suck fumes

Has anyone ever tried running a stock catalytic converter with just a straight pipe on the back with no muffler? I was just asking cause i am trying to figure a new way to make better torque on my 1999 spec miata. Everybody always runs the typical springfield dyno exhaust with the cylindrical maganaflow muffler and straight pipe afterward but i think there are better options out there, just need to know if anyone has tested this before i spend a bunch of money trying different parts and welding a bunch of systems together for dyno purposes.

By the way the stock manifold and downpipe have to remain untouched. cat back is open as long as it is 2.25" or less in outer diameter.

18psi

iTrader: (76)

I bought my car with that setup: stock header/cat/midpipe with a straight pipe in place of the muffler. It was loud as **** and I hated it. 2 hours of driving it back home and I was about to bleed through my ears. Its loud and obnoxious. And I DEFINITELY felt less torque than with the stock muffler.

Toddcod

iTrader: (8)

The stock cat is more restictive. I have ran n/a with a resonator and muffler. it ran prety stronge.
I have also ran no cat with sraight pipe. NO LOW END! SUCKEDSUCKED SUCKED!. i fixed it same day

I haven't dyno'ed anything. But the stock mufler ,with no cat ran pretty good.
But then again, I haven't really done much n/a stuff.

I would do what the other spec guys are doing.

apariah

So would this mean a turbo car would be faster with a cat and straight through muffler.

I ask because I'm having an exhaust system welded together this week, and the old school racer told me my car was going to fall on its face without a cat. Well he was dead wrong, since at the moment I don't have a cat, m exhaust system stop at the downpipe. Its loud and its going to be fixed this week, but clearly the car is much faster at 8.8 psi.

The only negative other than the noise is maybe I don't see boost until 4000 rpm or so, but when it hits it hits pretty hard. Should I see better spool or worse spool after the exhaust goes on. The plan is for 2.5" all the way to a straight through muffler.

I'll have a cat, and a test pipe with this system, I would expect better spool with the test pipe, but I realize I could be wrong.

Bond

iTrader: (6)

You ran open DP and didn't see boost until 4k? I think something is wrong there...

apariah

Nope still running, and don't see any significant boost until 4, but the transition from nothing to full is insane. So it lags then hits really hard. I'm running a Chinese t28 on a stock 1.6, I'm ok with the lag, for now, I'm just hoping it won't be worse after exhaust goes on.

Again the old guy from the exhaust place puts a v8 in everything. He thinks I should step things down to 2 and 1/4. Its not going to happen, but thats his suggestion.

Toddcod

iTrader: (8)

Forget the cat...... It is the most restrictive thing on the car. If you are going to have one. Get a 200 metal cell.

And a turbo car and naturally aspirated are 2 different beast. Opposite principles.

N/A needs backpressure. Hince mufler.

Turbo. the less back pressure the better. 2 1/2" 250hp, 3" more hp.

I like a reasonator and a magnaflow muffler.

Braineack

iTrader: (62)

with a restrictive exhaust you might see a few extra TQ down low, but a loss of top end HP. none of us here would dare use the stock exhaust or cat and want to be taken seriously. not the correct forum for your question.

curly

iTrader: (25)

+1 to what brain said. Go to specmiata.com (.net?). Everyone uses Springfield exhaust because that's what has been proven to provide the most power. If you can't afford it, do your best to copy it's design when you fab your own.

suck fumes

Well i have been running in spec miata for 9 years and i can tell you trying to get answers on specmiata.com is not the best thing cause those guys are your competitors and they are not going to tell what is better out there cause they don't want to get beat on the track.

I know the springfield is 2.25 in. diameter tubing and i know that a lot of people don't know the "best" exhaust setup cause people don't want to spend the money to find out. I have ran the springfield setup the whole time i have raced and i know that it is not the best out there. I have talked to some engineering friends and they say that using the stock diameter tubing with a good muffler or cat will create better back pressure which is what the 4 valve heads like, to make good torque.

Does anyone know if the stock diameter tubing on a 99 car is 2in. or 2 1/8in. ?

Also is there a certain year air flow meter that works better than others on 99+ cars? For example would a 01 meter work better on a 99 car vs. the one it came with?

Braineack

iTrader: (62)

I would not run a cat or muffler personally. if I had to run a muffler, I'd make sure it was a straight through design. Adding backpressure adds torque through AFRs and through velocity. When there's a restriction in the exhaust gas velocity out of the head are faster thus you make more torque because the there's less energy needed to push the exhaust out and more energy pushing the pistons down. However, once you start increasing revs, that restriction becomes a problem, because the motor has to start pushing the exhaust out of the head, wasting energy. This is why you'll see a loss in top-end HP. It's all a trade off. There's really not much power to be had, but I realize you want all you can get. The combo of stock diameter and a good muffler like a magnaflow 4" round might be a good setup. There's really not much to an exhaust.

webby459

iTrader: (8)

I think it's iffy per the rules to mix and match YEARS of afm, and you better not crack it open when you get one if you're running a 99.

I'm sure you know that the top guys, along with having mystery pro built motors, use bins of stock parts and dyno test and measure for the best result. This includes afm's, front and rear subframe assemblies, TUBS, control arms, etc. The pro engine builders use bins of cams, pistons, rods to mix and match for balance and tolerance optimization. The point is, without an open account at the local dyno or a pro builder, this is going to be an educated guess, at best. Smart to look here for guys with knowledge for an exhaust build, though!