I have been tossing around a few different ideas for cams that will go in my rebuilt L98 motor.

What I am trying to get a better understanding is how cam duration at .050 and advertised duration effect how the motor will idle and run.

For example. One cam I am looking at is the Comp 503 Cam. The specs on this one are:
Advertised Duration: 276 int/281 exh
Duration at .050: 224 int/ 230 exh

I was under the impression the duration was the amount of time in degrees during the rotation of the cam the valve stays open. Maybe I am wrong with this thought though. What is the difference between advertised and duration at .050 and what do they mean?

At the same time I am also now noticing the operating range for the 503 cam is 1800-5800. My car is a 5 speed so I am wondering if I should keep my LT4 cam which has an operating range fromn 1200 - 5200.

I appreciate all your input!

 

"Advertised" duration is how many degrees the valve is off its seat. Obviously you need a spec to describe exactly what is meant by "off the seat". Due to variations in individual hydraulic lifters, rocker arms, etc., it's impractical to measure ANYTHING at the valve, so all of the numbers are measured at the cam lobe. Unfortunately not all mfrs use the same figure. Comp for example uses .006" of lobe lift, i.e. they consider that valve motion begins and ends when the lifter has been moved .006" off of its rest position (the cam base circle), and I believe Lunati and Crower also. Crane uses .004" IIRC, and so any given cam would have a higher "advertised" duration using Crane's method. GM uses something like .001", so their "advertised" durations are always extremely high. For this reason, you CANNOT directly compare "advertised" numbers from one mfr to another, even on the identical cam (such as a factory cam copy); and it is NOT POSSIBLE to "convert" one mfrs number to another's for dissimilar cams, because the shape of the ramp can be quite different.

.050" of lobe lift is the standard method of describing when the valve is open enough to actually flow usefully. At lesser lifts than that, the valve is basically just "leaking", and the little bit of flow contributes nothing towards cylinder fill or emptying (making power). Obviously this period of time should be minimzed, but it can't be eliminated.

Cam duration at .050" determines the RPM range that the cam will work most effectively in. Seat duration will determine idle quality and vacuum, and to some extent gas mileage, and will also effect parts longevity, for a given .050" duration.

http://www.compcams.com/catalog/008.html

 

When comparing cams it's always advisable to use .050 duration numbers. Advertised duration is WAY too easy for a cam grinder to play around with an make the numbers come out wherever they want them to.

Duration @ .050 and LSA (Lobe Separation Angle) are the 2 most important factors in determining how a cam will behave (street mannners, operating range, etc.) Advertised duration and lift are almost meaningless by comparison.

 

With that in mind my motor is a TPI. Mods are in the sig. What would you, or anyone advise as the maximum duration for good street manners?

I like the lift I can get from the Comp 503 cam with a 1.6 rocker. But the duration is 224/230 vs the LT1 cam which is 218/228. It does not seem to be a big difference but I am not totally sure.

Any ideas?

 

heres a brief course on cams to get you up to speed on the concepts

http://www.oregoncamshaft.com/cam-basics.html

http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://www.auto-ware.com/combust_bytes/camspecs.html

http://www.rpmoutlet.com/camfac.htm

http://www.chevyhiperformance.com/techarticles/95298/

http://www.hotrod.com/techarticles/43418/index3.html

http://hotrod.com/techarticles/113_0209_cam/

http://hotrod.com/techarticles/113_9811_cam/

http://hotrod.com/techarticles/113_9804_cam/

http://www.ridgenet.net/~biesiade/camdegree.htm

http://www.mousemotor.com/cb_1.htm