Kimera

Hi guys, I am not super mechanically inclined. I bought 1 78 longtubes with a 3in catless x pipe. I want to get bullet exhaust as well but the piping reads as 2.5. Do I specifically need a 3 inch system to run these pipes?

NMMike2012GS

I have a set of Kooks with their 3" x pipe and they reduce down to 2 1/2 so they work with my stock axle back exhaust.

pyrodav

I ordered the 3" rear section off the x pipe so I could run z06 Fusion axle back. Who manufactured your headers?

Boomer111

You need to get a reducer. 3 inches at one end and 2 1/2 inches at the other end.

I made the mistake of letting the shop weld mine, I would recommended that you do it right and get the reducer in the right spot and use clamps. Just be sure that no exhaust leaks are present.

Have the reducer in place near the end of the mid pipe will not cause any problems. By the time the gases get this far back they have cooled and will not pose a slow down.

You might need to weld a short piece of pipe off the end of the axle over pipes to be straight into the mid pipe for the clamps to work.

Kimera

Oh thats great. I have an a6.

I heard that could be an issue?

schpenxel

Even the ZR1 exhaust necks down just before the mufflers anyways. Doesn't help with fitment on yours, but it's not a restriction worth worrying about.

NMMike2012GS

Kimera
I would think if there different systems for a 6 speed vs. an A6 all you need to do is be sure that whichever vendor you deal with is aware of which you have. I used Maryland Speed and had no issues with my set up plus when I purchased it they were running a special which included ceramic coating free of charge.

Unreal

Why not just buy the 3" bullet setup to mate with the 3" pipe?

HOXXOH

Although that sounds simple, the problem is due to the limited space as the pipes go around the A6 trans. Beating the crap out of the pipes to get more space causes turbulence and defeats the reason for headers. Reducing the pipe diameter prior to the over-the-axle pipe keeps the exhaust gas velocity high, which is the better solution regardless of the space issue.

Unreal

Never messed with a6 and exhaust. Good to know.

ez2spd

I found this quite interesting myself

HOXXOH

Yes, I did too, which caused me to research a bit more, since there was a HP increase at first.

It appears the headers were the 1.875 primary tube version, where the 1.75 dia. should have been the correct spec for that build. Once they started bashing and decreased the available size, the power went up. When they got to the final run and the power went back down to match the first one's power, it also happened at a higher RPM.
What they didn't realize is that if they had swapped to 1.75 tube headers, they would have had a power increase that most likely would have been even greater than the bashed in 1.875 one's that were creating turbulence. Then the power would have fallen off right at first when they started smashing tubes. Maybe they reviewed it later, but chose not to re-do the test.

Airflow inside a tube is no different than the airflow around a car body. Straight and smooth is best. With exhaust heat, the diameter needs to diminish as the temperature decreases to maintain the same velocity.

Dano523

Yep, you need to understand how the collector works, and if the ID of the header pipe size it too large to start with, then the collector does not start scavenging it's best until higher exhaust speeds and amount instead (upper mid range and up). Hence with the 1 7/8" piping your given up HP on the lower/mid range that 1 3/4" piping would produce better isntead.

Hence it not about pipe size for flow over all (which both sizes will out flow the top RPM of the motor to not be a restrictor at top end), but to get the collector on line doing it job scavenging with enough gas flow speed at the lowest rpm instead (why the smaller pipe size gets the collector scavenging it best from low mid up isntead on a stock'ish type LS engine).