am i wrong or can i just go to the mufler shop have some 3 in pipe bent to close to stock shape ...... smoothing out a few areas and save $250 over a cat back system ...... the good thing is we dont have state inspections so i dont need a cat ..... also if i run headers without air pump plumbing wont my o2 sensor be way out of range?

 

First off, sure that sounds great.
IF you can get mandrel bent pipes, you know tons of info about exhaust flow backpressure and the whole nine yards. But beyond that, have you ever priced it out? ITS NOT CHEAP even for an exhaust shop to do for you. That's the bottom line, i meanif you can find it for cheap go for it. They're going to have to spend a long time trying to make things fit and they may not even get it to fit in the end... That's just my personal opinion.
If you know someone who has a shop and you can get materials at cost go for it.

 

are you sure that the shop you are talking about even has mandrel press?? cuz i know that finding shops that do in fact have one are few and far between.... and like Paladin said, it would be way too expensive to do it yourself. Save money and time and just buy the cat back

 

ya, shops like to charge as much as they can to mandrel bend pipes, if they have a machine to do so. Most shops just have a large stock of prebent pipes in the most commonly used sizes that they use. or they send out to have the stuff custom bent, and its not cheap.

 

is mandrel bending a special process or is the machines most shops have here the same thing maby its the termonlogy im missing ........ for example i have had exhaust run several times and have never had prebent pipes the shops always have stright lengths of pipe and custom bend each section for the requirments of each car and owners wants or needs ..... and just from my experence i get this done relatively cheep ........ i have had duel exhaust run on older cars from manifold/header to exit under the bumper for usualy around $120 ...... this is the reason for my question why pay d@%n near three times as much for half as much pipe and work? now if there is a distinct difference between what i would get from a cat back system and from having a shop bend it for me then it is worth it ...... at the moment my car is as far as i can tell a stock tbi 305 but i will evently be building a stout carbed 383 and at a much later date i will delve into home made multiport with distributerless ign , and iwant it to be able to breath as good as possable ........ maby i am totaly missing the point let me know ......... i have spent to many years roding in the old school ....... i never dreamed you could make a car turn a corner. lmao David

 

The difference is thus: A normal bender is simply a rounded press that pushes the side of a pipe until it bends. This is the bender that all muffler shops have. It's fairly cheap to purchase and use. Obviously, that dimples the pipe where the bend is, though. A mandrel bender actually bends the pipe without dimpling it (it's the same diameter through the entire bend), but the bender costs over $16,000, and takes longer to use to bend a pipe. So they pass that cost (time=money as well as the capital cost of the bender) onto the consumer. I think there's maybe one or two shops in this entire tri-county area that has a mandrel bender...

 

jza, that's the best explanation for mandrel bending I ever read. I've tried to explain it to people with almost no success. That's also why the pipes cost so much. I've never seen a dyno comparison between regular and mandrel pipes but it doesn't seem like it could make that big a difference.

 

thank you jza for the explination . i had no idea what the difference was .but like black bird says it is hard for me to imagin it would make that big a difference espicaly if i use a larger pipe such as 3in it should compensate for the smaller diameter in the bends ....... not to mention i cant seem to bring myself to spend that much for a cat back. ....... but if anyone can convince me of the benafits im definatly open to facts David

 

The ripples in the bends of a normal bender create turbulence in the exhaust gasses... Just picture the gasses getting lost and not knowing which way is out.. I suppose it's an excellent way to create backpressure. Second off most shops dont even have a 3.5 pipe in that kind of a bend.

 

When you bend a pipe with that 'smash-n-bend' machine, you aren't only increasing turbulance, but you are also reducing the size of the pipe, at that location by about 1/2". Now you're 3" pipe has the flow caracteristics of a 2 1/2" pipe.

Ever hear the old saying that a chain is only as strong as it's weakest link? Well the same holds true for piping. It only flows as well as the smallest area.

On our cars, using dual exhaust isn't practical, so we resort to single exhaust. That limits us to using at least a 3" pipe for good flow. If you dent it, and make it 2 1/2", then you might as well buy a 2 1/2" mandrel bent pipe.

AJ

 

well you all have convinced me that a cat back is worth the investment i do apreciate the info and time ....... can anyone answer my question reguarding the headers without air pump lines will the absence of them cause my O2 sensor to read too rich i would like to ditch the air pump but still keep my tbi and computer operating properly?

 

I was just about to post a new topic, but saw this discussion on exactly what I wanted to ask. Mendrel U and J bends are in the Summit catalogue for about $20-30 and 3" straight tubing is even cheaper at $15. If you have a mig welder, some time, and these bends, can't you piece together an awesome exhaust with a Summit turbo muffler ($20) for quite cheap? Since your making it custom, you can even have a cutout before the cat for racing! I only ask because I got a buddy who has a mig welder and has done this before at least twice on his own 66 'Stang. (He was offering me his old 2.25" dual system, but I declined in favor of ground clearance for my daily driver.) Opinions wanted please, before I start buying piping...