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This article by Ray Sedman was originally posted to Virtual Vairs.
There have been some good posts on VV regarding camshafts, grinds and profiles. I have received private posts from many people on this thread, so I thought I would post some general responses to the list. The following is a very brief overview of camshafts. There are many liberties that are taken here so this note is best read with a grain of salt.
Corvairs respond very well to head work and performance camshafts. It is helpful to think about the heads and the camshaft as a single item, 'a set', not two separate items. Leave the heads and camshaft out of the mix and most stock rebuilt and performance engines will look pretty much the same. It is the camshaft and the heads that will have the major effect of determining the operating character of your engine. This post is about camshafts, but since we are thinking along the lines of the heads and the camshaft as a single item, lets talk about heads.
Corvair cylinder heads can be broken down into two groups: small valve heads and 140 HP heads. All small valve heads, from all engines will share the same valve size, port configuration, valve angles, intake plenum cross section and a single carb mounting pad on each head, etc. The differences between years and applications are basically limited to valve spring diameters and chamber shapes. 140 HP heads have larger valve size, different and larger port configuration, different valve angles, larger intake plenum cross section and two carb mounting pads on each head. All turbo heads are small valve heads. As a general rule, 140s will show larger percentage gains (flow bench CFM) on the exhaust vs. the intake for the same amount of port work. This is because the 140 intake side is actually too large for most street engines. Small valve heads will show comparable gains on both the intake and the exhaust ports based on invested port work. [Note: the above are our observations based on our port and flow bench work. These findings may vary based on port work and testing of other people and their experiences.]
Corvairs have unique camshaft requirements based on the head design. In the stock from, the exhaust port is fairly restrictive and the intake plenum is not a great help in making the intake side flow well. Most cam grinders have profiles developed for more common engines and their requirements. In this manner, you would not expect a camshaft profile which was developed for a 327 inch Chevy V-8 to be an ideal match for your Corvair. I am not going to list specific grinders or suppliers of camshafts, as things always change and even a camshaft with a specific grind number may have many different incarnations along it's development line. It is safe to state that if you look at the grinder's/supplier's camshaft list and a '270 Hydraulic' cam which is listed for Corvair has the same specifications as the same 270 cam for a 327 Chevy, then you can pretty safely assume that that profile was not designed specifically for your Corvair.
Camshafts are advertised with certain numbers that help describe their operating characteristics. Most common are the camshaft lift and the duration. Most people are familiar with a cam description of 270, 260, etc. What does all this mean? A 260 camshaft will have an advertised duration of 260 degrees. The key word here is 'advertised' and we will discuss this later. You will also see a number which is described as lift. Lift is a measured distance that the valve will lift off it's seat at full lift. A typical number for a 260 cam will be around .420". Another important number is overlap. Overlap is the amount of time, in degrees, that both the intake and exhaust valve are open. Overlap is important as more overlap generally causes loss of low end torque and poor fuel economy. Camshafts with higher overlap will tend develop more power in the higher RPM range. If a cam has the same advertised numbers for both the intake and exhaust valves, this is considered to be a single pattern cam. A dual pattern cam will have different advertised numbers for the intake and the exhaust valve. For example, a '260' single pattern cam will have 260 degrees of duration and .420" lift for both the intake and the exhaust valve. A dual pattern '260' cam may have 260 degrees of duration and .430" lift for the intake and 268 degrees of duration and .420" lift for the exhaust valve. Lift, duration, single and dual pattern are all important things to consider, but all of this is solely dependant on the actual shape of the camshaft lobe, commonly described as the lobe profile.
It is the lobe profile that heavily determines the actual camshaft operating characteristics. You may have three cams with very similar advertised lift and duration numbers, but they will behave markedly differently as it is the lobe profile differences between the cams that make them different. Lobes can be symmetrical and asymmetrical. A symmetrical lobe will have the same shape (mathematical curve) on both sides of the lobe. An asymmetrical lobe will have a different shape (mathematical curve) on one side of the lobe as compared to the other side. Asymmetrical lobes can have many advantages to symmetrical lobes. For example, an asymmetrical lobe can lift the valve quickly off it's seat to help establish valve flow earlier, but on the back side of the lobe have a more moderate lobe profile to gently place the valve on it's seat to prevent seat and valve damage. A symmetrical lobe will lift the valve off the seat, bring it to full lift and place it back on it's seat in the same manner for both.
The advertised lift and duration of the cam are the common specifications published for the cam. A more accurate way of describing the actual working lift and duration of the cam is measured when the tappet is off the base circle by a prescribed amount. The base circle is the round part of the cam which does not move the tappet to lift the valve. The lift and duration numbers when stated in this case would be called the 'checking clearance' or the effective duration of the cam. The checking clearance is important since it not only makes a more precise way to degree the camshaft, but it gives a more accurate 'yardstick' to compare different grind profiles of specific cams. Most camshafts use a checking clearance of .050", but some use a .020" checking clearance. Others may use a different clearance or will not give any checking clearance. A checking clearance, or when the tappet has moved (risen) .050" off the base circle will tell you more about the camshaft lobe profile then the advertised lift and duration. This is because it takes time, in crankshaft degrees, to lift the valve off the valve seat to establish flow. The valve does not magically pop to full lift and then back it it' s seat, but must be gently raised off it's seat and then gently placed back down. All these gymnastics are call the 'ramp' of the lobe.
A camshaft with long ramps will take more time to raise the valve to full lift and place it back down to it's seat. In this case the camshaft will have less effective duration than a camshaft with faster ramps. Two camshafts may have similar advertised lift and duration figures, but when the effective lift and duration numbers are viewed you may find a difference in effective duration between the two camshafts. The advertised lift should be the same as the effective lift since lift is the gross mechanical amount that the camshaft will lift the valve. This is a set number that is not affected by ramps or checking clearance, assuming the cam is ground correctly to it's published figures.
With the information lightly discussed in this note, you may have a better understanding of how to select a replacement camshaft for your engine. How much camshaft do you need for your engine? Well, this really comes down to how much head work has been done to your heads. Remember, we want to view the cam and heads as a set, not two separate items. The other factor is how much of the engine's stock character do you willingly agree to give up?
Generally speaking, Corvairs will benefit from a dual pattern, camshaft based on our prior discussion on Corvair head design. Turbos show gains with asymmetrical lobe designs. Just keep in mind that the most important factors when installing your new cam are: 1) degree it, 2) make sure your rocker arm geometry is correct. Even the most fancy, high tech, new camshaft will not perform properly if you gloss over these two very important steps. If you are not sure of what cam to get and you have narrowed your cam selection to two grinds of the same cam grinder, pick the smaller of the two. This assumes that you will drive your car on the street. If you are in serious doubt, stick with a factory camshaft. They are very well designed and are good performers. I wonder how many people will understand and follow this recommendation? :-)
We have invested a fair amount of time slicing, dicing and understanding Corvair head and port work. With this understanding we developed camshaft profiles for our solid tappet roller cams. We have also used this understanding and developed specific camshaft profiles for flat tappet hydraulic Corvair cams.
Cheers!
Ray
American Pi.