| SSDiv6 |
07-05-2010 01:31 PM |
Re: Why did nhra change the cam and valve spring rule in 1985?
Quote:
Originally Posted by Billy Nees
(Post 196013)
The trouble with setting a specific # for valve springs in this day and age is that the cam manufacturers are building cams that a flat tappet lifter just can't follow even with the best springs! I believe that some of the really sharp Stocker engine builders are actually getting away from "square" cams and going back to "softer" lobes at least on the exhaust as they're finding they're "cleaning up" the intake charge by not bouncing the exhaust valve around on the seat. Think about what the "harmonics" of that exhaust valve bouncing on the seat is doing to the intake charge not to mention the cylinder pressure.
Another good reason for "softening" the lobe is simply to use less spring. Heavy springs just "tie up" horsepower that can't be used to accelerate the vehicle.
I guess where I'm going with this is that as much as I'd like to see a "valve spring spec" rule put in the book it would make a lot of cams in use today unuseable and I don't forsee any of todays Stocker engine builders or cam manufacturers lobbying for a rule that will make their stuff unuseable.
But these are just the ramblings of a bored, crazy old fool living on a hill in Pennsylvania.
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Billy, there is a lot more in camshaft design and in the early days, it was mostly hit/miss. Camshaft technology has taken great leaps with the advent of computer modeling.
In the early days when cams were checked and with the early spring rules, cams where still being designed and built with "Dwell" lobes or what many call square lobes. Let's not forget the days when many ran General Kinetics Cheater cams. Almost all of Don's cam designs were dwell lobes. The cam manufacturers of the early days for cheater cams were General Kinetics, Lunati, Cam Dynamics and Eonics. Later on, other companies such as Crane, Isky and Comp Cams joined the fray when the spring rule changed. In the early days, if you had a Chevy, you ran Lunati; if you had a Mopar, you ran a GK cam; if you had a Ford, Cam Dynamics, if you had an Oldsmobile or Pontiac, you ran an Eonics cam.
Although there were spring pressure limits in the early days, the specification sheets sent by the manufacturers were quite liberal especially with Mopar.
The big difference on the new generation Cheater camshaft is they have faster ramps with bigger numbers in 0.200" lift area. |