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Fuel Pump Problems: Ask Anything

Cam timing tricks and the advantages of an S-60 rear are the topics this month.

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Taking this one step further, the Light Metal Clutch system uses a steel cover over an aluminum pressure plate using a steel friction surface (similar to what is used on the aluminum flywheel). This further reduces the total weight of the clutch package and can get you down to an overall weight of around 40 pounds. This compares with something closer to 60 pounds with a conventional steel flywheel and Centerforce pressure plate. The bottom line is a lighter flywheel and clutch assembly will result in more power transferred to the rear tires to accelerate as opposed to using that power just to accelerate the clutch assembly. Of course, this acceleration advantage doesn't come without a price. Just the 11-inch aluminum flywheel (PN 900120) prices out at $629.95 from Summit Racing. The Light Metal Clutch assembly for the above flywheel is also pricey. A standard Dual Friction clutch and pressure plate package (PN DF 271675) comes in at $356.95 from Summit. Of course, you could use the Dual Friction package with the aluminum flywheel and save a little money, and the ultimate clamp load performance would be similar. You should use an SFI-spec flywheel (which all Centerforce pieces are) for this application because you don't want to witness what happens when a flywheel explodes at 6,000 rpm. We've probably created as many new questions as we've answered, but at least you have more information with which to make a decision.

A Matter of Timing

Eddie Allison; Houston, TX: I have been playing around with small-blocks for about 20 years. I also have a mechanical engineering degree, so I'd like to think I know something about camshafts. However, there is something I cannot seem to get my head around. How can you get a camshaft that has set specifications when the manufacturer says it is ground 4 degrees advanced? I understand that I can install a cam advanced or retarded, but once it is ground the specs are set. I was reviewing an old Hot Rod article, and in one table, some of the cam companies recommend certain cam specs that are ground 4 degrees advanced. So are they saying they grind 4 degrees from a certain blank? Are the specs they give the actual specs--the blank specs--or do I need to factor in some math for the 4 degrees? I like to know exactly what I am getting before I purchase and install. I have asked a few of my friends, but they don't get it either, so I hope I am not the only basket case.

Jeff Smith: When a camshaft manufacturer grinds a camshaft, the relationship between the intake and exhaust establishes the lobe-separation angle. Let's use a typical cam as an example and give it a 113-degree lobe-separation angle. It may be coincidence, but I've noticed that the intake centerline and the lobe-separation angle are always the same number if the cam is not ground advanced. So in this case, the intake centerline would also be 113 degrees. However, cam grinders often decide to grind advance into the camshaft before you install it in the engine. This means the cam grinder moves the intake centerline ahead relative to the No. 1 piston at top dead center (TDC). This is done for a couple of reasons. First, it's a fair bet that cam companies assume most guys will not make the effort to degree their camshaft, so advancing the cam compensates for a possible retarded cam position. Perhaps more important, advancing the camshaft also improves low-speed throttle response, as car crafters are famous for ordering camshafts that are a little too big. Long-duration cams sacrifice low-speed torque and throttle response, so advancing the cam is an easy way to recapture some of that lost torque and idle signal for the carburetor.

As you've noticed, the easiest way to spot this built-in advance is by comparing the intake centerline number with the lobe-separation angle. What the cam grinder has done is merely advanced the intake centerline in relation to what would have been its original position. If you are going to use one of these camshafts and you intend to degree it, it's important to know if it's already been advanced. If it has, you would not want to advance the cam further.

Over the years, I've collected some obscure cam-timing math you might find amusing/helpful/geeky depending on your point of view. Let's condense it down to the essentials.

We'll use a Comp Cams XE 284 flat-tappet hydraulic cam for a small-block Chevy as an example. The Comp specs for this cam are 284/296 degrees of advertised duration, 240/246 degrees at 0.050, with 0.507/0.510-inch valve lift and a lobe-separation angle of 110 degrees.

To find duration at any tappet- checking height (0.004, 0.006, 0.050, or 0.100 inch tappet lift):

  • Intake Duration = intake opening + intake closing + 180 degrees
  • Intake Duration = 34 + 69 + 180 = 283 degrees (advertised duration at 0.006-inch tappet lift)

Calculate exhaust duration the same way.

You can also find the intake centerline with a simple equation:

  • Intake Centerline = (intake duration / 2) – intake opening
  • Intake Centerline = (283 / 2) – 34 = 107.5 degrees
  • Exhaust Centerline = (exhaust duration / 2) – exhaust close
  • Exhaust Centerline = (295 / 2) – 35 = 112.5 degrees
  • Lobe-Separation Angle = (intake centerline + exhaust centerline) / 2 (107.5 + 112.5) / 2 = 110 degrees

There is a discrepancy between our calculations and the published cam card, as the cam card lists the intake centerline at 106 degrees. That could be attributed to this being an asymmetrical lobe in which the max lobe lift point used to determine intake centerline might not be the true lobe intake centerline. Or it could just be simple rounding off of numbers. The numbers are so close that a 1.5-degree difference isn't critical.

So by our calculations (using the 0.006-spec numbers listed on the cam card), the intake centerline is 107.5 degrees after top dead center (ATDC) while the lobe-separation angle is 110 degrees. That means that the intake lobe was ground 2.5 degrees advanced relative to the straight up lobe-separation angle of 110 degrees.

2266 Southwind Drive
Prescott  86301
Strange Engineering
8300 North Austin Avenue
Morton Grove
IL  60053
Comp Cams
3406 Democrat Road
TN  38118
Weldon Racing Pumps
640 Golden Oak Parkway
Oakwood Village
OH  44146
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