Pretty much my exhaust stops right before where the cats began.
I know back pressure is needed, so i just want to put a reducer on the end and call it a day.

I'm not going to supercharge it.

Right now its 2 pipes both at 2.25' Outlet

What should I reduce it to to effectively create enough back pressure.

I was thinking bringing them both down to 1.5'
I plan on chipping down the road if that matters.

All opinions appreciated.

 

this is a common misconception. generally speaking, fourstroke engines do not need backpressure to operate, and having backpressure will only make it more difficult for the piston to push the exhuast out of the valves. two strokes are a whole other story, and i dont understand it well, so we wont go there. basically, go ahead and run whatever size exhaust you want. keep in mind the smaller the tube, the more back pressure there will be. i say 2.25 for each side with 3 inch from there would be just fine.

one of the ways i solve problems is by extrapolating the numbers. lets say for instance you run .25 inch pipe. think that will work well? no it wont. how about running no pipe? it will be loud, but it will run just fine. 2 strokes on the other need need back pressure so the intake gasses dont pass right through since they exhuast is open at the same time. back pressure is bad for 4 strokes

 

You don't need back pressure, you want to minimize back pressure as much as possible without making the pipe too large. What you want is to maximize exhaust velocity.

So choose sizing based on your power needs. The best explanation I've ever come across is comparing your exhaust to a garden hose. If the pressure isn't turned up, the water flows out poorly, kind of limply to the ground. So to maximize the flow's speed you can do two things. Either raise the pressure (increase power output) or alter the size of the hose, kind of like how you can spray somebody by putting your thumb over the end of the hose to make it shoot further.

So if you don't want to supercharge, choose the appropriate size for your current power. You want the exhaust to maximize exhaust velocity so that the cylinder doesn't waste energy pumping out the exhaust. The two options you have for flow are restrictive and unrestrictive. The ideal would be unrestriced exhaust would be open headers (no pipes at all!), but thats not exactly practical. So instead we have to use restrictive exhausts (the kind with pipes!), but we want the least restrictive that meets our engine output. For the 2.8 I'd go with 2.25" (good for 210hp) if your ECU isn't tuned (You would go from 190 to around 205-210) , and 2.5" (good for 250hp) if it is. If I had to guess you'd probably be sitting around 225hp in the later case.

But since you're running open downpipes, I'd say you'd want 1.75" each, based on the calculator below. Although since you have much shorter pipes, I'd say 1.5" each might be perfect depending on if you're tuned or not.

Exhaust Size Calculator
Basic Exhaust Tech 101 - 2GStratus

 

Please don't do straight pipes on a 2.8, it sounds like *** and is stupidly loud and droning. The stock S4 exhaust is 2.5" before the resonator where the dual pipes meet and 3' after, if that helps any. Also the dual pipes are closer to 2.1" on a 2.8 from what I remember, S4 dual pipes are basically 2.25".

 

Huge understatement

 

Ive had mine on straight pipe for about a month because of not having time to get to it and it is definitely loud and drowns out everything even the stock stereo and that's only at 4k.
I find myself speeding up to how fast i have to go, then dropping it to 2k rpm if i want a conversation, no phone calls at all.
My girlfriend hates it. I don't mind it except getting EVERYONE to look at my car..Even the cops..
So far been pulled over 5 times.
second time i got a ticket and pretty much used it to not get ticketed again and again.

Sounds awesome at the drag strip tho


okay I just ordered some pipe, I'm going to do a Y that's 2.25 in 2.5 out, to a 2.5 inch muffler (With cone inside to actually quiet it) with turn down pipe, I just have a few more questions

1. Where should i put the o2 sensor to get the most accurate reading.. I was thinking at the Y when in turns into 1 pipe so it wont uneven the back pressure.

2. With that would you think id run into problems splicing the 2 o2 sensor wiring into just one sensor?

3. What color wires do you run for a wideband since I plan on getting one down the road.

4. What can i use to hang my exhaust up? I can only weld at work, or i would just weld it to a piece of metal to the bare car, but since i cant, the only thing i can think of is to tap into the bottom of my car? or maybe use the heat shield mounting points since i took those off, but the threads and bolts holding it on felt soft almost like a tin can, and i don't think it can take the vibrations without me losing it.

 

Assuming you're talking about post-cat O2 sensors, you'll need both, there's no way around it. The ECU controls both cylinder banks separately, if you combine any 2 inputs there's a million different things that can go wrong. So either use 2 sensors or get them coded out. Why would you need a wideband for the 2.8? You're not going to be doing any datalogging or tuning. Why not hang it on the stock mounts? They're already there in the best possible position. If you can't weld with the exhaust on the car then take it to a shop that can, it won't cost much for them to piece it together if you have all the parts.

 

That makes sense, so I guess ill do one o2 sensor on each side.
The wideband I want because i want to get nos, and I can tell if i start running lean.
I don't know if the stock mounts are in the right position.
The shops around here wont do it. they say there's a fine if they do anything other then oem pipe routing.

 

yo kenny, im workin on the same ****, well almost, im welding a "test" pipe in place of my cats, cut the cats in half and gutted it and welded that over the pipe so it looks like i still have them in place (need them in my state).

also to ditch the 02 codes a buddy of mine did this; instead of coding the ecu

$6.00 DIY O² Sensor MIL Eliminators

 

I'm just going to go stock exhaust no cats and throw in 02 sensors since Ill have them.

Next things im doing is

Fuel pump, wide band, NOSSSS