Pretty minimal, maybe 2-3hp if you're lucky. Changes the sound of the exhaust as well. If whereever you are doesn't require emissions testing, I say do it.

 

Any performance gains seem like they would come from the engine being able to 'breath' easier. Same as putting on a 3" pipe and 6" fart-can. There's some formulas floating around (but I don't know them off-hand) regarding how much flow for your engine you should have. Just like too much back pressure can choke your engine, not enough is nearly as bad. As the exhaust gasses cool, they slow and become more dense. That can cause a restriction just like too narrow an exhaust. A smaller pipe, or even the cat help keep the pressure on, the heat up and those gasses moving fast enough. Very much a balance.

 

As Tanya mentioned: Emissions testing Yes/No?

 

emissions testing or not, I probably would remove it unless it was a race car. gains are minimal as stated. there are plenty of other ways to make more horsepower legally and without destroying the environment before you remove the cat.

 

I gained 6whp on my nissan sentra 2.5 liter 4 cylinder just from removing the cat....enough of a difference that I coule feel in the seat of my pants. Having more exhaust pulses per revolution means extra pressure in the pipe, so I'd be willing to bet you'll see more than 6whp in a 2.8 supra.

I say do it.

 

I thought you liked cats ?? :zzzzz:

Do IT!!!

 

Only the hollowed out shells of the cats

 

Nope Im lucky, no emmissions testing (woot!), and I understand what you guys are talking about with the exhaust velocity, less restriction isnt exactly better, But I figure the cat would only marginally affect the backpressure. My next question comes with the muffler...

Next year, when I finish school I want to go turbo, my friend can get me a deal on a 7m-gte so Im going to look into that. Anyways I want to buy the muffler now so I can have a little more sound from the 2.8, BUT I dont want to have to buy a second new muffler down the road when I go turbo. My question about the muffler is: Can I buy the 3 inch magnaflow Ive been eyeballing and use that off the 2.8 without any losses? The gains in this case doesnt really concen me, because my goal is to go turbo. Is the muffler too unrestrictive? Especially if I remove the cat?

OR

Do I keep the cat and change the muffler so I can keep some backpressure?

Also, the car wont be raced, per say. Maybe some night meets at the drag strip, what Im saying is that the cars purpose is not to be a racecar.

 

4 stroke engines do not require backpressure. I don't understand why this false statement pops up so often. 2 strokes require backpressure for proper operation, 4 strokes require velocity. You DO NOT WANT BACKPRESSURE IN 4 STROKES....just keep repeating that to yourself until it's burned into your memory!!!! When you see people say they didn't have enough back pressure and they list power because they put too large diameter of an exhaust (like a 4" exhaust on a honda civic), it's because when the pipe diameter is too large, the exhaust creates vortices around the outer part of the pipe, which in turn creates backpressure and reduces the exhaust velocity, also defeating any scavenging affect which comes from high velocity.

Be reducing/eliminating restrictions and keeping the exhaust tubing the optimum size for the highest velocity, you WILL GAIN POWER.

 

Large diameter exhaust pipes can cause reversion, which to the untrained ear will sound like backfiring. You have to find the happy medium where you get maximum velocity and minimum backpressure or restrictions.

First, 2.5 inches and up - is big for Normally Aspirated 5M or 7M. Stick with 2.25 It will keep your exhaust velocity high (a good thing), but still give better flow than the stock setup, with less backpressure (also a good thing).

Second, no backpressure is not a problem. You want as little backpressure as possible in order to get the most performance. However, most people reduce backpressure by getting bigger pipes, which like was said above, lowers the exhaust velocity and can actually hurt (robbing the low-end torque) you more than it helps for NA setups. For turbo setups like you are planning, don't worry about loss of torque because you're wanting to spool up quicker and negate that problem.

Third, (this is for another noob who asked a similar question to yours a while back) you will not backfire alot. Backfiring has nothing to do with your exhaust, headers, etc. You will get more reversion with lower backpressure, which makes a popping/gurgling sound when the cold air flows back up the exhaust pipes. This will occur even more if you run open headers.

If you are designing an exhaust system for NA setup, stick with 2.25" piping, or 2.5" MAX, and you will need a decent muffler unless you enjoy being harassed by the authorities. Add in a resonator in the middle and it will be loud enough, yet wont be a cop magnet. No backpressure is only a problem if you do so by putting bigger pipes throughout your exhaust system, because then the exhaust gases aren't able to move through your exhaust system fast enough. Yes bigger pipes will reduce backpressure, but it will rob you of lots of torque (especially in the low rpm's) in the process.

Open headers would be ideal, but the sound level is loud, and the reversion I spoke of causes damage to your valves. So while it is ideal for 0 zero backpressure, it causes other issues. Basically you need to find the point at which back pressure is minimized with the smallest amount of side effects. You want MINIMAL backpressure while MAXIMIZING exhaust velocity. That's the bottom line. Though unless you have $$$$$ to try out a million different designs to get it exactly right...just stick w/2.25" pipes for lightly modified M-series engines (I/H/E, equivalent, and less), then 2.5" for heavily modified N/A (or if you're looking to be loud) and 2.5"+ for Turbo.

 

You may do more harm than good just hollowing out a cat. Replacing it with a straight pipe is good.

 

Agreed. Hollowing out the cat actually makes a chamber where the exhaust can gather and create back pressure.

 

Great info, I was always confused with the larger diameter / sacrificing velocity deal, Now I get it. But in the case of my car, if I lose the cat, keep all the stock sized pipes (for now) and reduce the 3" magnaflow I want to the stock size diameter I should be in the clear? Like I said, I want the muffler now, but dont want to buy one to fit the stock pipes and then upgrade to a 3 inch later, I figure Ill just do it now, one thing already checked off the list.

sound good?

 

Yes, that will make power and sound nice.

 

I think you would be able to convince me to do anything :kekekegay LOL What muffler are you running on your car anyway? it looks slick

also I googled the topic quickly and got this, just like you described in the earlier post. Another question: according to this, if the car DOES start to run lean (lets just say) the computer should adjust the fuel mixture accordingly to compensate for the extra air being pulled in, if what this article is talking about applies to this situation?

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

 

Except that's it's louder than I'd like, it sounds GREAT!!!! Very sexy deep tone. I just painted it black the other day too....I''ll post pics once I roll it back out of my shop.

Here's a short vid with the sound of the car.

 

whats hilarious is that I have already seen that video, I LOVE IT, when my friends ask me what kind of potential an MKII supra has I show them your videos, I saw the ones where you did a donut and then booted it out on the street too. Amazing ride you got there

But I mean, normal driving, does it resonate like hek while your driving, can you hear the radio? I want it loud and deep when I step on it, but I dont want it to be obnoxious at highway speed, does it provide?

 

Yeah, I was looking for that one and couldn't find it....lol, I think I have it on my myspace vids. Too many places to keep vids and pics these days!!!!!

 

http://vids.myspace.com/index.cfm?fuseaction=vids.individual&VideoID=8783969

LOL


Sounds insane at WOT, but is it loud on the highway? If I put a resonator in place of the cat, would that tone it down and would I still have flow?

 

http://vids.myspace.com/index.cfm?fuseaction=vids.individual&VideoID=8783969

LOL


Sounds insane at WOT, but is it loud on the highway?

Not so much on the highway as when I'm accelerating....even if I'm not accelerating hard, it's a bit loud. Once I'm at cruising speed, it's not as loud.

 

I put 3" from the exaust manifold out with a 3" inlet 4"outlet can from hks and felt a hell of a diffrance espessialy in the top end and it sounds great if u watch my video at the bottom of my post u will see

 

Nice, I like where this is going. Soon Ill be under the car, cutting the cat off and snooping ebay for a deal on a magnaflow... very nice

bradmk2drifter thats the intake making the whooshing noise eh? If so what have you done to it, pipe with cone? Are you stock 5MGE or have you done something a little special?

 

it is a turbo 5mge check out my post http://forums.celicasupra.com/showthread.php?t=37811
this will show you what u need to know

 

do those ebay lights cut like real projectors? Where do you get what people call "real" projectors for the MKII anyways?

 

no they dont they are a big circle and i dont know were to get the real deal

 

people talk about them but Ive never seen any links or pics on peoples car. Im looking to ditch the stocks casue they suck. A lot of people suggested Hella E-codes and the cheaper Neolights versions, thats most likley what Im going to get, but its cool to find out there is still other stuff available

 

I don't have issue with my (hollow) cat, and the sound is very nice, but a tad too loud. E-codes are very good too (see my sign)

 

Hey thats the first time I've seen your car domain, I found the link in your profile, your car is so clean, I love that color! And Id also love to have my ass in those leather seats!

As for the loudness, do you have an aftermarket muffler too?

 

2.5in Brüllen from Toronto. Unfortunatly out-of-buisness

but now, you can have a Raptor Racing one:
(still available, $$ slightly higher than the GB)
http://forums.celicasupra.com/showthread.php?t=25004