3 1/2 inch exhaust (1 Viewer)

[CruisinEveryDay]

Hey guys I have a buddy with a 97 who wants an exhaust system. He has EMS Powered 2'' down pipes in hand and is debating between a 3'' and 3 1/2'' exhaust system. Purely from a power standpoint would the 3.5 have enough back pressure or would it loose power. I know that on a NA setup a 4 inch system will loose torque but want to know what you guys think about the 3.5.
I know 3 inch is well known to work perfectly on NA setups and a lot of people use 3 inch on here....
I am aware that the 3.5 will be louder but he would rather have that then a 3'' for the extra 5-10 hp that could be picked up.
Oh and he will be running without cats so the only thing giving the system back pressure after the merge will be the pipes and free flow muffler.
Thanks!

 

[Bear80]

A few pointers.

- Run a f'n a cat. Random Tech makes some great cats that are great flowing, or any brand he can afford. I cannot believe the ignorance of "hp robbing cats" still exists.

- 5-10 hp is what 5% increase? LOL how much is that new exhaust going to cost for that much added power? Where did the thought of a larger pipe makes more power come form? This isn't a Diesel. Which leads me to the next point.

- Physics: Colder air is heavier and hotter air is lighter. Make the pipe to big with cheap mild steal and the exhaust cools while in the exhaust system and actually causes a loss of power. This is common knowledge in performance / race cars.

If he's dying to tell people the size of his exhaust no one can help but otherwise go with a 3" and a high flow cat.

 

[thatvan]

Word up Bear! This is one of the basics.

Here's some learn'n for ya:
http://www.nsxprime.com/nsx-faq/exhaust-theory/
http://www.gomog.com/allmorgan/exhaustbackpressure.html

 

[CruisinEveryDay]

Cats do decrease flow...
I'm aware of how exhaust works! Don't talk to me like I drive a Land Rover Bear! haha
The hotter the air the faster the exhaust velocity...
If he is paying to have the exhaust put on it not much of an issue price wise going from 3 to 3.5...

You are right, Care Bear, larger diameter pipes don't always make more power which is why I asked the question to where that limit stops!
So in a nut shell you have concluded that in your professional experience 3 inch is the limit with a NA 4.5 (we all know a turbo 4.5 can handle 4'' haha)?

Give me a simple answer to the question: Will a 3.5 inch system, merging from 2 inch down pipes, create enough back pressure or loose power?

 

[scottryana]

I don't think you will gain much on a NA truck going bigger than 3" but the dual 2" will be your restriction

Dual 2" is 6.28" of area while 3.5" is 9.6"

 

[CruisinEveryDay]

You convinced me to go with 3''.
I don't understand what you mean though?

 

[ajax1]

By going from 2 to 3 or anything bigger you are creating a divergent duct, divergent decreases velocity and increases pressure, we do this in planes to raise pressure for compressors most except for supersonic planes use a convergent exhaust to use velocity to get the hot gases out of the engine and clear the airframe, this all goes back to bernellis principal, as far as a car going to 3.5 inch exhaust will do is make you look fast and furious

 

[084runnerltd]

I am not claiming this is true, but I was told that if you start with 2" downpipes, attaching pipes any larger throughout the system has no positive impact.

If you run a water out of a standard garden hose and then attach a 3.5" hose....it doesn't get the water out an quicker.

Just reduces back pressure.

Again, not claiming this is accurate, heard it in an automotive class years ago (not from the teacher lol)


...via IH8MUD app

 

[CruisinEveryDay]

Ok great 3'' it is!
But I still don't understand what scott meant when he said "
I don't think you will gain much on a NA truck going bigger than 3" but the dual 2" will be your restriction
Dual 2" is 6.28" of area while 3.5" is 9.6""
Thanks guys

 

[Douglas S]

Area of a 3.5" circle = 9.62 in2
Area of a 3" circle = 7.06 in2
Area of a 2" circle = 3.14 in2

 

[scottryana]

Exactly, area of the dual 2" y-pipes totals 6.28in2, the area of a 3.5" catback/midpipe is 9.60in2, so it isn't the exhaust behind the Y-pipes that will be restrictive it is the y-pipes.

 

[CruisinEveryDay]

Perfectly explained! Got it now!

 

[lugueto]

what mods does the engine have that require such a large exhaust?

 

[half k cruiser]

What scotty is trying to say is you need to remove those downpipes. In order to get maximum horsepower your going to want the exhaust to exit right out of the manifold.

 

[scottryana]

Haha no that's not what I am saying lol. The only time you need to run a huge exhaust is on a turbo car or a car with a huge cams that's looking for peak power up top. On the 1fz 3" would be plenty 3.5" would be overkill unless you're thinking forced induction down the road.