Performance exhaust sizes

[AeroV]

Does anyone know what's Saab's reasoning behind sticking to using 2 1/4" exhaust even in 260hp Aero?

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[Naranto]

I understand that it is the downpipe that makes the difference. The size of the remainder of the exhaust has no effect on performance. A friend of mine (SAAB garage owner and well known track driver) has a SAAB 9.3 track car with large downpipe, which has the standard exhaust fitted. Pushing out well over 400bhp it goes rather well, as many a Evo and Subaru owners will testify.

[ndale1000]

Quite possibly correct it is a turbocharged engine consequently exhaust tuning is less important

in a. Non turbocharged engine exhaust tuning is very important because the pulses leaving engine having gone down the pipe can help pull out those that follow

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[Digger]

When it comes to free flowing exhaust a mandrel bent 2 1/4 in will give you better flow than a standard bent 2 1/2 in. A standard bent 2 1/2 in pipe will give you better flow than a standard bent 3 in pipe.

When it comes to street legal the catalytic converter sets the flow.

[chalpkin]

When it comes to free flowing exhaust a mandrel bent 2 1/4 in will give you better flow than a standard bent 2 1/2 in. A standard bent 2 1/2 in pipe will give you better flow than a standard bent 3 in pipe.

When it comes to street legal the catalytic converter sets the flow.

Are you sure a 2 1/2 inch exhaust will flow better than a 3 inch?

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[Digger]

Are you sure a 2 1/2 inch exhaust will flow better than a 3 inch?

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Absolutely certain.
Only a mandrel bend will maintain inner diameter. The standard pipe bending machine as found at most custom exhaust shops makes a radios bend without inner wall support and when you make a right angle radios die bend in a 3 inch pipe, you end up with less flow than you would if you had used a 2 1/2. To have better flow with a 3 inch than a 2 1/2, every angle needs to be mandrel bent.

[chalpkin]

Absolutely certain.
Only a mandrel bend will maintain inner diameter. The standard pipe bending machine as found at most custom exhaust shops makes a radios bend without inner wall support and when you make a right angle radios die bend in a 3 inch pipe, you end up with less flow than you would if you had used a 2 1/2. To have better flow with a 3 inch than a 2 1/2, every angle needs to be mandrel bent.

In your post, you say that a standard bent 2 1/2 inch exhaust will flow more than a standard bent 3 inch, Sorry, I beg to differ, the 3 inch will always move more gases in a comparable standard bent system!

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[Digger]

In your post, you say that a standard bent 2 1/2 inch exhaust will flow more than a standard bent 3 inch, Sorry, I beg to differ, the 3 inch will always move more gases in a comparable standard bent system!

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Mathematically you are absolutely correct, but we aren't talking about performance exhaust for a Mack Truck or a motor home where you have room for a four foot radios on a 90 degree bend.

This is a link http://www.trick-tools.com/Huth_2008_Bender_HB2008_2282 to the type of bender found in most custom exhaust shops, this is the industry standard, top of the line, for custom automotive exhaust work and this does not do mandrel bends. Make a 90 degree bend in a piece of 3 inch, bend a piece of 2 1/2 to the same radios, turn on the fan and start making measurements.

[AeroV]

When it comes to free flowing exhaust a mandrel bent 2 1/4 in will give you better flow than a standard bent 2 1/2 in. A standard bent 2 1/2 in pipe will give you better flow than a standard bent 3 in pipe.

When it comes to street legal the catalytic converter sets the flow.

So, would it be safe (not the efficient, safe) to run a ~290hp tune on stock exhaust? The key hardware requirement for that amount of power is considered as the 3" downpipe but if the stock exhaust is going to flow better it should be fine?

[Digger]

So, would it be safe (not the efficient, safe) to run a ~290hp tune on stock exhaust? The key hardware requirement for that amount of power is considered as the 3" downpipe but if the stock exhaust is going to flow better it should be fine?

That really depends on what you plan to do with the extra horsepower. Everybody has their own idea of normal driving, so if you plan to continue your normal driving with the occasional tromp on the foot feed for passing your exhaust should "safe" from frying your valves. If you're asking about red line racing, legal or street, that's a whole different dog.