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Old 10-22-2009, 06:10 PM   #1
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Default Convert existing exhaust to 3"

I have a full FM exhaust from turbo back. I like the idea of having a custom 3" DP fabbed up, but they do come at a price. After researching the exhaust topic it appears that you don't really need 3" from the turbo back to realize the gains in earlier spool. If I were going to modify my exhaust (specifically the DP) At what distance from the turbo would you start the taper to 3"? I ask this because I could probably have my current dp modified and 3" pipe run to the rear bumper for the same cost a s a custom DP.

I hoping that this might be a serious option for those of us that already have a DP.
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Old 10-22-2009, 06:18 PM   #2
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Well, with BEGI downpipes, if you send them in to get modified for a 3" exhaust, they simply change out the flexpipe portion near the end to a 3" diameter section that's only about 4" long or so, and put on a 3" flange.
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Old 10-22-2009, 06:30 PM   #3
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That certainly seems like more supporting evidence that the main restriction is the cat and muffler. Is their way really the most ideal conversion method? I would think that the closer the taper is to the turbo the better. At what distance from the turbo would the velocity of the gas be zero if you could immediately close the exhaust while a turbo was spooled. I think this would indicate the point at which the turbo has to work harder to expel gas ( increased back pressure) . By increasing diameter you reduce resistance. Ideally the taper would begin between the turbo and the point of highest resistance.

Any guess on where this point is? I'm typing as I'm thinking so I could have this wrong.
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Old 10-22-2009, 06:39 PM   #4
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I've been wondering about this -- I see a lot of guys here going to 2.5" or 3" exhaust systems. But in Maximum Boost, there's a chart for optimum exhaust size based on power output, with the assumption being that pipes too large can decrease exhaust speed. If I'm reading it correctly, the 200-250 HP range would be suited best by a 2" - 2.25" (single pipe) system.

Is Bell's chart incorrect? Or is there something about the Miata's exhaust system that encourages a wider diameter exhaust than the chart recommends?
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Old 10-22-2009, 06:40 PM   #5
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I personally decided to keep the DP unmolested so I could sell it later. I just swaged from 2.5 to 3" right at the 2.5" flange coming off my BEGI downpipe. I dont think there would be any noticeable gain by complicating it in the way you're thinking.

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Old 10-22-2009, 07:00 PM   #6
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I just went straight to 2.5" at the flange from the BEGi 2.25" DP. Only a 1/4" difference, and stepped up at the flange, then 2.5" STRAIGHT out with minimal bends, no cat, and straight thru center-offset Dynomax oval with the outlet now on the driver's side.

The difference over stock exhaust (with hollowed cat) is ASTOUNDING.

I wouldn't bother modifying a DP...if you're starting from scratch it's one thing...but just go with what you've got, and get it done sooner. Buy the biggest you can afford, just make sure it's got mandrel bends and a Magnaflow or other straight-thru exhaust.

If you have to have a cat get a Metalcat 200cell. I had one I was going to install and I can tell you they are very well made and look like they would hardly be a restriction.
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Old 10-23-2009, 12:21 AM   #7
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Ok, so here is another exhaust related question. I want a really quiet exhaust. What type of muffler should I run? I was plannig on a 3" straight thru dynomax and removing the fm duel exhaust muffler. A perfect scenario would be no louder than stock. I'd run two in series if I need to. While I want quiet I'm not will to sacrifice power for it. My current exhaust is just too loud for me.
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Old 10-23-2009, 12:57 AM   #8
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1. The turbo itself is a sound muffler
2. The cat muffles sound
3. The muffler should be a high flow one.
4. Exhaust tip should have some sort glass pack as well as a slight downwards angle.

On my custom exhaust, i first did not have a tip on it and it was quite a bit louder. After i put a tip on, it sounded like money. Not too loud but deep and throaty.

Check my sig for the build.
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Old 10-23-2009, 12:58 AM   #9
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Quote:
Originally Posted by mgeoffriau View Post

Is Bell's chart incorrect?
yes.
you sure that chart wasnt for NA cars? when it comes to turbo exhaust the best exhaust is no exhaust
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Old 10-23-2009, 01:16 AM   #10
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Yeah, definitely for turbo cars. The section does mention that the best exhaust is no exhaust (least restriction), but also comments that if the diameter is too wide, exhaust gas velocity decreases and can increase resistance. I'm going to reread it to see if I'm missing something.
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