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Selecting Your Cam's Lobe Center

Other related articles: Selecting the Proper Camshaft, Static vrs Dynamic Compression.

While most of our customers know what lift and duration are, most have no idea what the term Lobe Center means, or how it effects the engines performance. Therefore the biggest obstacle to overcome when selecting a cam profile, is selecting the Lobe Center. While there are plenty of articles on the internet, you need a masters degree in physics to understand most of them, therefore I decided to see if I can simplify it.

Lobe Separation or Lobe Center is simply the distance between the peak opening points on the intake and exhaust lobes. A 110-degree lobe separation means that the peak opening points of the intake and exhaust lobes is 110 degrees apart. Lobe Separation or Lobe Centers, is another way of expressing the Valve Overlap, which was the term formerly used by cam manufacturers. Overlap is the amount of time that both valves are open in the same cylinder. All three terms are similar, as they all relate to the relationship between the centerline of the intake lobe and the centerline of the exhaust lobe. In this article we'll focus on Lobe Centers, as it is more commonly referred to by cam manufacturers when describing a cam profile.

Simply put, the lobe center of a cam controls where the power curve is applied. The tighter the lobe center, the lower the rpm range; the wider the lobe center, the higher the rpm range. The rpm range is also known as, or refer to as the power band. In general, two degrees of lobe center is equal to approximately 500rpm. Therefore, if a cam with a 110* lobe center has a power range between 2500-6000 RPM, the same cam with a 112* lobe center would have a power range of 3000-6500 RPM. Conversely the same cam with a 108* lobe center would have a power range of 2000-5500 RPM.

A cam with a 112 lobe center will idle smoother than a cam with a 110* lobe center, while a cam with a 108* lobe center will idle rougher.  A cam with a 112* lobe center will generally have a smooth idle, a cam with a 110* lobe center will be a bit lopey, while a cam with a 108* may have a rough or choppy idle. A camshaft with a 112* lobe center will work with either an automatic or a manual transmission, however a cam with a 108* lobe center normally requires a matched stall converter for the best results, especially as cam duration increases.

Longer Durations
(higher number)
Tighter Lobe Centers
(lower number)
more peak power
less peak power
rougher idle
rougher idle
less bottom end torque
more bottom end torque
peak power occurs later
peak power occurs sooner
widens the power band
narrows the power band
raises the RPM range
lowers the RPM range
higher emissions
higher emissions
higher fuel consumption
higher fuel consumption

Tests have shown that, for a given cam profile, a tighter (smaller) lobe center will produce more average horsepower and a quicker revving engine.  However it is at the expense of a small amount of bottom end power and idle quality, due to the increased overlap. In reality, a cam with 220º at 0.050" and a lobe center angle of 112º will have the exact same mechanical overlap as a cam with 217º at 0.050" and a lobe center angle of 109º.  If you compared these two cams side by side in identical engines, the first cam would actually have a slightly better idle and would produce slightly more peak power at upper RPM's. The second cam would have a rougher idle, produce more torque and would rev quicker. 

You just need to decide which profile works best for your application. If you want a smooth idle and do a lot of highway driving, where passing power is of concern, the 112* might be a better choice. If you do a lot of light-to-light driving, don't mind (or prefer) a slight lope in the idle, the 110* may be better suited.  But if you want to eat V8's off the line and can live with a lopey or choppy idle, a 108-109* lobe center might be just the ticket. Basically, it comes down to a compromise between performance and idle quality, base on your specific driving requirements.

Inches of vacuum may be of concern if you have vacuum assisted accessories, such as power bakes or steering. A cam with a 112* lobe center will pull approximately 16-20" of vacuum, which is good for power brakes. A cam with 110* lobe center approximately 14-18", is borderline. However a cam with 108* will only pull 12-16", which is not enough  for power assisted accessories. These numbers can vary a few inches one way or the other, depending on several other factors and your individual engine specifications, so you may want to check your vacuum once the cam is installed. If you don't have enough vacuum you can install a vacuum canister or reservoir, which works by storing the peak vacuum in the reservoir. If that's still not enough, then as a last resort you can use a vacuum pump.

The final step in selecting a cam profile, is learning about Dynamic Compression Ratios, how it effects engine performance, and ultimately your cam selection.

A last word of advise. Once you get your cam and are ready to install it, be sure to pick up a Cam Degree Kit and degree the cam to the manufacturers specs when your installing it. NEVER install it dot-to-dot. When you degree the cam, you are making sure the intake centerline is precisely where the cam manufacturer intended it to be. This is very critical for manifold vacuum, throttle response, emissions, and gas mileage. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber and take out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely decreasing the gas mileage.

Many manufacturers suggest installing the cam three or four degrees advanced, which lowers the power curve of the cam and increases low-end torque. While this sacrifices a little top end power, more is gained on the bottom end performance than is lost on the top end. It should be noted that some manufacturers, such as Comp Cams, grind their cams with a 4-degree advance automatically ground in, so you need to verify this prior to degreeing the cam.
All Classic Inlines / Clay Smith Cams are ground straight up, so we highly recommend degreeing the cam 4 degrees advanced for optimum performance.


Other related articles:
Static vrs Dynamic Compression - Cam Installation Tips
How to Degree Your Cam - Adjusting Your Valves
Clay Smith Cam Cards
- Clay Smith Cam Profiles

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