badamn

alright iv'e studied the cam guide multiple times.could someone just tell me what part of the grind in a cam is duration?I don't want to know how to calculate it.because in my mind all cams have the same intake and exhaust duration.....it's 180 degrees!no matter what size lift.

batboy

iTrader: (6)

Duration is how long the cam holds the valves open. It's expressed in degrees of crankshaft rotation. Cam specs are confusing because they are stated as "advertised" (measured at .004" or ,006" lift) and the more common spec of measuring at .050". Take my cam specs listed as 230/236, .591/.601, 112 LSA (at .050"). The 230/236 is duration degrees (intake is listed first and exhaust is listed second).

badamn

ok let me ask this another way.why does the lift of your cam lobes change the duration(230-.591)(236-.601)is it becuase you're that much further away from the(@.050)starting point?

batboy

iTrader: (6)

It's mainly a function of the lobe and ramp design.

MrBluGruv

I think you are misunderstanding him here, the maximum lift per intake or exhaust, respectively, never changes, because the intake and exhaust lobes are separate from each other.

I dont mean this as a dig, but have you actually looked at a camshaft? If not, take a good look at one, it might make things more confusing at first, but after seeing it and then reading more, it will probably help things to make more sense.

One approach is to think of the intake and exhaust sides as completely separate from each other until you are talking about overlap.

So in his listed spec from above, the intake side of his cam is open and above .050" of lift for 230 degrees, and it has a maximum lift of .591", while the exhaust side of his cam is open and above .050" of lift for 236 degrees of rotation, and it has a maximum lift of .601". The lobe separation angle (LSA) is telling you how many degrees of rotation the maximum lift points of both sides of the cam (intake and exhaust) are apart from each other. Knowing that number, as well as knowing how long your duration for each side of the cam is, you can then calculate the exhaust/intake overlap, or for how many degrees of rotation on the camshaft that both sides are opening valves in a given cylinder.

badamn

thanks for trying guys...maybe i'm making this too complicated.I know that duration is measure by degrees of the crankshaft but it also has to be some part of the surface of the cam lobe whether it be the nose the ramp or a combination of both so which is it?

Also are you trying to tell me that lift has nothing to do with duration because i think it does.Anyone?

garygnu

like stated above different lobe designs have different lobe ramp rates.you mite hear cam lobe intensity .the more rapid lift lobes need more spring pressure to avoid valve float.

MrBluGruv

Yeh, I think you are essentially looking at all the individual facets of a cam as one thing, sort of. You have to understand what the cam is actually doing, to then understand that the exhaust and intake side are separate of each other, to then understand that for each side the lift and duration are mostly separate of each other.

Working from the smallest point to the biggest point (at least, in my mind this is the progression of scale it follows):

Each cam lobe has two components: lift, and duration. Lift will tell you how high the valve will go when fully opened, and duration will tell you how long the valve will be open for at a given lift (in cam specs, that is why you will sometimes see two duration figures listed). This effective ratio between lift and duration tells you the ramp rate of the lobe; if you have extremely high lift and extremely short duration, there is no avoiding the fact that the valve goes from closed, to open, to closed again in very abrupt timeframes, which can be exceedingly straining to the valve springs. Realistically, I think that that ratio is somewhat limited, though.

Moving on, both the intake and exhaust lobes per a given cylinder have their own lift and duration specifications. Some cams are designed so that both the intake and exhaust lobes have the same specifications, but that is not a requirement of cam design; the point being, these two lobes are independent of each other completely aside from one facet: lobe separation angle.

Lobe Separation Angle (or LSA, as you will see a lot on here) refers to how many degrees of rotation the maximum lift points of both intake and exhaust lobes are apart from each other. When you know the duration of both the intake and exhaust lobes, as well as the LSA, you can then calculate, using a defined formula, the total amount of open-valve overlap, in degrees, that the engine will see from the cam. By design, decreasing the LSA for the same lobe durations means an increase in overlap. Generally speaking, higher overlap is what causes the rough, choppy idle sound that so many of us love, but the cubic inch volume of your engine also can affect how much overlap you can run before it starts getting choppy sounding.


Now as far as camshaft and crankshaft relationship, all you need to know really is that the camshaft spins half as fast as the crankshaft in a 4-stroke engine (which is what the LS1, as well as pretty much any gasoline-fueled car engine, is) and that they are always aligned to a certain relationship so that the valves open and close at the correct point in relation to where the piston is and at which stage of the 4-stroke cycle the given cylinder is at. The cam specs have nothing to do with the crankshaft as far as lift and duration are concerned.

badamn

i understand how a cam works...intimately in fact.but if you measure duration in time or degrees it's always the same number from the point of lift to the point of closing,it's 180 degrees!or 360 degrees for the crank.The only way you could get more duration by these measurements would be to have a cam lobe that planes outward instead of inward so that the cam lobes would look like they are upside down on the crank and maybe they do...i've never bought one or seen one with a 180plus duration.So either the upside down cam lobe is how they achieve that or the maximum lobe height is calculated into the duration.

MrBluGruv

Well, either you are extremely far beyond my level of understanding, or you are greatly over-estimating your own, lol. Either way, I'm pretty lost.

I thinking now, re-reading again what you've posted, you are asking how it is they achieve a bigger or smaller ratio between lift and duration? If so, the absolute duration is not dependent upon lift, however if you try to measure duration at a higher lift point, that particular point will show longer duration with an increase in lift. I'm not sure how you would go about minimizing or exaggerating that effect.

I was under the impression that cam lobes were never manufactured with concave portions, as it would be absolute hell on the valvetrain though given the effective ramp rate, but I could be mistaken.

I hope I'm at least kinda catching on to what you're wondering about...

bww3588

iTrader: (49)

Easiest explaination is this...

Think of it as a speed bump in a parking lot. The wider the bump, the more time you vehicle will spend not parallel to the ground. That's duration. The distance your car moves up in elevation relative to the flat ground is lift.

You can change max lift and not change duration at all. The lobe just has to get taller, more quickly.

Next time you're in a parking lot, look at the speed bumps and compare them to speed bumps in a different parking lot.

badamn

so you're saying that the duration of the cam is in fact the "width" of the nose of the lobe.yeah thanks mr blue guy too.thats kinda what i wanted to know.The reason i even made this thread is because when i was researching cams i noticed that the durations increased along with the lobe height(i've been browsing cams for a long time and never noticed that before).For instance you can't find a cam with a 230 intake duration without having the height of the lift be over .600.On every single cam I came across they went hand in hand but again they are measurements of two different things so what are they related?

badamn

Yes i am saying that the duration is dependent on lift at least by the way they make cams at thunder and tick and texas speed as stated below...i'm not sur what u mean by absolute duration?

MrBluGruv

Remember too that as valve spring construction has improved, the ability to run higher lift cam specs has also become more widely available, so generally speaking, off-the-shelf cams have been constructed with higher lifts. If you are able to find older cams even for the LS1, you will find cams with similar duration to what you see now, only with lower lifts.

From an even less complicated standpoint: generally you make more power by increasing duration and/or lift, so as the cams you're looking at get "hotter", both of those aspects will be increased.

badamn

thats kinda the answer i was looking for.The limitation of the spring heights i mean.

TheBlueKnight

iTrader: (4)

Hey Patrick G recommended a cam for us to go with. Ls1 hot cam. 219/228 .525/.525 lift 112 LSA. With the heads we are looking at it should be close to 400rwhp and better low end power without having to rev out the engine like with the LS6 cam. We can also use our stock pushrods and Ls6 springs that will last a long time as the lift is so small. Saw quite a few dyno graphs for the hot cam. Looks like it adds about 25-30hp. Then the heads add about 25-30hp as well so it sound about right on, 50-60hp gain.