Camshafts 101

In the world of hot-rodded VW engines, few parts create such mystery and hype as the high performance camshaft. In the quest for “the ultimate” in horsepower, engine response, and often, bragging rights, enthusiasts will find themselves losing sleep over the selection of the “right” high performance camshaft for their hot VW. While the science of how the high performance VW cam works can be tricky, the selection process doesn’t have to be. It is generally accepted that the camshaft is the component of the engine that brings all the ingredients together. With either the wrong “ingredients” or the wrong camshaft, the results can be very disappointing. However, if all the parts are selected to work in unison, and are assembled correctly the resulting performance and longevity can exceed most expectations. The “how” of the camshaft will be lightly touched on here, but more so, a guide to deciphering all the numbers and how to use the numbers to make your choice. There are some simple “rules” that should be followed for the best results.

First, here are some definitions to common “camshaft terms”:

“Duration”: period, measured in rotation of crankshaft (degrees) that either intake or exhaust valves are open. Note, this degree measurement is usually discussed and advertised at .050″, which refers to either valve having traveled .050″ of lift before degrees are counted.

“Lift”: Lift has two definitions, and it is important to know the difference between the two. “Valve Lift” is just that… the total distance the valve travels as the cam opens it. This, in a VW motor, is usually a larger figure than “cam lift.” This is due to multiplier factor of most rocker arms (especially the aftermarket, high performance “ratio rockers” which can multiply cam lift by up to 1.6 times. “Cam lift” is the distance the lifter actually travels, in direct contact with the cam lobe. There is no multiplication factor when discussing “cam lift.”

Now for the “rules”…

Rule # 1. Ask yourself “what am I going to use this engine for? Driving everyday and on long trips, any time of the year? Drag racing only? An engine that is in between these two extremes? Maybe a little more driver than racer? Or vice versa?” Be honest with your answer. Erring one way or the other can make or break the out come. Here are some guidelines.

A. Daily driver? Stay at the conservative end of the cam spectrum. Select a cam that uses the stock VW rocker arms (not 1.25 or 1.4 ratio rockers), in an effort to keep valve lift no more than .460″. Running a valve lift more than .460″ in an everyday driven car will result in parts wearing out very quickly and make the engine noisier. The duration selection is going to depend on a few more variable… engine displacement, carburetion, or intake system, and gear ratios/ring and pinion ratios.

– 1600 to 1776cc, stock carburetor, stock gear ratios: Stay under 235 degrees duration @ .050″ (actually best to stay with stock cam at this level!)
– 1600 to 1776cc, dual Kadron or Weber 34ICT or Solex 32mm, stock gear ratios: Use duration of 240-245 degrees @ .050″
– 1600 to 1835cc, dual Weber IDF or Dellorto DRLA, stock gear ratios: Stay under 250 degrees of duration @ .050″
– 1900 to 2300cc, dual Weber IDF or Dellorto DRLA, stock gear ratios: No more than 265 degrees of duration @ .050″

B. Going for a weekend hot rod? Not necessarily a daily driver, but still a VW to be used on the street, and driven on a regular basis? This type of engine has gained popularity in the last 10 to 15 years. While more powerful and lively than the daily driver engine, it is also more likely to need more frequent servicing and in depth maintenance (involving disassembling the top end of engine periodically). These “in between” engines can typically move towards the more aggressive spectrum when it comes to camshafts (often the reason behind the increased power and decreased service life). Engines of this nature can be relied upon, on the street, running about to about .540″ lift at valve if the valve train is designed and assembled to lift this much. With the wrong parts or assembly and this much valve lift, you will be asking for a catastrophe…SOON. Duration can also be moved up but remember, as you add duration, you are making the engine less forgiving and less drivable around town. All of the usable power will become available at a higher rpm. One special note here too… these types of engines will only run best with twin, two-barrel carburetors, like the Weber IDFs or IDAs.

Here are suggested durations (note the durations are listed as @ .050″!):
– 1600 to 1641cc, stock gear ratios: no more than 250 degrees @ .050″
– 1679 to 1776cc, stock gear ratios: 250 to 260 degrees @ .050″
– 1800 to 2300cc, stock gear ratios: 255 to 265 degrees @ .050″

C. Building a drag motor? If that is the case, it is best to talk directly to the engine builder or the cam grinder, or both. A drag only engine will be built with little concern for reliability or longevity. The focus will be maximum horsepower. Therefore, both valve lift and duration are going to be much greater when looking for a suitable drag race camshaft. Typically the VW cylinder head can accommodate about .600″ valve lift before extreme measures are taken (machine work to valve spring pockets to accept larger valve springs). This much lift can only be achieved with higher ratio rockers. To figure the lift you want to run, calculate approx 35% of your intake valve diameter inches. The result is approximately the lift you will aim for in a drag engine. To calculate approximate duration you should run (@ .050″), divide engine cc by 4, and then divide that result by 1.77. The result is approximately the duration @ .050″ suggested for a drag race engine. Again, these are approximations.

Rule # 2. Is the engine using stock 69mm stroke crank or a longer “stroker” crankshaft? If the engine is being built around a stock 69mm stroke crank, then it is advisable to err on the conservative side of the duration figures above. If you are planning your project around a longer, “stroker” crank, then you can increase duration accordingly, to the larger figures shown above. The longer stroke crank does more for increasing torque at lower and mid-rpms than any other part of the engine. The effects of increasing duration (weakening lower and mid rpm torque) are usually offset by increasing crank stroke. This means you can literally have the best of both worlds… gutsy bottom end torque, and searing top end horsepower! Engines with similar displacements, but different stroke crank sizes will need different durations. For example, a 1914cc engine is built around 94mm cylinders and a stock 69mm crankshaft, however a slightly smaller cc engine, a 1904cc, is built around 90.5mm cylinders and a 74mm crankshaft. The 1904cc can be tuned for drivability easier than the 1914cc, even with a wilder cam (all other variables being equal; i.e. cylinder head porting, valve diameter, carburetion, compression ratio, etc). So where the 1914cc beings to bottom end torque at about 255 degrees @ .050″, the 1907cc could keep a healthy and wide torque curve with about 262 degrees @ .050″. Crank stroke length and cam timing go hand in hand.

Rule # 3. What type of carburetion will be used on your engine? Carburetion, more than any other factor, can make or break your cam selection. The effect the correct or incorrect carburetion has on how your motor behaves (or misbehaves) cannot be overlooked! An engine setup with a “wild’ cam does not have to be cursed with the stereotypical rough idle and poor off-idle response. This is where the correct choice of carburetion comes into play. For the longest time in the high performance realm (of all cars, not just VWs) the ultimate in carburetion has been one carb throat per cylinder. This type of carburetion is better known as the independent runner system or IR for short. The reason it has been held in such high esteem for so long is that the IR system is a “does it all” system. While it may be obvious that a carb throat per cylinder appears to be the best solution for upper rpm horsepower, it is also the ultimate for a smooth idle, excellent fuel economy and a wide and full torque curve. The IR setup is especially well suited to VW engines running a longer duration camshaft. With that said, regardless of displacement, if you plan on running a camshaft with 245 degrees or more (@.050″) then for best all around performance, you should plan on running twin, two-barrel carburetors. Once you exceed about 252 degrees, the IR carburetor setup is necessary for the street.