[aerohead]

*I went all the way through my textbook for internal combustion engines.
*Back pressure was so monumentally important that it wasn't even listed in the book's index.
*The term 'back pressure' did occur,one time,on page 493,on an engine performance table from I.N.Bishop of Ford Motor Co.,published in 1965,showing back pressure as a function of rpm,for a 137 cid,V-8,CR=8:1.
^ @ 1,000 rpm,back pressure was 0.4 psig
^ @ 5,500 rpm,back pressure was 12.4 psig
^ @ 2,000 rpm (highway cruising),back pressure was 1.94 psig.
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*the author mentioned 'tuning' of both the induction and exhaust system to guarantee scavenging by preventing pressure wave reversals and the reversion it would cause,which could contaminate the unit air charge,responsible for the work the engine does.
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*friction losses in the induction system were deemed much more important than that in the exhaust.
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*Exhaust back pressure is designed in,and begins right at the exhaust valve by limiting the valves diameter,to prevent burning.
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*Most of the exhaust gas leaves the engine on its own during the blowdown process,leaving the pistons to push out a volume of gases essentially equal to the air mass which entered the engine.
*Mufflers are essentially an acoustic sound suppressor although they do not necessarily ignore the fluid dynamics of gas flow.
*The entire exhaust tract is designed to meet interior and exterior sound pressure levels specified for the vehicle.
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*Highest fuel economy occurs with Minimum Specific Fuel Consumption (MSFC) which occurs near the engines midrange rpm.
*On a 3,000 rpm engine,the best BSFC occurred at 2,000 rpm.
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*In a SAE two-book series on fuel economy,published between 1975 and 1979,no where was 'back pressure' mentioned as a viable criteria for improving mpg.
*Even in the paper dealing with the mpg secrets of the Shell Mileage Marathon, no where was 'back pressure' mentioned.
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*In SAE Paper # 690541,FORD's David Hwang mentioned "Increase in exhaust back pressure generally increases the power loss and reduces economy." (so you probably won't want to shove bananas up your tailpipe).
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*In SAE Paper # 780132,Edward K. Hanson of GM's Buick Motor Division mentioned an exhaust system improvement to the '78 model year cars.They removed 17-pounds of mass.
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*In the October 1984 HOT ROD' article,'Motorcycle Pipes',they warn that 'performance' pipes can:
^ Narrow the powerband.
^Peak power goes up at the expense of the bottom end
^You bought the pipe to hop-up the engine,but it hops it down.
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*According to HOT ROD's Todd Howard,OEM exhaust manifolds were tuned,tube-type headers on some cars by 1986.
*In the same article,they tested a set of Doug Thorley 4-into-1 headers on a Chevy small block and at 2,000 rpm (highway cruise rpm) the engine suffered a 2-lb loss in torque (which mirrors mpg).
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*Walker introduced it's Dynomax Super Converter,low-restriction monolithic catalytic converter,with 26% higher frontal area in 1991.
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*In 1996,in Sport Truck's article,"CHAMBER MUSIC",Kevin Wilson mentioned how a straight-through glasspack Cherry Bomb muffler actually hurt flow.
*Wilson also comments "...backpressure (sic),which needs to operate efficiently."
*Wilson also comments that most buyers of aftermarket exhaust are after a 'sound',regardless of performance.
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Here's a test case from HOT ROD,Feb.2008,'THE MANIFOLD MAN.'
*Chevy 410 cid smallblock V-8
*ported Edelbrock Super Victor intake manifold
*4-into-1 tube headers
*open collector pipe
*SuperFlow 901 dynamometer at Westech Performance Group
*SuperFlow turbine flow meter
*7,000 rpm
*611-bhp
*783 CFM air consumption
*391.5 CFM per bank
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*allowing 3" I.D. collector and no reducer
*Hooker Aero Chamber muffler with 3" I.D. inlet/outlet
*0.049087 sq-ft outlet
*7,975.572 ft/min gas flow (after blowdown) velocity
*478,534.35 ft/hour gas flow velocity
*90.631 mph exit velocity @ 7,000 rpm (210 mph)
*25.9 mph exit velocity @ 2,000 rpm HWY cruise (60 mph)
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*I went out to the truck and grabbed my anemometer.On the MPH scale,and with pursed lips,I was able to blow a 26-mph air column at the vanes (ala birthday cake style).
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*So a 410 cid V-8 produces a 26-mph breeze out it's tailpipes at 60-mph.
I can see why the SAE isn't that excited about back pressure.
*We've had electronic noise cancellation technology for a couple of decades.
*In theory,we could create an open pipe exhaust which was totally silent.
*Is there a good reason why we haven't?
PS,
*The 'defense' dept. used to be the 'war' department
*The 'commemorative' air force used to be the 'confederate' air force
*The 'Specialty' Equipment Manufacturers Association' (SEMA) used to be The 'Speed' Equipment Manufacturers Association.
*Read your racing equipment catalogs very carefully.Very few manufacturers even mention fuel economy in association with their products anymore.The two I've seen may have extremely contextual data to present,which they haven't so far.