Cutting out Cat, Noise difference...?

[JuXsA]

You can also put some sound deadening in your trunk floor and that will help a lot. I plan on doing that in the next few months.

[TurboSedan]

it's not the same exhaust system. i went from the factory sized pipe to much larger 3" mandrel bent pipe.

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Joshua Mitchell
'91 Dodge SPiRiT R/T
'87 Shelby CSX #667
'98 Buick Regal GS

[turbovanman²]

Anyone have any theories as to why the car would be quicker with the addition of a cat and muffler? My understanding from reading the post is that this is the same exhaust system, turbo, same boost, etc. just the addition of a muffler and cat, so there has to be more restriction. Why would the engine be making more power, especially being a turbo car?

Why? Less backpressure than stock and being a turbo car, that is good, and larger pipes equals more flow than the stock system.

[BadAssPerformance]

Anyone have any theories as to why the car would be quicker with the addition of a cat and muffler? My understanding from reading the post is that this is the same exhaust system, turbo, same boost, etc. just the addition of a muffler and cat, so there has to be more restriction. Why would the engine be making more power, especially being a turbo car?

more restriction = more backpressure = more torque and throttle response off the line, but less overall hp at WOT

[thrashercharged]

Anyone have any theories as to why the car would be quicker with the addition of a cat and muffler? My understanding from reading the post is that this is the same exhaust system, turbo, same boost, etc. just the addition of a muffler and cat, so there has to be more restriction. Why would the engine be making more power, especially being a turbo car?

it's not the same exhaust system. i went from the factory sized pipe to much larger 3" mandrel bent pipe.

Ok - that explains it. I misread the post (my fault - just reading it too quickly and not paying attention). I thought you had the same exhaust system and added the cat and muffler so it didn't make sense. I didn't realize you went from a factory setup with no cat and muffler to a 3" mandrel with cat and muffler.

I believe the stock pipes are 2.5" downpipe into 2.25" pipes with 2 90 deg mandrel bends, then over the axle with 2" or 2 1/8" pipe? It's interesting to know that a complete 3" mandrel with cat and muffler will outflow the stock system w/o cat and muffler.

more restriction = more backpressure = more torque and throttle response off the line, but less overall hp at WOT

In my experience I haven't seen this to be completely the case on a turbo car, except for maybe initially before the turbo spools while the engine is operating under normally aspirated conditions. But more restriction always increases turbo lag, so it's probably debatable whether some restriction does a turbo car any good. Increased restriction might gain some torque on initial take off before the turbo is spooled, but it'll take a bit longer to spool too, so the trade off may not be worth it.

I agree that increased restriction always equals less overall HP at WOT, especially on a turbo car.

Not trying to be picky, but just so someone doesn't get the wrong understanding, I'd like to clarify that more restriction = more backpressure = more torque isn't exactly true. More restriction does sometimes make more torque at a specific RPM, but the reason increased restriction can sometimes produce more torque on a normally aspirated engine (at a certain RPM) is not because of the increased backpressure, but because of the tuning effects (cylinder scavenging) that increased restriction can sometimes cause.

The point at which a pipe ends into open air (i.e. the length of the pipe) can have a pronounced scavenging effect on the cylinder. This is especially effective on 2-cycle engines and why different exhaust pipes can have such drastic effects on 2-cycle power, and why open header race engines vary the size, length, placement etc. of their pipes and collectors, all to tune for max efficiency at a particular RPM.

Pipe diameter also has a big effect on tuning, because as the exhaust gases travel away from the combustion chamber, they start cooling drastically and start decreasing in volume, so if the pipe remains too large, the velocity slows down. Decreasing pipe size will help maintain velocity and affect scavenging. This is a big benefit to OEMs because this allows them to tune their exhausts and still decrease the pipe size as they get toward the back of the car saving weight, cost, reducing sound levels, and making it easier to bend over the axle as well.

Adding a cat or muffler, or going with smaller pipes often does increase torque at lower RPMs, but it's due to this tuning effect, not because of the increased restriction or backpressure. The increased restriction and backpressure is often associated with increased torque but it's really a (unwanted) side effect that's a necessary evil. If one could, you'd try to achieve the same tuning effect without that unwanted side effect of increased backpressure, but that's not easily done, perhaps impossible.

[BadAssPerformance]

^^^ agreed on all of the above, plus good info Thrasher

factory exhaust on most turbo mopars is 1-7/8" over the axle

and yes, for a turbo car at WOT HP, the bigger the better

3" seems to work nicely for most 4cyl turbo cars even with larger turbos, on my shadow, I get the same mph with open downpipe or full 3" which just means the restriction is elsewhere...

[Tony Hanna]

In my experience, all other things equal, a quieter car seems to "feel" faster. Running my old Daytona with an open downpipe vs. a full 2.5" manderal bent exhaust w/muffler was a really good example of this for me. I know the car should have been faster with the open DP but it didn't seem that way. It's gotta be a psychological thing... The extra noise diverts some of your attention away from how hard the car is pulling or something goofy like that. I don't know if that makes sense to anybody else, but it's something I've noticed.

[amcpacer]

I just installed a 3 inch mandrel bent system with a 3" straight through FloPro muffler. I installed a 3" high flow cat converter and am sure glad I did because the car is so much quieter with it. The cat eliminated the droning that I had when the system only had a muffler and 3 inch pipes. I did not feel any loss of power but my car is only at 12 psi of boost right now. Perhaps a cat only hurts performance when running super high boost.

[89boostedminivan]

i just removed mine the other day, 2.5" DP into 3" then into a glasspack then exits out the pass side under the slider door, gonna go back in a week or so and get a 3" DP but as it is now it sounds like a diesel truck,loL! its def louder but id say mainly in lower rpm's is were its the most noticeable. acceleration has greatly improved and thats with 18psi of boost