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All About That Base Circle

base-circle-illustrationOf all the things I do here at COMP Cams, nothing gives me more satisfaction than helping a customer achieve success with an engine building project.  Sometimes I get the call when the project is in the idea stage, or often, the customer has already purchased a few parts such as block, crank, head castings, etc.

One area I feel customers are too quick to compromise is camshaft base circle size.  No thought is given to what happens when you ask for that “small base circle cam.”  We have been conditioned to ask for this when building a Small Block Chevrolet with crankshaft strokes north of 3.500 inches.

I’m not sure that anyone knows, or even remembers, when performance enthusiasts started adding stroke length to the Small Block Chevrolet. Through the years, we have seen the 383 gain popularity.  Some years later, we started to see some enthusiasts opt for 4-inch stroke cranks.  I think it safe to say short track racers are the ones who started the trend of looking at crank stroke to increase the CID of the Small Block Chevrolet.

From an engineering stand point, I think you should look at how GM chose to deal with the rod-to-cam clearance with the 400 CID Chevrolet.  As you may know, this engine featured a bore size of 4.125 inches and a stroke length of 3.750 inches.  In this case, the engineers decided to look at the connecting rod rather than give the cam a smaller base circle. Unlike the previous small blocks that featured the 5.7 inch rod length, The 400 was equipped with a shorter connecting rod, 5.565 inches. This rod also featured a reconfigured bolt for cam clearance. My point here is that GM engineers made the decision to redesign the connecting rod rather than make the camshaft have a smaller base circle.

My recollection of small base circle cams comes from my time of working with local dirt track racers. Most were building 406 CID engines using junk yard blocks; at the time, the only other options were a new OE replacement block from GM or a Donovan aluminum block. Most of these were 4.1550-inch bore, 3.750-inch stroke, with a 6-inch aftermarket “H” or “I” beam connecting rod. A few of these, for our low budget customers, were what we called “Super Stock” rods.  These were stock 5.7 inch rods with polished beams and SPS bolts.  These required what is called a small, or .900-inch base circle cam. Cam lifts on these rollers was around .420 inch with a 104 to 106 lobe separation angle. I bring up the cam specs to serve as a base for why I think things are as they are today, and how exact terms came to be generic terms when discussing base circle.

Many times today, a customer will think or throw out the term “.900 base circle”.  The thinking here is whatever the cam specs are for my stroker engine build up, I need a .900-inch base circle cam. This is not always the case. Have you ever thought about what determines the base circle size of a camshaft? The two main things are lobe lift and journal diameter. For example, a .420-inch lobe lift roller cam should finish around 1.028-inch base circle size. When you request a .900-inch cam, you are taking .128 inch of base circle diameter away, dropping the lifter down an additional .060 inch in the lifter bore.  Below is a chart that shows the different base circle size for specific lifts.  These apply to a 1.868-inch, standard journal Small Block Chevrolet.

chart

In terms of clearing the connecting rod, the actual base circle is not what interferes with the rod.  It is the flank/nose, peak lobe lift area that is of concern. Shrinking the base circle pulls this point away from the rods.

There are a lot of options out there that will allow not having to sacrifice base circle. Most, if not all, of the connecting rod manufacturers offer a “stroker” rod. These will already have the shoulder of the rod clearanced to clear a standard base circle cam. These are often available at little additional cost over the standard rod. Most block manufacturers offer raised cam blocks, and COMP even carries timing chain sets for the raised cam blocks.

Dean Harvey has been a member of the COMP Performance Group family for 34 years, currently as an Engine Builder Sales representative working with customers in the performance street, drag race, circle track, road race and tractor pull markets. He lives in Memphis, Tennessee, and has been on various circle track and drag racing teams through the years.