Chevy Small Block Cam - Separation Of Power

Does Lobe Separation Really Matter?

Mike Petralia Feb 1, 2002 0 Comment(s)
Sucp_0202_01_z Chevy_small_block_cam Chart 2/9

Did you know that you could take two cams that are ground with EXACTLY the same lobes, even from the same cam company, and still get one to make about 20 more horsepower than the other? It's the lobe separation that does the trick, and if you picked the wrong separation figure, you could be losing power. We think the cam manufacturers do this just to keep us guessing because it's an area where camshaft design and technology can get very confusing.

What Is Lobe Separation?
Lobe separation angle is NOT the same thing as the lobe centerlines, although it does directly involve them. Lobe separation is the number of degrees between the point when the exhaust valve reaches maximum lift and the point that the intake valve reaches maximum lift. Those two max lift points are the lobe centerlines and usually fall somewhere in the 102- to 122-degree range.

You can calculate lobe separation by adding the intake and exhaust lobe centerline figures together and dividing the sum by 2 (Ex: 110 intake center line + 114 exhaust center line= 224/2= 112 lobe separation). This also is the figure needed to determine how much the intake lobe may be "advanced." To calculate intake lobe advance, simply subtract the intake lobe centerline from the lobe separation angle (112-110= 2). The difference is how much advance is ground into your cam, 2 degrees in this example. It's also important to remember that you cannot change lobe separation or intake lobe advance after the cam is ground. Lobe separation is not the same as valve overlap either, as some people mistakenly identify it. Overlap is simply the degrees that both valves are open at the same time (see diagram).

Sucp_0202_06_z Chevy_small_block_cam Solid_roller_camshaft 3/9

Since testing five cams back-to-back is a huge chore, we elected to use solid roller camshafts with 234/240 duration at 0.050 and 0.496/0.504 lift to eliminate any valve-to-piston-clearance problems and downtime due to cam break-in. Consequently, we had to be sure that each cam was properly lashed to a tight 0.015 inch for both intake and exhaust.

Typically with street cams, the farther you spread the lobe centers apart with a wide lobe-separation angle, like 116 degrees, the less overlap you get. Although both wide (112-116) and narrow (102-110) lobe-separation angles do have some effect on overlap, it's still possible to get tons of overlap with a very wide lobe-separation angle. Race cams like those used in Pro Stock and Pro nitrous applications usually have a very high lobe-separation angle of 116 degrees and above, yet still have lots of overlap because the intake vale opens very early in the cycle while the exhaust valve has not closed very far yet.

It's the scavenging effect caused by this that pulls the fresh fuel/air charge into the chamber. Why is all this important? It all has to do with the cylinder heads, exhaust system, and how well the engine will breathe. If you've got inefficient cylinder heads or a poor-breathing exhaust system, the wrong lobe separation angle might not pull enough fresh charge into the cylinder and you'd lose power

The Power Of Separation
How much effect a wide vs. narrow lobe-separation angle has on power is precisely what we wanted to test, so Comp Cams ground five identical-duration solid roller camshafts with a different lobe-separation angle on each for us to try. The test was simple: Install all five cams straight up in the same engine with no changes in between. Since we wanted to cover the broadest separation spectrum for street cams, we asked Comp to grind 106-, 108-, 110-, 112-, and 114-degree lobe separations to see how that alone would affect power.

Our rather mild small-block was a 0.030-over 350 using a few good-breathing parts to make power. Basically, our tests showed that most of today's cam manufacturers have it correct when they grind their cams with a 108-110-lobe-separation angle, because it was within this range that our test motor worked the best. The difference in peak power, made between the best and worst cams, was more than 20 hp with the 108-lobe cam making the most at 475 hp and the 112-lobe cam making the least at 452 hp.




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