Crankshafts: Evolution of the Flat Four Crankshaft

To counterweight or not to counterweight? That is the question.

We have had a number of e-mails asking why we counterweight the cranks for our 2.3 and 2.4 liter engines, but not for the base 2.2 liter engine. Here is the scoop!

The waterboxer is a horizontally opposed, 4-cylinder, 4-cycle engine just like its older Type 1 and Type 4 brothers, except it is water-cooled instead of air-cooled. If you were to look down at one of these engines while it was running, and you had X-ray vision, you would see the forward two pistons (#1 forward right and #3 forward left) and the two rearward pistons (#2 rear right and #4 rear left) reciprocating back and forth away and toward each other, but slightly off-set from each other front to back. It's actually like two 2-cylinder engines joined by one crankshaft, or like two BMW motorcycle boxer engines bolted together.

Because it is a 4-cycle engine, when the opposing pistons get to top dead center, one is just about to fire (compression/power stroke) and one is exhausting (exhaust/intake stroke, or "overlap"). At that moment, the other two pistons are at bottom dead center. Every time a spark plug ignites and an explosion takes place above the piston, the force is transmitted down the connecting rod to the crankshaft rod journal causing a "racking" or twisting force around the vertical access, over and over, again and again, billions of times over the life of the crankshaft. All of these twisting forces pass right through the center main bearing. It is for this reason that the crankshaft in early VWs would eventually crack right through the center main bearing journal, usually at about a 45-degree angle. Interestingly, since the break occurred at an angle, the engine would often continue to run for some time even with a crankshaft broken clean in half! If it did not break there, it would at the #1 journal where the flywheel bolts on because that is the next weakest point. The Type 4 engine design addressed the #1 journal problem by making that journal significantly larger than the rest, but the center main failure persisted.

The standard 2.1 liter waterboxer crankshaft is not known to fail in that way. As a matter of fact, we have NEVER seen one crack or break in any way. It is a superb design, way more sophisticated than any crankshaft used in the two earlier VW horizontally-opposed engines, the Type 1 and Type 4. The waterboxer crankshaft design overcomes the twisting problem by implementing two massive sections, one between each pair of opposing rod journals, that are not present in either earlier design. These two hunks of mass act kinematically to absorb the momentary impact of the ignition process, thereby alleviating the stresses to the center main bearing. The waterboxer crankshaft also employs the identical #1 main journal design that VW figured out worked well on the Type 4. This is just one example of how VW drew on its many decades of knowledge, which culminated in the waterboxer design.

Counterweights are not added to horizontally opposed engines to make them more "balanced" or "smoother." Horizontally opposed engines are already harmonically balanced due simply to their physical layout. Counterweights are added to reduce dynamic stresses, so the engine lasts longer. In the case of our GoWesty 2.2 liter, the crankshaft is not modified in any way. Additionally, our all-aluminum forged 96mm piston is actually 5% lighter than the original 94mm steel reinforced cast aluminum piston. That means the crankshaft in our 2.2 is actually dynamically stressed lower than stock. So, there is simply no need for counterweighting a stock crankshaft.

The reason we counterweight the crankshafts in our larger engines is to decrease dynamic stresses and increase longevity. The crankshaft we use in our 2.3 and larger engines is increased from the standard 76mm. In the case of the 2.3, we increase it by 2.65mm; the 2.4 gets 6mm more stroke; and our latest 2.5 liter gets 8.5mm more. Even though the lower mass of the pistons we use in these larger engines reduces dynamic stresses, the increased stroke has the reverse effect. Since we don't know how much margin there is in the original design (that is, how much more stress it can handle and last) we decided to play it safe. We choose to add counterweights to offset the greater stresses created by the longer stroke just for good measure. And since the crankshafts are getting modified anyway to increase stroke, the cost to add the counterweights is minimal, making the process cost effective. We could save about $100 per engine, eliminate the counterweights, and it would work fine. Indeed, it would probably make it past the warranty period. Our goal, however, is to make an engine that performs better, is more efficient, and lasts longer. While it is true that our warranty is 48 months/48,000 miles, which is four times greater than any of our competitors, we all know that should not represent the life of the engine.