What's more restrictive on NA?

[90redmiata]

The stock CAT or the stock muffler?

[ben91]

cat, by far and large

that said, removal of a non-clogged cat on n/a or s/c system produces little to no benefit.

[metalman]

A properly functioning cat does little to nothing in
reducing net horsepower on a stock car.
An aftermarket cat-back system can actually gain 'significant' net whp.

[darknessprevails]

All I know is with my turbo I can tell that the cats on you feel the loss

[90redmiata]

I've been trying to do some research on exhaust upgrades and came to the following conclusion:

Exhaust upgrades are worth very little HP on S/C cars but can besignificant on turbo cars.

Net/Net is that the cat-back upgrade on a S/C car is more for sound and less for performance.

Anybody actually experienced a verified significant net rwhp gain from a cat-back? I was disappointed with the results of my own investigation as I wanted to believe that I would pick up some good power from an upgraded cat-back.

Please give me some hope! :)

[metalman]

Anybody actually experienced a verified significant net rwhp gain from a cat-back? I was disappointed with the results of my own investigation as I wanted to believe that I would pick up some good power from an upgraded cat-back.

Please give me some hope! :)

Turbos etc are another matter entirely.
Actual dyno tests I have been part of and witness to netted 6-8hp with cat back on a stock Miata. On a 90-105 hp car I would call that 'significant' and plus the nice sound factor is also a bonus.

[Atlanta93LE]

I'd sell you the cat-back on mine right now if I had a true turbo exhaust to put on in its place :lol:

[90redmiata]

6-8hp is worth it. I'll keep looking for a good aftermarket cat-back.

[oldgrayfrog]

Jason at Enthuza makes some very nice stuff and he's local. There is some talk amongst the Fast Forward Supercharger set that putting a "test" pipe in place of the cat is worth 8-10 hp on a sc car. Given that Grassroots Motorsport found only a 1 hp difference between the best cats performance and a "test" pipe, I would suggest buying a high quality cat. The header on all but the 99-00 is very good, don't go looking there. Making sure you have a good source of cold air for your intake track (TSI turn signal intakes) or the like help. Once you get beyond 5 psi, I strongly suggest an intercooler.

[90redmiata]

If a cat-back picks up 6-8 hp and there is little to no gain between a cat and a test pipe, I would argue that the most restrictive piece of the system would be the stock muffler, not the cat.....No?

If that is the case, then I would think a good straight through muffler such as a magnaflow (which I have just laying around) even with the stock pipes and cat should still provide an improvement in sound as well as power...

[oldgrayfrog]

I think the point was that there isn't that great a loss to a high flow cat. The tests were performed on a naturally aspirated car. Going sc or turbo is diffferent can of worms. Certainly the biggest restriction NA is muffler. Go as loud as you, and local law allows, after that in a sc situation and certainly a turbo one, get the restriction of the stock cat out of the way.
Anyways, on a sc engine your greatest gains come from pulley changes, not mufflers.

[90redmiata]

I hear you on the pulley change. I already looked at that but the cost snowballs real quick from there for not a whole lot of gain.

Change pulley/belt ~$120 (cheap) for ~7-7.5 psi
Then intercooler to cool the hot air (~$1,100)
Now need bigger fuel system (injectors, engine management) ($1,000)

That's close to $2,300 more for around 30 more rwhp.

I'm thinking I'll start with maximizing what I have (colder air into the blower, better flowing exhaust). Maybe a basic timing retard too so I can get the base timing back up and get some more low-end grunt.

After that, maybe a small boost increase....

I need to hurry up and find a good, cheap NA body to sway my aluminum headed 306 and T5 into so I'll leave this car pretty tame :)

[Atlanta93LE]

No way should an intercooler setup cost $1100...TDR isn't the only way to do it.

[Msidd]

If there is a lot of pressure coming out of the tail pipe, does that mean it's restrictive?

[ben91]

NO!
The most restrictive part of the exhaust is the cat. I absolutely can not state this any clearer.

I'm sorry, but unless your cat is in poor shape, replacing it with a test pipe will result in 0 or near 0 gains.

To make power with the 45 you need to spin the crap out of the blower and cool the charge air. I'm not saying that replacing the entire exahust system with mandrel bent aftermarket won't make more power than stock, but it won't make much more. Spend the money elsewhere.

[90redmiata]

I hear you loud and clear....just makes no sense to me. Can you explain how the cat is the most restrictive part of the exhaust but when replaced by an open pipe it picks up zero HP?

Think about it and let me know your thoughts.....

Thanks.

[ben91]

You are working with bad data and incorrect assumptions all around. A little bit of research and reading will quickly change your mind.

As applied specifically to your set up, if you have a good cat and replace it with a test pipe, you will gain no appreciable performance benefit. You will however make the car louder and pollute more. Look, I ain't exactly a tree hugger--I've not run a cat on my miatas in years, but they are turbo charged... which is a different ball game entirely.

Feel free to pressurize the exhaust and test yourself. You'll find pressure loss of 4-6 psi before/after cat and 1-2 psi before/after muffler.

Also when replacing the cat-back, the big difference is going to nice mandrel bends. Stock crush bends aren't great for flow. For the most part, a muffler is a muffler is a muffler.

[90redmiata]

So your answer to my original question really is that neither the cat nor the muffler are restrictive on an NA but rather it is the pipe between the converter and muffler that is the bottleneck? The power gain comes from replacing the crush bent pipe as there is no power gain from replacing the cat and a muffler is a muffler?

Correct?

[Stealth97]

your thinking way, way too far into this. just pick up a nice catback. I have an enthuza, and its awesome.

you can not expect gains from just replacing one thing, like just that cat, just the piping or just the muffler alone. Everything is part of a system, and its all got to work together.

[90redmiata]

Cool.

[oldgrayfrog]

The 1 hp gain between test pipe and high flow cat, not between I/C was bought off ebay for 150 bucks, piping was another 50 or so . It reduced 260 degree temps at 12psi to 110 degrees, that's 92% efficient. My 155 crank pulley was $200, my 62.5 nose pulley was $100. This took me from 5 psi to 12psi. Most folks say that the 5 psi on an 1.8 NA car is about 150 rwhp, at 12 psi figure about 200-210. We'll see what my car does in about two weeks, that's when DIY will be done and its dyno tuned. BTW it looks like I will probably be going Turbo in the next couple months, so those who are looking for cheap sc stuff stay tuned, all of my supercharger, pulleys, the DDM cold air intake, will be up for sale soon. Nothin like takin a perfectly good car and tearin it apart...again.

[ben91]

So your answer to my original question really is that neither the cat nor the muffler are restrictive on an NA but rather it is the pipe between the converter and muffler that is the bottleneck? The power gain comes from replacing the crush bent pipe as there is no power gain from replacing the cat and a muffler is a muffler?

Correct?No my answer remains that the cat is the most restrictive point of the exhaust system, by far and large. As stated originally.

Your M45 set up won't gain much/anything power wise by removing your cat if it's in good shape. Your M45 set up won't see much/anything power wise by replacing the cat back. Doing both might give you a little bit more. Also getting a header will be a bit more. Maybe 10 whp for header to tip. It'll certainly be louder. Figure that will cost ya $800 for reasonable stuff. You'd be better off putting money in an intercooler and engine management. And then a bigger pulley when you get bored.

[Stealth97]

The 1 hp gain between test pipe and high flow cat, not between I/C was bought off ebay for 150 bucks, piping was another 50 or so . It reduced 260 degree temps at 12psi to 110 degrees, that's 92% efficient. My 155 crank pulley was $200, my 62.5 nose pulley was $100. This took me from 5 psi to 12psi. Most folks say that the 5 psi on an 1.8 NA car is about 150 rwhp, at 12 psi figure about 200-210. We'll see what my car does in about two weeks, that's when DIY will be done and its dyno tuned. BTW it looks like I will probably be going Turbo in the next couple months, so those who are looking for cheap sc stuff stay tuned, all of my supercharger, pulleys, the DDM cold air intake, will be up for sale soon. Nothin like takin a perfectly good car and tearin it apart...again.

no one has ever gotten 200whp out of the JRSC. The blower just can't flow enough.

[ben91]

You can get pretty close to 200 if you put enough $$$ into it. But that same money would get you to 300 with a turbo, and have more power everywhere.

[metalman]

To utterly disregard the cat back is bad advice IMHO.
It's a part of the puzzle when adding performance to a Miata as Stealth indicated.
Obviously any gains will vary with your setup and with the type of cat back chosen and sometimes other changes will indeed net larger gains ie. engine management etc.
The very best design exhaust would likely run straight out the left side of the car from down pipe or header and have no significant turns. As already indicated, crush bends on small piping do nothing good to performance. Pipe size can be a factor too depending on setup. Mufflers can make a difference too, theyre not all the same at all, but again, it depends on the setup.

[90redmiata]

Thanks guys for the feedback and the advice. I'm still learning these cars (in fact, I just started about 5 weeks ago) but I do learn quick. I ask alot of questions, question a lot of answers and ultimately choose what I think will best serve my own personal goals. Thanks for your patience and help along the way.

I always start out with a goal for every car even before I buy it. Your knowledge and experience is certainly appreciated as it will help me make better decisions along the way toward my goal with this car.

Thanks again guys....