My truck flows more air than your truck

[gmctd]

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 ]

[rjschoolcraft]

I respectfully disagree.

Sand and flour are not fluids. They are solids... Just lots of little ones.

[Spindrift]

I'd rather error on the conserative side so I fabricated a new fresh air intake scoop that mounts on the underside of my bumper and plumbed it direct into the K&N airbox using 4" flex hose. I want that darn filter to see all the pressure mother nature, and 65 mph can throw at it. Maybe I should even close off the old opening thru the fender well? A pressure wall at the filter can't be all that bad, can it?

[catmandoo]

i agree with ronniejoe,try pumping fluid in a cylinder and see how far you can turn the crank.air on the other hand compresses.but your theory has some merits,how about say an 8 inch inlet at the grill tapering to the 3 inch into the turbo.will the loose of space create a higher pressure,thus helping boost, preturbo?

[gmctd]

Correct, ronniejoe, but in that form each satisfys the definition of fluid - able to fill a container without deforming it.

While conforming to the shape of the container.

Six of one, half dozen of the other - still, being hit by a balloon of water would probably hurt less than a balloon filled with sand.
Wet sand would probably compound the hurt.

spindrift and cat - plans are, mounting the late round filter box to the front bulkhead, using 3" diameter ducting to the compressor.
Straight and simple.

The intake system, filter thru compressor, will be pressurized at any forward vehicle movement, at reduced temperatures.
Reducing the base temp into the compressor will reduce the outlet temp, correspondingly.

[ 08-07-2004, 08:11 PM: Message edited by: gmctd ]

[rjschoolcraft]

GMCTD is right on air being a fluid...

Note, I didn't say liquid! Gases and liquids are both fluids by definition. Gases are compressible, liquids are not (within rational limits, of course).

[markrinker]

There was a period in the '70s where every musclecar manufacturer produced a variation on forced air induction to imitate the drag racers of the times. 'Ram Air', 'Cowl Induction', 'Ram-Jet', etc. all became high school lore.

Most were cosmetic by the time they reached production - all victims of form vs. function and a conservative approach to work at all altitudes and under widely varying weather conditions - i.e. the carbs could not be factory tuned to be dependent on the extra air flow, so then could not effectively use the extra air flow when it was available.

A turbo diesel application like ours is a much better candidate for 'using' available pre-pressurized air - although available COOLER air is probably way more important.

I would take any (marginal) performance gain provided by better air flow - on a nice day. However, I don't want a system that will gulp water the first time we cross Beaver Creek this fall hunting pheasants, get jammed full of grasshoppers like my grill does in summer, or pack full of snow in the winter plowing snow.

So, I'll probably continue to suck musty fender air through a never ending supply of clean Fram filters at every oil change.

[ 08-08-2004, 04:45 AM: Message edited by: Mark Rinker ]

[Billman]

All of these 'Hood Scoop', 'Ram Air' ideas sound great until it rains out. It now makes the trucks a fair weather ride only.

What would be useful would be to have a similar set-up that we have on some of the Airport Snow Removal equipment. They have an air operated slave cylinder that opens a duct for outside air(Not Underhood) when applicable.

Maybe something like the Chrysler 'Air Grabber' mounted on top of the hood, directly over the filter housing that opened with a swicth, and closed off the fender inlet. Once it starts raining, switch back to the factory ducting.


I'm now thinking of leaving the lid off the air filter housing. Even with underhood temps, although I don't think they can be that high at 65 mph, the intercooler will be doing its job.

[gmctd]

Good points -

Where GM provided a direct path thru the bulkhead in various vehicles, a baffle was used as a basic centrifugal filter - air navigates abrupt changes in direction easier than water (in it's various forms), bugs, dirt, etc.

The 6.2L series had a plastic duct molding - air in at the top, 90deg turn down, drop ~8", 180deg turn back up to the top, flap-covered drain at the bottom for the heavies.

Earlier trucks also had a sheet metal baffle spaced out from the bulkhead opening about two inches.

Later, 89 up, passenger-side radiator support (boxed) had air ports in the front wall, outlet ports in the pass-side wall, providing a 90deg turn into the above-mentioned molded centrifuge.

The new Ford PS 6.0 has a huge dedicated opening designed into the drivers-side grille (matching design on passenger-side), with a maze-type plastic baffle and huge oval filter box attached directly to that opening, and large ~4" duct direct to compressor.

Anyone in front of those trucks can hear the turbo sing any time the accel pedal is nudged, at any speed.
Well, I had my windows down for the cool weather, and the new Ford chase truck was seldom far behind.

As Mark pointed out, re-work should not create any problems which defeat the advantages.
With 95,000cfm to work with, a simple baffle or two won't be a problem.

Fluid - some confusion may result from modern idiom -

We have transmission fluid, power steering fluid, dry-cleaning fluid, lighter fluid, etc.
All liquids.

When defined, these become light oil\additives\coloring, light oil\different additives, petroleum naptha\soap, benzine (iirc).

Modern lighters use butane, a liquid at hi pressure, a gas at Baro.
Butane is fluid in both states.

Camp stoves, BBQ pits, and Propane - liquid at hi press, gas at Baro, fluid in both states.

The term fluid, as defining a property of a substance, means that it is flowable, pourable, storable - it can conform to the shape of a container without deforming that shape.

So - fill a cylinder full of oil, piston won't move.

Fill a cylinder full of sand, piston won't move.

Both have fluid properties.

Fill a cylinder full of rocks.......well, you probably have too much time on your hands.

[norm]

gmctd is correct, but it depends on what the meaning of the word "a" is (remember Slick W?). Think of "fluid" as a property of a substance not a substance. Sand could be called "fluid", while a grain of sand could not(depending on the size of the "container").

Pressure (air/gas) is definitely what to think of here. In residential HVAC, we need a 6" pipe (duct) to move only about 100 cfm, but that is at the pressure a typical furnace fan and duct system works. Our 3" or 4" exaust pipe is moving a lot more cfm, while at higher pressures. Same with the intake before turbo except at negative pressures (vacuum) typically. Efficiency, effectiveness, and practicality are what we balance with these specs and power requirements.
If we wanted the absolute best volumetric effeciency, we probably would be using superchargers, not turbos...

[CleviteKid]

I am in complete agreement with jd's theories about ram-air at speed, and it is experimentally apparent with my NA engine. I have a 4" intake hose directed thru the radiator support at the original GM location on my '84. Only a piece of window screen glued to the back of the grille keeps the bugs and rain drops out of the hose. I do have a 3/16" hole at the bottom rear of the air cleaner, outside of the filter contact zone, to drain water that might accumulate there. The oiled Amsoil air filter will repel water to some extent also.

On the trip to and from Montana for the 2004 Rendezvous, I found that uphill performance was better and EGT lower the faster I went, up to my self-imposed limit of 85 mph. I attribute this mostly to the increased ram-air effect, which should go approximately as the square of the speed, by the way. Even over the 7000 foot passes, if I was going fast enough I did not perceive the need for a turbocharger (remember, I was towing nothing, and my gear and I did not exceed 500 pounds payload).

However, don't think you can get all 95,000 available CFM into the engine at 1000 psi or so - by making a big funnel shaped scoop.

Imagine pouring Jack Daniels from a big pitcher into your hip flask thru a funnel: if you try to pour too much too fast, the excess will just fill the funnel and pour over the edge onto the counter and the floor, where you will be competing with the dogs to lick it up.

jd is correct in reminding us that our engines are just big air pumps that add some fuel to extract power. Adding enough fuel is not really the problem, moving enough air thru the system is the challenge, and jd is describing sound ways to improve the performance of the big air pumps we are operating.

And for the record, even though I don't have one, from a thermodynamic perspective ALL DIESELS SHOULD BE TURBOCHARGED, and from a practical perspective, all diesel pickups carrying a heavy load or towing a substantial trailer should be turbocharged and intercooled. And you can quote me, even though The Clevite Kid don' neeeeeed no steeeeeeeenkin' turbo on his truck!

Dr. Lee

[grape]

have you ever filled a glass under your faucet? Did you pay attention to what happened when it got full, as in water running over the edge instead of building enough pressure to blow the glass apart. Air does the same thing, something like less the .5" of water of pressure under 200 mph, anything else and the air will stack up and spill around the air inlet. This is why they keep shrinking the air inlets on the IRL cars, more area doesn't gain performance but it does create drag. As far as cfm goes, I've done the math, but inlet flow at the compressor mouth needs to be roughly 630 cfm for enough air for 300 hp on a 6.2 litre diesel. All has to do with the efficiency of the compressor wheel.

[JeepSJ]

Based on personal experience, I tend to agree with Grape in that you are not going to generate much pressure. But, pressure, any pressure, no matter how small, is better than a vaccuum. We used to run a LSR car at El Mirage and Bonneville. We were in a naturally aspirated class and we tried all kinds of scoops and ducts to try to get more air into the engine. IIRC, the best luck we had was actually pulling air from the cowl area. The best we ever saw was a little over 1psi in the air box. We picked up several hundred RPM on the top end.

[gmctd]

0.5"h2o at 200mph?

Those figures right, grape?

Less than 1" water, where 27"h2o is 1psi would seem to indicate vehicle dynamics were creating a low pressure 'void' area at the pick-up point, where the engine flow rate was beginning to exceed available forced-air supply.

Most men wouldn't even be able to hold an arm out the window into the slip-stream at that speed, right?

[Barry Nave]

A finger,yes.
Hand,NO

[JohnC]

150 MPH will raise a column of water 11.4 inches for a pressure of .338 psi.

YMMV

(hmmm, upon further reflection, there seems to be something wrong with those figures.)

Let's see, a cubic foot of fresh water weighs 62.4 lbs, so a column of water 12" high should exert a pressure of .433 psi.

Airspeed (MPH) = (inches-of-water x 1980)**1/2

MPH**2/1980 = inches-of-water

(150 x 150)/1980 = 11.364

So, 150 mph = 11.4" of water = 0.411 psi

QED

[ 08-09-2004, 08:53 AM: Message edited by: JohnC ]

[CleviteKid]

As a freshly certified A&P mechanic, JC is the authority for the pressure vs. speed function, in a static tube.

When you are sucking air out of the back of your tube, conditions may vary, and the effective pressure and resultant flow rate may be different that static conditions would predict. But do I know the answer? No.

The Clevite Kid.

[JeepSJ]

GMCTD brings up an interesting point about vehicle dynamics. One of the teams running a roadster had the grill blocked off and had a hole in the front of the grill to feed their air box. They got the best results when they extended the duct about 12" out in front of the grill. Not being an aerodynamic guru, I would imagine that it would have something to do with an affect similar to the transition layer - while you are travelling 60mph, if your hand is resting on the outside of your door, you barely feel anything from the wind, but if you raise it slightly then you feel the force of the wind. We theorized that there is a dead spot immediately in front of the grill on their racer, and extending the snorkle got the opening out into the air stream.

[TurboDiverArt]

Originally posted by JeepSJ:
Based on personal experience, I tend to agree with Grape in that you are not going to generate much pressure. But, pressure, any pressure, no matter how small, is better than a vaccuum. We used to run a LSR car at El Mirage and Bonneville. We were in a naturally aspirated class and we tried all kinds of scoops and ducts to try to get more air into the engine. IIRC, the best luck we had was actually pulling air from the cowl area. The best we ever saw was a little over 1psi in the air box. We picked up several hundred RPM on the top end.

That doesn

[TurboDiverArt]

Originally posted by JeepSJ:
GMCTD brings up an interesting point about vehicle dynamics. One of the teams running a roadster had the grill blocked off and had a hole in the front of the grill to feed their air box. They got the best results when they extended the duct about 12" out in front of the grill. Not being an aerodynamic guru, I would imagine that it would have something to do with an affect similar to the transition layer - while you are travelling 60mph, if your hand is resting on the outside of your door, you barely feel anything from the wind, but if you raise it slightly then you feel the force of the wind. We theorized that there is a dead spot immediately in front of the grill on their racer, and extending the snorkle got the opening out into the air stream.

Ahh, I think you are onto something. That's probably why the Pro Stock cars I mentioned above have the scoop 10 inches above the hood and a good 2 feet in front of the windshield.