crush bent =
bad exhaust flow
shitty welds =
bad exhaust flow
dual exhaust =
unpainted bumper =
He's right you know, that rough welding and the crush bends are doing NOTHING for your exhaust flow- neither is having the muffler so far removed. Definetly needs some tips, but I would have just gone dual muffler. That split point is causing major backpressure probably- and backpressure AFTER the muffler is a very bad idea.
Dual Exhaust does not cause HP loss in theory. What it can lead to is a lack of pressure needed for gas scavenging to take place, which can lead to the celica feeling bogged down low in the RPMs.
Done right, dual exhaust presents the car with the ability to handle a large amount of exhaust gases with minimal loss in gas scavenging pressure- unfortunately, problems like bad welds, too many bends, piping circumference not calculated before and after the split point; all these things add up to duals, more often that not, stealing hp away from a car than giving it. Finally, a 4 banger usually doesn't generate the sheer volume of gas pressure needed to make dual exhaust a really viable option.
Also, because the nature of dual exhaust system is that it's usually not done by professionals, you get more exhaust gases coming out of pipe than the other. THis looks really shitty at night and it's usually because the pipe is split after the tail box, so the gas is forced down one or the other channel (again, possibly increased by crushed bends or crummy welding). And again, because it's a none professional job, the pipes are going to be of different lengths, so it will take longer to come out of one pipe.