At the very center of a reciprocating automotive engine is the crankshaft. Due to the unique configuration of a horizontally opposed engine, the crankshaft is a major concern to engine life and power. A good crankshaft can determine the nature of an engine. A cheap one will cause a lot of problems and a well built one will last for decades. It's a matter of finding something that works well and is within your budget.
When you start shopping for a crankshaft, there some things that you need to know. We'll cover this issues one at a time.
This is probably the most critical aspect about picking a crankshaft. The construction of the crank is critical, as it will determine foundation upon which your engine is built. It understand it's importance, think about the Empire State building. Would you build it on a wood frame?
To understand how this happens, let's look at how a forged crank is constructed. The forged steel crankshaft begins life as cast chunk of steel with the general shape of a crank. It is then placed into a machine that hammers the metal down to the finished size, losing no metal in the process. Only as the final step, the jorunals are machined. As a result, the forged crankshaft has a higher metal density, and this results in greater strength and resistance to flexing.
We are fortunate enough to get a forged crank straight from the factory. All stock Type 4 crankshafts are forged. VW knew that this was the only way to get a long and reliable life from a boxer engine.
A cast crank is formed with a mold, and by pouring molten steel in this mold, the rough casting is formed. After cooling the crank is machined down to specifications.
What has been discovered with Type 1 engines is that when using a cast crank in a high performance and/or high stress application, the crankshaft has tendency to flex. Gene Berg states in his technical article about crankshafts, his experience has shown that a cast crank can require over twice the radial clearance to allow for growth when compared to properly forged crankshaft in a Type 1. Given a situation like this, using a cast crank can cause piston-to-valve collision, excessive blow-by, and wrist pin/keeper failure.
Scat Enterprises is the only company currently offers a billet crankshaft for the Type 4. The availability of the billet crankshaft is a relatively new process, so we don't have decades of experience to draw upon.
Billet parts are made from a similar process to casting, but the initial material is different. Billet parts begin as a large chunk of metal, steel in this case. A CNC machine then follows a pre-determined program and cuts the parts down to the specs. This allows the manufacturer to change dimensional specs quite easily, as they don't need a new mold; it's as easy as changing the computer program.
Like I said above, this is a new process. Some tuners believe that billet crankshaft are stronger than a forged crank and only use them. Others believe that billet crankshafts are inherently weaker and only recommend forged. For my money, I'm going to stick with a good forged crankshaft for my engines. I'll wait to hear how the billet cranks work in the VW engine.
Counterweighting is used to balance the forces of the connecting rod journal. As the crankshaft rotates, the weight of the rod journal on one side is unbalanced. Counterweighting counteracts this weight and smoothens out the crankshaft. This makes the engine run smoother, spin to a higher RPM regularly without a failure.
Any time you increase the stroke, or drastically increase the RPM range of an engine, counterweighting becomes necessary. This helps to negate the flexing of the crank. The flexing can cause the same problems as the cast crank above. If you are in doubt whether you need counterweighting, then I say go with it.
Luckily, all of the aftermarket stroker cranks are only available with counterweighting. If you are building an engine with a stock stroke crank, I feel that it's only necessary to add counterweights to the crank if you intend to regularly drive in the 6,000+ rpm range. If your target rpm range is lower (which most street engines are), then stick with the stock crank.
The piston bore and the crankshaft stroke are the key variables in determining the displacement of an engine. The stroke also determines the torque. As the stroke is increased, the power potential increases. This results in the torque band moving lower, so the engine makes more power sooner in the RPM band.
If you keep the camshaft mild, this can make an engine a stump puller with a LOT of bottom end torque and very little top end horsepower. This works well with a heavy Bus, like the Westy campers. If you go with a larger camshaft, you can broaden the RPM band and have a lot of power from down low to up high. This type of engine feels like it will never run out of steam; it just keeps accelerating.
The 1.7 and 1.8 crankshafts had 66mm of stroke, and the 2.0s were 71mm. Stroker crankshafts are available in many different lengths, generally starting from 74mm, increasing in 2mm increments until 90mm!!
To get more stroke out of an engine, you have to either buy a brand new crankshaft with more stroke, or have a stock one welded on and then machined to acheve the offset. In the latter case, the machine shop welds extra material onto the outer edge of the rod journal. They then grind down the journals to achieve the increased stroke.
It takes a specially equipped shop to do this type of work, but if done correctly, it can be as strong as the original crankshaft. Most of the stroker crankshafts available for the Type 4 are welded stock cranks.
Rod journal selection is a critical factor in building an engine. If you select a rod journal that is too small, the crankshaft will lose it's rigidity and flex. If the journal is too large, you'll have clearance issues with the case and camshaft.
The 1.7/1.8 rod journals are quite large and make for a solid crankshaft. The 2.0 rod journals are 5mm smaller (50mm in diameter) and are generally considered to be the smallest journal to use in a high performance engine.
The Type 1 1600cc rod journal and the Type 4 1.7/1.8 journals are the same diameter, but the Type 4 is wider. Other common journals are the Porsche journal and Chevy 2" (50.8mm). Some tuners have used rod journals from a VW Rabbit (Golf) engine, but due to their narrow diameter (46mm), some have found them to be too small and were causing the crank to flex. The journals were just too small.
If you decide to run different rod journals or if you decide to increase the stroke of the crankshaft, you will inevitably run into clearance issues. There are many points of interference to check when you are building a Type 4.
If you run a stroker crankshaft or are using connecting rods from another car, you might run into clearance problems at the crankcase, the piston at BDC, and the camshaft lobes. If you use stock 2.0 rods, clearanced on the big end, you can run strokes up to 78mm with reduced base circle camshafts.
It's this author's opinion that every engine, regardless if stock or not, needs to have these point of interference checked. It doesn't cost any more, except time, for you to assemble the parts and check to make sure that the parts will clear each other. It will surely cost you money if you assemble it unchecked and run the engine.
In the past, a few crankshaft options have been offrered that, for one reason or another, are not ideal options for any Type 4 engine in this day in age.
Roller crankshafts: Initially tested by the Porsche racing team and longed believed to be great for turning fast RPMs. Let's take a look at the roller crankshaft. The connecting rods rolled on roller bearings, the rod journal pressed into large discs. Basically the crankshaft is made of a series of discs pressed together to make a crankshaft.
Full circle counterweighting: This option has been offered cyclically for decades in the Type 1 world. This concept takes counterweighting to the next step, but instead of solving a problem, it creates more problems. The full circle adds weight to the crank shaft not on the side opposite of the rod journal, but adds weight to crankshaft were it's not needed. In fact, it's been reported that a full circle crankshaft suffers from the same problems as a stock, non-counterweighted crankshaft.
Every engine is different and has different requirements. I urge you to really study this article and select the crank that is best suited to your project.