exhaust - what size

[16bit]

I have a 2 1/4 inch system at the moment - I was wondering if I should increase size as my car is supercharged and increasing boost soon. Would 2.5 be enough?

[Okibi]

What headers are you running?

[16bit]

loch stewart

[Okibi]

hrmz .. what's the size of the flange that connects to the cat?

Perhaps a high flow cat and a mandrel bent system the same width.

[16bit]

already have hi flow metal cat (cost me 400 bucks). It is press bent so maybe mandrel is the way to go. do you think that going to a 2.5 inch system would be ok though. I am not going to re do it with the same width.

[Okibi]

Id ask on the miata.net forum its a bigger net to throw out and i'm sure you'll catch some good answers from people who've tried different combinations with their supercharger.

[tk421]

Hi Mate,

With the coldside Tom from FFS put it to me like this. A belt drive blower (mine, yours as far as I know, anyones really) has a linear relationship between engine speed and boost. We are picking pulleys in order to spin the blower at a certain amount of rpm to make a certain max boost level at redline. When you put a bigger zorst on obviously you are removing a restriction (so intuitively more power). The problem is the blower is still pumping in the same volume of air all things equal. Therefore what you get is a rise in power from less restriction in exhaust, then a fall in power due slight a drop in boost. To give you an example the first time my car hit the dyno with boost on and a standard exhaust it was running 12psi at the top. With a 2 1/4 custom mania system fitted (high flow cat, xforce headers and a big resonator) the boost dropped to 10.8 psi.

I know you are thinking about raising boost (as I think you have mentioned it before - I have a bigger pulley all ready to go myself) but personally I think you would be wasting your time with a 2 1/2 if your happy with your 2 1/4. Tom's view was that at the 160rwkw level there are no gains to be had even going over 2inch as he has dynoed his with an open exhaust vs the full one and seen no difference. I'm sure there is a point (a certain level of boost) that a smaller exhaust becomes just too much of a restriction for a supercharger setup but I don't think any of us are going to reach it on the street - I will be at 13.8psi after my change of pulley personally. Someone mentioned headers which can make a difference for sure, however you already have good ones so thats out of the picture.

The main reason I put my 2 1/4 on was getting a new cat, bigger resonator to manage the noise factor better and the rather nice engine note that has resulted. Obviously you are running a centrifigal but If your boost rises in a straight line to redline like mine then I think the ideas I have mentioned might hold true for your car as well.

As far as I know a turbo in this example will simply spin faster to compensate to continue making the same boost level vs the wastegate or boost higher because of quicker spool up etc.

The other thing to consider is exhaust noise. I don't know about yours but mine is reasonably loud, loud enough for me to feel mildly self concious when giving the car heaps in the Syd or Nth Syd CBD as it reverberates off buildings (sounds GREAT though ). I think my boost is a pound or so higher than yours at the moment so you might find the higher boost will give you a bit more exhaust volume like mine even with your existing system, when I go to higher boost I expect it to be magnified again. Certainly I wouldn't contemplate going 2.5\" just for that reason. No turbo to supress the noise remember.

Just my 2c as they say, I think I'm at least comparing apples to pears when considering our relative setups (rather than oranges )

A lot of comments I have read on mnet seem to support this idea that 2.5 is just louder and not more effective with a blower. I would be playing with your boost first and seeing how you go from there personally...

Cheers

Steve

[16bit]

very good point - i will leave it as is!