In respect to duration, the approximate relationship between advertised duration and the duration at .0050 is usually relative. For example, a cam with an advertised duration of 260 might have a duration of 210 at 50 .0050, and cam with an adv duration of 290 might have a duration of 240 at .0050.

So my question is what happens (results in performance) when comparing two cams that do not follow this relationship?

Cam A: Larger adv duration than cam B, but less duration at 50 than cam B.

Cam B: Smaller adv duration than cam A, but more duration at 50 than cam B.

Which cam is bigger? How will each one drive as well as perform?

Thanks in advance to those who respond.

 

IF you are comparing apples to apples - where the advertised duration is measured at the same lift point - the difference between AD and D50 MAY mean something about the shape of the lobe.

Take a look at a manufacturers lobe catalog. The way Comp's is organized is useful. See page 50 (40 of 68) here: http://www.compcams.com/Technical/Ca...obeCatalog.pdf and look at the listings for the .406 and .420 rollers. The 406 all have 32 degrees difference between AD and D50. For the 420's, the comparable number is 38 degrees. So, these "families" of lobe all use the same initial profiles.

The difference between AD and D50 is sometimes referred to as lobe "intensity". It represents how rapidly the lifter is moved from the base circle to 0.050" lift. In this case, the 406's are more "intense" than the 420's. Of course, it only is in reference to this one, small part of the lift profile. Going a little further, you can also see that specs for duration are also provided for 0.200" and the total lobe lift. Comparing these numbers gives you further insight into the shape of the lobe. If you were to comapre two lobes with comparable total lift, the one with more duration at 50 and 200 and less advertised duration is likely going to be more agressive. So, in very general terms, lobes with a quicker rise from AD to D50 may also steeper to other points on the lift curve. But this is far from always or necessarily true.

It's unfortunate that more detailed descriptions of lobe profiles are not published. If they were, WAGS such as using specs as described above would be unnecessary. OTOH, how many people would be able to really make sense of the information? In very generic terms, it's easy to figure that steeper lobes provide more "area under the (lift) curve" and more power potential but require some combination of more spring, less weight (of the valve, retainer, etc.), or lower rpm, etc. than more gentle shapes. But as to how steep is too steep for a given application, that is something much harder to define or determine.

Short answer is that the difference between advertised duration and duration at 0.050" doesn't mean much. But it does give hint about the general "aggressiveness" of a lobe.

 

Rskrause, I'm sending you a PM of actual cam specs.

Thanks for the response.



I recently picked up my professionally built LT4 stroker street/strip motor from my engine builder. The D50 is 248-254-a big cam I'm thinking (110 LSA). I also know someone who bought Comp's biggest XFI shelf grind for an LT1 with a D50 at 242-248 and an Ad suration of 292-300 (113 LSA). When asked of my Ad duration, I simply asssumed it would have an Ad duration of 300-306...or something close.

I looked at my cam card and learned that the Ad duration was 286-292! I'm sure there's a logical reason for it. I'm just wondering if the cam is acutally smaller or bigger?

Wanting to know what all that meant, I called Comp Cams. The tech guy said said it represents a difference in lobes but did not elaborate beyond that. The tech guy on the phone was really short with me and it was obvious he was looking to terminate the call. He also mentioned that my cam would fine on the street-my motor is 388 cubes.

So I'm wondering, is my cam bigger or smaller than the XFI I mentioned. How will my cam perform and what should the driveability be like?

 

To see what Rich is talking about, figure you are in high school geometry class. You have to plot points on a xy graph which has 360 points to represent a circle. X would be 0-360, and y is .000"-.500" representing lobe lift. Now if you were to take all the numbers that you are given on a cam and plot those on the xy graph where .006"(y) = 280(x), .050" = 230, etc - you would see the bell curve that represents a cam lobe profile.

http://www.iskycams.com/degreeing.php

^ has some good pictures.

 

Most likely, your cam is "bigger" (more area under the curve) but you really can only guess. There simply is not enough information.

Rich

 

I understand now what people are referring to when they say "area under the curve." The closer the Ad dur is to 50 the more aggresive the ramp-hence the fact that my cam is a solid roller.

On my old version of Desk Top Dyno 2000 however, the instructions prefer that the advertised duration be used to best approximate an engine graph. While this would fail to reveal the true potential of a cam with a greater area under the curve such as mine, it seems that it would give a fairly accurate hypothesis of the operating range.

So if I understand this correctly, my cam would not rev too high for the street and the driveability would be reasonable (considering it's 388 cubes rather than 350) considering its advertised duration of operational range. It would just run substancially better in that same power band (read power under the curve) when compared to a cam with the same advertised duration and LSA but a much smaller duration at 50? Is this a correct interpretation/understanding?

 

Hold on just a second. I see where you are going with this but DON'T get ahead of yourself. What happens at extremely low lift points is NOT a good indication of how "aggressive" a lobe profile is. There are a LOT of factors involved. And what happens at .050 and higher is really a lot more important than what happens below that point, as far as power production is concerned. The fact that you are talking about a SOLID lifter cam only makes things more complicated- there are "lash ramps" invoved in the cam grind that can really throws apples-to-apples comparisons out the window.

There is NO WAY to accurately assess how "aggressive" a cam is ground just based on the advertised vs. 050 duration specs (and knowing max lift doesn't help fill in the gaps much either!). You would need to know the entire lobe profile to compare one cam to another- the kind of info they don't publish on a website. You just can't get there from here.

You can't just extrapolate the acceleration rates and such in a linear way starting with advertised duration points and 050 duration points. The acceleration rate changes with lift and that's where the real "magic" happens in a good lobe profile. A cam card will only tell you so much- adv. duration points, 050 duration points and max lift point. There a LOT of stuff happening in the shape of that lobe between those few points that you aren't seeing.

That's why grinding cams is as much art as it is science.

When using a simple desktop engine simulation program like DD2000 you DEFINITELY want to use the 050 duration points for best/most accurate results. (I'm talking about best results as far as predicting WOT power, not idle qulity, fuel economy, driability, etc.) 050 duration points are consistent across ALL cams- it's a standard. Advertised duration points are damned near meaningless. They are chosen at an arbitrary lift point that can vary from manunfacturer to manufacturer and even from grind to grind. Want proof? OK, I can do that.....

Here are two cams, both from Crane, that I happen to have some experience with.....

http://cranecams.com/?show=browsePar...tType=camshaft

http://cranecams.com/?show=browsePar...tType=camshaft

You'll notice that one is spec'ed at 290 advertised duration while the other is spec'ed at 272 duration- that's an 18* difference!! But look more closely at the specs- 050 duration, 050 event points, max lift, LSA, etc. Notice anything funny? Yeah, they are all EXACTLY the same. Want to know why? Becuase they are BOTH EXACTLY THE SAME CAM!!! Just that one is spec'ed at .001" lift for advertised duration while the other is spec'ed at .004". How's that for a noodle twister?!

The best you can do evaluating cams as a shadetree mechanic is to know a few simple things: Cam type (hydraulic vs. solid, roller vs. flat tappet), 050 durations, Lobe Separation Angle (LSA), Intake Centerline (ICL) and max lift. Anything beyond that you simply can't get the hard data you need to make a meaningful comparison. That's why cam compaines spend MILLIONS of doallrs every year developing new lobe profiles and staffing technical support lines with people who can (allegedly, and only if the stars align properly) help you make the best choice for your motor.

 

Comp publishs some additional info on lobes in their "lobe" catalog.
http://compcams.com/Technical/Catalo...obeCatalog.pdf

It also has .200" duration listed. Knowing the advertised duration and checking height, .050" duration, and .200" duration can tell a lot about the profile as to power production.

But I wish they'd just give me the Cam Doctor output instead so I could really tell. Of course, having a simulation program that could USE that data to make good estimates of torque with a detailed flow table for the heads would be really nice - and probably something I couldn't afford!

 

You cannot compare a hydraulic cam to a solid roller this way because of valve lash in the latter (usually .016-.020"). Also advertised durations could be measured at different lifts. In short, you're comparing apples to oranges.

 

Another thing to consider is modern cam lobes may be asymmetrical. They are often more "aggressive" on the opening ramp than on the closing side. So, even if you compare duration at multiple lift points, two cams with identical numbers may actually have different lobe shapes and work differently. One reason for slower closing is obvious - it may ameliorate valve bounce. I have also noticed that there is supposed to be a specfic benefit to slower closing of the exhaust valve. I've not been able to reason this out, nor have I seen any explanation. If anyone has one, I'd like to know about it.

Rich