I've been told by some people on pavementsucks.com that aftermarket mufflers do affect performance a good bit. I told them about me wanting low end and some recommended getting rid of the flowmaster and in fact the stock flows just as good if not better than the flowmaster. Anyone hear anything like this before?

 

When I put on my Dynomax cat back I noticed a difference on the highway. It smoothed out and was actually a slightly lower in RPMs when going 75 - 80. Didn't notice any 'seat of the pants' power gains though.

 

I know a loud muffler will affect the butt dyno

when I put my magnaflow dual exhaust on my dak. I noticed a big difference. but I also went from a clogged rattely cat to a high flow cat. I would say if your just changing the muffler you wont notice that much of a difference. but I think almost anything will give you better flow then the stock muffler.

 

Ditto....

I run a similar system with a Vortec muffler, and didn't notice a huge increase in bottom end torque. Theres a nice mid range peak useful when climbing from 2000 to 4000 feet when I leave Phoenix
Most exhaust mfrs rate thier large tube single systems as havng the most bottom end torque. Scientificly, it has to do with how the actual exhaust pulses are handled. Single in, dual out systems tend to be midrangy as pulses don't scavenge well out of a dual outlet muffler. They also tend to be noisier because pulses don't spend as much time in the muffler. Obviously, a muffler with fewer baffles is going to expell the pulses before they can be quieted down, however , louder is not always better. Why?
Just like intake systems, exhaust sysytems need to be tuned to produce maximum flow (power) in the real world range you operate your truck. It may suprise you to know that the factory system on later gen two trucks is suprisingly efficient and still quiet.
Still, I read your post over in PS.com, and didn't comment there for a reason. I will heartily agree with several of the posters who suggested you need a torque multiplier (read gear change) to get the kind of "kick" your looking for. The only other way to "add" gobs of tire smoking torque is to add things like a supercharger, or nitrous, or...have your PCM custom reflashed. I believe your truck is programed with a very high torque management system.

 

Find the Jan '99 Vol 39 No1 issue of Popular Hot Rodding. Page 70-77 has a great write up on mufflers and pipes. Muffler volume relative to engine displacement is key too.

 

from this excellent article

http://www.superchevy.com/technical/...exh/index.html

also this quote from the book '21st Century Performance' by Julian Edgar:

"Few tests have been done that clearly show the effect of changing back
pressure. Most muffler and exhaust comparison tests change more than one
parameter simultaneously, making the identification of exhaust back pressure
as a culprit difficult. However, Wollongong (Austraila) mechanic Kevin Davis
has done extensive testing of varying back-pressure on a number of performance
engines. These range from turbocharged Subaru Liberty (Legacy) RS flat fours
to full-house traditional pushrod V8s. In not one case has he found any
improvement in any engine performance parameter with increased exhaust back
pressure.

The tests came about because Kevin has developed a patented variable-flow
exhaust that uses a butterfly within the exhaust pipe. He initially expected
to use the system to cause some back pressure at low loads 'to help torque.'
However, he soon changed his mind when any increase in back pressure proved to
decrease torque on a properly tuned engine. What increasing the back pressure
does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on a modified 351 4V
Cleveland V8. Following the extractors he fitted a huge exhaust that gave a
measured zero back pressure. Torque peaked at 573Nm (423 ft-lbs) at 4700 rpm,
with power a rousing 329 kW (441 hp) at 6300 rpm. He then dialed-in 1.5 psi
(10.4 kpa) back pressure. As you'll see later, very few exhausts are capable
of delivering such a low back pressure on a road car. Even with this small
amount of back pressure, peak torque dropped by 4 per cent and peak power by 5
per cent. He then cahnged the exhaust to give 2.5 psi back pressure. Torque
and power decreased again, both dropping by 7 per cent over having zero back
pressure.

Figure 6.1 {which is a graph with three lines showing HP against rpm} shows
the power curves gained in the tests. These results were achieved on a large
engine with a large overlap cam - one of the type some people suggest is
'supposed' to like back pressure.

If, in fact, power does increase with increased exhaust back pressure, it is
most likely the air/fuel ratio and/or ignition timing that are no longer
optimal for the altered state of engine tune."

 

My experience has been that just about any exhaust mod I did (headers, highflow cat, highflow muffler), the most noticable gains were always at high RPM. I noticed improvements when I would hit steep hills, or try to pass cars at highway speeds. Low end never seemed to change.

 

I gutted my catalytic converter and have a single glass pack, and the "seat of the pants" felt pretty significant.

 

with a highflow cat 3" out to a flowmaster dual 2.5" out with 3.5" SS tips seems to run good ..sure is louder..as far as any lost in the low end ....i couldnt tell.

 

I'd simply like to state that anyone on this forum who has a 5.9 magnum v-8that makes peak torque at the flywheel at 4700 rpm ( and runs it there all day)needs a 4 inch straight pipe!
The rest of us can do better with far less...the "mechanic" even suggested that. It's a state of tune.
By the way, to keep from rejetting my carb and changing my ignition advance,my old Roadrunner drag carused a short piece of 3 inch pipe on a collector. Length could vary, and usually was a bit of trial and error (later I learned melted crayon marks could determine its length.). Well, I coulda ran more gear...