Proper cam selection......

We touched a little on proper cam selection in the last post, but I thought I'd expand on it more to help people not "overcam" their cars and actually have worse performance than stock, rather than better. Here's two general rules that will save you a ton of grief. # 1. Larger engines can tolerate more "cam" without ill effects because they generally have more low-end torque to begin with, and can afford to lose some in exchange for a mid-range and top-end power increase.  Here's a couple of good examples-the old standby-the "350 hp / 327 Corvette cam  ( p/n 3863143 ) ( how's that right off the top of my head? )  will absolutely kill a 305, works pretty good in a 327 with a 4-speed, better in a 350, and is really sweet in a 383 / 400 regardless of transmission. The vaunted Pontiac "Ram Air IV" cam ( p/n 9794041 ) "hits" like gangbusters about 3,000 rpm and pulls hard to 6,500 in a 400. The truth is, you give up quite a bit of low-end and mid-range torque for top-end rush.  It only makes 10 inches of vacuum and has a badass lope at idle in a 400.  It works best with a stick and stiff gearing. That's why they were only available with 3.90:1 or 4.33:1 gears from the factory!  Automatic versions had a 2,500 rpm converter from the factory.  Put this same cam in a 455, and it smooths out quite a bit. It still has a noticeable idle, but it will work with an automatic with a stock converter and 3.23 to 3.73 gears, and pulls hard from 1,500-5,700 rpm. The factory initially put this cam in 1973 SD-455 T/As with 3.42:1 gears with either a 4-speed or a TH400. ( The 308 / 320 duration cam barely passed emissions, but it was too close for Pontiac and the EPA's liking. It was swapped for the slightly milder 301 / 313 duration RAIII cam, and the hp was down-rated from 310 to 290.)  # 2. A lot of manufacturers use "advertised duration" to make the cam look "bigger" than it really is, and obviously sell them to the unsuspecting, ininformed public. Don't go by advertised duration. The industry standard measures duration at .050 lift, and these numbers are more "apples to apples". For example the above mentioned RAIV cam has 308 / 320 degrees advertised duration. At .050 it measures out to 231 / 240 degrees. Duration is the length of time the valve stays open. Since an engine revolution is 360 degrees, you can see that 240 degrees is a lot of duration. A good rule of thumb is this-if you have 350 or more cubic inches and your cam has less than 225 degrees of duration, then you don't need a high-stall torque converter. The other factor is manifold vacuum at idle. For most cars with power brakes and other accessories you need at least 10-12 inches of vacuum at idle to operate properly. 14 or more would be better for a street machine. Most cam manufacturer's catalogs will list these specs and tell you-"Works best with 3.55 or stiffer gears"  or "Requires 3,000 rpm converter with automatic" or whatever.  # 3. Consider a modern aftermarket cam. You have to realize that GM , Ford or Chrysler, most factory performance cams and their exact replicas ( Crane "Blueprint" Series ) are based on 1964 camshaft profiles and are not computer optimized. Competion Cams Xtreme Energy series is excellent, as is Lunati's Voodoo and Streetmaster series. Sometimes a short-duration, higher lift design is the way to go. The Edelbrock Performer / Performer RPM / Torker-Plus series is an excellent choice as well if you follow the catalog's recommendations. The right cam can make or break an otherwise great engine's performance. Mastermind