Surely you'd get turbulence effects where it steps down and emphasise the forced noise from the back. I'd have guessed it'd also make more back pressure having a smaller pipe after your downpipe.
Anyway, I just did a search and found this:
"Pipe diameter does have an effect on flow rates as well, but again it is not the major factor in most cases. 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par. There are also full 3.5" systems and those that start out at 4" and taper down. Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage. There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" as we want our system to flow enough to be capable of coping with a customer's changing goals. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club.
The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster.
However having smaller pipe towards the end has less effect than having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better……..if you decide to neck it down."
This was from this page:
http://www.seatcupra.net/forums/showthread.php?t=291217&highlight=downpipe+2.5"+system
It still doesn't
help me much though as I can't find a system, unless it's a Blueflame, that'll give me 3" all the way through. I'll still get a 3' downpipe though
![[B)]](/forums/styles/seatcupra/oldforumsmileys/shades.gif "Cool [B)]")
