Quote Originally Posted by 4 l-bodies View Post
Asa,
What I have found is running aftermarket camshafts is there is a definite loss of low end torque in lower RPM band, then of course "hopefully" there will always be a crossover point (RPM) where larger cam will be overtake the smaller camshaft and make it worthwhile.

For example it was found that Taft S2 gives up like 20 lb/ft down low (below 3500 RPM), approaches stock cam torque by 4000-4500 RPM and adds another 20 HP and 10 lb/ft above 4700. The larger S3 lost 30+ lb/ft below 4000 RPM. Then above 4700 RPM, it crushes the stock cam!

Many times what happens in not well thought out powertrains is powerband becomes tighter or more narrow because rest of powertrain isn't up the the task of increased RPM with the larger duration camshaft. Could be rod ratio, could be valvetrain limitations, turbo, gearing, converter, Say for example running a large duration camshaft in an engine that doesn't want to run much RPM, or tiny duration camshaft in a high RPM engine. Probably not a good combo, either way.

A perfect example of a not very well thought out powertrain was an old friends 351 4bbl (4V) Cleveland Mach 1 Mustang 25-30 years ago. Put in much bigger camshaft, and slightly higher stall converter, single plane manifold, along with not very deep gear. The vehicle was real dog until 4800 RPM. Way too big of ports for that size motor, not enough gear, and probably needed a looser converter. Very narrow powerband. From 4600 to 6400 it pulled pretty good, but then it was over. Made for a boring and lazy car to drive on the street. His buddies 351 Cleveland 2BBL headed Grande Mustang would eat it alive on the street. At the track, the 4V headed 351 would prevail, because he was always in that very narrow powerband. My 69 Z-28 was also another low torque dog. Huge duration factory solid lifter camshaft. At least that had a much larger powerband. Not much below 4000 RPM, then WOW! The factory knew it was going to need deep gearing, so they came with 3.73 gears from factory. It needed an even deeper gear. That 302 needed 4.56/4.88 gears to really wake it up. Needed RPM to keep in correct powerband. Same deal with the Ford Boss 302 and the Boss 429 Mustangs.

IMO, when selecting a camshaft, there are definite compromises to be considered, just like turbo selection. Bigger isn't always better. It will almost always effect something like low RPM torque, ecomomy, driveability, emissions, powerband, valvetrain life, etc. It also obviously has many positives too. That is one reason why turbo and NA used a different camshaft profile. Compromises...

Todd
interesting..is there any explanation as to why torque is lost with by opening the valve longer or further? or is it that the location of the valve event relative to the crankshaft was changed and thats what impacted the low rpm performance?