Why do they even have "advertised duration"??

[Rich Schmidt]

Advertised duration means noting. Zip Zero Nada. You should see how some companies rate their cams. A stock GM cam for a smog era tax cab engine is listed at about 300 degrees advertised duration. Thats the actual amount of time the lifter will be at any other position other then totally down. A 1969 L-88 cam like I have in my 427 is something like 360 degrees advertised. Of course they dont take into account valve lash, which takes up about 45 degrees of that duration before the valve ever moves.

The fact is that very few flat tappet cams have gotten any more agressive over the last 40 years,the companies just play numbers games. One company tells you their 290 cam is 230 @.050 and a 300 degree cam that has 240@.050. On the next page of the mail order catalog is another cam company offering the "next best thing" in cam grinds. They have a 290 cam that has 240@.050 and a 300 cam that is 250@.050. But you may notice that the first company's 300 degree cam has about the same lift as the second company's 290 cam. It sure sounds impressive,until you realize that they are the exact same cam,but just use 2 different numbers to get their advertised durations. Its mostly smoke and mirrors.

The same goes for solid vs hydraulic cams. You may see a 280/230 hydraulic cam with .500" lift. You then see a 280/230 degree solid with .520 lift and think the solid is the hot setup. You would be disappointed to see that after you removed the solid's valve lash from the figures(something that the cam companies usually try and hide) the solid is about the same lift and 10 or 15 degrees smaller at .050 then a similar speced hydraulic.

 

[dubs74camaro]

Boom!

 

[Cardinal]

Back when I first got started with building modifed engines (cam kit, aftermarket intake, aftermarket carb, headers, etc.) there were terms then that didn't mean anything other than marketing value. Terms like three quarter cam (does that mean that it had 12 lobes rather than 16? LOL), and full race (as opposed to not racing?).

Then there was (and still is) "advertized duration" which ASSUMES that a cam's measured duration is accureate from the base circle (zero cam lift) back to the base circle (zero lift) again.

Then a man named Ed Iskenderian (also known in my day as The Camfather) found that attempting to measuere a camshaft's specificaitons based an the base circle method was USELESS! He found that by measureing it from .050" lift on both sides of the cam's lobes (up and down sides) the cam's specifications could be ACCURATELY and REPEATABLY be measured. He didn't invent degreeing a cam in, BUT he perfected it by the above method of measureing the cam's specs at .050".

 

[Cardinal]

Now for my opinion on cam selection.

IMHO, less is more. I've been there, done that, and HATED it when I selected a cam that was "too big" for my setup OR wasn't streetable. TRUST ME, less cam can make more USEABLE power than too much cam.

And I will NEVER understand why people resist degreeing a cam in. How do you know where the cam is in relation to the rest of the rotating assembly unless the cam is degreed in? Plus, when it is degreed in, it will produce the maximum power output because IT IS IN THE CORRECT AND BEST ROTATIION PHASE OF THE MOTOR!

 

[69CHVL]

Lotta theory here, little real-world experience.

The Magnum cams run good, but are "old" and have alot of seat timing compared to the newer lines like the Voodoo and Xtreme. The more agressive cams allow you to have an engine that makes similar power/vacuum/idle quality than an older cam with more duration. I've heard all these stories about harder on the valvetrain, more noise, etc. Bottom line - the quietest cam I had was a Xtreme HR .590/.600 lift cam. Quieter than the Magnum I had, quieter than the 60212, and quiter than a Cammotion - all in the same motor. I like the Xtreme cam the most.

Say what you want about Comp, but the fact is, they test each grind extensively via in the field and Spintron before releasing them - this I learned from a few builders who have direct contact with the higher-ups at Comp. Some cam co's just use old race grinds and call them a "street cam"

And laugh again all you guys want, the Thumper cams are even making more power than the Xtremes - unless you want to believe what a simulator program says.

Yes - I like Comp

 

[Thunderkyss]

Has anyone ever figured out how to translate cam specs to horse-power?

Without using a program like Desktop Dyno, how do I know if I'm "matching the cam to my application"?

I know by looking at the cam specs, approximately where my hp & tq peaks will be. I know how to approximate how much lope I'll have. I can even grab a cam spec card & approximate what kind of stall I would need.

But as far as power output, all I can think of is referencing another engine someone else built. "So & so put such & such a cam into his 355 & made xxx power with 10:1 compression ratio, so it should produce xxx power in your motor"

know what I mean?

 

[dubs74camaro]

Yes - I like Comp

Well someone likes them. They are the largest. I think the fact that you have worked for them for 42 years might have something to do with it!

LOL....just kidding.

 

[Rich Schmidt]

Now for my opinion on cam selection.

IMHO, less is more. I've been there, done that, and HATED it when I selected a cam that was "too big" for my setup OR wasn't streetable. TRUST ME, less cam can make more USEABLE power than too much cam.

And I will NEVER understand why people resist degreeing a cam in. How do you know where the cam is in relation to the rest of the rotating assembly unless the cam is degreed in? Plus, when it is degreed in, it will produce the maximum power output because IT IS IN THE CORRECT AND BEST ROTATIION PHASE OF THE MOTOR!

I guess hey lots of street guys just use stock steel timing gears like I used to. with a stock timing set after about 500 miles the set has about 20 degrees off slack in it and it only gets worse from there.so do green in a cam with anything other then a good double roller set is a waste of time.I took a well worn sloppy stock set with a ton of slack out of my 7500 RPM race engine and put in a brand new steel roller set and the car ran exactly the same

 

[Cardinal]

There is NOTHING wrong with a link belt timing chaing gear set. GM used a nylon faced cam gear to eliminate mechanical noise and harmonic pulses from the timing chain gear set. Most people replace them with a steel faced timing chain gear set (steel cam and crank gear with a link belt chain).

You can degree in any new timimg chain gear set be it link belt, roller, or true roller. That ASSUMES that the chain & gears aren't worn out.


There should NEVER be 20 degrees of slack in a new timing chain gear set. The only way that I can think of that there could be that much slop in it is if the main caps of the block were align bored which would make it manditory to buy a special timing chain gear set that is shorter to compensate for the center to denter distance from the crank bore to the cam bore.

 

[sooner]

Comparing .050" duration and advertised can tell you something about the aggressiveness of the lobes. Not the whole story, but something. Comparing Two cams with the same .050" but different advertised, the cam with the shorter advertised will have less overlap and likely more cylinder pressure. The aggressive cam will be harder on the valve train, most likely, too.

Build on the .050" numbers, though, then narrow in on advertised if you like.

^All of this and a bag of chips.

Advertised duration DOES matter, and it tells a story about how aggressive or slow a given lobe is

Ol PDQ67 may have been about half silly, but as he used to say "you can't trap a charge until the intake valve closes" that's the way I look at it too.

When I'm looking at cams, I try to figure out which way I need to go with my cylinder pressure to get what I want out of it. Like Todd posted, I decide on what how much .050 duration I want to run and then after that I look for a cam that has enough seat duration for what I'm wanting to do.

I'm no professional engine builder, but the concept just makes sense to me on how to choose a cam based on your cylinder pressure needs.

AJ,not to harp on the DD2003 thing, but the xe274 would be the cam to compare the 280 magnum to since they have the same intake duration @.050.