exhaust
09-24-08, 08:24 AM
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exhaust
I know this is likely a redundent thread, but i am curious. I have a na 12a with a racing beat dual outlet header to a dual-pre-silencer centers section with a double outlet right in front of the axle. i have two-into-one over the axle to 3". I need a finish to this exhaust and i was wondering what you all have had good luck with. i've heard that it is necassary for this set up to join in front of the axle to provide the proper scavaging effect (approx 53" form exhaust ports on the block) forgive my lack of knowledge, i've been out of the rotary game for some time.
09-24-08, 04:21 PM
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it was a a thought at one time that the cfm that 3" exhaust would flow would be equivelent to the dual outlet center section that dumps out in front of the axle. so i fab'd up the pipes over the axle and ran an open 5" supertrapp with a 3" core out the back. Hine sight being 20/20, i wish i had gone a different route.
09-24-08, 05:31 PM
#6
Yeah, or 1 7/8".
My Cosmo has a 2" system for a while after exiting the engine. Then about a foot or so before the rear end, it goes into a shorty preslincer. Yes, a stock presilencer (I thought that was pretty cool of Mazda) after which point it exits emmidiately into a 1 7/8" or 1 3/4" OD pipe that curves up over the rear end like a 1st gen and into the large stock muffler. This exhaust system actually flows quite well and has adequate power up top without being noisy. It's actually quite pleasant and suits the car's luxury personality.
Part of the good power of this system is due to the reduction in pipe diameter right after the preslincer as the exhaust has lost some of its energy at this point in heat and noise, especially this far from the engine. Plus the temperature differential in rotary exhausts to the outside air is quite substantial as well, compared to piston engines anyway (we're like 500 degrees hotter with the outside air being equal, equals more heat lost through the pipes). All this equates to the need to reduce pipe diameter to keep exhaust velocties up and thus keep power levels up.
3" is inadvisable on an NA and difficult to muffle.
By the way, each pipe of the RB dual pipe system is only carrying the flow of one rotor. It's really not that much. It's just really, really loud. Why? Because in a collected system, the rotors being 180° out of phase, tend to cancel each other out acoustically and thus are quieter without hurting flow (think woofer box and how bass is killed when the woofers are hooked up wrong). Infact with a proper collector, exhaust scavenging is put into play actually increasing flow over two seperate pipes. Anyhow as I was saying the rotors are 180° apart which means when one is flowing, the other basically isn't. Therefore you CAN get away with a collected 2" system if you want. I would recommend a 2" system on a 12A anyway. Anything larger and your low end will suffer more than your high end would suffer with a 2" system.
A 2 1/4" system would be fine too, but I don't know where to find thickwall pipe in 2 1/4" locally. I like 2" because RB sells 2" thickwall which is 1/8" wall thickness (.120") 1 7/8" ID. They also sell other stuff in 2" so it is more convenient. The next step up is 2.5" thickwall but that's too big in my opinion. They also sell 3" in 1/16" (.063") but being stainless is probably more soundproof than mild steel at that thickness - I don't know - I've never messed with it.
My Cosmo has a 2" system for a while after exiting the engine. Then about a foot or so before the rear end, it goes into a shorty preslincer. Yes, a stock presilencer (I thought that was pretty cool of Mazda) after which point it exits emmidiately into a 1 7/8" or 1 3/4" OD pipe that curves up over the rear end like a 1st gen and into the large stock muffler. This exhaust system actually flows quite well and has adequate power up top without being noisy. It's actually quite pleasant and suits the car's luxury personality.
Part of the good power of this system is due to the reduction in pipe diameter right after the preslincer as the exhaust has lost some of its energy at this point in heat and noise, especially this far from the engine. Plus the temperature differential in rotary exhausts to the outside air is quite substantial as well, compared to piston engines anyway (we're like 500 degrees hotter with the outside air being equal, equals more heat lost through the pipes). All this equates to the need to reduce pipe diameter to keep exhaust velocties up and thus keep power levels up.
3" is inadvisable on an NA and difficult to muffle.
By the way, each pipe of the RB dual pipe system is only carrying the flow of one rotor. It's really not that much. It's just really, really loud. Why? Because in a collected system, the rotors being 180° out of phase, tend to cancel each other out acoustically and thus are quieter without hurting flow (think woofer box and how bass is killed when the woofers are hooked up wrong). Infact with a proper collector, exhaust scavenging is put into play actually increasing flow over two seperate pipes. Anyhow as I was saying the rotors are 180° apart which means when one is flowing, the other basically isn't. Therefore you CAN get away with a collected 2" system if you want. I would recommend a 2" system on a 12A anyway. Anything larger and your low end will suffer more than your high end would suffer with a 2" system.
A 2 1/4" system would be fine too, but I don't know where to find thickwall pipe in 2 1/4" locally. I like 2" because RB sells 2" thickwall which is 1/8" wall thickness (.120") 1 7/8" ID. They also sell other stuff in 2" so it is more convenient. The next step up is 2.5" thickwall but that's too big in my opinion. They also sell 3" in 1/16" (.063") but being stainless is probably more soundproof than mild steel at that thickness - I don't know - I've never messed with it.
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