Muffler, Resinator or straight?

[Ed Hackenbruch]

I  saw a 5A that has dual stacks that run up the outside of the bus to just above the roofline. Now i am wondering how he did it without blocking his tail lights etc. or blocking the engine doors.  Don't know if he has mufflers or not.  When i had to replace my muffler a couple of years ago, MCI wanted $1000 for the one for my 5A.  I went to the Donaldson site and could not find one with a 5 inch inlet and a 4 inch outlet on the same end. I ended up getting one with the same dimensions except that it was shorter and had a 5 inch inlet on one end and a 5 inch outlet on the other end. This meant that my tailpipe is now just behind my rear tire instead of back by my bumper which actually is a little better because now i am less likely to scrape it going through a dip.  Had no idea of the flow rate of the old one so just picked one towards the higher end. First time i took off down the road it sounded better and i thought that it felt like it had a little more power on takeoff.  Cost was about $240- $250  and about $30 shipping if i remember right.

[JohnEd]

DD has a min and max and that is based on what injectors your engine has, their not going to make any more power with a straight pipe as they do with a muffler,if your going to do anything with a 8v71 use duals with mufflers like the Eagle bus did.

And there we have it.....the holy grail.  No joke!  You want to know when you are finished screwing with your exhaust system and squeezing that last drop of power out of the mill?  Measure the back pressure and compare it to the DD "min spec".  No sarcasm here in the least.  I did this sort of thing with the Lex.  I made a Manometer and welded in ports and measure the ex pressure at different components before and after to determine what back pressure they were contributing. I found that the center resonator was the culprit and that neither of the down stream mufflers were hurting a thing.  My fix cost $25 but you can spend thousands for the "real" high performance system.  I make it 1/2 psi after the cat so I have arrived and am finished.  My question is the shape and size of the pressure pickup as all those gasses rushing past do strange things.  How and with what does DD measure the back pressure?  I would think we all would have that analysis on our list of "must do"s and make an entry in the log.  The psi of back pressure has a HP number associated with it and that is a loss that can be avoided.  I don't have your book Clifford and I suspect others don't either.  How is it done the right way.  My testing has validity in terms of relevance only.

That part of my heart that will forever live in the 1960s is tickled pink with the sound of that Lex V8 I helped create.

Can that part of the DD manual be posted without breaking a law?

John

[Lin]

I think that we are missing the point.  We all know that there are people in the world that will do anything that gives them even an insignificant benefit no matter how it effects others.  There are those that use ridiculously bright headlights knowing that they are blinding everyone coming at them, but they just don't care.  On our way back from Death Valley last month, it was the end of a holiday weekend.  There were lots of RV's on the road on their way home from destroying the landscape with their ATV's.  Who can blame them, they find it to be fun, right.  Well, one of them in front of us that was pulling his ATV's behind, opened the side door and threw a large bag of trash onto the road.  We can certainly understand that; why should he have to carry around his garbage?

I guess the muffler question seems similar to me.  I will assume that even those that run without them would not support the idea that everyone should run without mufflers; that they would not want to be listening to all that massive noise all the time.  Yet somehow it is okay for them to do so.  The question is not whether you can save a dollar by making a public nuisance of yourself, but rather why would you want too.

[artvonne]

  I dont like noise, but then, it depends on what you call noise. I like the sound of Harleys. I like the sound of LOUD Harleys, but I dont like REALLY loud Harleys. Straight pipes with high comp pistons and hot cams? Too loud.

  I dont mind the sound of a big diesel barking, unless its obnoxious. But a nice bark is welcome. Same with Jakes. Jakes with big pipes make too much noise.

  Jags with open pipes? MMMMMM. Ferraris with straight pipes? YEAHHHHH. Chevy Truck with blown exhaust? Not so much. 650 Norton Commando with straight pipes running 65-70 mph? TOO LOUD!

  Somewhere in Ohio the Muffler started coming apart on the Bounder. I stopped and found it wasnt even connected to the header pipe. I stopped at a shop and they yanked it off and put on a straight pipe. The Turbo kills the sound quite a bit, but the pipe was quieter than the straight header exhaust whistling against the fallen muffler. It still has a straight pipe, and no one has ever complained in the 4 years its been like that.

  But is makes the loudest whistle/chirp sound when you let off the throttle from full power in the mid range. WHOOOooooop! But I hope the -5 is quiet. Im really ready for quiet.

[TomC]

Donaldson makes a 5" muffler that has the intake and exhaust on the same end and is a oval muffler about 20" long.  It is made to use with a turbo engine.  When I first bought it, I looked inside and all that was inside was one baffle.  Needless to say it flows well and quiets it-but not completely.  Going through a tunnel-you can still hear a nice exhaust note coming from the 8V-71.  Highly recommend some sort of muffler-some cops get testy over no muffler.  Good Luck, TomC

[Iceni John]

The old Jags had a "split six ex that had two runs each serving three cylinders each and remember how they roared with that peculiar note?

Yup, that's exactly what my old Mercedes had on its M130 straight-six engine, and supposedly it was done that way to get exactly the right back-pressure for each cylinder at all revs, and hence the most power without burning the exhaust valves.   Mind you, it also had sodium-filled exhaust valves (do our Detroits have them?) and mechanical fuel injection, and it was designed to run all day long at full throttle, just how Germans drive on the Autobahn!   Even Mercedes' legendary rally cars had mufflers and/or resonators, because MB found they made less power without them, and the exact diameter and length of the exhaust systems were critical to extracting the last drop of power.

Some years ago Perkins found that an unmuffled exhaust needed to be at least 20 feet long and as straight as possible, to avoid power losses while still being reasonably quiet.   Obviously this setup is impossible with rear-engine buses like ours.

John

[JohnEd]

John,

The Chry Hemi Hi Performance had those sodium filled valves stock.  They are supposedly available for any engine with hi compression heat levels associated with stuff like 12 to 1.  Yeah, we do that.  In the 70's the cost was $30 per valve but it must be through the roof today.

I once read where a captured ME 109 or Messerschmidt was captured by the Brits.  Ho humm stuff till they measured the displacement of the engine and weighed the plane.  No way it could perform as they knew it did.  The compression was so high it was certain to burn the valves in minutes and the buggers were sticking around till the very end of the dog fights that were called on account of ammo.  Everybody had supercharging of one sort or another.  They checked the metallurgy of the seats and it wasn't all that much better.  Cutting open a valve they found that the center was sodium metal.  SM conducts heat away from the valve surface much quicker than steel so that was the answer.  It was long ago and I am only repeating that lie....if it is one. 

John

[RJ]

JohnEd-


Exhaust Back Pressure:  5.0"HG or 2.5 psi*


*From DA BOOK:  X-5910, Diesel Engine Maintenance Manual, GMC Truck & Coach Division, pg 175.

From pg 177 in the same book:

"A slight pressure in the exhaust system is normal.  However, excessive exhaust back pressure seriously affects engine operation. It may cause an increase in the air box pressure with a resultant loss of efficiency of the blower.  This means less air for scavenging which results in poor combustion and higher temperatures.

"Causes of high exhaust pressure are usually a result of an inadequater or improper type of muffler, an exhaust pipe with is too long or too small in diameter, an excessive number of sharp bends in the exhaust system, or obstructions. . ."

So there you go, straight from the horse's mouth, so to speak.

On a side note, Chevy used exhaust back pressure to limit the output of the turbocharged Corvair engines back in the '60's.

FWIW & HTH. . .

[TomC]

2 stroke Detroits (for that matter most any engine) do not have sodium filled valves anymore.  With the electronics on engines now, exhaust temp is just not an issue.  In fact, rare is the truck that has a pyrometer (exhaust temp) gauge.  2 stroke Detroits flow so much air that getting exhaust temps over 900F degrees is nearly impossible.  Good Luck, TomC

[luvrbus]

The last I will say about the back pressure is get yourself a copy of the DD engineering bulletin no.39 dated Sept 1987 will tell you thing you need to know on pages 24 to 28 it also addressees the air filter vs the exhaust system plenty of charts and graphs showing changes in performance factor then you have it straight from DD no bs  lol


good luck

[artvonne]

The last I will say about the back pressure is get yourself a copy of the DD engineering bulletin no.39 dated Sept 1987

  Any way you can post it?

[FloridaCliff]

When I was replacing the muffler on my 4905,  I did a little experimentation.

First ran two duals out the side(straight pipe)  Way too loud, even from the drivers seat.

Second add resonators to the straight pipes.  Still very loud, even on the inside.

Next added two mufflers, offset in/out, much better.  Louder than stock, but a nice DD sound.

YMMV

Cliff

[Alan Baker]

Hay Geoff,

You're the 6V92 turbo guy. What say you?

Alan