|05-07-2005, 02:28 AM||#1|
Join Date: Sep 2001
Location: At the end of the longest line
Tech File: Cams cams cams!
A lot of gearheads (or "tuners"...whatever) out there don't tend to do their homework when purchasing aftermarket cams. This is generally why so many cams have become marketed as "stage I" or "stage II". This cookie-cutter strategy is not only crap from a logistical stand point...but it insults the buyer's intelligence. There are really only a few things to understand when looking at a cam and deciding if it's right for you.
This one is really pretty simple. Duration is just the amount of time that a given valve spends off of it's seat represented in crankshaft rotation. For instance, if an intake valve was open and off of it's seat for half of the cranshaft's rotation, the cam's duration would be 180 degrees on the intake side. Generally in the performance industry, however, duration is measured "@.050 inches". This simply measure the crankshaft rotation between the point when the valve is opened to .050" and when it gets back to that point as it closes. It's important to be sure and to know if the duration you're seeing is "advertised" (this essentially means total duration to SAE specs) as opposed to duration at .050" since both mean different things to your engine.
Duration will effect your engine's vacuum as well as your engine's rpm capabitlity. The more duration you use, the higher your powerband will be moved upward. Likewise, you'll also get less vacuum since the valves are opened longer. The safe limit for cam duration vs. vacuum on a street vehicle is generally around 225 degrees @ .050"...although this can change from motor to motor. But generally you'll have problems with things like vacuum assisted brakes without use of a cacuum booster of some sort.
I don't think much energy needs to be spent here. Lift is simply the distance that the valve will be away from it's seat. Always be sure to test for any piston-valve interference when installing a cam with a high than stock lift.
Lobe Seperation Angle
This is something virtually no-one pays attention to when ordering a cam. Lobe seperation angle (or LSA) essentially refers to the distance from one lobe's peak to another's. For instance, if you were to physically draw 2 straight lines out from the centerline of a cam...one to the very peak of the exhaust lobe and one to the peak of an intake valve...and then measure the angle where those two lines intersected, you'd have the cam's LSA. Most cam LSAs fall into the 104-118 degree range. The lower you go numerically the higher the powerband becomes since it's directly tied into duration. Most factory cams tend to be in the 115 degree range. This gives you good driveability that won't fall flat on it's face if you choose to stand on the loud pedal. Someone looking for more kick in the mid-to-upper range would probably look for something in the 110-112 degree range. ranges as low as 104 degrees do exist, but almost always in serious race applications.
Dual Pattern Cams
A pretty simple premise. Dual pattern cams get their name from the fact that the intake and exhaust durations and/or lifts are different. In almost every case it is the exhaust duration that is greater. This causes the exhaust valve to be opened for a short time while the intake valve is opening. The result is that as exhaust is dragged out, it's helped along by incoming air and incoming air benefits as well as more of it is pulled in due to the pressure effect. This effect is especially present at higher rpms.
Again...pretty much what the name implies. This is the ramp that the lifter/follower/rocker arm (depending on the engine setup) will follow to open the valve. Performance cams usually use more aggressive ramps on the opening side and softer ramps on the closing side. A more aggressive ramp will generally improve idle quality and throttle response since it facilitates better vacuum. But buyer beware...anytime you see "aggressive opening ramps", read it as "beef up your valvetrain" since the valvetrain will be put under more stress when the valve snaps open.
These are merely what I feel to be the more important aspects of cam selection. There so many factors to consider when selecting a cam that to list them all would involve a seemingly endless post and most likely a mild heart attack for me. Always be sure to talk with a specialist when ordering a cam to decide what is going to work best for you so you're not stuck with a poor investment.
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