The way I understand it, the higher the numbers, the longer valves stay open. Explain to me the advantage/disadvantage with for example a cam listed 270/270 with .218@050 vs one listed as 284/284 with the same .218@050 . School me.

 

There's a lot more to cam design than most people think and in most cases simple generalizations simply don't cut it so i'll try to avoid making any. With the two examples you gave what you're going to see is a difference in the shape of the cam lobe, the one with less advertised duration will be narrower overall and assuming they both have the same total lift that means the ramp rate will be steeper. What that means is the shorter duration cam can be considered more agressive towards the valvetrain, opening and closing speeds are higher and lifter side loads are higher which may accelerate wear and create more valvetrain noise.

As to how the difference in advertised duration affects the powerband, the one with more total duration will peak at a higher rpm or maybe produce a slightly broader powerband but the difference won't be huge.

 

Appreciate that. I usually associated higher duration numbers with higher rpm cams. Was having trouble getting a grip on the fact that the 284/284 cam was advertised as 1500-5000 rpm high torque/low rpm cam. Still try to learn at least one thing a day, getting harder at 60+.

 

I would generally suspect about 260 degrees of duration seat to seat for a 3" stroke motor and maybe longer duration for a longer stroke motor like 4" or more. I would suspect your 256 degree cam to provide that little Bronco with great throtel response and I am impressed that you gave that I6 an aluminum cylinder head.

-Lucky 200 I6.

 

My liitle six took about three months of research before building and it did pay off. Runs exactly as I planned. Super response, more than enough power and really broad torque range. Only thing different I wish I had done was a tranny with closer gear ratios. I've got a fresh built 302 that I picked up for another project and trying to figure out their reasoning when they made their parts choice.

 

The word "aggressive" is not exactly correct when speaking of camshaft lobe rates and shape from what I have read, been told & taught. Harvey Crane put it in words the best using the word intensity when describing certain parts of the lobe. A lot of people get sucked into a "important" Lobe Seperation Angle for their particular application. When in fact the LSA simply is not a really important piece of the puzzle. Duration, however, is in the top aspects when designing a camshaft and choosing one, along with the obvious others that are required for optimum operation, which I am sure you found out in your research.

The amount of ways you can manufacture a camshaft is immense.

 

40+ years in the high performance industry and we still can't get a consensis from different MFGs. Some rate their advertised duration at .007 lifter rise some at .006 and some at .020. Cam ground in advance and lobe spread along with [ascymitrical or scymitrical] "spelling" valve event curves{ sometimes the lift side has a different curve than the down side} Total area under the curve. Short duration, aggressive ramp speed with high lift will put the valve in to the high flow region of the heads flow curve for a longer time than a longer duration less aggressive low lift cam. You flow a lot more air at .300 lift than at .006. Advance and lobe seperation become important when you are fine tuning the package. Usually tight lobe motors are considered more peaky and the power drops off sooner say 6500 rpm while a smoother ramped cam with say a 112 to 114 lobe spread would be a much smoother idleing motor get better mileage and have a lower but broader longer torque curve. Lots of generalities here. As was posted before there are a lot of variables to picking the right cam. I always picked the cam after I knew the Heads Flow curve on both intake and exhaust. Intake manifold, carb size, compression and fuel, car weight converter and gears and tires. If the cam is the brain you need to know what it has to work with to make the right choice.
Bob

 

Who lists a cam @.007" lift?

Listing a cam at .020" is still not as reliable as comparing cams at .050 because of valve lash affects.

What gazguzzler said here is what you should be thinking about
LSA is a calculated characteristic.

 

GM lists cams at .001 lift and at .050. I believe crane listed at .007. Comp at .006 as did Isky. I may be wrong on who but advertized duration is all ove the place to sell cams. Most of the Rollers are at .020 which eliminates most of the lash and gives a truer picture. But Like you said the area and shape of the curve is what makes a cam rock. No use putting a 600 lift cam in a Pontiac stock head that port stalls at .450. Less lift more duration. Same motor Edelbrock heads. Flow to .600+ Shorten duration, tighten lSA, More lift. Hemis right find the info on the heads if you can, be honest on how your going to drive it 95% of the time and cam it for that.

 

Never really noticed that Gaz, advertised duration I guess would be like cfm numbers with heads, big doesn't mean much without a purpose.

Look at the type of lobe you (OP) want to run as well, flat or roller.

If anyone ever wonders about the seat durations, just take your open duration (actual duration) and subtract it from 720 degrees.

 

Advertized lift/duration/lobe separation is just that- advertized and not the specific grind including ramp profile that is used on the cam.

 

Good stuff guy's. Appreciate it.