What dictates the cam's success for maximum area under the output curve along with highest peak torque is not always the duration involved. The most important factor is actually the overlap and the Lobe Centerline Angle (LCA).  First, and of prime importance to a street-driven car / truck, is manifold vacuum. (Please keep in mind that this is relative to the discussion of our HEMI which is a 90* V-8) As far as idle quality and vacuum are concerned, it degrades rapidly with increasing overlap. The reason it does so stems from the fact that a V-8 has an induction phase every 90 degrees. This means that when one piston is moving rapidly on its induction stroke (about 90 degrees down the bore) there is another piston stagnant (TDC or at dwell) at the top of its stroke with the valves in the overlap position, i.e. both open.

If we are using a single plane intake manifold (90* design), gross amounts of overlap can actually pull exhaust gases out of the exhaust manifold, out the intake valve of the exhaust stroked cylinder and into the intake-stroked cylinder. If we simply replace the intake manifold with one of a dual-plane design (180*) we make it impossible to draw exhaust gases from one to another. As a result, vacuum increases by as much as 50% or more.

Given that we are not discussing intake manifold choices, lets only consider camshaft selection and we can see that gross amounts of overlap are not good for a street driven / daily driven car / truck.

Assuming we are choosing a cam for a streetable engine, how much overlap can we use before it becomes a problem?

The answer here is that it depends on the valve sizes in relation to the cylinder displacement, compression, and exhaust flow. If the heads have small valves in relation to cylinder cubes, then the amount of overlap we can use is significantly more than the same cylinder with much larger valves. If compression is raised, we can also assume the use of more overlap. Big (but not excessive) overlap is a prime key to big power numbers, but only if your exhaust system flows well.  If a big-overlap, big-cammed engine has an exhaust system with any measurable backpressure, the price paid is a big drop in output.

Spreading the LCA is a way of cutting the overlap while still retaining the duration. Unfortunately, nothing comes without a price. The downside of spreading the LCA to reduce overlap so a decent idle and vacuum are achieved has two major strikes against it. First, the piston comes further up the bore before the intake valve closes. At low speed this pushes the intake charge back into the intake manifold. The result of which is low-speed torque is reduced. But so long as it drives well, this may be okay if it helps top-end output. Mapping intake, cylinder and exhaust pressures throughout the cycle indicates that getting the first half of the induction stroke right is of paramount importance toward making the second half optimal. In other words, if the first half of the stroke is not optimal, there are no means of redemption on the second half.

This forces us to the conclusion that for a given duration, there is only one optimal opening point and one closing. This, in turn, means, within a small window, only one LCA gives optimal results. If the LCA is spread to preserve the idle and vacuum, the price paid is reduced torque and hp. Some of our competitors should have gone to a shorter cam on the correct LCA, as it would have produced better results however the moral here is that if the cam had been selected on the basis of overlap and LCA first, then the duration would have been decided by these two factors, not some arbitrary decision on the part of the "CAM EXPERTS".

Assuming the compression ratio remains constant, longer duration  moves the torque curve up the rpm range. Peak torque itself usually only increases a minor amount. The additional hp comes from the fact that the torque delivered happens at a higher rpm and power is directly proportional to torque times rpm. (HP = TRQ x RPM). However, if you don't feel inclined to run a compression to match a longer cam's requirement, then stick with a shorter one, as it will produce better results. For most engines with reasonable heads, the ability to raise low-speed torque with compression increases holds good to about 285-290 degrees of cam duration. After that, low-speed torque will drop off faster than further compression increases can recover it. A cylinder's breathing ability is not only dependent on the duration of valve opening, but also the amount of valve lift involved. The type of heads typically used on domestic V-8s are much more responsive to lift than, say, a four-valve OHC designed engine.

Our competitors have made mistakes when specifying cams with their "larger valved heads", "stock compression heads", and "big lift / duration cams controlled by reduction in LCA". Without doing their homework for them, Frank Racing Inc. is explaining to you, the customer, a brief peak into the science that goes into our cams as well as into our head design.

Frank Racing Inc. studies our ported heads on a flow bench and considers both gross volume at a given lift but also port velocity and we also consider what the cam is doing and why, as it relates to these numbers discussed prior. Customers have commented that the Frank Racing Inc. cam's seem small and commonly ask for something, "Just a little bit bigger" and say, "I want the most radical cam you have."

We try to explain to our customers that we made these cams for specific applications and to please define their application and choose the Frank Racing Inc cam that best matches their wants and desires. Please read the specifications on the website which clearly indicate where the cam makes power and what you need to consider when buying complimentary parts with the cam.

Written by: R. Lee Graham with special thanks to David Vizard and Comp Cams for their support