A couple months ago I switch my exhaust from a 3" Y and a 3" Borla catback to 3" true duals with a X pipe into Magnaflow 4x6 mufflers dumped before the axle. I went to the exhaust shop first, told them what I wanted and if they could do it. Since the car has a body mounted torque arm I wanted the shop to see what they had to work around and they said yes they could fit the 3" tubing and the X around the torque arm no problem. So I went ahead and bought the 3" mufflers and dropped them off with the car. I didn't measure the pipes when I picked the car up from the shop I assumed they were correct since they never said there was any problems. I just looked under the car to make sure nothing was hanging too low to the ground. Well last week I pulled the engine along with the transmission and exhaust out of the car because I'm rebuilding the engine it spun a rod bearing. I got to looking at the exhaust and it didn't look right so I started measuring the pipe and it is 3" diam. from the headers back to right before the X. Then turns into 2 1/2" diam. and is 2 1/2" back to the mufflers. The X looks to be 2 bent pieces of 2 1/2" cut and welded together the center of the X is about 4" wide and 2/1/2" tall. The end of the 2 1/2" pipes were expanded to fit inside the 3" mufflers. I called the shop and they said they had to reduce the pipe size to make the X fit through the torque arm cross member and the exhaust pipes around the torque arm. I didn't notice any performance difference while the engine was stock but now I'm stepping up the engines performance with a cam and head swap. I was wondering what are your guys thought. Do you think the different sizes of the exhaust will have any serious adverse effect on the cars performance now?

 

Unfortunately I can't answer your question technically...

...but I'd be wary of a shop that said they were going to install 3" pipes but instead installed 2.5" pipes and didn't consult you first and never told you after they did it. Kind of shady.

 

I agree with above its more principle. If you paid for 3" that's what you should have received.

I believe it does not restrict you until you are making over a certain horsepower level. I believe I read that somewhere while researching true duals. It took me a while to find a shop that would run the 3" true duals, most said its impossible. They did a great job on mine and I love it.

 

At this point I'm a lot less worried about the integrity of the shop that did the work I'll take care of that later. I'm more concern with the cars performance my guess from everything I've seen from looking at other peoples builds I should be around 380rwhp on motor and around 500-550rwhp with nitrous.

 

The true dual sticky at the top of this sub forum says that you should go with 3" pipes if you have over 500HP.

See this about halfway down the first post: https://ls1tech.com/forums/generatio...-pics-too.html

 

Well if your were going to step down thats the best way to do it. But of course its not a optimum flow deal .The worst part is using a 3" muffler the flow will tumble a little due to step up 2 7/8 on the inlet then stepped back down to 2 1/2 so the flow has been choked down twice. If you were going to use two different size pipe it would be better to step up than down . And that type of X pipe actually flows pretty good , better than a stamped one. Just take it back and get it done right 3" all the out the back.