Just for info purposes, there are two specs for cam duration, the well know
"advertised" duration and the actual duration. The actual, or true duration
is the duration measured from .050" of valve lift at opening, and .050" of
valve lift at closing (off the seat). There could be two cams listed as
270/270 advertised duration, but have durations of 221/221 at .050, and
224/224 at .050". The 221 would have a slightly rough idle and the 224
would have a slight lope to the idle in a 318. Don't always go by the
advertised duration, the actual, or .050" of valve lift duration is the
actual valve event duration. That is what you want to look at when
selecting a cam.
Anything less than 270 advertised duration, or 221 at .050" duration will
not give a "racy" idle. The 221 at .050" would give a slight hint to a hot
cam. It is noticeable, but not pronounced. A 224 at .050" would be a good
choice if you were looking for the race car like idle with an improved top
end. Make sure you get a high lobe center (centerline) like 112 or 114
when you get up in the duration. You will loose a little more bottom end
with the higher centerline, but it won't confuse the computer too bad.
With the cams listed, you must have proper rear end gearing to get te
engine in the cam's power band. A 224 cam in a 318 would have power from
approx 2-6K RPMs. Gear appropriately. Also, if stock exhaust is on, get a
dual profile cam, like the one's you listed. A 218-220 to 224 at .050"
would be a good choice. If you have headers, true duals, and god flowing
mufflers/cats, then go with a single profile, straight 224 at ,050" or
whatever you like.
All cam companies will tell you the duration at .050" of valve lift, it's
just not as popular as the advertised duration. Most of your racing cam
suppliers list both as well as valve lift, overlap, and lobe center.
Mike
At 09:10 PM 2/14/2000 -0600, you wrote:
>Jeremiah,
>
>I'm in the same position as you, except I'm looking for the "hottest" cam I
>can run without freaking the computer (after I heat it up with a Hypertech
>III power programmer this summer!). A little "lope" in the idle would be
>music to my ears. According to my 97 owner's manual, the stock cam in my
>5.2L is: 250/264 (int/exh) duration and the valve overlap is 31 degrees. It
>doesn't list the lift and lobe center angle.
>
>The Mopar performance cam is what you quoted and has 270/276 duration, valve
>overlap is 55 degrees, lobe separation is 109 degrees, and the lift is
>.458/.457 (int/exh). I priced this kit today in Austin, TX and was quoted
>$513 for the cam and lifters!!! Ch-Ching!!
>
>I've been looking at Crane and Competition Cams. You can get to their
>websites at www.cranecams.com and www.competitioncams.com
>
>Crane has two cams listed. The hotter one is part number 704121 and has the
>following specs: 260/264 duration, valve overlap of 34 degrees, lobe
>separation of 114 degrees and lift of .458/467 (hmmm...same lift as the
>Mopar cam).
>
>Competition cams has three listed. I talked with a guy and he recommended
>part 20-608-8 and this one has the following specs: 254/262 duration, valve
>overlap is 34 degrees, lobe separation of 112 degrees, and lift of .480.480.
>
>Based on the hotter Mopar cam, Competition Cams has another, part 20-604-8
>with the following specs: 264/274 duration, valve overlap of 45 degrees,
>lobe separation of 112 degrees, and lift of .512/.512.
>
>I plan on calling Competition cams tomorrow and seeing if that last one I
>listed above will work on my 5.2.
>
>No doubt that an aftermarket cam will cost less. Mopar sells the whole kit,
>cam and roller lifters. The lifters on the 5.2L magnum engine are already
>rollers. They guy at Competition cams said my stock lifters should work
>fine with the part 20-608-8 so that saves some $. I think what I will wind
>up doing is getting the hottest cam I can get from Competition Cams, use my
>existing roller lifters, and also purchase a set of Competition Cams roller
>rockers (part # 1425-KIT).
>
>By the way, they usually don't list valve overlap on the cam spec sheets,
>but you can calculate it by adding the BTDC intake valve opening degrees to
>the ATDC exhaust valve closing degrees to compute the valve overlap. I
>belive that as the valve overlap gets larger, the more pronounced and
>rougher the idle will become. Valve overlap helps the cylinder "scavenge."
>
>Hope this helps.
>
>Mike
__________________________
mschwall@flash.net
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 11:48:07 EDT