Cam specs intrepretation, please

[Jim Weatherall]

Here are the numbers for my cam. I would like someone to interpret them for me.

248/248 duration, I think I understand this... degrees of duration for intake and exhaust, right?

521" lift, is this a half inch of lift?
lobe #602/602 108deg lobe center, what's this?

[Cobra20646]

Jim,

The 248/248 duration is as you guessed. The only problem is that there is no way of knowing EXACTLY what the duration is unless they make it relative to a certain amount of tappet lift. For example, your cam may have an advertised duration of 248 degrees, but at .050" of tappet lift, it may only be 215 degrees. Most cam grinders will offer the specification at .050" lift, but if they don't mention it, than you have no way of comparing one cam against another. Sort of like apples and oranges. More duration increases torque in the upper RPM ranges at the expense of lower RPM operation. Affects idle quality too.

The lift of .521 is once again apples and oranges. But I would assume that this figure is measured at the valve stem with 1.6 ratio rocker arms. But then again, if it ain't specified, how do you know?

The 602/602...... no clue here

The 108 degree lobe centers is the amount of crank shaft degrees between the centers of the intake and exhaust lobes. It is also related to valve overlap, or the time that both valves are off their seats. (The intake valve opens before the exhaust valve is completely closed)
A cam with lobe centers of 112 degrees is in the catagory of a good street cam; good idle and good vacuum at idle. One with 108 degrees will be in the catagory of a performance cam; lopy idle, with an increase of peak torque and horsepower.

Hope I got that straight...

- Jim -

[Jack21]

Jim,

The major cam grinders, Competition Cams, and Crane Cams go into this in great detail both in their catalogs, and on their web sites. If you're selecting a cam for your motor, you need to do some reading.

I can tell you from experience that for a street driven car, even a Cobra, that you'll be happier if you go a little conservative on the cam. Somewhere between 218 - 228 deg on the intake lobe is plenty. With a dual pattern cam, you can go 8 - 10 degrees more on the exhaust side.

[Anthony]

The #602 may refer to that cam manufacturer's own internal designation of that particular cam lobe profile, as they can use the same cam lobe shape for cams of different engines. You can call Crane cams and custom design a cam for your engine and pick from hundreds of cam lobe shapes, no matter your type of engine.

[Jack21]

Another thing to consider in cam selection is flat tappet vs roller tappet. Most OEM's are putting hydraulic roller cams in their engines these days. Most cam grinders, Crane, CompCams, etc., have hydraulic roller retrofit cams for virtually every popular engine. It's worth the difference in cost for the performance benefits. Since the lifter rolls, rather than slides, the engine uses less torque to turn the cam, and makes that recovered torque available at the rear wheels to push the car. This alone is substantial. 40 - 50 ft/lb. Like adding another 30 - 40 cubic inches to the motor. Second, the rollers open the valves faster, and hold them open longer. More air/fuel in the engine. You can use a milder grind for better off-idle and mid range, and still have the top end when you need it.

I used a CompCams hydraulic roller on a 351W. Spec's are 224/224 & .533/.533. It pulls 16" vacuum at 750 - 800 RPM idle. (I was expecting 11" - 12" from previous flat hydraulic grinds.) Motor pulls like a diesel at low rpm in OD, and feels like I've added 4 more cylinders from 3000 RPM up.

Street Rodder just finished a 390FE build up. They used a Crane hydraulic roller retrofit. The numbers will astound you. Unlike the 427 side oiler, Ford made millions of 390 FE's from about 1958 to around 1976. They're still lurking in Galaxies, T-Birds, and Station wagons. People are almost giving them away free. All you need is the block. Hmmm. Maybe there's life left in the old FE after all.

[Cal Metal]

Check this out: www.cranecams.com/camvtfaq.htm. Will answer many of your questions.