At least enough to produce the HP being demanded by the driver's right foot and then a little more if you don't want to blow smoke! Here's the "thought process" for figuring out how much CFM is needed for a given HP. I'll first give an example calculation where 300 FWHP is being demanded by the driver and then I'll "capture" this "thought process" as a "general equation" which can be plotted and used for a wide variety of other situations.

If a given GPH flow of #2 summer blend diesel fuel combusts with enough air in the cylinders the "chemical ENERGY" in each gallon releases 51.08 "heat ENERGY equivalent HP". About 65% of this "heat ENERGY equivalent HP" is converted into "piston power stroke HP" and about 57% of this "piston power stroke HP" is converted into usable FWHP.

So a 1 GPH fuel flow produces a "piston power stroke HP" equal to (51.08)(0.65)=33.2 HP. This 33.2 HP is the "average HP" over the entire "piston power stroke" from TDC to BDC. The difference (51.08)-(33.2)=17.9 HP is the HP lost to the cylinder head and cylinder walls and into the coolant during the "piston power stroke". This means that 17.9 HP of the initial 51.08 "heat ENERGY equivalent HP" in each gallon of fuel is lost to the coolant during the "combustion process" and then the radiator transfers this lost 17.9 HP to the atmosphere.

This 33.2 "piston power stroke HP" in turn produces a FWHP equal to (33.2)(0.57)=18.9 HP. The difference (33.2)-(18.9)=14.3 HP includes the HP lost to the coolant due to "piston friction" with the cylinder walls during the other 3 strokes, and other rotating frictional losses, and includes the HP lost due to parasitic losses such as the oil pump, water pump, and alternator. The 14.3 HP also includes the "pumping loss HP" which is the "airflow HP" that's required to force the air into and out of the cylinders. Finally the 14.3 HP also includes the net amount of "heat ENERGY equivalent HP" which goes out the exhaust valves and isn't used to power the turbo.

Note that some of the "heat ENERGY equivalent HP" which goes out the exhaust valves is used to power the turbo to make the boost to make the airflow into the engine to combust the fuel to make the initial 51.08 "heat ENERGY equivalent HP" to begin with and that the 14.3 HP above includes a "net loss" which accounts for the turbo having used some of the "heat ENERGY equivalent HP" going out the exhaust valves which would otherwise have been wasted and just exited the tailpipe into the atmosphere.

The bottom line is that 18.9 FWHP is produced by each 1 GPH of fuel flow and since each GPH produces 51.08 "heat ENERGY equivalent HP" the overall net conversion efficiency is (18.9)/(51.08)=0.37 which means that 37% of the fuel's "chemical ENERGY" is ultimately converted into usable FWHP. Well actually the FWHP needs to be applied to the road to be usable by a driver but that's another story!

So since each 1 GPH of fuel flow produces a net 18.9 FWHP if you want to produce 300 FWHP you need a (300)/(18.9)=15.9 GPH fuel flow. Since each gallon of #2 diesel fuel weighs 7 lb a "volume fuel flow" of VFF=15.9 GPH is equal to a "mass fuel flow" given by MFF=(15.9)(7)=111.3 lb/hr=(111.3)/(60)=1.86 lb/min.

Since it takes 15 lb of air to combust each lb of fuel you need a "mass airflow" given by MAF=(1.86)(15)=27.9 lb/min to make 300 FWHP! If the air temperature at the air filter inlet is 70 F then the "air density" is AD=0.075 lb/ft^3 and this gives a required air filter inlet CFM=(27.9)/(0.075)=372 ft^3/min.

The "air fuel ratio" is given by AFR=MAF/MFF and in the above calculation AFR=(27.9)/(1.86)=15.0 however this AFR is the "theoretical" AFR for "perfect combustion" and that requires "perfect mixing" of the fuel with the air and that doesn't quite happen in an actual engine.

It requires an AFR of 18 or more to avoid visible smoke due to incomplete combustion and this in turn requires more air and the MAF to get an AFR=18 is MAF=(1.86)(18)=33.5 lb/min and this in turn requires a CFM=(33.5)/(0.075)=446.7 ft^3/min.

So what kind of an air filter do you need to support the CFM=446.7 ft^3/min that's required for a "smoke free" FWHP=300? Well the ISO-5011 test results for the stock Motorcraft FA-1750 (Ford P/N 2U2Z-9601-AA) filter in the stock air box gave the following results...

CFM......Inches H2O Restriction
000.0.. .0.000
314.5. ..4.436
471.1.. .9.598
627.7...16.660
786.5...25.775
929.8...35.705

So from the above a CFM=446.7 ft^3/min can be obtained at about a 9" H2O restriction for a new filter and this leaves plenty of "restriction overhead" to allow the restriction to increase by 17" H2O as the filter loads with dirt until the restriction reaches 26" H2O on the "filter minder" at which time it should be changed, the filter that is not the minder. As you can see the ISO-5011 testing included up to a 930 CFM at a 36" H2O restriction but a 26" H2O restriction was used as the termination or cut-off restriction during the dirt loading test.

Now if your air filter inlet temperature doesn't happen to be the 70 F used in the example you can calculate your actual AD from AD={(2.70325)(AFIAP)}/{(AFIAT+459.67)} lb/ft^3 where AFIAP=Air Filter Inlet Air Pressure psi and AFIAT=Air Filter Inlet Air Temperature F. For example at sea level AFIAP=14.7 psi and if AFIAT=70 F you have an AD={(2.70325)(AFIAP)}/{(AFIAT+459.67)}={(2.70325)(14.7)}/{(70+459.67)}=0.0750 lb/ft^3.

Well the wife says I have to quit having fun and get ready to travel tomorrow so here's an opportunity for anyone who wants to practice their analysis and equation skills. I've received several PMs that mentioned some members have trouble with or don't understand at all my analysis and equations so here's a chance to try on your own to come up with a "CFM equation"!

First study the above approach I gave for calculating the CFM that's required for a "smoke free" 300 FWHP and do the calculations for 375 FWHP. Then define some general terms of your choice like DFW for Diesel Fuel Weight because not all diesel fuel weighs 7 lb/gal and terms like PCE for Piston Conversion Efficiency because the PCE=0.65 I used varies some with the FWHP, etc... and then redo your calculation for the CFM to get 375 FWHP but use your general terms instead of the numbers and you'll have written an equation for CFM as a function of FWHP, DFW, PCE, AFR, etc...

I'll eventually come up with my version of the "CFM equation" but in the meantime if anyone wants to post theirs I'll be happy to answer questions and help you figure it out on your own. Working with equations is just like balancing your check book except you're first using symbols to represent parameters like "deposits", "interest earned", and "checks paid" and then you substitute actual numbers for the parameters and use your calculator to get the desired result.

Using a "CFM equation" saves you the thinking process of having to work through the logic trail I used above each time you want to calculate a new CFM value for a different set of parameter assumptions. Once you understand what the equation means and how it was derived you can just substitute new numbers into it and use it as many times as needed without ever again doing all the "drudgery thinking" of all the logic steps on which the equation is based!

The above approach is called "cookbook engineering" and it's what greatly increased my productivity over the years. Of course if you happen to apply the wrong equation to a given problem because you never really understood what the equation meant to begin with you're called a "bad cook" and you likely got fired!

 

Bump.

 

Quick semi off topic question for you Gene. I was putting some new shelves in the barn today and ran across an air filter for my old (really old) truck. It's got a 327 gasser that was built up to around 300 hp. The surface area of that filter, as well as most other old carburetored gas filters I remember, is much smaller than that of the 6637. Is that just from the different BTU content of gas vs diesel?

From your posts, it seems a motor needs "X" amount of air per volume of fuel to produce that kind of horsepower. If that's the case, then fuel injectors and turbos won't have an affect on air flow required to produce a given HP. Am I comparing apples to oranges here?

 

When I complete the above "CFM equation" I'll make sure it includes the "FEC=Fuel Energy Content BTU/gal" as one of the "variables" but until then the short answer is no it's not... "the different BTU content of gas vs diesel"!

The BTU content of gasoline fuel is 125,000 BTU/gal, the BTU content of diesel fuel is 138,700 BTU/gal, and the BTU content of bio diesel is 126,200 BTU/gal so diesel has 11% more BTU/gal than gasoline and bio diesel has only 1% more BTU/gal than gasoline, and I'm assuming you "think" you see a much larger difference in the "surface area" of the gasoline air filter vs the 6637 filter which BTW is also recommended for gassers according to the OEM's website.

When it comes to comparing the "effective" surface area of air filters "looks" can be somewhat "deceiving"! For example below is a picture of a stock 7.3L air filter and according to the advertisement I "stole" this picture from its dimensions are 7.3"x13.4" and it has pleats 3.3" deep.

The stock filter appears to have a surface area of only (7.3)(13.4)=98 in^2 but as you can see from the picture air flows into the pleat openings at the top and then down into the deep "V" shaped pleats and out their sides. This greatly increases the overall effective filtering area.

I hope someone will take the time to count the number of pleats and post the results here or send me a PM because I want to calculate its effective filtering area so I can compare it to the effective filtering area a 6637 which because of its dead en cylindrical shape only uses about 40% of its actual area at any given time.



Here's the bottom-line factors which... "have an affect on air flow required to produce a given HP".

When considered as a ratio of the gross weights of the air and the fuel involved the number of Oxygen molecules required to combine with a given number of hydrocarbon fuel molecules works out to be 14.7 lb of air to 1 lb of gasoline fuel, and about 15 lb of air to 1 lb of #2 diesel fuel, and in both cases this is called the Stoichiometric Air Fuel Ratio. So in terms of the AFRS=Air Fuel Ratio Stoichiometric diesel only requires 2% more weight of air compared to gasoline.

However for a gasser the gasoline is premixed with the air in a "homogeneous" mixture prior to combustion and this allows the mixture to burn completely at an actual AFR=AFRS=14.7 so a gasser only needs an airflow that's sufficient for providing a AFR=14.7.

For a diesel the fuel is injected and as can be seen in the video below this results in a "heterogeneous" mixture of the fuel with the air and this means the fuel doesn't mix perfectly with the air and the flame doesn't burn evenly so that if the airflow in a diesel is only just sufficient to supply an actual AFR=AFRS=15 you'll get too much smoke to pass the EPA diesel emissions spec.

EDIT... The FTE software screwed up my link so try pasting this in to your browser window...
Ok now it works so just click the link and hit replay.

Diesel Injection Flame
http://videos.streetfire.net/video/D...ame_190502.htm

That's why I used an AFR=18 in my above calculation for a "smoke free" 300 FWHP! An AFR=18 requires 20% more airflow by weight than for a AFRS=15 and 22% more airflow by weight than for a gassers AFRS=14.7 but even this AFR=18 won't pass a 2003 EPA diesel emissions spec for NOx which requires an "EGR free" AFR=25 which is 70% more airflow by weight than for a gasser.

My CAT C7 meets the 2003 EPA NOx emissions spec without using EGR by flowing more clean air to the tune of about 40% more airflow than is actually need to combust the fuel and the "heat capacity" of the extra air is used to absorb enough heat from the combustion process that the peak cylinder temperature stays below 2,000 F and this results in less NOx production.

Before I continue with the "CFM equation" I'm going to add a post here called "The A, "Bee", C of Airflow, Air Temperature, and Air Pressure" because I think some more background information is need to really understand the issues involved with the "CFM measure" of air flow, so please stay tuned to this channel for the interesting "Bee" story to follow!

 

Ummm errrrr I have a cat, her name is Jady

 

The A, "Bee", C of Airflow, Air Temperature, and Air Pressure...

The "A" is for airflow, the "Bee" is for "Honey Bee" because a swarm of bees is a good way of visualizing the behavior of "molecules" in an air mass, and the "C" is for "CFM" which turns out to be a very confusing way of quantifying airflow.

On another forum I did a post called... "The Roach Air Motel... Air checks in, but it doesn't check out!" ...to try and explain why there's a much larger "CFM airflow" going into an engine's air filter than the "CFM airflow" going into the engine itself. The "Roach Air Motel" effect also occurs in reverse because for any air filter there's more "CFM airflow" coming out of the downstream side of the air filter's element than the "CFM airflow" going into the up stream side!

This time around I'll try and explain this seemingly "mysterious CFM effect" by using "bees" as an analogy for "air molecules". Imagine you've got an entire enclosed football stadium filled with "bees" and that the "bee density" is so thick you can't even see your hand in front of your face. The bees are buzzing around in a random chaotic manner and bumping into each other and into you and into the walls of the stadium and into everything else they come into contact with.

Say you need to transport some bees from inside the stadium to a customer who's got a large trailer waiting outside and you've got a 1 ft^3 box with a removable lid to do the job with. So you bring the box inside the stadium and remove the lid to let some bees inside and then close the lid and take the box out to the waiting trailer and then open the lid to let the bees out.

If you repeat this exercise once per minute the box is flowing 1 ft^3/min or 1 CFM of bees but the customer doesn't care about that because he's only paying for the number of bees getting delivered per minute and that of course depends how many bees are in each boxful which in turn depends on the "bee density" inside the stadium.

Well the wife is pestering me again so I'm going to post this much just to see if anyone's actually interested in hearing the rest of the bee story including why "atmospheric bee pressure" doesn't depend on "atmospheric bee temperature" but "atmospheric bee density" does depend on "atmospheric bee temperature" whereas "stadium bee pressure" depends on "stadium bee temperature" but "stadium bee density" doesn't depend on "stadium bee temperature" and how to relate the CFM delivery of bees to the number of bees delivered for various situations?

If there's no interest in bees I'll just move on with completing the "CFM equation" which is actually the equation for the CFM airflow into the engine's air filter that's needed to provide the customer "engine" with the number of bees it needs to "eat" a given amount of "fuel" or in the case of my C7 "CAT food"!

 

I'm currently hanging out in Santa Rosa and our next stop is Amarillo so we can go eat a couple of those huge steaks that you guys give away for free there! I don't plan on eating much between now and then so at least I'll have a "gluttons" chance of getting my name on the "wall of fame"! Or maybe I should just keep stuffing myself so my stomach doesn't shrink?

 

Funny, I was in Santa Rosa Friday night. Stayed at the Santa Rosa RV Park. Yea and good luck with the steak, its more like a large roast.

 

darn, wrong Santa Rosa.

I think you are slipping up, because I understood all that on the first try through, and I know I AIN'T getting any smarter.

Do like the bee theory of atmosphere, but thought them stadium bees t'were a different breed altogether.

 

I see where you're going with this, but by all means continue the bee story. It's easier to follow than the equations.

As for the gasser air filter. I wish I hadn't thrown it away. I understand looks can be deceiving, but I'd be willing to bet your favorite beverage that the surface area was smaller than the 6637. Think back to the old round filters that sat on top of a carb. About 3" tall, 12" - 14" in diameter, with a pleat depth similar to the 6637.

With regards to the OEM filter, will a wix drop in work for you? I've got one at the office. If I don't get sidtracked tomorrow, I'll count the pleats as well as take any other measurements for it you may want.

 

Since the weather has improved the wife is placing more demands on my time to do some sight seeing so the bee story will be told in several installments and then I'll complete the "CFM equation".

If you think bees are... "easier to follow than the equations" ...then you might be surprised when I start explaining how bees posses a "Quantum Electronics" property that's analogous to the one possessed by actual O2 and N2 molecules which have various internal vibrational and rotational degrees of freedom which act as mechanisms for absorbing heat ENERGY from the combusting fuel at just the correct cylinder air temperature of 1,200 F to avoid melting aluminum pistons and then holding this absorbed heat ENERGY like a "sponge" so that it doesn't contribute to further increases in cylinder air temperature and then releasing the absorbed heat ENERGY as the cylinder air temperature falls below 1,200 F so as to keep the cylinder pressure from decreasing as rapidly as it otherwise would and thereby increasing the piston's power stroke HP!

That pleat count and dimensions would be a helpful input but it's quite possible the WIX version of the stock Ford filter isn't built to the same specifications as a "genuine" Motorcraft FA-1750 (Ford P/N 2U2Z-9601-AA) filter because over the years Ford has continuously updated and improved their stock filter element. For example the 2002 owner's manual for the F-350 specifies the FA-1680 element and then later this was updated to the FA-1720 element which has filter pleats about double the depth of the previous FA-1680 element and then additional improvements resulted in the latest FA-1750 element which I showed in the previous picture.

The S&B website discusses ISO-5011 testing they did on the "made in China" versions of their S&B filters and those tests showed that the "third party knockoffs" failed miserably and I suspect the same might be true in general for any third party version of an OEM air filter!

While you're at it measure your 6637 as well because here's an example of the above "third party version of an OEM air filter" issue for the 6637.

Below is the WIX specification for their 6637...

Part Number:46637
UPC Number:765809466371
Principal Application:Air Filter Assembly for Various Equipment.
All Applications
Style:Air Filter
Service:Air
Media:Paper
Height:12.380
Outer Diameter:8.500
Inner Diameter Top:Closed
Inner Diameter Bottom:4.000
Ends:Metal CFM:425

...and the specification below is for the OEM Donaldson version of the 6637???

Part Number:42790
UPC Number:765809427907
Principal Application Donaldson Disposable Housing
All Applications
Style:Air Filter
Service:Air
Height:11.880
Outer Diameter:10.500
Inner Diameter Top:Closed
Inner Diameter Bottom:4.000
Ends:Plastic CFM:680

As you can see Metal vs Plastic Ends, Height slightly different 12.38" vs 11.88", and a 2" difference in Outer Diameter 8.5" vs 10.5"! So what are the correct dimensions to use for calculating the outer surface area of a 6637?

Since everyone seems to use the WIX 6637 and not the OEM Donaldson I'll use the WIX dimensions which give a "physical surface area" of AREA_6637={(Pi)(D)(L)}={(3.142)(8.5)(12.38)}=330.6 in^2.

For the gasser you get a "physical surface area" of AREA_gasser={(Pi)(D)(L)}={(3.142)(14)(3)}=132 in^2.

For the gasser all of the intake suction is applied uniformly to all of its "physical surface area" all the time so the gassers "effective surface area" is equal to its "physical surface area" so for the gasser filter the "effective surface area" is AREA_gasser=132 in^2.

Due to the dead-end cylindrical design of the 6637 filter the intake suction isn't applied uniformly to all of its "physical surface area" all the time and for a clean 6637 element very little suction reaches all the way out to the dead-end region of the cylinder and Tenn proved this by measuring very little suction there on his clean 6637!

As the portion of a 6637 that's nearer the neck loads with dirt the suction reaches farther out toward the dead-end region of the cylinder and Tenn proved this by measuring more suction there when he wrapped a towel around the inner portion of his 6637.

So the "effective surface area" for the 6637 at any given time is only about 40% of its "physical surface area" so the 6637 "effective surface area" is equal to (0.4)(AREA_6637)=(0.4)(330.6)=132.2 in^2!

So it looks to me like "looks are deceiving" and that the gasser filter has exactly the same "effective surface area" as the 6637 filter has!

I hope everyone recognizes that I didn't intend for this thread to even mention the "dreaded 6637 topic" and that you brought it into the conversation by making claims... "The surface area of that filter, as well as most other old carburetor gas filters I remember, is much smaller than that of the 6637." ...which as far as I can tell aren't substantiated by the facts!

BTW since you made the offer... "I'd be willing to bet your favorite beverage that the surface area was smaller than the 6637." ...I drink anything that's cold and has at least a 5% alcohol content!

 

Hmmm. And are you positive in your assessment that the 6637 only has the 40% efficiency? Looks like if it's at least 40.1%, then I was correct.

I'll get the counts and dimensions for you. I may even have an early 99 filter I can measure as well. Never have seen the metal ends on the Wix filter. I'll need to look closer and make sure I'm, not missing something.

 

I'll get you the filter data if you delete a couple of old PM's.

 

Good to hear from you again Gene. Now to the matter at hand. Who cares what the effective filtering area is in the first 1/4th or 1/2 is as long as they suck through the same sized hole known as an intake? Are you saying the total flow capacity of the stocker is better than the 6637? I think we would need to calculate the total flow of both, though we already know the max flow/cfm of the 6637, I will await the #s of the stock panel filter. I would have to postulate that the panel's area would be smaller just from looks alone. Just wrap it around into a circle and see how small it is then.

Please continue the Bee thread.

 

Yes I am but please note that I started this thread for the sole purpose of discussing the topic in its title but if people persist in posting things about the 6637 on my thread that aren't substantiated by the facts then I'll have no choice but to let this thread digress into another one like this... " 6637 question"... https://www.ford-trucks.com/forums/s...3&goto=newpost ...which perhaps you haven't read?

If you put a box around a 6637 with an air inlet tube running to the front of the grill area like the stock air box has so that both air filters ingest intake air at the same temperature then yes when it comes to net CFM vs Inches H2O restriction the stocker is as good as or even better than a 6637!

Also as I discussed here... "Best cold air intake and exhaust."... https://www.ford-trucks.com/forums/s...1&goto=newpost ...I think there's some good reasons why the stocker's overall "Dirt Filtering Efficiency" and especially its "Dirt Filtering Efficiency" of "invisible" particulate matter with very small um diameters is probably better than for a 6637!

I'm basing the above net CFM vs Inches H2O restriction claim on the ISO-5011 test results for the stock Motorcraft FA-1750 (Ford P/N 2U2Z-9601-AA) filter in the stock air box which are discussed at length in the first link above and are the numbers I quoted in my first post on this thread for the sole purpose of showing that the CFM needs of a mildly tuned 7.3L can be easily meet with a stock filter. If you look at those numbers you'll see the stock element in the stock air box provides a 471.1 CFM at a 9.598" H2O restriction.

The WIX 46637 that's used and recommended by many here on FTE is cross referenced to a Baldwin PA2818 and to a Donaldson B085011 and even though I doubt the WIX 46637 is of equal quality to a Donaldson B085011 the only CFM vs Inches H2O restriction data that I can find is for the Donaldson B085011.

Here's a link to the Donaldson site... http://www.donaldson.com/en/catalogs/engine/033621.pdf ... and if you look on page 2 in the 4th row of the table for the B085011 you'll see the following restriction data...

ECB Initial Airflow Restriction
CFM @ "H20 Air Cleaner
4".......6".......8 "......Model
280....400....470......B085011

So the stock Motorcraft FA-1750 element in the stock air box provides the same 470 CFM flow as what's probably a better version of the WIX 46637 but the stocker has 1.6" H2O more restriction at this same 470 CFM flow. However if you put the 6637 in a box with an air inlet tube the air flow velocity through the air inlet tube that's required to supply a 470 CFM flow to the downstream 6637 element causes an additional restriction of at least 1.6" H2O and possibly more so that based on net CFM vs Inches H2O restriction the stocker is as good as or even better than a 6637!

BTW if you look at the link to the Donaldson site on page 3 in the 3rd row of the table for the B085011 you'll see the dimensions as 8.50" diameter and 11.00" length so the previous data I showed with an Outer Diameter of 10.500" was in error and that's for one of the larger diameter Donaldson filters!

Hopefully the "6637 dust" will eventually settle down and I'll get on with that story which will not be as "simple minded" as some might think!