Home » Public Forums » GMCnet » Mufflers, crossover and gas mileage (How much restriction matters)
| Mufflers, crossover and gas mileage [message #307526] |
Wed, 21 September 2016 08:07  |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
Karma: 7
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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, 1978 Royale rear bath
Raleigh, NC
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| Re: Mufflers, crossover and gas mileage [message #307527 is a reply to message #307526] |
Wed, 21 September 2016 08:28  |
lqqkatjon
Messages: 2324
Registered: October 2010
Location: St. Cloud, MN
Karma: 5
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I don't think a hot crossover is any indication of muffler blocking. It just plain gets hot and will burn off the paint in normal operation.
that is not to say you do not have muffler blockage.
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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| Re: [GMCnet] Mufflers, crossover and gas mileage [message #307533 is a reply to message #307530] |
Wed, 21 September 2016 09:30 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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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
On Wed, Sep 21, 2016 at 7:06 AM, Espen Heitmann wrote:
> Did you take a look at this tread ? http://gmc.mybirdfeeder.net/
> GMCforum/index.php?t=msg&goto=304111&rid=3866&srch=Exhaust#msg_304111
>
> 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
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| Re: Mufflers, crossover and gas mileage [message #307540 is a reply to message #307526] |
Wed, 21 September 2016 11:29 |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
Karma: 7
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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, 1978 Royale rear bath
Raleigh, NC
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| Re: [GMCnet] Mufflers, crossover and gas mileage [message #307552 is a reply to message #307541] |
Wed, 21 September 2016 13:52 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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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.
On Wed, Sep 21, 2016 at 9:48 AM, Espen Heitmann wrote:
> 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@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: Mufflers, crossover and gas mileage [message #307630 is a reply to message #307526] |
Thu, 22 September 2016 10:23 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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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
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| Re: Mufflers, crossover and gas mileage [message #307633 is a reply to message #307526] |
Thu, 22 September 2016 10:56 |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
Karma: 7
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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, 1978 Royale rear bath
Raleigh, NC
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| Re: Mufflers, crossover and gas mileage [message #307652 is a reply to message #307633] |
Thu, 22 September 2016 15:40 |
lqqkatjon
Messages: 2324
Registered: October 2010
Location: St. Cloud, MN
Karma: 5
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Senior Member |
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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
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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| Re: [GMCnet] Mufflers, crossover and gas mileage [message #307654 is a reply to message #307633] |
Thu, 22 September 2016 16:23 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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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.
On Thu, Sep 22, 2016 at 8:56 AM, Tyler wrote:
> 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@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
_______________________________________________
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: Mufflers, crossover and gas mileage [message #307662 is a reply to message #307526] |
Thu, 22 September 2016 17:46 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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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
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| Re: Mufflers, crossover and gas mileage [message #315592 is a reply to message #307526] |
Fri, 07 April 2017 08:47 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Fix that 2" section to 2.25" aluminized pipe. Area on 2" is 3.14 and area on 2.25 is 3.97. Enough to matter I think. With old crossover unblocked enough to cook the intake long term. I think 2.25 is the minimum you want to allow a good compromise between flow and velocity (torque and drivability). Though the long pipe at 2.5 is less than 2 x 2.25s combined, the gas volume becomes less as it cools with distance. For example on the 455 Olds 442 GM used 2.25 exhaust pipes and 2" tailpipes after the mufflers. Same on GS 455 and pre cat years Corvettes.
John Lebetski
Woodstock, IL
77 Eleganza II
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