GM 8v71 muffler sizes

[fortyniner]

Ive noticed there seems to be two different muffler sizes on the GM
hwy buses. A short one and a taller one. Is one preferred over
the other for any reason?

-Tom P.

[TomC]

Or- there are numerous mufflers that you could use.  Some with 8V-71 non turbocharged use two 4" mufflers (one for each side) like what Eagle buses did.  Donaldson makes some rather compact free flowing 5" mufflers.  If I remember right, my 5" oval same side in and out turbo muffler was only about 22" long.  Go to the Donaldson muffler site and take a look.  Good Luck, TomC

[luvrbus]

I have noticed that also Tom P maybe the GM guys could give us a answer only GM would stand a muffler up you have to remove to work on about anything on the front

[fortyniner]

Right, and the taller one is really in the way  but I wonder if the increased size provides better flow or something.

[FloridaCliff]

Clifford,

Of all the great engineering GM had on there highway coaches, that one stumped me too!

As you know, I replaced my stand up one when it tried to burn me out..

Put on duals with smaller mufflers and never looked back.

Actually going to build a two-one adapter and go back to one muffler(just easy to get at), to gain a little ground clearance.

Cliff

[OneLapper]

I have physically taken both the short and tall mufflers apart. There was no difference in the internal construction.  I played with the idea of re-skinning the rotted muffler as well as trying to increase the size of the outlet. It didn't work.

I ended up buying a 4" inlet/4" outlet Donaldson muffler. I ordered the 5" like TomC is using but Fleet Pride shipped the wrong muffler 4 times. I used the 4" for a couples years and although it's much better than the OEM, it still doesn't flow enough for a 8V71. That muffler is designed for smaller 4 strokes.

I just purchased the 5" Donaldson muffler TomC is using. FleetPride has since fixed the part number issue. I'm in the process of installing this new muffler but it'll be a few weeks before I can finish up the project.  I posted a video on YouTube of Part One already.

Part 1; Installing new muffler on a GM PD4106 Bus Conversion

I can't seem to post the link properly. I'll edit this post tonight to get the link working.

[fortyniner]

So the original GM muffler, short or tall is too restrictive for optimal 8v71 operation?

[OneLapper]

So the original GM muffler, short or tall is too restrictive for optimal 8v71 operation?

To keep it in perspective, "No". The stock muffler on the 4106 was designed to be first and foremost quiet.  The 6" inlet and 3.5" muffler outlet was adequate with N60 injectors, 1900 to 2100 governed rpm's on the 8v71.  I think that engine was rated at 250hp at 1900 rpm and 270hp at 2100 rpm.

For my bus, that stock muffler creates too much back pressure which "may" contribute to high water temps on long pulls. For reference my new engine has N65 injectors, Advanced timing, and 2450 rpm governer.  I don't know what HP my engine has, but I'd assume at least 304 (or 318 depending on which manual you read).  At 2450rpm it mighy be more.

Mark

[HB of CJ]

If you have a turbo DD 2 stroke, have you considered just running a big USDA, USFS approved spark arrestor per side and no true muffler at all?  They do cut noise somewhat and helps prevent even the notion of being responsible for fires that never happen?  Just a thought. FWIW, guys had a 453T years ago that had just a shorty spark arrestor with no muffler at all and it sounded sooss coool.  HB of CJ (old coot)

[fortyniner]

Looking forward to your muffler swap results. My  old 4905 has n65 injectors with advanced timing and its really fast even with stock muffler, Wonder if splitting outlet into two pipes would cut noise a bit.

-Tom P.