How does volumetric efficiency factor in to all of this. I think the 7.3 is rated at 85% V.E., does that mean subtract 15% from the figures given in the earlier posts.

On another site a few years back there was a big debate about the 7.3 and airflow. An actual rocket scientist did the math and came up with 540 CFM out of the exhaust pipe at 4000 RPMs.

 

Volumetric efficiency is only a ratio of ACTUAL volume of ingested air as a percentage of the 100% THEORETICAL volume available (think in terms of internal clearance "tightness", like some of the gases passing the rings and either coming out through the PCV or recirculating through the engine and consuming some of that theoretical volume).

EXAMPLE... if the engine could theoretically utilize 1000 cfm, then if it's VE is really 85%, it would actually only ingest 850 cfm of fresh atmospheric air in the real world.

Also, because of the effect of temperature (hotter gases either take up more volume or create higher pressure in a fixed volume environment like our engines), the 540 cfm "out the tailpipe" is not equivalent to 540 cfm going through the air filter, and cannot be compared as such.

 

And elevated air temperature is the achilles heel of the 6637... sucking in hot underhood engine air.

Take hot underhood air, make it hotter by compressing it, then cool it off a little to try and restore some air DENSITY back into the volume of air pushed into the cylinder while the valve is open.

Or, start the process off right in the first place, with cooler, denser air from outside the engine compartment. The cooler the air is to begin with, the less net temperature rise from the compression. The lower the inlet temp is of at the intercooler, the lower the outlet temp will be. The lower the temp of the air injested into the engine, the denser the air is that gets crammed into the cylinder, and the more oxygen is available to combine with fuel to make power.

There is a reason why commercial trucks often have their aircleaners outside of the engine compartment, tucked under and behind the cab in cabovers, or hanging on the outside of the cowl in old school conventionals, or completely baffled under the cowl in modern aerodynamic conventionals. The desire is DENSE air, not just high pressure air. The high pressure is only an accelerated means to an end. The end is air density.

 

Yep... the best combustion (power gain and/or economy per unit of fuel) is always achieved with excess O2, and that rarely happens by accident.

 

I don't recall if the 7.3 has a MAF sensor. I know on my little 6 cylinder iirc I've seen it hit over 1000 MAF in cfm easily. I don't know how accurate that is or if it really pertains to the situation. I'm running S&B and I know it's Apple's to oranges

 

The "readings"you see, in a scan tool, from your MAF sensor, is NOT CFM. It is what is known as "ad counts". It basically tells the ecm how many "beads" of the MAF sensor's abacus to count. Then, in a "transfer function"/lookup table in the fuel map, the ecm knows how much fuel to inject per ad count, along with cross referencing other tables for things like tps, ect, iat, calculated load, rpm, etc....