Well I'm sure theres nothing I can say to change your mind, but theres a lot of engineers/physicists I've talked to who agree with me, and it was engineers/physicists who designed the internal cumbustion engine. But this is the last thing I'm going to say about the subject cause I'm not trying to start an argument.

 

ok dude

 

unless someone has a solution that costs less than a full catback sys... ima stick a tip on it and call it a day

 

i dont care what anyone says, an exhaust on any vehicle needs some backpressure. I lost some low end on my 3.9 when i installed my 3" catback and never gained it back. When we put a manifold back exhaust on my buddy's chevy 350 we drove it around the block with strait manifolds for the sake of being loud, and he couldnt even spin his tires anymore because he had lost so much low end. Theres a reason automotive engineers design exhausts the way they do, to get them to perform in a matter so they are friendly to the environment and for drivability. sure if you have a cummins or a gas engine thats already a torque monster, backpressure may not be as important and getting rid of backpressure may in fact help you perform better. But for engines like a 3.9 and other relatively small displacement engines, backpressure is a necessity.

 

a tip wont change anything, and fyi a catback system does not mean you replace the cat. its just everything after the cat. i got mine for like 200$.

 

i know a tip wont do anything...all for looks homie.


will the rest of the system help out? or is the cat a necessity?

 

Ok, I lied... I'm not done.

You guys are repeating exactly what I'm saying. A 3 inch exhaust system is unneccessary on that small of an engine because the surface area of pipe to volume of gas ratio is so high. That large of a pipe slows the velocity of the gas trying to exit due to increased friction and therefore makes the engine work harder to push it out (i.e. backpressure). That extra work the engine is doing is robbing its performance. I am confident that if you replace that 3 inch exhaust with a 2 inch exhaust your truck will perform better; not because it has more backpressure but because it has less.

As for the running open manifold, the exhaust had nothing to do with performance, your o2 sensors cant get a reading if there is no exhaust going by them so your computer had no idea what mixture to put in the motor.

 

alright, I understand what you're saying now. And if you're right, we've all just been misusing the term "backpressure", but we're still right about exhaust size. Yes, 3" may be a bit overkill for a 3.9, and a smaller size pipe (say stock 2.5), is ideal. And in hindsight, when i was talking about my buddy's chevy 350, it was an older carbed 350 with no EFI, so the computer had nothing to do with it. But the carb probably just needed to be adjusted.

 

lol, that was my original point, most people have a misconception of what causes backpressure in a pipe, which leads to the idea that engines need backpressure. I was simply trying to share some of what I'm going to school to learn. I was not trying to imply that you guys have no idea what constitutes a good exhaust system. I have a 5.2L in my truck and I'm building a 5.9 for it and I won't go bigger than the factory 2.5" pipe.

As for your buddy's chevy, sorry I was still in the mindset of EFI. Its back to what I said about carburated engines, when you remove the backpressure that an exhaust system has (cause every exhaust system has it, the idea is to minimize it) it leans the motor waaayyyy out.

 

so from what i understand im not going to loose any low end if i gut my cat?

i have a 2.5" on my 3.9 going to a flowmaster 40 single exit from there into a y pipe befor the axel then duals up over the axel and out the back sounds pretty good at idle and has a wonderfull sound at 1500 rpm doesnt sound to bad up over that but def not the sound of a v8