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Im looking at cams. Im under the old belief that the higher the lift the more lopey the idle. I know that is wrong. I have learned that a cam with a 114 LSA is more tame than a cam with a 110 LSA. Is this correct? Can someone please explain how to figure out which cams are choppy and which are tame?
You're on the right track. Lift has nothing to do with lope, it's all about lobe separation angle and the timing events of the intake and exhaust valves.Quote:
Originally Posted by sefrayser
LSA is figured this way: If a cam is cut with an intake centerline of say....106 degrees ATDC and an exhaust centerline of say...114 degrees BTDC, you add the two together and divide by two....106 plus 114 is 220, divided by two is 110 lobe separation angle.
The smaller the lobe separation number, the snottier the idle, the lower the manifold vacuum and the motor wants to make power in the lower half of the rpm range more than it does up high. The bigger the lobe separation angle, the smoother the idle, the higher the manifold vacuum and the motor wants to make power in the upper half of the rpm range more than it does down low. Most hot rod cams will be ground on a 108-110 degree lobe separation angle and most hot factory cams will be ground on a 114-116 degree lobe separation angle....likely has something to do with manifold vacuum and power brakes.
Here's a degree wheel showing the timing events of a cam in advertised degrees....this is a 280/280 grind....This cam might be 220/220 to 230/230 @ 0.050", depending on ramp design.
http://www.imps4ever.info/tech/camsh...ing-schema.gif
This wheel shows the intake centerline at 102 ATDC and the exhaust centerline at 102 BTDC. Add the two together, get 204, divided by two, you get 102 lobe separation angle.
http://www.alfabb.com/bb/forums/atta...1&d=1189535925
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Best explanation yet!
I made several changes after I first wrote it, so go back and read again. :3dSMILE:Quote:
Originally Posted by sefrayser
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duration has more to do with lopeyness ( spelling ? ) if you have an engine with a stock automatic trans and converter then you will not be able to run much more than a cam with a duration of 280 or so .. and it will not rattle windows as you wish .. if you have a high stall or a stick shift and want a rumpity idle then go for a duration more than 300 degrees .. i`ve never ordered a low duration high lift cam so i dont know what it would sound like .. i always order high duration cams and the lift is whatever it is ..
Tech- any thoughts on split duration cams?
The Camfather (Ed Iskenderian) says it better than I could.....Quote:
Originally Posted by firebird77clone
Tech Tip - 2003
Longer Exhaust Duration: Is this really necessary?
Most stock camshafts from American production V8, V6 and 4 cylinder engines manufactured today are ground with the longer exhaust lobe duration. Or, another way of looking at this is that they are ground with shorter intake durations! The former embraces the viewpoint that either the Exhaust Ports or Exhaust Pipe system is somewhat restrictive, and is in need of an assist. The latter suggests that the intake system is rather efficient and cam timing can be trimmed back a bit with out much sacrifice in power, in order to maximize throttle response and cruising efficiency.
Take your pick here. There is no absolutely correct viewpoint - because both are probably true! In a stock engine running at conservative RPM levels, for the sake of overall efficiency, fuel economy and a quiet smooth running engine, this staggering of intake and exhaust duration is quite common and appropriate.
However, High Performance is another thing entirely. Change one factor, let's say in this case, the exhaust system (installing headers and larger pipes) and you have just negated in most cases, the need for that longer exhaust lobe. Now couple this change with a different intake system and camshaft and you have really scrambled the equation. But, wait just a moment. Why is it that so many people (racers & cam grinders alike) insist on running a cam with longer exhaust duration regardless of what equipment is employed? The answer is "habit". Most of them have been somewhat successful in doing it their way and will probably never change unless virtually forced by circumstances to do so.
Before we go any further however let's review what it actually is we are trying to do with an engine when we attempt to make more power. Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!
Example #1 (Oval track racing) Here, I have often observed that the most experienced drivers are those who are most likely to run a single pattern (equal on intake and exhaust duration) cam. Why? Because such cams always, I repeat always make more torque! These veterans have a more educated foot and greater experience in feathering the throttle in the corners. They can therefore, utilize the benefit of added torque, in the lower to mid RPM range, to their advantage.
Their counterparts, the younger drivers on the circuit, generally are not as experienced and may at times actually get "crossed up" in the corners especially with a lighter car or when they are learning the ropes. In their case, a longer exhaust duration is often the more appropriate choice. It will often help them to drive better, more "flat footed" if you will, without consequence. But please for the sake of accuracy, let us be truthful. The benefit comes from an actual bleeding off of low to mid range torque, which is always what happens when Exh. Duration is lengthened, not from any improvement. The improvement, (if any) would come because of an improvement in scavenging at the extreme upper end of the power curve and would usually be marginal at best. Yet the so-called "extra power" potential of a longer Exh. Duration cam is most often why they are touted - power most people are backing away from at the end of the strait away!
Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well.
One of my favorite expressions is how "The Drag Racing mentality has infiltrated the ranks of Oval Track". Many have crossed over and made the switch in the past 10-15 years and some have brought their preconceived notions about how to cam an engine with them. A few may actually read these concepts and if they do so will at least come away with a better understanding of what they are doing. On the other hand they also could find that this information might actually help their cars to run just a bit faster!
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Wow, all I wanted and more.
Thx Tech.