Duration is measure from that point opening to that point closing.

IE: Take the ,050 figure.... .050" when opening , all the way to .050" closing.

Not the actual time it is at that lift.

http://image.stockcarracing.com/f/te...lift_curve.jpg

Here is how I try to interpet this info, because of the quicker valve speed caused by the ratio, the engine will effectively "SEE" a degree to degree and a half of duration, just as a bigger diameter lifter wheel will do the same but to a lesser degree. Now in my mind there are downsides to this with a hyd. lifter at higher rpm range that can be a power robber! if you are useing a hyd cam with a solid lifter and go 1.8 rocker I see that as no downside and a benifet to aid in cylinder filling/exhausting. Again thats just how I try to interpet this data!!!

Way to go Rookie. I new this would get fun. Honestly I've read a lot on this and right when I understand what I believe to be correct something else comes along and completely changes what what I thought I knew to be true. Kinda like the Big Bang theory. Makes my head hurt. Hah.

More importantly I think we need to get back the remark MT posted about Jim v's engine. Jims ports obviously flow with great velocity and with the valve timing of the cam it only requires so much to charge the cylinder. With more cam duration in his case would only back up in the intake manifold runner and beyond creating an inefficient engine. This difficult to explain and I'm not good at it however look at an engine like a little factory and assembly line. It doesn't take much to throw things off whether it be computers or workers running it. Abones touched on this a few times in the past but efficiency is everything to have a good end result.

look at a cylinder like a pvc pipe Bxxb that gets filled with powder then capped and ignited and off to the next. To little of powder isn't maximizing the blast but overfilling and igniting before it's capped isn't a good ending result either. Ok. Go ahead and laugh. Haha

I think the aftermarket heads have pretty much forced the cam manufactures to change their grinds over the years. Old gm heads that don't flow particular well maybe needed that extra duration, lift, valve timing etc. Most generally gm exhaust ports sucked over the years so you wonder why so many shelf cams have at least .020 more exh lift. Just sayin...

A lot times including myself most generally look at lift and duration but in reality maybe we should attempt to understand the opening and closing events. When we saw the similarities in bobs so called custom grinds it really bothered me. They weren't build specific at from what I could see. With different cubic inch, heads, cc's, intake manifolds and so on the valve timing events were very similar.

I'm just here trying to understand also but it's possible sometimes we do overthink the little things that really matter and make a big difference.

MT I know you and others have utilized the software a lot. I'm assuming it has the ability to change the opening and closing events. You ever find yourself going one way or the other real quickly regarding end results. Just curious how much ability those programs have.

i totally get how the wheel dia can make a minute duration change because it changes when lift starts and ends but i can,t wrap my head around how changing nothing except the rocker arm ratio can change the amount of time the valve is off the seat.what is meant by [effective duration]?

Again, duration figures are from the set point opening to the same closing.

Example:
.050" duration is from when it is .050" going up and then counting all the degrees till it takes to get to .050" again when closing
.100" duration is from when it is .100" going up and then counting all the degree till it takes to get to .100' again when closing

Those #'s are used based off of lifter rise and fall.

Those #'s don't change due to rockers because nothing effects it. They can't as the lifter rides directly on cam.

Now, the effective duration is what the actual valve see's. This is lobe lift x rocker ratio

.050" lobe lift x 1.7 = .085" valve lift
.050" lobe lift x 1.8 = .090" valve lift

So, obviously the 1.8 rocker get's the valve open to the same .085" lift earlier than the 1.7 rocker. It will not return it, when closing, to the .085" until a little later than the 1.7 does.

So, this is why the duration at the valve is longer with the 1.8's vs 1.7's.

Not to be confused with duration at the lifter. Which again, is how we typically 'talk cams.'

It doesn't change when a valve starts to open, but it will change duration at all lift points. .004" seat, and .006" seat is not when the valve start to move. That would be like .0000000000001 , lol. .004" and .006" are standards to compare cams/ As we know, Crane is pretty much omly performance cam company to use .004". Comp cams and pretty much every one else uses .006" to make the lobes 'look faster/"

Ok, lets say you had the "perfect" camshaft design. Lets say it spec'd a 246.5* lobe, for the intakes duration number, with a 1.7 rocker arm. (rocker ratio should be factored in before a cam is ground typically).

Anyhow, according to my software, that cam lobe would have

246.5* at .050
166.5 at .200

Going to a 1.85 ratio rocker arm, would change the cams design, to

243.5 at .050
158.5 at .200

Thats assuming, that you want the valves to open and close at XXX degrees. Like SB said, the rocker ratio, can change what happens at the VALVE, and that is what the engine sees.

this is a copy and paste from another form. When or if I'm ever asked by anybody this will be my reply.


Postby PackardV8 » Mon Jun 16, 2008 10:14 am

Hi, CamKing,

Good, simple explanation.

I have a distant memory of being lectured by some amateur cam geek, "The correct terminology is not more degrees of duration at a given lift, but, 'Increased area under the event curve,'" or something close to that. This was too complicated for me to calculate back in the pre-computer days, but today, with computers doing it easily, do you pros ever/still work with that as a comparison?

thnx, jack vines

Its almost like adjusting a solid roller, to different lash settings. That can change the power curve a bit