Asymmetrical Cam Lobes---Why???

[DeputyDawg]

I am trying to understand the thought behind a camshaft having asymmetrical lobes and I need some help. It is my understanding that it simply means that the opening and closing ramps are different in shape allowing different opening and closing speeds on the same lobe. Is there more to it than that? Is this just another way for example to crutch a bad flowing exhaust port by allowing the exhaust ramp closing speed to be slower than the exhaust ramp opening speed?

[steelcomp]

Sort of...It's not really about poor port vs. better port, it's about the relationship between the two ports. (Intake vs. exhaust) In general, I think you're looking for the ex port flow to be about 75-80% of the intake. For every percent of differential varying from that, you adjust the cam to compensate in either direction. You baseline your intake lobe based on all the particular paramaters of your engine. Then, if the ex port flows more than 75% of the int., then you nock off a little flow on the ex lobe. If the ex port flows less than 75%, then you increase the flow a little. Obviously that's an exagerated oversimplification, but you get the idea. It's why it's imperative to know at least the advertised flow numbers for your heads, your engine specs AFA bore, stroke, rod length, rpm range, c/r, etc. That way you can build the information for a cam that's precisely what you need, instead of guessing, and either getting lucky...or not.
Go back and read the "selecting a camshaft" thread here in Gearheads. It's pretty informative.

[DeputyDawg]

I may not have worded what I was trying to say properly. I am not talking about a cam being dual pattern. What I am talking about is a cam that has asymmetrical lobes(most modern hi-po cams) wherein one side of the lobe is a different size and shape than the other side of that very same lobe. What goes into figuring the different shape of each side of a camshaft lobe? I agree that having your heads flowed is an absolute must if you really want the right cam, I would just like to try and understand the design relationship of each side of an individual lobe itself. I am trying to understand the whole camshaft selection thing on a little deeper level, but here lately I have been thinking about it so hard that I can't see the forrest for all the trees so to speak.

[steelcomp]

I may not have worded what I was trying to say properly. I am not talking about a cam being dual pattern. What I am talking about is a cam that has asymmetrical lobes(most modern hi-po cams) wherein one side of the lobe is a different size and shape than the other side of that very same lobe. What goes into figuring the different shape of each side of a camshaft lobe? I agree that having your heads flowed is an absolute must if you really want the right cam, I would just like to try and understand the design relationship of each side of an individual lobe itself. I am trying to understand the whole camshaft selection thing on a little deeper level, but here lately I have been thinking about it so hard that I can't see the forrest for all the trees so to speak.
I got that after I re read your post. I guess I was surprised ro hear anyone even talk about an assymetrical lobe. I'm gonna sit on this one for a few. See if anyone else knows. :D

[DeputyDawg]

No problem steel,
Hell I don't usually come across the way I want to when using a keyboard. I am probably over thinking this camshaft stuff, but it is one of the areas I would really like to understand a lot better outside the basic "Big cam goes rumpity rump" stuff. Ya know what I mean?

[Raylar]

As for assemetrical cam profiles and lobes, the technology has been around for quite sometime. The idea is that when one looks at valve openings and closings you need to think about the lift profile as a curve as it gets plotted. The amount of flow is not just proportional to the time a valve is open, but how much it is open at each portion of the valve event. In other words, if you can throw or lift the valve open very quickly and keep it open more under this curve and then still close it slowly enough to keep it from bouncing off the seat or getting into some uncontrolled motion, then you can with a good breathing head and cylinder sometimes pack more fuel and air into a cylinder and hence if you can burn it make more power. The asemetrical lobe is basically one that has quick opening rates and a slower controlled closing, hence the lobe is not the same on each side of center.There are a lot of other technical factors here but the idea of asymetrical lift cams is a good one, we use this technology in our new cams, but you have got to be careful with valve train mass and spring control with a special valve spring and pressure to safely use it in a chosen motor.
Valve & cam science gets complicated doesn't it!
Ray @ Raylar

[steelcomp]

Bingo! Now lets talk about degreeing an assymetrical lobe cam. :D
ICL or by events? :idea:

[DeputyDawg]

When degreeing an asymmetrical lobe cam you must use the opening and closing figures at .050" tappet lift off the base circle of the cam. The problem with using the intake centerline method is that it has you finding the theoretical centerline of the intake, and or exhaust lobe. That system will work on a symmetrical lobe cam but when you try and do it with an asymmetrical cam when you try to find the centerline of the lobe there is an automatic error that may be as little as 2 degrees or as much as 6 degrees depending on the actual lobe design. Also the centerline method does not really indicate if your camshaft was properly produced, as no confirmation of the duration occurs. Since the opening and closing at .050" lift method is not affected by the lobe design, it is more accurate to degree using the opening and closing at .050" tappet lift method. This method will also verify your actual duration figures.
The above recommendations were gathered from several cam grinder sites.

[DeputyDawg]

Thanks Raylar.
Come on Info, Fiat, 058 and company. Some of y'all gotta have something to ad.
I wish we could get cstraub in on this one.

[steelcomp]

I've gone round and round with guys on this one, and as much as I agree with what you said here, you also need to be able to check where the max lift is on the lobe, not necessarily the C/L. With an assym. lobe, it's as important to know where max lift is, since thats critical in relation to piston speed. Events can be off several degrees from the factory...my Bullet was off 4 1/2 deg. total on the in lobe. NOt a lot, but definately a lot when moving the point of max lift.
More later.
OK just to add this...how do you know what your base circle is?? and how do you know that if it's not a stock base circle, did the mfgr compensate for the different dia. AFA degrees of duration? MOST cam grinders will give events based on standard GM base circle dia. If base circle different, numbers no longer work. :coffeycup

[DeputyDawg]

steelcomp posted--OK just to add this...how do you know what your base circle is?? and how do you know that if it's not a stock base circle, did the mfgr compensate for the different dia. AFA degrees of duration? MOST cam grinders will give events based on standard GM base circle dia. If base circle different, numbers no longer work.
Very true.

[steelcomp]

There are two purposes for degreeing the cam so it depends on your objective as to how you approach this. If the valve opening & closing points are your priority, set them where you want them and don't worry about the rest.
If you're targeting peak lift at a specific point of crank rotation/piston position, use the .050" before & after max. lobe lift method. This is usually how you'd go about phasing any asymetrical lobe cam.
Just a thought.

[Infomaniac]

Bingo! Now lets talk about degreeing an assymetrical lobe cam. :D
ICL or by events? :idea:
By Events is the only way I degree one in.

[cstraub]

Thanks Raylar.
Come on Info, Fiat, 058 and company. Some of y'all gotta have something to ad.
I wish we could get cstraub in on this one.
I've been watching it. Years ago lobe profiles took hours upon hours to map out with calculations. In our hot rodding infancy we wanted to "get the part to the track" so early on it was easier to design the opening side of the lobe then just invert the numbers and wall la you had the closing side.
As our industry evolved and we learned about air and how it moves in an intake path or exhaust path, then asymetrical profiles only made sense.
I use 95% asymetrical on my intakes for the following reason. I want to pack the cylinder with as much air so when opening the valve I want it to open quick (get the party started). After max lift, lets slow the valve down closing because the RAM effect is taking place with our fast moving column of air still coming in the cylinder even with the piston coming up. Let's pack as much into it as we can. Then closing the intake valve. Lets set it down easy, not shut it quick and chance a "bounce".
Ray made some excellent points in his post. Area under the curve is critical in making power.
Degreeing, events only. And pay attention to the cam card. Some companies like .020" over .050". Some feel it is more accurate.

[058]

Thanks Raylar.
Come on Info, Fiat, 058 and company. Some of y'all gotta have something to ad.
I wish we could get cstraub in on this one.Nothing I can add to what Ray and Chris have posted. Simple, direct and concise. I have dealt with assymetrical cam lobes on Cummins diesel engines regarding injector timing and the fixture thats used to time the injector is what could and should be used to time the cam on a automotive engine. IMO it would be more precise and save some time.

[ECeptor]

OK, so the asymmetry is because you can push a valve open faster than the spring can push it closed...that makes perfect sense.
Now, how fast can you open and close a valve? What's the minimum number of degrees needed to ramp a valve open from 0.050" lift to, say, 0.500" lift? Assume stock 496 heads and springs. How fast can you close it? Assume 5,500rpm max.
I'm curious about these numbers because I'm trying to write out a general formula for a cam profile based on a few specs. I'd then use the cam profile along with the head bench flow numbers to project engine power. I know there a programs to do this for me but it helps me understand things better by doing it myself.

[cstraub]

Eceptor.
Now you have to factor tappet diameter if it is a flat tappet or wheel diameter if it is a roller.
Chris

[ECeptor]

Eceptor.
Now you have to factor tappet diameter if it is a flat tappet or wheel diameter if it is a roller.
Chris
Good point. I finished up my spread sheet yesterday but need good ramp up data to put into it. Given ramp up rates for both flat and roller it should show the flow difference between them.
Anyone have any good numbers to use?