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Author Topic: Muffler, Resinator or straight?  (Read 3947 times)
Alan Baker
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« on: March 27, 2011, 11:34:41 AM »

Hi,
I think the muffler on my 84 Eagle 6V92 TA is nearing the end of its life.
I noticed several caoches at Bussin' this year with a straight pipe. I remember Dave Galey mentioning that he had to be sure to cut off his Jake when entering his neighborhood as he never ran a muffler.
I think I'm going to try out a straight pipe, all I have to loose is a couple of hours of my own labor.
What do you think of this idea? Has anyone got first hand knowledge of a 6V92 running sans muffler?

Alan Baker
1984 Eagle 10
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« Reply #1 on: March 27, 2011, 12:02:20 PM »

We ran our 8V71 without a muffler and we would set off car alarms as we passed, too loud for me.
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« Reply #2 on: March 27, 2011, 12:18:02 PM »

The turbo in your setup will help but as Bob stated, I also tried the straight pipe on my 8v71n and it was unbelievably loud. I put a straight through 4" muffler on it intended for race cars and now it is unbelievably loud.... Wink Luckily for those staying near me I am never the first to leave!
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« Reply #3 on: March 27, 2011, 12:30:09 PM »

Alan:   When my muffler went bad I replaced it with one from AERO company, I think.   It was a 5 inch straight thru and muffled some but the Ohio State Police didn't like it.   Before I installed it I talked to a bus buddy with a 4107 with a 6v92ta and he had one on his and it wasn't that noisey.   The turbo will cut the noise down some.   Saw that same muffler at Perry this year.
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Kenny
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« Reply #4 on: March 27, 2011, 12:38:00 PM »

Some what related to this, is there much of a gain in power when running a straight pipe on a 6V92?  Kenny
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« Reply #5 on: March 27, 2011, 12:43:09 PM »

Why not just leave the present muffler in place, even though it may not be working well?   As long as its interior baffles don't get dislodged and block the exhaust flow, what harm would result from running a muffler that's at the end of its life?   It should still muffle slightly better than having no muffler at all (as long as its baffles don't buzz and resonate at certain revs, like what happened with a previous old car of mine that would make a delightfully fruity exhaust sound at 3000 RPM!).   Just make sure that exhaust gases are still being directed where they should, and not out through rust holes and against anything flammable.

John
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« Reply #6 on: March 27, 2011, 12:45:02 PM »

Better check your DD bible they require some back pressure

good luck
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« Reply #7 on: March 27, 2011, 03:13:55 PM »

I must have enough back pressure cuz the bus seems to run better. Maybe it seems that way because nobody follows me anymore!!!! Grin
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« Reply #8 on: March 27, 2011, 03:27:34 PM »

If you decide to buy a muffler I have a new old stock 5" inlet on one end and a 5" outlet on the other. Not sure what it will fit. 45.00 + shipping, and it is heavy.  Tom Y
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Alan Baker
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« Reply #9 on: March 27, 2011, 04:31:29 PM »

Its my understanding that DD don't need too much back pressure.
My muffler is still muffeling, I'm starting to leakage at seams.
I know the 8V71's make a lot of noise. I tried the same thing Larry did when his bus was mine and went back to stock.

Alan
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Alan Baker
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« Reply #10 on: March 27, 2011, 07:58:40 PM »

When I transplanted an 8V71N into my GMC transit 5301, there was only room for two shorty mufflers and dual exhaust(without going wild), and it was definitely loud, but not obnixious.  I did notice when the throttle was open wide and I was near the jersey barriers on the highway, that I could easily hear the exhaust rumble in the drivers seat w/windows closed!  BUT--IT DID SEEM TO HAVE MORE POWER THAN IT DOES NOW IN MY GMC 4905(same engine now transplanted into newer coach).

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« Reply #11 on: March 27, 2011, 08:06:09 PM »

Better check your DD bible they require some back pressure

good luck

I have heard this many times about DD 2 strokes.  I have also heard that less flow resistance to flow will yield more HP and MPG in the INLET and EXHAUST.  The statement that a DD needs some back pressure and won't run "right" without it seems to fly in the face of that "other" logic.  I have heard both so many times from credible sources I won't argue with either.

Mt only experience is with 2 stroke gas engines.  The straight thru exhaust is whjat we would have called a "blue-eeey" pipe.  It made a sort of bluey sound and it was basically a megaphone on the end of a short piece of exhaust pipe.  It was very loud but not ear splitting.  It was discovered that a properly designed "expansion chamber was a far superior performer to the Bluey pipe.  The thing about the Bluey was that that megaphone part, with it's expanding size, would actually create a "negative back pressure' wave to be reflected back to the ex port.  If properly designed and sized that wave would "scavenge" the cylinder and more completely flush all the spent ex gasses out and allow the engine to breath a fresh and fuller charge with every stroke.  The Bluey had a big advantage over the straight pipe cause the straight pipe never sucked.

THEN.... along came the expansion chamber.  That was a "system" that scavenged the ex gases and "overcharged" the cylinder both. The Ex was pulled out of the cylinder by the megaphone section of the chamber and then the reverse-megaphone section would create a brief positive back pressure pulse that would "stuff" the last puff of scavenged air, fresh by that point in the flow, back into the ex port.  Between the inertia of the intake air rushing into the cylinder and that brief positive (back pressure?) pulse, the air was stuffed into the cylinder and when the ports were closed there existed a supercharged condition in the cylinder.  Voila, and you got a 50% power boost...maybe. All depended on the timing of those suck and blow impulses and their relative intensities and how far apart they were.  There is more to it...isn't that always true....and the math involved is daunting for anyone that isn't well versed in "The Calculus"....but it is comprehend-able. But, "some back pressure is needed and good"....that, take alone, doesn't make sense.  And not that I can't accept a truth without full comprehension cause that would be a lie.  Just look at all the stuff I don't understand but accept as true about "women" that is based on results alone. Huh Roll Eyes

So as I understand it:  Free flow to the max....largest diameter ex pipe you can connect....freest flowing muffler you can buy and a straight pipe is the freest.....min bends in the pipe....shortest pipe length you can accomplish and get the gases out of the engine bay.  OK?  Now in conclusion "SOME BACK PRESSURE IS GOOD AND DD SAYS SO".  Huh Tongue Tongue Tongue  Feels like an Abbot and Costello skit..."Who's on first?".  This keeps coming up for me and I hope there is an answer lurking out there if Clifford can't help.

And the expansion chamber sound level?  Absolutely ear SPLIT  TING and illegal and should be...at least without a sound suppressing end....that I never used until forced.  Outlaw! But quick about it.

Be gentle with me but WT?...over.

John
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« Reply #12 on: March 27, 2011, 08:15:33 PM »

Steve,

The lower the freq of the ex noye the more difficult it becomes to "silence" the thing.  Witness the V8 with itsd clever little "cross-over pipe right behind the engine.  Its only function is sound suppression as far as I know.  What I am certain of is that if you leave that little jewel out of your system the engine is defining and illegal in all 50.  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?  When you ran a split 8 system you created two 4 cylinder systems and my math says that would be very loud and difficult to control.

Brings up the question of "were the old 4 cylinder and IL 6 cylinders much more noisy in the day?  Where does the term "scream'n Jimmy" come from and why?  I am barely old enuf to remember the noise along the highway but little else about it.

Anybody?

John
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« Reply #13 on: March 27, 2011, 08:19:38 PM »

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.
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« Reply #14 on: March 27, 2011, 08:35:41 PM »

I am a believer in all things "Clifford".  Duals sounds good to me...pun intended.

Thanks

John
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« Reply #15 on: March 27, 2011, 08:41:08 PM »

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. Huh  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. Grin  Cost was about $240- $250  and about $30 shipping if i remember right. Smiley
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« Reply #16 on: March 27, 2011, 09:00:37 PM »

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
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« Reply #17 on: March 27, 2011, 09:20:50 PM »

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.
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« Reply #18 on: March 27, 2011, 09:38:35 PM »

  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.
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« Reply #19 on: March 27, 2011, 09:54:36 PM »

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
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« Reply #20 on: March 27, 2011, 10:43:54 PM »

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

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« Reply #21 on: March 27, 2011, 11:59:20 PM »

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
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The price of apathy towards public affairs is to be ruled by evil men." Plato
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« Reply #22 on: March 28, 2011, 12:25:36 AM »

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. . .

 Wink




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« Reply #23 on: March 28, 2011, 07:51:47 AM »

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
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« Reply #24 on: March 28, 2011, 10:17:29 AM »

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
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« Reply #25 on: March 28, 2011, 10:40:15 AM »

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?
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« Reply #26 on: March 28, 2011, 04:59:20 PM »

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

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Alan Baker
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« Reply #27 on: March 29, 2011, 04:10:19 AM »

Hay Geoff,

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

Alan
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Alan Baker
Lake Placid, FL
61 PD4106-00038 for 23 yrs
84 Eagle-10 10" roof raise
6V92 turbo 90 injectors Allison 740
since 2000
Every ride is a new adventure
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