This is probably so ridiculous to ask you guys that it won't surprise me if there are no responses, but I really don't understand the technical relationship between Big Cubes and Short/Long Duration Camshafts......so here goes:

Why is the rpm range of a large displacement engine limited when running a short duration cam as compared to running a longer duration cam? Is it simply a matter of less time or more time to fill the cylinders?

example: 496 BBC using a 270 adv. dur. cam may only make maximum power at about 5,000 rpms, when a 290 adv. dur. cam in the same engine will make its' maximum power at around 6,200 rpms (assume both cams are hydraulic flat tappets).

Thanks, Charles

It's in the stroke b/c a longer stroked motor will trap more charge in the cylinder then the same cammed shorter stroked motor.

Thus, the cam will appear to be smaller in the longer stroked motor due to the piston being lower at the time the intake valve closes all things being equal except stroke.

pdq67

Charles, you have it right. The bigger the cubes of the motor the bigger cam it needs to fill the cyl, sort or like a fat guy and a skinny guy running a race while beathing through a straw, the big guy is gonna need a whole lot bigger straw diameter to keep up with the little guy.
You can offset it a little by putting in bigger valves if the bore allows it, but a bigger cam is cheaper.

Doesnt duration dictate RPM operating range and CID dictated lift ?????? ( among MANY other varibles ... just a guideline though )

Originally posted by pdq67:
It's in the stroke b/c a longer stroked motor will trap more charge in the cylinder then the same cammed shorter stroked motor.

Thus, the cam will appear to be smaller in the longer stroked motor due to the piston being lower at the time the intake valve closes all things being equal except stroke.
This is totally false. It doesn't matter what the stroke is. It takes the same duration for the piston to go up and down. In a longer stroked engine, the piston has to move faster, which is why it gets hard to rev it high. An RPM is an RPM.

Scott,

I beg to differ here!!

Take two SB's, a 4"b x 3"s, 302 and a 4"b x 4"s 400 and use the same cam AND then calculate where the piston will be at in the cylinder for both motors AND that will verify what I am saying!!

AND to make it easy, go to this sight and try it!!

http://www.rbracing-rsr.com/comprAdvBMW.htm

I just inputted a 302 and a 400 and the 302 using a 60 degree intake valve closing number has a "dynamic stroke" of .8000 of it's 3" long stroke!!

The 400 has a "dynamic stroke" of .8175 of it's 4" stroke so therefore the longer stroked motor trap's more charge then the short stroked motor by .0075 x 4.0" = .030" all else being held identical except for stroke!!!

Right Pat K.!!!

Now I know a .030" longer stroke at the intake valve closing point doesn't seem like much until you add it on top of the already larger volume cylinder the long stroked motor has and this is why a longer stroked motor makes a cam seem smaller then when it is installed in a smaller, shorter stroked motor!!

AND believe it or not, the same cam that is installed in a 267, a 305, a 350 AND a 372 with the same stroke will run the same except for ther differences the bore displacement increases makes and not the stroke!!

I.e., the hp and torque points should all be about at the same rpm levels for all four motors!!! but the Hp and T numbers will increase due to engine displacement increasing!!

This is shown to be a fact in Ed Staffel's book, "How to Build Max Performance Chevy Rat Motors" on page 51 in a table!! Ed, (or somebody he knew), did a study on this by holding the bore of a stout BB motor constant at 4.5" and then varying the strokes from 3.76", 4.00", 4.125", 4.25" and finally 4.5" and that is what he found happened!!

pdq67

Gotcha. I partially misinterpreted what you were saying. It seems TDC would occur at the same point regardless of stroke, but piston speed is not contant because the crankshaft is pushing the rod a lot sideways at first.
The longer stroke's piston would be behind at first but then catch up to the short stroke's piston.

Scott,

I hope I didn't appear to come on too strong here but I wanted to try to set straight a "motor myth" if you want to call it that with respect to all this.

I have to give Pat K. and his great DCR sight plus Ed Staffel's book credit for educating me on this subject.

I still wished I knew more about the numbers and why about 8 seems to be the number to shoot for if running pump gas with an iron-headed motor? I figure it is b/c you have to subtract the chamber's volume out of the total cylinder/head volume to get to the number 8 but don't know for sure???

Ed went on to say it is a fuel octane thing if I recall right?(?) AND he gave credit to Mr. Dema Elgin, so it may just be an "emperical number", if you will??

Pat, I am using the Boost Calculator site b/c my machine got dumped and I am not computer literate enough to get yours to load again so that I can use it!! Heck, I have to admit my second son loaded your great program for me the first time..

I asked my wife to help me AND she just gives me that "you dumb-sh-t" look and mutters something under her breath that sounds like I spend too much time doing something????

pdq67