I’ve been looking at cam specs for the Summit Racing Pro LS Truck camshafts and I have a question. I’ve noticed that many of the specs now list the lobe separation angle (LSA) spec with a plus number added. The cam I’m looking at is the Pro LS Truck 8718R1 cam. The LSA specs are listed as 112+2. But the actual specs say the LSA is 112 degrees. So what does the +2 refer to? It’s something new that I’ve never seen before and I’m not sure what it means.

J.L.

This is a great question with, for once, a simple answer.

But don’t worry, we’ll expand our answer just so everyone understands what we’re talking about. The short answer is that the +2 refers to how the cam is ground relative to the LSA and the cam’s intake centerline. The +2 in this instance means it has been machined with two degrees of advance built into the cam, placing the intake centerline at 110 degrees BTDC.

But let’s take a closer look to make sure everyone understands this explanation. Lobe separation angle is the angle between the intake and exhaust lobes for a given cylinder and is expressed in camshaft degrees. Using the COMP Cams illustration at the bottom of this article, you can see that the intake lobe centerline (the left lobe) and the exhaust lobe centerline (on the right) are both displaced 116 degrees away from the TDC centerline.

The lobe separation angle is the sum of the two angles divided by two or 116 + 116 = 232 / 2 = 116 degrees. If we advance the intake lobe two degrees to 114 degrees BTDC, then the LSA would be 114 + 116 = 230 / 2 = 115 degrees. This is accomplished during the actual grinding of the camshaft and cannot be altered. Cam companies often add this advance because this improves low-speed torque. Not all cams are ground with advance built in, but it is very common.

Before this shortcut description, the best way to figure out if the cam had been ground advanced was to check the intake centerline figure. If the intake centerline number and the LSA number are both the same, then the cam is not round with advance. But if the intake centerline is 110 degrees and the LSA is 112 degrees, then the cam has been ground with 2 degrees of advance built into it.

In the case of the Summit Racing Pro LS Vortec Truck Swap Camshaft, the LSA is expressed as 112+2. As you suspected, the cam is ground with a 2 degree advance with a LSA of 112 degrees. This means that if you were to degree the cam, you would find the intake centerline would measure 110 degrees BTDC. In this case, there is no reason to further advance the camshaft because the factory has done it for you. The new listing as 112+2 merely points out that the cam has been ground with two degrees advance in the cam. This shorthand description tells you everything you need to know.

Summit Racing Pro LS Truck Camshaft (SUM-8718R1) Specs

Advertised
Duration		Duration at
0.050"		Valve
Lift		LSA
Intake2792300.625"112+2
Exhaust 2912420.605"
Intake Centerline in this case is 110 degrees BTDC.
This illustration from COMP reveals how the lobe separation angle (LSA) is determined. In this case: 116 + 116 = 232 / 2 = 116 LSA. If we were to advance the intake lobe (move it toward the centerline) by two degrees, this would change the LSA:  114 + 116 = 230 / 2 = 115 degrees. A tighter (smaller) LSA number also increases valve overlap. (Image/COMP Cams)
The LSA cannot be changed once the cam is ground. However, degreeing the cam will indicate where the cam is relative to intake centerline. In the case of the Summit Racing cam SUM-8718R1, the +2 indicates the intake centerline will be at 110 degrees BTDC. (Image/Jeff Smith)

Keywords
Ask Away, camshaft tech, Lobe Separation Angle, LSA
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.