[CougarAOD]

When looking at cams, they are usually listed with the
advertised duration measured @.006 tappet lift and the duration
@.050. The comparison of the two durations will show the ramp speed
or intensity of the cam lobe.
An example of a slow ramp cam would be the Edelbrock Performer
cam for the 351c, it has an advertised duration of 282int/292ex, but
the duration @.050 is only 204int/214ex. This cam has a very slow
action to it.
An exapme of a fast ramp cam would be an Ultradyne Cam I was
looking at, it had a 276int/276ex for advertised duration and the
duration @.050 was 221int/221ex. This is a fairly fast ramp for a
hydraulic lifter cam.
The comparison of these two cams is in the single lobes, not the
comparison of single pattern versus dual pattern cam. Ultradyne
appears to no longer be in business.
I feel that the fast ramp would give you "the best of both
worlds" so to speak. In the way of it having the smaller advertised
duration close to stock cam status, but with the .050 duration closer
to a high performance cam. The longer "open" time would provide more
air flow, yet still have the drivability because the "closed" time is
near that of a stock cam. The bad effect of a fast ramp is with the
higher intensity, rate of wear will increase and the longevity is
reduced.
Now other then the longevity from an "easy on parts" slow ramp,
what application would a slow ramp be better suited in then a fast
ramp? I am wondering which profile would be better for say big or
small exhaust, big or small ports, or big or small carbs? How would
the ramp speed compliment the resonance of the air/fuel charge with
the different big/small ports, exhaust, and intake?
Cam companies make both style cams, they have big cams in fast
and slow ramps and small cams in fast and slow ramps. That leads me
to believe that application would determine one over the other. I'd
think the focus would be to have the valve move faster, so more time
can be spent in the open time without reducing the closed time and
spend less time getting to either in the transition time. It seems
the trend is to make the ramps faster, but the slow ramps are not
outdated and still widely offered by many cam companies---which leads
me up to this question.

Let me know what you think,
Mark Piechowski
70 XR-7 460 AOD
69 SS 351c 2v AOD
70 XR-7 conv 351c 4v FMX
69 Sunroof Eliminator 351w 4v FMX

 

[Logan]

Woooooooooooooooooooooooooooooooooooosh!!!!

(That's the sound of the previous post going over my head at high speed)

 

[Cougrrcj]

Cam science = Voodoo

There are many, many considerations to picking a cam. Weight of the car? rear gearing? intended rpm usage? -do you want power down low or high rpm? Intake efficiency? headers? port work? exhaust backpressure? power adders? -what kind? valve size? Compression?

OK, as a general rule, conventional hydraulic lifter cams have long slow ramps. This is to make them quiet and to help with longevity. The valves are slowly opened and closed.

Mechanical lifter cams have a 'lash' or clearance betweed the rocker arm and the valve stem. This way, the cam can slowly push the lifter like a hydraulic, but then, since there is that gap or lash, the valve stem itself gets spanked hard and opened rather quickly and closed rather quickly. A mechanical lift cam is noisy, and the lash must be set frequently to account for the increased wear due to the stresses involved.

The problem with both those conventional flat tappet cams is they are limited by the geometry of the lifter/cam juncture. The cam slides the entire diameter of the base of the lifter. This causes wear. The 'faster' the ramp, the more wear. If you want to go even faster, the dynamics if the flat tappet prevents you.

A roller cam eliminates the slide. It has lifters with a roller at the end of it. This way, the cam rotates and the roller lifter literally rolls along the profile of the cam. Less stress and wear. Also, the contact point of the roller lifter is easier to control. With a roller you can really spank the valves open since the stress is relieved at the cam/lifter. 5.0 liter HO Mustangs have used roller cams since '85. All 5.0s used roller cams since 88. The reason? high lift/short duration gives high MECP (Mean Effective Cylinder Pressure), thereby increasing efficiency.

I could go on and on about cam science, but that's just the basics.

 

[modifiedmark]

I will say that in most cases fast ramps work well. I have bought a lot of cams over the years, every thing from Chet Herbert regrinds to blue racers to Crowers and you cannot beat a big cam company for there recomendations. I know crower used to not be shy about grinding one just as you need. As far as durability goes, dont worry about it, in your street cruiser motor you cant hurt one if it is set up properly with the right valve springs. Fords have a advantage over gm by having a larger dia lifter which gives it more surface area to work with. I once run a class that did not allow roller cams and ran a crower with a 603 lift and if I remember right the @.050 duration was around .256 with a 105 deg lobe center. The most the chevys could run was about 575-580. I used to run the little 377 windsor at about 7500 rpm all night with no problems at all. It kicked its share of 406 chevys asses also.mm