B2 Motorsports - Jeff Lipton 487 FE Build

[Luce]

Let me take a stab at the high velocity port explanation....

In the first half of the intake stroke, there's a vacuum in the cylinbder and the intake charge is accelerating. Toward the end of the intake stroke, the piston is decelerating, and the intake charge has built up momentum. The inertia of the intake charge continues to ram itself into the cylinder, even though there is no more vacuum in there. Actually, between the resonant frequency of the intake runner, and the inertia of the charge is there, the cylinder can actually be "supercharged", in the sense that the engine will have better than 100% volumetric efficiency.

Inertia is proportional to velocity squared, so you really want the velocity without sacrificing too much flow.

[blykins]

Some can explain things a lot better than I can. I can see it in my head, but my fingers don't type what I want sometimes.

The momentum and inertia comments are pretty much dead on and that's why it's best to differentiate between a flow bench and an engine.

If some of you guys can remember the Boss 302 Mustangs, you have a very small engine that is relying on a huge head to supply an intake charge. A little engine sucking on a huge port (that has huge flow capability) can't muster enough vacuum to get the intake charge moving quickly. At low rpms, these cars were turds and it really took some rpms to get them to make an efficient pull on things.

[olddog]

Originally Posted by Luce

In the first half of the intake stroke, there's a vacuum in the cylinbder and the intake charge is accelerating. Toward the end of the intake stroke, the piston is decelerating, and the intake charge has built up momentum. The inertia of the intake charge continues to ram itself into the cylinder, even though there is no more vacuum in there. Actually, between the resonant frequency of the intake runner, and the inertia of the charge is there, the cylinder can actually be "supercharged", in the sense that the engine will have better than 100% volumetric efficiency.

Excellent explanation.

Originally Posted by Luce

Inertia is proportional to velocity squared, so you really want the velocity without sacrificing too much flow.

This is wrong.

First inertia historically had several definitions that were not correct, but today is generally accepted to describe Newton's first law (in layman's terms) a body in motion tends to stay in motion, unless acted upon by an external force.

Inertia is fine to describe the phenomena, but when you want to show relationships or calculate the amount, the term you should have used is Momentum.

Momentum is not equal to velocity squared, just mass times velocity.

Momentum=P Mass=M Velocity=V

P = M * V

Energy=E is equal to Mass times Velocity squared

E = M * V^2

Back to the port. The Mass of the air times the Velocity provides the Momentum that will compress the air into the cylinder at the end of the intake stroke. CFM is the volume per unit of time. The mass is proportional to the volume and the density. The velocity is proportional to the volume and inversely proportional to the area of the port (which is not constant throughout the port).

Therein is the conundrum. The easiest way to get more flow is to increase the area, but at a given flow more area results in less velocity.