Mufflers, crossover and gas mileage

T

tyler

Active member
Jun 22, 2013
518
98
28
The last thing on Jim K's bad gas mileage fix list is blocked mufflers. I have indication of exhaust restriction on my intake manifold crossover
passage, as the paint is definitely overheated.

Banging on the mufflers with a rubber hammer only sounds like rust scale, no loose plates or baffles. They look good from the outside.

So, looking to replace, reading on the forum, saw Walker 18425 muffler as an option. Crawled under coach and hit the pipes with a micrometer thing.
2.2 inch OD pipe going into each muffler (original config two mufflers, Y after). One muffler pipe is 2.2 coming out, other muffler is 2.0 coming out,
and welded to the 2.2 from the other to make the joiner/Y. I wuould assume this is some local shop's solution to a cheap muffler system?

Question is: is that enough difference to unbalance the sides, and push exhaust through the crossover? This input matters because if I can figure out
how to put two new 18425's on myself it will be under a hundred bucks. If I need a new Y to get the flow right, it will be at a shop anyway and I'll
just let them do mufflers and Y

--
1975 Glenbrook
Raleigh, NC
 
These coaches do not produce a great deal of any kind of pressure at the
RPM that we run them most of the time. That being said, the 2.5" exhaust
works just fine with cast iron manifolds. The primary job of the exhaust
system is to remove heat at a faster rate than the engine can produce it.
(prevent overheating) The exhaust system has to have the capability or the
engine is not long for this world. Headers that are of the proper design
for low RPMs will aid in heat removal. Smooth, mandrel bent exhaust pipes
are a help. No 90 degree bends if they can be avoided, and no restrictive
mufflers either. Stainless steel is the material of my first choice. Does
not have a tendency to vibration crack or rust out. Lasts a long time. Do
not go larger than 3" diameter on the pipe after the mufflers, or you will
have to enlarge the holes in the frame to facilitate the installation of
the tailpipe. Dual exhaust all the way back? Don't waste the time or money,
you will not gain anything except noise. GMC did a pretty fair job of
exhaust design on this coach. Jim K has a very good 3" Stainless exhaust
kit that contains most parts you will need and it fits the coach. Or, a
competent exhaust shop can build whatever you need if you don't like to
spend money on shipping. If you are really looking for performance
upgrades, spend your money on a lower geared final drive. Best bang for
your buck out there.
Jim Hupy
Salem, OR
78 GMC Royale 403

> Did you take a look at this tread ? http://gmc.mybirdfeeder.net/
> GMCforum/index.php?t=msg&goto=304111&rid=3866&srch=Exhaust
>
> That is one way to get rid of the Y joint and back pressure/restrictions
> --
> 1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Mean Green Machine,
> And just sold my 1973 26'-2 # 581 White Canyon lands under the name
> Dobbelt trøbbel
> in Norway
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
I'm not really going for performance, average would be good. 5 and 6 mpg not good enough for me...especially since I already have a 3.42 final drive -
should do better.

First question: "Is the difference between 2.2 inch and 2.0 inch muffler exhaust exit pipes enough to be a problem?"

Additional question is "If I am measuring 2.2 max O.D. (including long run to the rear), and old forum posts and responses within this thread are
talking 2.5 inch, am I under stock to begin with?"

I have a local guy who is going to put it on his lift Thursday morning. He said he had a back pressure tester that should tell us something. Anything
else I should ask him to test?
--
1975 Glenbrook
Raleigh, NC
 
I guess it is 2.25 or 2 1/4" and that is standar size out from the manifolds through the mufflers and to the Y, then it is 2 1/2" from the Y and back
--
1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Mean Green Machine,
And just sold my 1973 26'-2 # 581 White Canyon lands under the name Dobbelt trøbbel
in Norway
 
You need to understand that this is not a car/pick up.
The amount of exhaust and high temperature your pushing out is tremendous
as the wind drag is greater than you think.
There is a difference between a car muffler and a truck muffler.
The ruck ones cost more and last.
Most muffler shops install the cheapest ones on our MH and you have exhaust
issues that end up creating problems..
Customers that invest in our system never have problems for 10-15 years
and the performance is greater.

> I guess it is 2.25 or 2 1/4" and that is standar size out from the
> manifolds through the mufflers and to the Y, then it is 2 1/2" from the Y
> and back
> --
> 1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Mean Green Machine,
> And just sold my 1973 26'-2 # 581 White Canyon lands under the name
> Dobbelt trøbbel
> in Norway
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
2.25 before the Y and 2.5 after the Y is stock. I'm thinking the .05 error is in measurement or rusting to iron oxide or both. At some point they
become Swiss cheese so the material went somewhere. Put a Y on your garden hose as you put your thumb over one side the other side shoots further. In
a reversed Y situation the differential would try to equalize at the intake. This increases flow above the 18436572 pattern
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First
 
My guy this morning welded in oxygen sensor style bungs on both down pipes, engine side of mufflers. Then he put air pressure gauge on each bung one
at a time. Idle, and 2500 rpm. He saw no back pressure or unevenness across sides, but there was more of a pulse on the passenger side, with some
jumping around pretty good on dial at first start up. When I questioned him for what it would cost and what benefit it would be to replace from
manifold to 2.5 straight pipe (that size starts along passenger frame rail) he basically said "Let's do that when you need it, you don't need it
now."

So my next steps, with exhaust guys input - compression check, valve adjustment, block intake crossover, carb remanufacture by one of the experts. He
thought timing could play a role, but less likely.

Input on what he found, what I am proposing for next steps, etc. is appreciated...
--
1975 Glenbrook
Raleigh, NC
 
Start by verifying what your timing is doing. an adjustable advance timing light, and make sure it is where it should be at idle, and getting full
advance at rpms. If not- get a dick patterson distributor.

getting the carb rebuilt by a professional is also a good route to take. I had good experience doing that, and mine starts instantly and runs nice
now.

how many miles on your engine, and check the timing chain. Make sure it is not stretched.

as far as blocking the crossover is a recommended good thing to do, I don't think that will get you any more MPG, but pulling the intake, having it
cleaned up and resealing it properly will assure that you are not dealing with a cracked intake or other unknown vacuum leak. Keep in mind it is a
very messy job.

lastly, install a vacuum gauge, that will help you figure out what your engine is doing. Cheap and easy to install.

--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/
 
That is not the proper way to check.
One need s to run it under at least half or full throttle to get valid
reading of delta P.

> My guy this morning welded in oxygen sensor style bungs on both down
> pipes, engine side of mufflers. Then he put air pressure gauge on each bung
> one
> at a time. Idle, and 2500 rpm. He saw no back pressure or unevenness
> across sides, but there was more of a pulse on the passenger side, with some
> jumping around pretty good on dial at first start up. When I questioned
> him for what it would cost and what benefit it would be to replace from
> manifold to 2.5 straight pipe (that size starts along passenger frame
> rail) he basically said "Let's do that when you need it, you don't need it
> now."
>
> So my next steps, with exhaust guys input - compression check, valve
> adjustment, block intake crossover, carb remanufacture by one of the
> experts. He
> thought timing could play a role, but less likely.
>
> Input on what he found, what I am proposing for next steps, etc. is
> appreciated...
> --
> 1975 Glenbrook
> Raleigh, NC
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
Yes WOT is the test but need road load to do the test
2.25 is fine before the Y. Going to 2.5 will loose some torque
Two 2.25 pipes have more cross sectional area than one 3". So the 2.5 long run is the more restrictive part.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First
 
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