darkwaveboi

do the 89-91 NA's use the backpressure sensor that goes into the cat to actuate the 6-ports? I thought that the VDI did all that shizat and backpressure was a feature on the Series 4's.

What do you all think of this:
2 RB 3" ID flanges
2 RB 3" gaskets
THEN: get an exhaust shop to Fab a 3" pipe and weld on the two flanges to make my own cat-replacement straight pipe?

Any downsides to this? (i live in VA so noone checks for cats except during emissions)

thanks
jeff

darkwaveboi

too late to edit: will the pre-silencer bolt up to the stock dp (pre-cat, yadda yadda) or do you need the RB downpipe/header for it to bolt in. thanks

aZizzo

Yeah, as far as i know the auxilary ports rely on backpressure, but i heard u can get aroumd it with an air pump.

blu_gxl

the 89-92 doesn't work off of exhaust back preasure it works off the air pump i believe any who the vdi has nothing to do with it as for the rest someone else will help you out i like to keep emitions complyant

selanne8

i don't think the 89-91's use backpressure either...but i could be wrong.

john

NZConvertible

Enough guessing!
The 86-88’s used exhaust backpressure to open the aux ports (and had no VDI).
The 89-91’s used pressure from the air pump to operate both the aux ports and the VDI via ECU controlled solenoid valves.

Jeff, unless you plan on fitting a set of properly tuned extractors, 3” is too big for an NA. 2.5” max. But the cat replacement pipe is a good idea.

Listed

what meters the exhaust back pressure and how can it be tested? seeing as how it regulates the aux ports I'm guessing it could be the reason for the 3800 hesitation?
thanks,
Chris

NZConvertible

It’s not “metered” as such, it’s just generated within the exhaust pipe, just like in every other car. It can be measured just like any other pressure, by teeing a pressure gauge into the exhaust pipe at the point you want to test.
For the S4, Mazda merely exploited that pressure as a means of operating the aux port actuators. All of the parts were selected/tuned to operate the ports at just the right time. Anything that alters the backpressure will affect when the ports open. The more free-flowing an exhaust is, the less backpressure is generated. The lower backpressure, the later the ports will open. If the backpressure is reduced enough, they won’t open at all. You can see why the S5 system is so much better! No, that’s an electronic problem with fuel delivery.

darkwaveboi

jason (NZ): im planning on going through a teeth pullin and doing a TII swap (already got the harness, ecu, and tranny..no going back now ). honestly..is 2.5" all you need on a TII when your goal is around 350rwhp? I was always disillusioned from turbos and that when you have a turbo, you want as much of an open exhaust as possible becuase the turbo itself creates the backpressure needed. If 2.5" is fine, then hell, i have no problem with that.
Thanks

RylAssassin

Hey i got a real simple question talking about all this backpressure and ****. Why is it so bad to have no back pressure? Should i re-consider getting a stright pipe for my N/A?

NZConvertible

OK, that’s a bit different. Yes, as a general rule you want the biggest exhaust you can get behind a turbo, so I’d recommend a 3” system.

BUT...

Don’t forget about the wastegate probs associated with these cars. If you want to run a full 3” system, you’ll need to make sure you can control the boost. You’ve been warned...