Is there really any truth to this business I keep hearing of losing power, acceleration and gas mileage, by removing cats and installing larger pipes and low restriction mufflers?

Maybe a slight alteration in the torque and horsepower curves on a dyno read-out...but anything the average driver can actually FEEL in their "seat-of-the-pants-meter"?

I was always taught (old skool stuff) that it is a GOOD thing to get the exhaust to flow better. Like serious drag racers certainly don't put on mufflers and restrictive little exhaust pipes to increase back pressure, so they can go faster, do they?

What is the straight scoop here?

No kidding. Just people not understanding things. The exhaust gases need to leave the combustion chamber. Any restriction will limit how much leaves, and how much power the motor will have to use to push out what's left when the piston comes up. If one uses headers, the inertia of the moving exhaust gases creates a low pressure wave that moves back from the collector, towards the combustion chamber. The timing of this wave depends on the volume of the combustion gases, and the RPM of the motor. If one has too large of a primary tube, the pressure wave will hit the combustion chamber at a higher rpm than if one had a smaller primary tube. This would reduce the scavenging affect at lower rpms, and cause one to think that they need "back pressure" to make power. Increasing the exhaust size after the collector has little affect on the tuning characteristics of the exhaust system, so go for it.

I would say maybe......, with cat gone , all depends on what the o2 sensor / oxygen sensor's reads .

What you want in a perfect exhaust system is no back pressure but retain or increase velocity at the exhaust port to help with the scavenging effect (the high speed of the exhaust leaving will create a vacuum in the cylinder and help the next fuel charge coming in)

so scavenging is increased by the reduction in back pressure (why people that race don't run mufflers) but can be hurt if the headers are too large at the exhaust port opening! This is especially important the first foot of header tubes are too big. this has led to step headers that retain the velocity while working the 0 back pressure angle to the optimum.

This is simplistic as a race motor with a 5500 stall and a shift point of 7500 has a much different requirement in a header size and design than a stockish motor running in the 1000 to 3000 area with a tight converter requiring throttle responce and early torque

use the ideal pri tube dia/length and any reduction after the header collector flange is costing you. 3" mandrel from there on back w an X pipe and dynomax super turbos w resonator muffs of some sort in the rear tailpipes (make sure the inlet/outlet nipples on the resonator muffs of your choice aren't necked down & louvers punched to the outside) and someone added sideways standpipes to the tailpipe curve over the axle and made a significant diff in the noise & yes tune the carb for what you have.

Good stuff all of it for sure! Thanks much. Now let's see if this "dummie" got it straight.....

On a basically stock vehicle, modifications to mufflers/cats, such as removing them, or adding low restriction mufflers...and increasing the overall diameter of the tail-pipe, is not necessarily going to affect the power/torque output.

EXCEPT, the A/F ratio MIGHT need to be re-adjusted to compensate on a non-computerized motor. Otherwise, the on-board computer will make the necessary adjustments itself.

Do I have that about right?

People have been giving me crap about having installed a fresh-air intake, along with a low-restriction air filter on the intake side....

...and that, plus the addition of a pair of 32" Thrush glasspaks (for that real old skool sound) in place of the granny quiet mufflers that were on there...

...and 2 1/2" tail pipes in place of the 2" factory exhaust....

...will cause a significant drop in backpressure, causing the 440 in my Plymouth to actually run slower and get even worse gas mileage than I am already getting.

I tried to tell them that they are FOS, but I just wanted to make sure and check with you guys just to be sure.

This thread reminded of days gone by. I have installed headers on many vehicles since 1976. I have done 304 javlins,350 novas,307 nova, 302 cougars, 340 challengers, 340 darts,360 RVs and many others I can't remember. This was back in the day, 76-86. I was the neighborhood speed tech, did it for many guys. In every case the car pickup HP. Those were the days for sure...

The car that stuck out the most in my mind was a 307 nova with a 3 speed manuel trans. My buddie Billy drove that car and it went from a grandma car to one of the quickest cars in the neighborhood, that thing scooted with headers and a 4 barrel.

here is a quote from page 139 of the Austrailan book
"21st Century Performance"
by Autospeed.com website founder Julian Edgar

begin quote

"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."

I have EFI now and it will adjust the AFR for my changes.

Having said that I ran my car on the dyno with exhaust - ( 2" mufflershop pipes, crimped bends and cherry bomb glasspacks ) for a total of


266 HP at the wheels - 352 Ft/ Lbs Torque

I then opened my cutouts ( shorty headers ) and ran it with the same AFR - 12.8:1 WOT

I got :

317 HP at the wheels - 358 Ft/ Lbs Torque

While the TQ numbers stayed close - open headers moved the TQ curve ahead in the RPM range.

Thats a 51 HP increase from my junk exhaust system!