Flow vs. Velocity

[gijoe985]

What do we want more from a head/intake? Flow or velocity? Boostin21 made a very good post in another thread promoting velocity over the total flow. This is a dilema that I have encountered with the Ford 351c with 4v heads.

After doing some math, the amount of air being drawn in to our cylinders at 6000rpm should be 180cfm. 195cfm @6500. Therefore, the highest velocity would occur when our flow meets the drawn in requirement, right? Do these factors get thrown out because of the forced induction nature of our engines?

On a side note, how do you get over 100% VE? Doesn't make much sense to me, but I know it's common in race engines...

First question is more important. I'd love to hear some discussion on this topic. I've read up on it and have been turning wrenches for years now, but I still feel pretty green when I get the chance to brain storm with some of you guys. Ther's a lot of knowledge on these boards. Thanks everyone.

 

[a_flint87]

Exceeding more than 100% VE can be accomplished with the addition of a turbocharger or a supercharger. The following website has some interesting info about volumetric efficiency on it. Fuel for the discussion!

CFM - (Cubic Feet Per Minute) As It Relates To Your Engine

 

[gijoe985]

Exceeding more than 100% VE can be accomplished with the addition of a turbocharger or a supercharger. The following website has some interesting info about volumetric efficiency on it. Fuel for the discussion!

CFM - (Cubic Feet Per Minute) As It Relates To Your Engine

I thought that even NA engines could produce 100%+ VE... Which I believe was produced by increasing flow velocity.

 

[dsm-onster]

Flow is king. Not 'flow potential' or what head gurus spout. One uses velocity as a tool to achieve the most flow. One can use velocity to achieve over 100% VE in a naturally aspirated engine. Match your air flow speed to your port diameters and lengths. There you'll achieve the most flow.

Higher velocity accompanies higher rpm. More flow potential accompanies higher rpms. Matching port diameters and lengths to those velocities yields more VE at those rpms. High VE at high rpms yield more power than high VE at low rpms.

. . . Super over-simplified.

 

[a_flint87]

How then is it possible to have a N/A engine exceed more than 100% VE? While maintaining natural aspiration that is?

 

[gijoe985]

Flow is king. Not 'flow potential' or what head gurus spout. One uses velocity as a tool to achieve the most flow. One can use velocity to achieve over 100% VE in a naturally aspirated engine. Match your air flow speed to your port diameters and lengths. There you'll achieve the most flow.

Higher velocity accompanies higher rpm. More flow potential accompanies higher rpms. Matching port diameters and lengths to those velocities yields more VE at those rpms. High VE at high rpms yield more power than high VE at low rpms.

. . . Super over-simplified.

So then if your ports are too large, you will minimnalize the amount of velocity/flow, correct? I guess I am trying to find the connection/distinction between what an engine will flow ont he bench and what that will do when it's bolted on to an engine.

 

[dsm-onster]

So then if your ports are too large, you will minimnalize the amount of velocity/flow, correct? I guess I am trying to find the connection/distinction between what an engine will flow ont he bench and what that will do when it's bolted on to an engine.

That is the debate for the ages! Look at it as relativity. Know what your setup does in hp before a head mod the flow bench the head. Mod the head and note the percentage increase. That will likely be the percentage increase in hp after reintalled.

. . . Assuming you have a properly sized cam, turbo, etc.

 

[Defiant]

How then is it possible to have a N/A engine exceed more than 100% VE? While maintaining natural aspiration that is?

Air mass inertia.

 

[Locke]

Air mass inertia.

Which is the feeding force behind the (Helmholtz) resonance effect designed into a "tuned" IM.

 

[a_flint87]

Which is the feeding force behind the (Helmholtz) resonance effect designed into a "tuned" IM.

I had never even heard of the Helmholtz resonance effect before, I had to look it up! Thanks Locke. I learn something new every day!

 

[dsm-onster]

Here's a paper on it. . .

 

[a_flint87]

Thanks dsm-onster! Here is some more info on the theory of Helmholtz resonance.Helmholtz resonance - Wikipedia, the free encyclopedia

 

[gijoe985]

When you do the calculation for a NA engine, for how much the reguire air flow would be, you factor in volumetric efficiency. Since a turbo engine reaches over 100% VE, could you use that number in the equation to figure out what port flow would be optimal at different RPMs? Just an idea...


And thanks for that great PDF. This is good stuff. I can pass a little bit on to my students.