This is a hypothetical question, and I guess it doesn't have to be just for TPIs.....

...but what is the practical difference between adding air via a cam with more lift as opposed to more duration?

I've been looking at cam specs, and asking friends of mine about their cams, and i see LOTS of high HP numbers coming from a lot less lift than my ZZ4 cam, but LOTS more duration.

What would better suit a TPI? A little more lift or a little more duration? Not looking for absolutes here, just trying to learn a little engine theory here.

Thanks!

 

I feel a smaller duration and more lift is better for TPI, Hense the Lingenfelter cams esp the LPE219 cam. 219 duration with 560 Lift

 

duration tends to lead to rpms as i see so far..... tpi likes low duration cams... like 208-219 on intake duration... anywhere in there seems to work very well

 

I'm a fan of big lift and smaller duration cams. You will notice the the "new" Comp Cams XFI series has high lift. These are meant to be used on fuel injected engines. They are also on 113 lobe centers.

 

The theories at play in the selection are that a TPI is tuned for resonance at fairly low RPM. The TPI runners will use a pulse (pressure wave) created by a valve opening event, carry the pressure wave along its length to the plenum, then use the reflected pressure wave to carry extra air charge into the cylinder. That charge needs to be trapped in the cylinder quickly to take full advantage of the TPI. A higher lift cam will provide more of an opening for the air charge to pass more easily. A duration closer to the stock figures will maintain the timing of the valve closing closer to the optimal point to trap more air in the cylinder.

If you extend the duration and maintain the stock LSA, the initial pressure wave may come a little too early, and the valve closing a little too late to take best advantage of the induction tuning. If you advance the valve timing to keep the valve closing closer to original and trap more air, the initial valve opening may occur too soon and the timing of the reflected pressure wave may arrive well before the valve closes. A similar phenomenon will occur if the valve timing is late, in that even though the initial (valve opening) pulse is "on time" the valve closing will be too late, allowing some of the cylinder pressure to bleed back before the valve fully closes.

More opening, either via more lift, larger valves, or both, while maintaining the relatively short duration is better for a TPI. This is where roller cams excel. Due to the smaller contact area on teh lobe, they can generate appreciable lift and yet have fairly steep ramps to keep duration shorter, and all without creating excessive lifter side loading.

Just remember that the whole thing is a symphony of duration and timing, and that the timing affects the induction system. If you can provide more opening area and less flow restriction, yet maintain adequate charging velocity and valve timing, you'll get more bang from each combustion cycle.

Mkos and Orr89 are right. I just thought I'd offer some of the background.