I was just a Mad Hatter custom Exhaust shop getting a muffler hanger re-welded, and i asked about the U-bend delete.

he said, "its a waste of money, you will se no gains what so ever. The Cat controlls the backpressure, and everything behind it simply carries the exhaust out the back. The U-bend is there for a reason, GM tested it and had a mathematical and logical reason for putting it there. Even a high flow cat could reduce power because of lowered backpressure."

NOw im a bit confused... everyone here says off with it, but the exhaust expert says keep it...

Am i understending him wrong? it seemed pretty straight forward.

Im with everyone here on the issue, but im wanting to understend what he meant by this because i know it is a widely discussed topic on this site...

 

The U bend is there to make room for the O2 sensor, not for extra backpressure. GM put the O2 sensor above the exhaust to protect it, but in order for it to fit, they had to step down the exhaust like they did. Many people have had it done with no problems. When you have it deleted, just have the bung welded in off to the side instead of on the top so the sensor will fit. The guy seems like just another person who believed every backpressure myth anyone told him. Its not likey you are cutting off the exhaust and welding in straight pipes. All you are doing is removing a very restrictive part of the exhaust with a better flowing piece.

 

Then he obviously doesn't know much about these cars. Thats also why people don't usually ask exhaust shops for mod advice- they're just there to weld on exhaust parts that you want installed, they're not paid to think.

I totally disagree with this. So he's saying that nothing behind the cat adds back pressure? So if you have 10 psi worth of backpressure (just throwing out a number) measured before the cat with the engine running, that it doesn't matter if the exhaust dumps after the cat or runs through 5 mufflers before exiting? Of course more stuff behind the cat increases the backpressure.

Here is an actual article from an old Wbody place that directly disproves this:

http://www.thrashercharged.com/tech_htm/exhaust.shtm

They measured 28-30 in Hg with the full exhaust on the car and only 13-14 in Hg with just the cat.

Usually, I agree with the notion that all of GM's knowledge and engineering power generally overpower the general ideas of a shadetree mechanic. However, the U-bend is simply there to protect the O2 sensor from damage. My guess is they originally had it hanging straight out the side, and it got snagged on a branch or roadkill or something in a test car. There are many cars that use the exact same O2 sensor without the Ubend around it- so it obviously is not required for that O2 sensor to function properly. Plus, there are many people who have deleted the ubend and the rear O2 still reads like normal.

Its pretty common knowledge that bends in a pipe restrict flow inside it, and you essentially have 4x 90 degree elbows in that short region- thats not even taking into account the fact that the pipe greatly reduces in size in that area due to the bending process. So obviously this area is a hinderance to flow in the area. Its tough to relate this directly to horsepower loss due to the restriction because I have never seen anyone do a before/after dyno with just the ubend delete- however on an L67, you can easily monitor before/after KR (knock) and it has a pretty noticeable effect on KR reduction by a couple degrees.

I also disagree with this. I'd love to see some sort of proof of anyone replacing a regular cat with a high flow unit and losing horsepower. Thats ridiculous. Changing the amount of backpressure in the engine by a small amount is not going to drastically effect power output at all. If you keep a full exhaust, especially with mufflers and such- I wouldn't worry at all about back pressure- you're never going to reduce it by a very drastic amount. It would be one thing if you cut off the catback and cat, and just let the rear header collector dump to the ground- but even then, I kinda doubt you'd have a loss of power across the entire RPM band. Think about all of the aftermarket downpipes sold- not a single one has a ubend in it, and no one has ever had a problem with them "losing all back pressure and power"- even completely catless.

If they won't do the mods you want, or give you a hard time- then just go to another place. People who can cut and weld exhaust pipe are a dime a dozen. As long as the mods you are doing are proven for your engine setup- as verified by others online with the same setup, I wouldn't worry at all about it.

It reminds me of the time I went into autozone and asked for 6x Autolite 103's. The guy asked for my car info- I asked for the plug again, he said he needed the car info to verify they would be correct- so I gave it to him- a 96 monte carlo. Then he asked which engine- I said "3800 supercharged", he said he couldn't find that option (they only came with a 3.1L or 3.4L DOHC)- I then asked for 6x Autolite 103's and he went and got them, lol.

 

It reminds me of the time I went into autozone and asked for 6x Autolite 103's. The guy asked for my car info- I asked for the plug again, he said he needed the car info to verify they would be correct- so I gave it to him- a 96 monte carlo. Then he asked which engine- I said "3800 supercharged", he said he couldn't find that option (they only came with a 3.1L or 3.4L DOHC)- I then asked for 6x Autolite 103's and he went and got them, lol above post taken from bumpins post! not sure how to do the " un" thing for a post, sorry


priceless bumpin, glad your so knowledgeable on these topics, thanks

 

I believe ALL muffler shop tech's are idiots! That is one of the stupidest things I've heard in a LONG time!

 

I had a shop pretty much tell me the same thing....just went to another and they deleted it without any drama or a bunch of lame comments!

 

Well, Thank you wery much. Everything was exactly what i was after. and im sure many others were wondering the "why" behind the u-bend delete also.

 

if i were the owner of a muffler shop, i would endorse this idea and do the job if the customer wants it done. it translates into making money for my buisness in this weak economy.

 

Its kindof a toss up though; I used to play another angle when I worked at Circuit City, and I can kinda see how that would translate here a bit. When someone comes in with an obviously dumb idea that they found on the internet or from a friend (be it some exhaust mod, or installing a sound system a certain way)- if you can prove that there is a better, cheaper, or more effective way to do it- then they will consider you somewhat of an "expert" of sorts, and will come back to you again down the road with other vehicles they want to mod. So I can kindof understand the angle they're coming from, but you have to have factual information to back it up- you can't just make random crap up like the guy the OP was talking about.

 

the dee da deee award goes to that tech that said that's bs