal Driver

Damn harmonicas!