---Backpressure in exhaust; good for city bad for highway!---

[ksa8907]

Originally Posted by Frank Lee

This isn't about extremes, and there is a case for throttled exhaust in engine management.

My point is, there is no "one size fits all". An exhaust system has to be tuned for the engine it is going on and the engine speed that is desired.


On a side note, how did we get on a 7 year old thread?

[Frank Lee]

Nobody was talking about a "one size fits all" solution.

Perhaps I'm misunderstanding something.

Re: old thread: Looks like Aerohead came across the muffler ad and/or the EXUP bit and decided to post it in an existing thread vs starting another.

[ksa8907]

When i mentioned taking a theory to the limits. That is what is done with science to determine if the theory holds up at the extremes. This will tell you if your hypothesis is correct or if there are other factors.

Pressure obviously plays a role but it is not the primary concern. Maximizing the benefits of the exhaust pulses and the effect on scavenging are most important.

[darcane]

Originally Posted by aerohead

Both have engineered backpressure into their products for low speed economy,then provided short-circuiting for high speed efficiency
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Here,YAMAHA (read Ford SVO engine builder) designs it's 'smart' exhaust,with computer-controlled valving to maintain optimum backpressure under all load conditions

Sorry Aerohead, but you are a little off here. Both of these systems restrict flow to alter the pulse timing causing a negative pressure wave that travels back down the header tube and hits the cylinder head when the exhaust valve opens. These systems actually reduce pressure at that RPM.

Backpressure is never desired.

For a good semi-technical talk about exhaust design see:

https://www.youtube.com/watch?v=RWTARjxiqlo

[Frank Lee]

Originally Posted by ksa8907

When i mentioned taking a theory to the limits. That is what is done with science to determine if the theory holds up at the extremes. This will tell you if your hypothesis is correct or if there are other factors.

Pressure obviously plays a role but it is not the primary concern. Maximizing the benefits of the exhaust pulses and the effect on scavenging are most important.

Right. We are talking about a system that operates at many frequencies and flow levels... not just one. It stands to reason that it will be difficult if not impossible to optimize a non-adjustable exhaust system for whole spectrum of operation. What we have now is just a rather crude compromise that delivers acceptable but not optimal results across the whole range.

[HiFlite]

Statements regarding the advantages of backpressure may have had some merit in the good ol' days. Take a "performance" small block for example. To get high power for ad campaigns, there had to be considerable overlap between the intake and exhaust valves at high rpm. With a short time to fill and exhaust the cylinders, both valves were open simultaneously for part of the crank rotation. This was okay for high rpm because the intake charge didn't have time to migrate out the exhaust. But at low rpm, there is time, so part of the intake charge would go out without being burnt, hence lowered mpgs and power/torque. If one reduced the rate at which the exhaust left however, via a restrictive exhaust, that effect was reduced. Add in the total carb-chaos caused by low-speed valve overlap and the tendency for car makers was to mount a fairly restrictive exhaust to avoid stalling and other uncivilized behaviors at low speeds.

Gearheads, of course, would put on cherry bombs, reduce exhaust restriction and get much more top end power. The result? Loping at idle, missing/dying with light throttle application, etc.

All this goes away when there's no valve overlap. In the 1980's, emission considerations required little or no overlap but the top end power went away. Now, with VVT, we can have both, no overlap at low speeds, and a measured amount at the higher rpms. With it, high-restriction exhausts have no advantage or use whatsoever. (Though of course, resonance effects must also be managed).

[aerohead]

Originally Posted by darcane

Sorry Aerohead, but you are a little off here. Both of these systems restrict flow to alter the pulse timing causing a negative pressure wave that travels back down the header tube and hits the cylinder head when the exhaust valve opens. These systems actually reduce pressure at that RPM.

Backpressure is never desired.

For a good semi-technical talk about exhaust design see:

In the common vernacular,if one observed a manufacturer wittingly installing a longer path,or mutilated structure within the pathway of a duct,they might presume that some thought went into that extra cost and difficulty of manufacture.
When they claimed a benefit to that more tortuous pathway they would be implying an advantage with respect to performance.
Since an internal combustion engine must operate under transient conditions,from idle,low speed operation,up to redline,under full load,it appears that considerations are being made to optimize performance as best they can,for a market basket of different operational environments.
My internal combustion textbook mentioned 'tuning' of the exhaust system by the manufacturer,to optimize the full spectrum of operation.
Back pressure was intentionally designed into the system for low speed operation.My personal racing experience has demonstrated that 'backpressure' (in the common vernacular) was essential to top engine performance.

[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.

[gone-ot]

...neither bananas nor potatoes (wink,wink)!

[darcane]

Looks like a pretty exhaustive study to me.


Sorry, I couldn't help myself.

But other that the one magazine article, I see nothing that counters my assertion that back pressure is never desired. And I don't put much authority in magazine writers.

Originally Posted by aerohead

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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?

A German OEM parts supplier has been making functional prototypes for about 5 years years:
https://www.eberspaecher.com/en/prod...echnology.html
They market it to either silence exhaust or generate an exhaust note for electric/hybrids, but I don't know of any applications where it is actually used yet.