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Turbo Camshaft Guide - The Truth About Camshafts And Turbochargers

Tech Editor Smith debunks some long-held turbo cam myths

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Overlap Chronicles
Overlap is defined as the number of crankshaft degrees of rotation established between when the exhaust valve closes (EC) and the intake valve opens (IO). This is established by several factors. We discussed this idea with Comp Cams’ lobe designer Billy Godbold. Godbold says the popular belief is that the lobe-separation angle is responsible for the amount of overlap, but that is only partly true. The important other half of the equation is the length of both intake and exhaust lobe durations. If either intake or exhaust duration increases, it will affect overlap. The accompanying Comp Cams illustration makes this easier to understand. If we move the intake and exhaust centerlines closer together, the angle gets smaller—as from 114 degrees to 110 degrees. When this happens, that small triangle that indicates overlap increases in size.

With a given lobe-separation angle, overlap will increase with added duration. We’ve included a short explanation of how to calculate overlap from the opening and closing points given on the cam card. In our case, the specs for these three Comp cams are offered at 0.006-inch tappet lift. As you can see, increasing the duration from the smallest to the largest cam increases the overlap by an amazing 12 degrees, even though the lobe-separation angle remains at 110 degrees.

How to Calculate Overlap
Comp Cams XR276 HR hydraulic roller cam PN 12-423-8
Duration: 224/230 degrees at 0.050 at 0.006-inch tappet lift
Cam installed at 106-degree intake centerline

Overlap = Exhaust Closing (EC) + Intake Opening (IO)

Intake events: IO = 32 BTDC; IC = 64 ABDC
Exhaust events: EO = 75 BBDC; EC = 27 ATDC

Overlap = 27 + 32 = 59 degrees Now let’s look at three hydraulic roller camshafts with three different intake and exhaust durations yet with the same lobe-separation angle of 110 degrees. All overlap figures are given at 0.006-inch tappet lift.

Comp XR 270: 218/224 degrees duration at 0.050
Overlap = 24 + 29 = 53 degrees overlap

Comp XR 282: 230/236 degrees of duration at 0.050
Overlap = 30 + 35 = 65 degrees overlap

Comp XR294: 242/248 degrees of duration at 0.050
Overlap = 36 + 41 = 77 degrees overlap

Even though the lobe-separation angle of 110 degrees did not change with these three cams, the overlap increased a total of 24 degrees because both the intake and exhaust durations increased by 12 degrees with each larger cam.

SOURCES
Comp Cams
3406 Democrat Road
Memphis
TN  38118
800-999-0853
http://www.compcams.com
Kurt Urban Performance
Commerce
MI
248-345-8169
www.kurturbanperformance.com
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