2.5" vs 3"

[Jericon]

What are the advantages and such of each? I don't want to just go with a "Bigger is better" mindset and want to make an educated decision about what to get.

Short term (< 6 mo), I am staying NA and looking to redo the entire exhaust. My Long term plan is to Supercharge, so I want something that will work well with that application.

[ecko04]

Originally Posted by Jericon

What are the advantages and such of each? I don't want to just go with a "Bigger is better" mindset and want to make an educated decision about what to get.

Short term (< 6 mo), I am staying NA and looking to redo the entire exhaust. My Long term plan is to Supercharge, so I want something that will work well with that application.

http://bit.ly/1dmCBhN

http://www.ft86club.com/forums/showthread.php?t=45645

http://www.ft86club.com/forums/showthread.php?t=49972

http://www.ft86club.com/forums/showthread.php?t=48704

http://www.ft86club.com/forums/showthread.php?t=44562

http://www.ft86club.com/forums/showthread.php?t=47919

http://www.ft86club.com/forums/showthread.php?t=40715

http://www.ft86club.com/forums/showthread.php?t=49966

http://www.ft86club.com/forums/showthread.php?p=1155442

[Jericon]

Originally Posted by ecko04

http://bit.ly/1dmCBhN

Yup... didn't even think to search. Thanks.

[ecko04]

Originally Posted by Jericon

Yup... didn't even think to search. Thanks.

It's all good. 2.5", 2.75" and 3" have been discussed pretty extensively.

In a nutshell, for the NA vehicle 2.5" is fine.

Any form of FI will benefit from larger diameter piping.

If I were even thinking about FI, I would go with 3" or at the minimum a 2.5" overpipe, larger and less restrictive front pipe and a 3" catback section.

Then it's a matter of what sound you like.

The benefit of larger diameter piping with a FI setup is that you're adding more exhaust gas and it needs to go somewhere. The larger piping keeps excessive exhaust gas temps down and will build boost, on a turbocharged setup, quicker than smaller diameter piping. There may be some weight savings as well. Oh...and it sounds cool.

[boredom.is.me]

2.5" vs 3"
She's going to be disappointed either way.

[FR-Sizzle]

Perrin recently did a blog post about there 3 inch system vs the 2.5 inch. Go with 3 inch whether you go FI or not. There dyno's during there R&D showed that the 3 inch system made more power than the 2.5 inch.

2.5 inch is good if you want to keep the sound down as the 3 inch system is a bit raspier. I personally would go with the 2.5 inch system if you decide to get headers front/over pipe in future since headers make the exhaust VERY loud. If you decide to just keep the headers stock but change the front/over pipe I would go with the 3 inch system as it will be slightly louder than the 2.5 inch system but wont be too bad since you dont have headers.

[ZionsWrath]

Originally Posted by boredom.is.me

2.5" vs 3"
She's going to be disappointed either way.

This thread is in regards to diameter not length...

[boredom.is.me]

Originally Posted by ZionsWrath

This thread is in regards to diameter not length...

Who would have guessed. I'm sure a lightened up the day for a few people. Something funny never hurts.

[ZionsWrath]

Originally Posted by boredom.is.me

Who would have guessed. I'm sure a lightened up the day for a few people. Something funny never hurts.

I'm just sayin, diameter matters more than length.

[boredom.is.me]

ah, I see what you did there.

[ramiram1984]

Originally Posted by ecko04

It's all good. 2.5", 2.75" and 3" have been discussed pretty extensively.

In a nutshell, for the NA vehicle 2.5" is fine.

Any form of FI will benefit from larger diameter piping.

If I were even thinking about FI, I would go with 3" or at the minimum a 2.5" overpipe, larger and less restrictive front pipe and a 3" catback section.

Then it's a matter of what sound you like.

The benefit of larger diameter piping with a FI setup is that you're adding more exhaust gas and it needs to go somewhere. The larger piping keeps excessive exhaust gas temps down and will build boost, on a turbocharged setup, quicker than smaller diameter piping. There may be some weight savings as well. Oh...and it sounds cool.

The only thing that confuses me is that, yes the diameter is larger so exhaust can flow better.
But what about where it leaves from? The diameter of that is still smaller than the larger piping header if you have one. So, the exhaust must just bottle neck somehow, no matter if it's FI or not.

This is the only thing that confuses me.

[FR-S Matt]

Running my 3", the only thing I am dealing with is the loudness and some droning on N/A. If anything, I'll be dynoing soon to see how well it does perform on N/A. If anything, the exhaust feels great during WOT like the engine has all kinds of freedom, but I really think that's the header doing that more than anything.

[ecko04]

Originally Posted by ramiram1984

The only thing that confuses me is that, yes the diameter is larger so exhaust can flow better.
But what about where it leaves from? The diameter of that is still smaller than the larger piping header if you have one. So, the exhaust must just bottle neck somehow, no matter if it's FI or not.

This is the only thing that confuses me.

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.