Don:
Are you aware that NAPA has a 13' length of 3" aluminized pipe available
? I have a part number in case your local NAPA can't find it.
I used the original donuts that center the pipe in the cross members.
However, when installing the pipe, from the rear, I had to drive the
last 6-8 feet with a block of wood and sledge hammer, due to the
tightness of the donuts. Should have tried some slickum there!
If I understand you, your new exhaust piping will follow the original
routing from the engine and elbow over to the Y-pipe and right side of
the coach and then rearward (same way mine is). At some point near the
rear, it will angle inward at a 45 degree angle to inside the area
between the two frame C channels with the muffler outlets angling 45
degrees again, so that they are then parallel to the length of the coach
and then I assume running under the center of the rear bumper? If the
outlets run under the bumper, you're going to run the risk of flattening
the ends while transiting entrances and exits of businesses, gas
stations, etc., having depressed curbs?
Regarding gas flow, the hotter the gas temperature the faster the flow,
which reduces exhaust gas back pressure on the engine. However, it's
generally acknowledged that some back pressure is desirable. That
principle is one of the reasons I had my system (except for the piping
running along the side of the coach because it was too long for the
coating process) ceramic coated inside and out.
Was also curious where you're located with all that race engine building
capability available to you?
Paul
> From: Donald W. Miller [SMTP:millerdw]
> Sent: Thursday, July 09, 1998 2:22 AM
> Subject: Re: GMC: Muffler
>
> Paul,
>
> Thanks for your nice detailed report. Sounds like I'll still be able
> to hear the stereo OK.
>
> My header outlets will be combined by a Flowmaster 2.5" to 3" "Y"
> pipe. A single 3" pipe will go aft following the original routing
> through the present holes until aft of the frame cross member behind
> the holding tank. At that point a 45 degree elbow will be followed by
> the single muffler. The muffler will be mounted at a 45 degree angle
> to the frame. Muffler outlets will have 45 degree angles, with dual
> 2.5" pipes running aft to near the rear bumper. BTW, have a
> macerator, so the big holding tank drain pipe was removed which
> increases space in this area.
>
> Looks like it will fit, but the pieces are now laying on the floor.
> When we get to the muffler shop the layout aft of the holding tank may
> change.
>
> Exhaust temperature near the valves can be over 1500 degrees. The
> temperature difference between inside and outside the pipe is great
> and the pipes are long so some energy is lost in the form of heat.
> Physics says a gas has less volume when cooled so the muffler will
> have less exhaust passing through it.
>
> Sounds like magic but based on solid physics. Measuring temperatures
> at the mufflers on both systems while underway would tell how much it
> is worth.
>
> The guys giving me this advice are serious race engine builders who
> design with computers, flow benches, computerized dynamometer, etc.
> Many late model car exhausts are similar, so I thought it was worth a
> try.
>
> Don
>
> From: Bartz, Paul
> Date: Thursday, July 09, 1998 04:28
> Subject: RE: GMC: Muffler
>
>
> Don:
>
> You say "...located way aft where the gases had cooled somewhat".
> Somebody is dreaming. The reason I say that, is on my coach, the
> tailpipe outlet at the rear of the coach is so hot that you couldn't
> grab hold of it at idle after the engine warmed up without burning
> your hand. Imagine what the temperature is like closer to the engine.
> On the other hand, while moving down the road, there would be some
> degree of heat exchange between the exhaust system and the ambient
> air, how much I don't know, but doubt that it would be very
> noticeable?
>
> One of the design features Flowmaster claims for their mufflers is
> that their design creates a negative pressure situation which pulls
> the hot exhaust gasses out of the engine into the muffler.
>
> If you haven't already, I recommend you call Flowmaster (800-544-4761)
> and get their opinion on what you are proposing. Talk to Juanita
> there. She is their RV expert and will get you an answer from their
> staff if she needs to.
>
> Up to 40-45 MPH, there is a mellow exhaust tone you can hear, but
> after that point I don't hear too much. I certainly haven't found the
> sound objectionable at all. I especially like what I call the "marine
> engine at idle in the water" sound I hear while standing at the right
> rear of the coach exterior, at engine idle. Very pleasant.
>
> I would still appreciate a description/drawing/picture, etc., of your
> one muffler installation showing the piping arrangement and routing
> when you have it installed.
>
> Using the two outlet muffler, are you planning to have dual pipes
> running to the rear? I assume you know that the original single pipe
> system goes through a multitude of openings in the frame cross members
> and not like most other vehicles, suspended from overhead. Is it
> worth the trouble to bore additional holes for a second, parallel
> pipe? Or are you considering shortening and exiting the tailpipe(s)
> to the side of the coach? If to the side, what about exhaust gases
> being sucked in through the windows (especially when opened going down
> the road) and causing carbon monoxide poisoning potential, due to the
> aerodynamic effects of the gas flow along the coach exterior moving
> through the air???
>
> Paul
>
> From: Donald W. Miller
> Sent: 7/8/98 5:30:34 PM
> Subject: Re: GMC: Muffler
>
> Paul,
>
> They suggested one muffler would be quieter than dual mufflers.
>
> They said the single big block Flowmaster with 3" input and two 2.5"
> outlets would have very little back pressure in this application,
> especially if located way aft where the gases had cooled somewhat.
>
> There is room on mine for this setup so we will give it a try.
>
> Is yours much louder than the factory setup?
>
> These days, I like lots of power without the noise, so I am hoping it
> is not too loud.
>
> Don
>
