If the rotating assembly is the heart and soul of your engine, then the camshaft is the brain. The camshaft controls the movement of the entire valvetrain. The right cam will produce excellent power right where you want it, maintain good drivability, create decent vacuum for power brakes, and even deliver the loping idle you crave.
Like the human brain, the camshaft is a highly complex entity, and we could write entire books dedicated to the theory of camshafts. However, there is some very basic information that anyone should know when choosing a camshaft:
- Cam Lift vs. Valve Lift
- Advertised Duration vs. Duration @ .050”
- Lobe Separation and Centerline
We’ve put together this Summit Racing Quick Flicks video to help you launch down the road to finding the right camshaft for your vehicle.
Hi, I’m Mike, and on this installment of Summit Racing Quick Flicks we are going to discuss the specifications of the camshafts and how to use them to determine what camshaft is right for your application. Before we can get into discussing the numbers, it is important to understand a few things about the camshaft and how it operates.
For starters, we have to understand what a cam lobe is, and a cam lobe is a part we know operates a lot of the valve train components, but what it also does is it determines everything about the cam meaning that is going to go ahead and effect the duration and lift and all of those numbers that we see on our cam card, it’s also going to go ahead and control certain other aspects of our engine—mainly the mechanical fuel pump, if we have on, and the distributor that is inside the engine.
The first specification we are going to discuss is cam lift. There are two types of lift produced by the camshaft. One is lift at the cam, and the other is lift at the valve. Lift at the cam basically means that we are going to take the difference between the heel of the cam—or the backside of the cam—and measure it against the peak of the lobe which is the high side of the lobe. The difference between those two is going to be the lift at the cam specification. This number will always be much smaller than the lift at the valve spec, due to the fact that it is not taking rocker ratio into consideration.
The reason why we do the multiplication at the rocker arm on most push rod V8 engines is so that we can actually get the camshaft into the engine. Because what will happen sometimes is the lobe can get too large. If we were just depending on lobe lift or cam lift, the lobes would be larger than the journal inside the block and then the cam would not be able to get installed inside the engine. This diagram illustrates what lobe lift is and how it relates to the base circle of the camshaft. The next camshaft spec that we are going discuss is duration.
Duration is the amount of time measured in crankshaft degrees that the valve is open. What this means is we are actually not taking this measurement out of a standard 360 degrees like you would think because there are only 360 degrees in a circle. We are actually taking it out of a 720-degree base. This is due to the fact that the camshaft is going to spin one revolution in comparison to two revelations of the crankshaft. For example if we had a 320 degree duration camshaft well we still have 400 degrees of rotation left at the crankshaft due to the fact that we are taking out its 720 degrees in relationship to the 360 degrees rotation of the camshaft this is what makes it possible to still build cylinder pressure with a duration this high with a camshaft that large.
You will notice when you look at the specs on your camshaft that there will be two duration specifications listed: advertised duration, and duration at 50 numbers. The advertised duration number used to be called duration actually and when you bought a camshaft there was one duration spec listed and this was kind of the old way of doing things because that cam manufacturer determined that they were going to measure duration from lift point, whether it be 6,000ths, 12,000ths, whatever that number might have been, and rate it at that duration spec they came up with from that lift point. The problem with this was you couldn’t do an actual apples-to-apples comparison between cam manufacturers when comparing, say, small-block Chevy camshafts with similar lift specs and similar duration specs it didn’t mean they were actually the same cam shaft. So what they came up with was the at 50 standard or the at 50 duration number and what this means is they are now required to list a duration spec from at 50,000ths to peak lift, back down to 50,000ths again. This gives us the ability to go ahead and do a comparison between cam manufacturers so we can determine what cam is right for our application between those different manufactures.
Now that we have determined what lift specs and duration specs are right for our camshaft, the next number we need to look at is lobe separation. Lobe separation is often misunderstood. What it is, is the amount of degrees between the peak lifts between both the intake and exhaust lobes on that camshaft on the same cylinder.
This diagram illustrates how duration is measured. It is important to understand what lobe separation is because it can have a drastic effect on that camshafts characteristics and how it’s going to act in the engine. Then main things it’s going to affect are engine idle quality, engine vacuum, and the engine RPM range. Here you can see how lobe separation angle is measured. For example, the lower the lobe separation gets, say at like the 106 to 108 range, means that engine is probably going to have a rough idle. It’s going to make very little vacuum at idle and its going to have an RPM range where the engine is going to make power from the mid-range to the higher end of the RPM band of the camshaft, whereas in comparison we look at a cam that has a 112 to a 114 lobe separation range. What it’s going to do is its going to have a smoother idle quality, a better engine vacuum for things such as power brakes, and it’s going to make power sooner—meaning it’s going to make more low-end torque, come on quicker, and have a broader RPM band.