Except when it isn't.

there is a great discussion on the merits of larger dump pipes, even on street driven cars, here:http://www.ausrotary.com/viewtopic.php?f=4&t=163139

If things are selected appropriately and you use quality mufflers, you can achieve significant improvements in response and powet without increasing noise.

What is an appropriate selection will on your turbo, boost levels, porting and whether you are using innternal or external gates.

how much of a difference does a 3 -> 3.5" make in spool time? are we talking enough of a difference to warrant upgrading if i don't need to?

According to Turblown's website, on EFR systems, the difference is around 10% on boost threshold rpm (ie around 200-300rpm) - not heaps. But I imagine there would be a lift in peak power output as well. Add to this, potentially lower EGTs.

The downside of the bigger exhaust is noise and cost. If you were starting from scratch the cost difference is negligible, but if you already have a 3" system and it is working fine, it's hard to make a case to justify the upgrade IMO, unless you have a large frame turbo that was choking up. If you have an EWG with a screamer pipe, then there is even less of a case to made for upgrading since the dump isn't sharing flow with waste gas from the EWG.

In terms of noise, if it tapers back down to 3" a metre or greater after the turbo, noise should be around the same as a straight 3" system. You can get a full 3.5" system down to acceptable noise limits but it requires very capable mufflers specifically designed for rotaries, like the AES mufflers available here in Australia.

there's no such thing as too much

If I could find mufflers for a 5" or 6" oval to make this car quiet as fuck and maintain a 3" height (yay for oval!) to go over the rear solid axle and between 4-link, I would...

https://cimg7.ibsrv.net/gimg/www.rx7...32bab3e96d.jpg
Going V-Band to 3" -> 4" Downpipe to 4" Inlet, Dual 3" Magnaflow XL Muffler + As many other mufflers as I can fit...

Honestly though, I wouldn't run over 700whp on a Rotary without going bigger. Drop that EMAP, gain efficiency (lose a bit of high RPM boost control, but oh well -> more backpressure allows for better boost control).

Listen to David, he knows better. 5" now, right? :lol:

^ the thing about that is the low point will still be the resonator (which you will need at least one of if you want to be quietish). You might as well just use round pipe and as many oval resonators/mufflers as you can stuff under there while takig great pains to package it all nice and tight.