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!