Mufflerectomy Issues?

Hola peoples,

I've read about getting the spent exhaust gasses out of the car as fast as possible post.
I am seriously considering the straight though TT stainless steel pipe and wanted some input from y'all. I've some questions about longevity.
I am pretty sure that i am not going to chip my car. I understand that this mod is a pretty good bang for the buck. I soon will be moving to a place where the elevation is 5,300 feet. With the current opinion of high elevation and chipped cars' (and a few stock ones too) turbo's going bye bye, i do not want the added stress the chip would place on my turbo. I've a lead foot and i know i wouldn't stay out of it much. Anyhow, would a bigger, less restrictive exhaust be better up at 5,300 feet or will it make that much of a difference? What will this do for me in the long run? I'm pretty sure anything is better than stock, but i'm no expert! That's why i'm here! How could VW nail me if something engine related went wrong, with this str8 through pipe? I'm using great oil so that's not a concern.

What do y'all think?

DF


[ April 10, 2002, 19:37: Message edited by: Diesel Freak ]

I am also considering the downpipe from tttuning.com too. My only complaint is that they do away with the Cat. <shrug>
I do not want to chip my car becuase of the clutch. I don't want to have to replace it in the near future.

Comments, suggestions? anything?

DF

Exhaust system mods accomplish very little on a car with no mods otherwise. Stock chip? might as well leave the exhaust system alone. Maybe straight-pipe the muffler if you insist on doing something.

Downpipe ... I know someone here who did all sorts of experimentation with the downpipe including dyno testing, and ended up with a stock one!

With a turbocharged car, the overwhelming majority of exhaust backpressure is from the turbine of the turbocharger. Post turbine pressure drop is inconsequential.

The turbine is tiny and the exhaust has to go through there. The turbine is like the smallest cross sectional flow area in the exhaust path by a bunch.

Doing anything post turbine will be negligible. Keep it stock.

With a turbocharged car, any backpressure is really really BAD.

Unfortunatley, with the older wastegated A3 design, the turbine is always in the way creating excessive back pressure all the time.

With the newer A4 design VNT turbo this is simply not the case and good strong power can be gained through the use of a non restrictive exhaust system post-turbo to take advantage of the decreased backpressure the VNT turbine design provides!

Remember, turbochargers always hate backpressure..........

*Disclaimer* - purely a newbie in the performance tuning field, but...

I really do feel a difference when I cut out my muffler and replaced it with a straight pipe. Of course my biggest gain was with the Upsolute chip, but after the mufflerectory the power seems more immediate. Someone mentioned that the reduced back pressure allows the turbo to spool up quicker...maybe that's the reason.

I'm as skeptical as the next guy, and I hate mods that don't "feel" like they add anything. At $30, the "bang for the buck" on this mod is high IMHO.

On a downside, my first tank mpg is *way* down, probably because I punch it at every possible opportunity.

-Dan

Remember -> You can improve the turbine pressure differential by installing an exhaust system with a higher flow capacity than stock. It's impossible to have "too much" exhaust flow downstram of the turbo.

It is a common misconception without bounds that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's tow pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transfered comes from a different source.

Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.

So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.

As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in the housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the heat of the exhaust, that otherwise would have been lost.

This is a measureable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature, sometimes over 600-700*F lower after the turbine.

All else being equal, The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet (in english, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

Raising the inlet pressure is possible, but tough. Lowering the outlet pressure is easy - just bolt on a bigger, free flowing exhaust. I've seen a couple of posts from people who added aftermarket exhausts, who report "my turbo spools up faster now" Well, that's because by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster. You should also see less boost drop at redline, because if an exhaust system is flow-limited, once you pass the flow limit of the system, any additional gasses you try and force through it only raise the outlet pressure. Higher outlet pressure, lower pressure differential, less work, less boost.

1) Turbos reclaim energy that would otherwise been dumped overboard in the form of heat, by using exhaust gasses to spin a turbine, which in turn spins a compressor, which compresses the intake air.

2) Compressed intake air makes more power, because it allows you to burn more fuel per power stroke of the engine, and because it helps scavenge the engine. (The new, compressed, intake charge "blows out" the remaining exhaust gasses)

3) The amount of work done by a turbo's turbine depends on the amount of gas flow through it, and the pressure differential across it

4) You can improve the turbine pressure differential by installing an exhaust system with a higher flow capacity than stock. It's impossible to have "too much" exhaust flow downstram of the turbo.

5) The intake compressor works best when it has been specifically sized for an engine's flow requirements and boost levels.

6) The best way to choose a compressor wheel and housing is to call the manufacturer of the turbo, and answer all their questions.

7) Intercoolers are our friends. They reduce the temperature of the compressed intake charge after the compressor has heated it.

8) An intercooler is only as good as the air flow into it and out of it

9) Wastgates limit boost levels by acting as a "rev limiter" for the turbo.

Conversly, if you want to make sure that your turbo never surges, just use the OEM stock muffler and shove a potatoe in it, then drill out a 1/4" hole in the middle of the potatoe.

Success -> No chance of any surge ever happening and you can eat the sooty baked potatoe when you get home for dinner too (just be sure to peel the skin off first) !

now Fred is editing my messages for me man this place is going down hill fast

[ April 11, 2002, 18:23: Message edited by: garrettp ]

Originally posted by SkyPup:
With a turbocharged car, any backpressure is really really BAD.

Unfortunatley, with the older wastegated A3 design, the turbine is always in the way creating excessive back pressure all the time.

With the newer A4 design VNT turbo this is simply not the case and good strong power can be gained through the use of a non restrictive exhaust system post-turbo to take advantage of the decreased backpressure the VNT turbine design provides!

Remember, turbochargers always hate backpressure..........

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">Hmm... I s'pose us B4 owners are in the same boat as the A3 guys...
Patrick

So where is the biggest restriction coming from on our cars? I'm assuming it's the muffler...but could be the down pipe or somewhere else even.
And how's the sound with a straight pipe?

[ April 11, 2002, 20:07: Message edited by: bryantdi ]

The biggest performance restrictions are boundless.

Once you get over that, the straight through mufflerectomy sounds nice and sweet and is next to unoticable unless you tune in to the turbo whine.

I am also considering modifying my exhaust.
I am thinking of a 2.5" tt catback but with a straight pipe (no muffler). Would this be advisable? Also, while talking to tt, they said they also have a 2.5" down pipe.
Sounds interesting, though I want to keep my cat in place. Would it make sense to go to the 2.5" downpipe into the cat and out to a 2.5" cat back without muffler? Your feedback are appreciated.

As mentioned before, I removed my muffler last week.

Besides the turbo whine (hard to detect inside the passenger cabin), the main difference is a noticable droning sound around 2600 RPMs. Again, it's nothing like a ricer (which I loathe), but you *will* notice it.

All in all, I would do it again without hesitation.