Need help in trying to build exhaust

[coderedk]

i want to go to a muffler shop to get a custom exhaust. I dont know too much about it but i want to know what to say to the technician. I guess im looking for the biggest piping with most airflow to get maximum hp and a nice deep tone. ALso ive heard something about a restricitve flat piping on the coupe?how would i get rid of that? thanks guys any help is greatly appreciated

[el_blacky06]

all you gotta do is look under the stock exhaust and right away youll see the flat part half way to the exhaust. idk exactly where it will be on the coupe but its very noticable. just tell the welder that you want a custom exhaust and tell em wat kind of exhaust you want like lets say magnaflow, flowmaster, etc. if they have the piping then they should be able to do it no problem.

[dldjros69]

You dont want to go to big, but you dont wanna go to small. Its all about back pressure and what not. Low end torque vs high end power.

Somebody will chime in here but its something like 3.0 inches for a turbo application and 2.5 inches for a naturally aspirated engine. Something like that.

I definitely want to get rid of that flat spot though, that ish looks ugly

[el_blacky06]

You dont want to go to big, but you dont wanna go to small. Its all about back pressure and what not. Low end torque vs high end power.

Somebody will chime in here but its something like 3.0 inches for a turbo application and 2.5 inches for a naturally aspirated engine. Something like that.

I definitely want to get rid of that flat spot though, that ish looks ugly

[ItsEcko06]

Somebody will chime in here but its something like 3.0 inches for a turbo application and 2.5 inches for a naturally aspirated engine. Something like that.

i've heard the same. what size is stock piping though? 2.25"?

[coderedk]

What if I got 3 inch pipe but my car isn't turbo? IM going for a deep bass tone rather than just loud raspy tone

[dldjros69]

What if I got 3 inch pipe but my car isn't turbo? IM going for a deep bass tone rather than just loud raspy tone

You would loose a ton of torque. Im sure the tech is gonna tell you the same thing. Instead of big you want to go even. You will have a header and then just delete the cat and the resonator or one of the last 2. That should help with the sound.

[Hussain]

the cat has a sensor next to it so it'll throw a check engine light on the 2.5s if you take off the cat. I was under aaron's car (chiapetchua) a while ago. honestly, you really really do not want to go bigger then 2.5 pipping. look at it this way, the V6 altimas dont even use piping bigger then 2.5". on V8's they use true dual exhaust with 2.5" OR 3". like its been said before, you will gain HP but lose torque.

this means that if you go with big pipping, your car will be dramatically SLOWER! then once you get hella high in the RPMs it will be a decent amount faster. honestly just go 2.5" and get the stillen headers if you want maximum sound....

OH YA!!! if you want a nice sound, you could try a resonator instead of a muffler. that will give you a good flow and give it a really deep sound

[el_blacky06]

actually no its not gonna throw the CEL if you take off the cat. i took my cat off 2 weeks after i got my car and all i did was did a custom downpipe/straight pipe 2 1/4 from the headers and did a hole on the downpipe to keep my sensor on. i could take pix to see how it looks like to help out

[coupecrazy1232]

actually no its not gonna throw the CEL if you take off the cat. i took my cat off 2 weeks after i got my car and all i did was did a custom downpipe/straight pipe 2 1/4 from the headers and did a hole on the downpipe to keep my sensor on. i could take pix to see how it looks like to help out

could u post a vid of how it sounds? i have a 2.5 and am wanting to go this same route! thanks

[Hussain]

actually no its not gonna throw the CEL if you take off the cat. i took my cat off 2 weeks after i got my car and all i did was did a custom downpipe/straight pipe 2 1/4 from the headers and did a hole on the downpipe to keep my sensor on. i could take pix to see how it looks like to help out

ya I didnt think about it. if you know how to mess with the sensor then ya you could do that.

[el_blacky06]

YEA SURE THING ILL POST IT UP TOMORROW SINCE ITS ALREADY DARK OUTSIDE

[coupecrazy1232]

YEA SURE THING ILL POST IT UP TOMORROW SINCE ITS ALREADY DARK OUTSIDE

thank u very much!!

[el_blacky06]

http://www.youtube.com/watch?v=7q-S2qedhR4

[dldjros69]

Pics of all the pipes if possilbe?

Did you take the resonator off too?

Does it sound deeper in person?

[coupecrazy1232]

thank u very much!!

[el_blacky06]

it sounds deeper and back fires more in person so most of the time i gotta keep it in low rpm just to keep it quiet if theres a cop around lol the hotter the engine is, the more it will back fire. and ill upload the pix later on

[el_blacky06]

http://www.youtube.com/watch?v=dWKhTxNloRs

heres the vid i just found from my old vids of straight pipe cat delete and no resonator. i only had a short ram intake and stock headers at that time i love the sound of it!

currently i have the intake, stillen headers, stillen pulley, and cat delete. i put back my resonator a month ago when i got pulled over and got a ticket for my exhaust "being too loud".

[dldjros69]

Wow that sounds really good

[XXplosive990]

how much would a custom exhaust cost? oh and blacky did you get your exhaust done in fontana? if so where at?

[el_blacky06]

a custom exhaust may vary by the brand of exhaust you want like lets say magnaflow for your car is gonna cost you about 250 to 300 bucks at the most. and i got it done nearby my house on whittram(i think thats how you spell the st. name) and cherry nearby the california speedway. the place is called ralphs muffler. they do good jobs there i reccomend you to go there

[XXplosive990]

lmfao!!! ralphs muffler is exactly where i was thinking of going. most of my team goes there to get their exhaust done. and yeah, i've heard good things about that place

[el_blacky06]

team? just curious is your team called team nemesis or am i mistaking your car. and yeah i wont be suprised if everyone goes there for their exhaust lol

[XXplosive990]

lol yep yep thats them...you've seen my car around?

[el_blacky06]

oh alright i only know Luis with the white srt4 and yeah i believe ive seen your car twice at his house you have a team nemesis sticker in your front windshield right?

[XXplosive990]

haha luis with the srt is my cousin.

[el_blacky06]

oh what i had no idea lol tell em i said wats up! and do you guys still go to the meets every friday on pepper by the 10 fwy?

[XXplosive990]

nah, cops stopped those meets. now we're looking for a new place to meet up

[el_blacky06]

coderedk: so did you end up getting a custom exhaust done to your coupe?Xxplosive990: ahh that sucks. the last time i was there was like more than 5 months ago when my friend with a g35 almost got beat by that 08 civic si. that car is a beast to be keeping up with my friend

[LongBeachCoupe]

The survey says.....

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.