Say I have a head that intake flows 239 cfm @ .500 lift, and 229 @ .500 lift.

Do I want a cam that has under, over or exactly .500 lift? Generally speaking...

 

What head is it? Give me a lot of specs on what you got and I can run if on DD2000 FWIW.

AJ

 

I don't get the question. How could it flow two different numbers at the same lift?

 

but.. get one with the most agressive lobe you can at the max lift.
in otherwords, find the cam with the shortest duration at the max flow number


__________________
89rs w/a 400, some aluminum things and a 4.10 gear

 

My mistake...it should be 239 cfm @ .500 lift, and 229 @ .600 lift.

I found these specs for the 855 casting Vortec head. I have the 062's but I figured they'd be about the same.

I have DTD2000 and I've been trying different cams. Seems like I make the most power in the RPM range I want with the cam I already have. I just need to add 1.6 ratio rockers for more lift.

I remember someone saying that you want .060" clearance from retainer to valve guide for safety. I measured .511" from open until the retainer touched the guide. If I plan on having .481 lift that leaves .030" clearance. Maybe I could try a set of Comp or Crane retainers? Any thoughts? My cam specs are in the signature, it's .431/.451 lift.

Thanks for the replies...

 

ttt

 

Flow never goes down from opening the valves further, unless you use undercut valves and you open them so far that the non-undercut section ends up in the port. So beyond a purely theoretical question, don't worry about it ever happening to you. Those specs are in error; just like you won't reduce the flow of 2 pieces of 1/2" garden hose by replacing one of them with a section of 1" hose.

The numbers given on cam cards are peak lift numbers. The valve actually spends very little of its time at anywhere near that lift. Don't try to evaluate flow that way. A hypothetical cam with very steep ramps that snaps the valve instantly open to .400" and holds them there would outflow a cam with typical gentle street profiles and .550" of peak lift. It's "area under the curve" that counts. That's why the newer designs of cams like Comp's Xtreme series are so much better than older designs: they used computer modeling methods to design ramps with the maximum "intensity" possible while keeping the valve acceleration below the threshold of beating the valve train to death in a short time.

 

Select a cam based on it's .050" timing specs and intended rpm range. Not peak lift. Leave the design up to the cam grinders.
There is more to it than numbers and ,more is usually not better.
All the cam manufactures have tech lines and tech email to help you select your next cam. They are the experts on their stuff
and want you to get the right cam the first time. It's in their best interest. Give 'em a call.

 

Yeah, I was confused by that, too. And thanks for referencing the acceleration rate of the valve train. I thought that maybe I was the only one who ever considered that, or at least tried to discuss it. That's always important when designing the valve train, or especially when considering springs, and one of the main reasons factory springs aren't acceptable for use with almost any aftermarket cam.

 

Can you run these through dtd2000

 

Here it is.

 

the heads have 2.02\1.60 valves, but its still pretty good

 

They'd progressively add 10 or 15 hp to the top end of the curve. So around 355 hp. Good motor!

 

what with 1.6 magnum rockers and a 625 demon carb