Let's talk air flow -

Air is a fluid.

Ok, I feel all those raised eyebrows, out there - but, no, I'm not any further around the bend than previously, so rest easy, folks.

Definition of fluid is ability of a substance to move thru, or fit into, a container without altering the shape of the container.
And, yes, I am familiar with the concept of a balloon, for all you hecklers.

Water, sand, flour, Diesel fuel, oil, air - all fluid substances.

Fortunately for us, air is right in there, as a fluid.
We can channel it, direct it, compress it, heat it, cool it, store it - most of us even breathe it, every chance we get.

As does the big stinky air pump we all are so addicted to.
Displacing 400 cubic inches every other 360deg crank rotation - it is an engine requiring 4-distinct cycles to do that deed, two per rotation - , it can pump ~400 cubic feet per minute at 3000rpm.

Air Flow is measured per unit volume, usually in cubic feet - a dimensional area 12" by 12" by 12".

Flow Rate is measurement of unit volume - cubic inches, cubic feet, cubic yard, etc - per unit of time - second, minute, hour, etc.

400cfm per 3000rpm indicates a minimum requirement for ducting and filtration which permits maximum attainable efficiency.
Simple math would indicate this pump cannot move 400cfm at idle, but we would not want to size an intake system based on the minimal flow.

Or, restrict it to only 400cfm - we have a turbine-driven compressor stuffing air into the intake at up to 2.5x pressure ratio, derived by (Boost + Baro) / Baro.
Which means, at maximum power and rpm, we require a system allowing ~1000cfm flow rate.

Would seem to require an enormous filter and ducting, right?
4", maybe even 6" diameter required here?
No girly-man 3"dia ducting gonna hold this big hoss back, that's fer sure!

Well - remove the rubber compressor inlet duct, and take a gander at the actual compressor intake diameter, where the blades are.
"Durn!! Whur's my tape maisure - thet cain't be right!!"

Remember - that's 1000 cubic feet flowing over a 60 second interval, about 17 cubic feet per second.

Now, think about this - if the flow rate of the supply air pump - the turbo compressor - is only equal to the demand air pump - the engine - no pressure will be developed in the system, and no advantage will be gained.
400cfm in, 400cfm out, 1:1 pressure ratio - may as well remove the compressor, as that's just Baro input, same as naturally aspirated.

However, if the flow rate of the supply pump exceeds the demand flow rate, excess flow will stack up in the intake as pressure - the greater the excess, the greater the pressure.

The GM-8, in recent tests, easily generated 20psi Boost on oem 63mm fuel rate, up to 2500rpm, where tests were limited to prevent DTC-78 and resultant power loss.
Factory '95 air box and curved intake duct, AC filter element - all stock.

So - 17cuft/sec - 1000cfm - doesn't seem to be limited much by that 2" compressor inlet diameter, so where's the need for 4" ducting? 6"???
The 3"dia where the inlet ducting is mounted\clamped should suffice for straight ducting, but a path requiring several sharp bends could benefit from the larger 4" diameter for less flow restriction.

Unless - the 3"ducting were plumbed up to the radiator bulkhead to take advantage of the cooler air available there, under high pressure.

Flow rate can be improved by increasing the diameter of the duct.
3"dia provides an area of 9.5sqin, and 4"dia provides 12.5sqin, where sufficient real estate is available for the increase.

Flow rate can also be improved by increasing the flowing pressure.

These trucks have about an 18sqft frontal area, hood to bumper, headlite to headlite - that pushes quite a lot of air out of the way, at any speed.

Move the truck forward 1 foot -12 inches - and it has displaced 18 cubic feet of air.

At 60mph - 1 mile (5280ft) per minute - it will displace 95,040cfm.

Now, comparing the truck's effective 'flow rate' of 95,040cfm to the 6.5L absolute maximum flow rate of 1000cfm, it can be seen that, again, where the supply is more than the demand, it will stack up as pressure.

Stick your hand out the window, palm forward, at 30 - 45 - 55 - 70mph, to get a sense of the available pressure.

We can use that pressure to increase the flow rate of the oem factory intake system, simply by providing a dedicated path from the hi pressure area at the bulkhead to the air filter box.

No worries about twists, turns, curves, bends - it will get there, at most any speed, with no loss of effectiveness.

I mean - come on, now! 95,040 cfm vs 1000cfm. A measurable loss of effectiveness?
I don't think so.

And, much cooler and denser than the flow thru the radiator.

(reader interpolation required for other speeds, rpm)

[ 08-08-2004, 05:25 AM: Message edited by: gmctd ]