Talk to me about exhaust...

[P71]

I picked up a real RB 12A collected header for the FB. I come from big honking V8's or big honking turbo cars, so all of my stuff has had huge exhaust on it. Why not a single 3" on a 12A? Does the 2.5" flow sufficiently?

Here's what I'm thinking parts wise, though still undecided on size:

RB Header > Magnaflow Welded Ultraflo Bullet Muffler (it will look like a cat and act like a resonator) > custom mandrel bent piping over the axle > Magnaflow Sport muffler (either dual outlet, or if maybe a big single, or maybe a single outlet muffler with a dual tip added on before the valance so it looks like the original dual-outlet).

Anyways, need thoughts, facts, and opinions.

:driving:

[josh18_2k]

2.5" at most. 2.25 would be better.
too big slows exhaust velocity, creating backpressure. you dont wanna use pipe any bigger than the outlet of the header. also, you dont want the pipe to get bigger or smaller as it travels along. thats why crush bent sucks (smaller volume in the bend). you dont wanna use pipe any bigger than the outlet of the header.

[P71]

So 2.5" it is then I guess.

You going to be at ORPCA tomorrow? I have some Boxter's to beat up... ;)

[Whizbang]

i have a magnaflow muffler i got from Summit on my Turbo II. Its the only thing in the system and i like it. Could be because i had a straight pipe for awhile...but at anyrate...

[Rogue_Wulff]

The collector on the header is 2" OD, so no need in going any larger for the rest of the system.
Many people have used Magnaflow mufflers, with mixed results. There's just not many options for controlling the sound, and having a long life.
My 12A car has a RB header, and a single borla clone generic stainless muffler. A bit louder than I would like, but not obnoxiously loud.
My SE has the RB header/presilencer and some generic muffler. I think the muffler died a long time back. It's obnoxiously loud. Even the harley guys around here think it's too loud....

[P71]

The collector necks down to 2" then back out to 2.5". It's used and the PO may have done that (it came off of an IT racecar). 2.5" should work well both in cost and fitment.

I had a 3" single on my ported 13B FC (all custom as usual) and it ran great (mid-13's at 104 in the 1/4 with a bad 3rd gear so I had to short-shift into 3rd). It did make quite a bit more power then the 12A, but the exhaust is not the only mod that's going to happen.

The basic 12A will remain totally stock but I'm ditching the fan for an e-fan, losing the air pump/emissions, rebuilding the carb, installing a custom intake up to the carb (cone filter in a ram air box in the nose, carb hat), the header/exhaust, and a bunch of cooling mods (radiator/coolant/fans/t-stat). I'm going to try and hit 150rwhp on a stock port before an eventual on purpose rebuild with port work to go for 200rwhp NA.

[one320fc]

On my GSL-SE I have stock manifold to 2.5" piping w/ gutted cat and a Magnaflow muffler. Its pretty nice, but pretty loud.. I got a ticket last month for it:banghead:

[PercentSevenC]

Here's my recommendation: use 2.5" RB pipe. It has an ID of 2.25", which should work pretty well while still containing the sound much better than standard 1/16" wall piping. For muffling, I recommend an RB presilencer and a Borla ProXS in back. If this is too loud for your taste, put a Magnaflow round muffler after the presilencer and before the axle.

[BigBadWolf]

I ran 2.25" inside 2.5"(thanks Matt, your a genius) off a RB header with a 14" Magnaflow Race Series for resonator and 20" Magnaflow Race Series for muffler with a 4" rolled tip. 1 ticket from St. Augustine Beach cop in 3 years ago. Produces a deep, rich sound. My biker buds always think there's a huge hog coming down the road when I keep the revs down.

[RotaryProphet]

2.5" at most. 2.25 would be better.
too big slows exhaust velocity, creating backpressure.

To correct/clarify your statement: To large of a pipe will not cause back-pressure. What it will cause is a low exhaust gas velocity, and an obnoxiously loud buzzing exhaust note as the noise reverberates in the overly large pipe.

Gas velocity is important. Think about it this way: In a properly sized exhaust system, your exhaust port opens, and the pressure in the combustion chamber equalizes with the pressure in the header, and sends a high velocity pulse down the pipe. This column of gas moving down the header has momentum, which a properly tuned engine will use to it's advantage.

Toward the end of the exhaust phase, when the intake is opening, there isn't really any more exhaust to push out; the pressures are equal. However, that column of gas moving down the pipe isn't going to immediately stop, it's going to continue traveling down the pipe, which creates a negative pressure area that, upon reaching the exhaust port, pulls out some more spent exhaust gas that would otherwise have made another trip around the cycle with the intake charge. The real beauty is that now there's negative pressure in the chamber, which works against the intake port and pulls some of the air/fuel mixture into the chamber, and helps get that column of gas moving.

Too large an exhaust pipe will prevent the gas from reaching a high enough velocity to create enough momentum to pull this off; the gas has very little energy and stops soon after the exhaust pressures equalize, and in extreme cases, will even backflow after the momentum is spent.

On the other hand, too small a pipe will prevent the gas from equalizing pressure with the outside atmosphere before the exhaust port closes, resulting in a diluted air/fuel charge. In other words, back-pressure.

The same concepts apply to intake tuning; that column of intake air charge also has momentum that can serve to ram more air into the chamber after it's "full", achieving over 100% VE in some cases. All of this relies on properly sized runners, both in diameter and in length. It should be obvious to anyone versed in engine dynamics that the effects I've talked about rely heavily on the time between the gas equalizing pressure and the closing of the exhaust port/valve. That's where the runner/system length comes into play; the longer the system length, the lower the RPM where you'll see your benefit from this sort of tuning. If the system is too long then the system won't have time to make use of the vacuum created before the exhaust closes, and if the system is too short, the momentum will be gone before the intake is opened. In general, tune all of your runners towards your peak power band. Some race engines have made use of varying runner lengths between cylinders to "spread out" the power increase (one cylinder reaching it's peak effect at 6000, the next at 6250, the next at 6500, etc) to provide a wider torque peak, but peak power from this relies to some extent on individual cylinder fuel/spark tuning, and is therefore out of reach of most of us mortals.

The specifics for your application are, as usual, left as an exercise for the reader.

[BigTeddy]

I have gone through many different types of exhaust setups, and to my findings anything smaller then 2.5 just will not work well for free flowing, no restrictions on rpm performance...If you want to get the max power out of your car...Go with 2-1 header collector and then merge those pipes to 2.5 inch dual or to 3 inch single...The rev will be clean, there will not and won't be no restirictions on rpm...Just a little loss on back press. and a little noise...

[warwickben]

hey guys havnt been around for a bit. my sa has the whole rb exhaust setup on the car.(stock port setup). the car is pretty loud now(or was since i bought a rx8 and the 7 is off the road) when i really get on it sounds like a swam of angry bees. and if i was on the high way, and in 4th gear doing 80ish if i put it in 5th and took my foot of the gas you would hear a puff sound sound when the car got to about 65mph. any one else had this happen.

also when i got the car it had the stock setup on it, the stock muffler was so badly rusted out i had to replace it. i was tight on cash so i bought a cheap ebay carbon fiber wrapped muffler, to replace the stock one. hey 30 bucks shipped lol. every time i would shift gears it would shoot flames. but wasnt that loud. when i put the car back on the road this year. i had the rb header and mini silencer put on only. the car was twice as loud then before, but no flames. after a week i got the rb muffler, and when i took the ebay one off the top of the casing (carbon fiber was burnt). my lessons these car's react in strange ways to what you do with the exhaust.

any ways enough with my stupid stories.

[PercentSevenC]

I posted in your other thread, but reading this, it sounds like you have an exhaust leak.