cam-fused

[cazsper]

I found a chart on cranecams.com that gave a bunch of cam specs, and now I am more confused than ever. For a hydraulic roller cam for a 351c in the 2000-6000 rpm range, the cam specs are 224/232 586"/609"-112'. Looking at a cam for a 460bbf, in the 2200-6200rpm range, the cam was 228/238 590/614-114. Four degrees on the intake and 6 degrees on the exhaust with similar lift on a difference of 110 cubic inches only changes the RPM range 200rpm.

On a Cleveland cam in the 3000-7000 range, the cam is 236/240 621/631-112. For the 460 in the 3000-6600 range, the cam is 238/246 614"/636"-112. That's only 2 degrees (intake), 6 degrees (exhaust) and similar lift. With 110 more inches, it only changes the RPM range on the top end.

I keep hearing the additional displacement will "absorb" more cam. But according to this, it doesn't really. Can anyone help explain this?

 

[Jeff73Mach1]

Numbers lie. What you can't see from the numbers is the profile of the cam. Opening and closing and lift are all great starting points, but the curvature of the lobe can make a big difference in the effective time and amount the valves are open.

While I think the general premise that more cubes will calm a cam down is true, I don't think it can be fairly extrapolated across engine families. Heads breathe differently even with the same sized valves.

 

[mustang7173]

Hello Cazsper,

Here is a great article from Hot Rod Magazine on the how to choose camshafts. I often will re-read this article just to keep the basic understanding.

http://www.hotrod.com/techarticles/113_9604_choosing_the_right_camshaft/

mustang7173

 

[cazsper]

Thank you. I understand what they are saying, but how do you use that information to pic a cam? I seen a few guys here with 460's or even 460 based strokers with cams that are supposedly ideal for a 393 Windsor.

And as for picking a cam from a company, it seems like their cam recommendations are useless. Or ballpark figures at best right?

 

[Don C]

One other thing you have to consider is the rocker arm ratio. Windsors are 1.60:1, 429s are 1.73:1.

 

[cazsper]

But don't the cam specs calculate for rocker arm ratio? When it says the cams lift is .535/.545, isn't that assuming you're using 1.6:1 on a Windsor?

What's confusing me is my '68 has a 393w has 10:1 with AFR heads an Air Gap intake and a hydraulic roller 232/240 @ .050 with about 540/550-112. Now, I have been so contemplating a 520 bbf with SCJ heads, air gap intake and about the same compression. According to the cam companies "charts", the cam in my 393w is about the size cam I would want for a 520 inch motor.

 

[TommyK]

At the risk of grossly oversimplifying, David Vizard chooses LSA first based on engine displacement and intake valve size. Overlap is specified based on intended usage. Duration is computed from those two specs.

I don't understand it all but I believe Dan Jones designs cams in Dynomation on Vizard's principles with good success.

http://www.popularhotrodding.com/tech/0607phr_camshaft_basics/

 

[Don C]

Yes, that cam spec shows the total lift, but the ratio affects overall profile and the opening and closing speeds.

 

[cazsper]

So two cams with the same overall lift but using different rocker ratios will be different?

 

[TommyK]

So two cams with the same overall lift but using different rocker ratios will be different?

As measured at the valve, yes.

 

[cazsper]

Now I'm even more confused..lol

I guess that's why they have people to call about this.

 

[Finmach]

For example, if you have a 1.73 rocker arm ratio and a camshaft lift 0.600". If you have the clearence between the piston & valve, you can increase the lift by swapping the 1.73 rockers to 1.80 rockers, that way you'll increase the lift to 0.624".

Divide the lift by rocker ratio = 0.600" / 1.73 = 0.346821" = overall cam lobe lift.

Multiply the 0.346821" with the new rocker ratio = 0.346821" * 1.80 = 0.624" = new lift.

Not so difficult as it might think.