[91TB78]

I've heard many good things about the STi muffler/exhaust as far as power/exhaust flow, but I have not heard much about headers on either the WRX or STi.

Would adding a set of nice headers help out the STi and is it worth the cost? I've always been under the impression that exhausts typically don't add that much (Non aspirated cars particularly), but that headers add significant power and torque.

What's the word?

 

[jbhebert]

In an ideal world, equal-length (all runners are the same length) headers will add high-en HP at the cost of some low-end torque. Torque is often helped by having some back-pressure in the exhaust. However, we don't live in an ideal world and have to deal with cramped engine compartments and convoluted turbo plumbing. This makes it hard to create a header that flows much better than the factory manifolds. I would say in the case of the Subaru, you'll find better gains in a better flowing up-pipe like the Vishnu 321 or similar.

 

[Agent Chobos]

levelling required by law...

Torque is often helped by having some back-pressure in the exhaust.

Actually, this is a pretty common myth. Torque is increased by improving the velocity of intake/exhaust gases. Exhaust gases will generally flow faster through a smaller diameter pipe (like the river analogy; fluids speed up when the channel narrows). So a bigger exhaust relieves back-pressure, which is good for top end, but at the low end, the pipe is too big for the gases to get going very fast. The effect of a smaller pipe is faster flow and therefore better torque, but it's not the back-pressure that does it.

It all depends on the ultimate goal. Huge power with a turbo engine will almost always require large diameters. Witness the insanity of the Dahlback Golf: 4.5 inch (inner diameter) exhaust, 900 hp from a 2.1L at 45 psi. Not exactly streetable.

 

[Agent Chobos]

Oh yeah, back to the original subject. HKS makes headers for the current WRX:

http://www.hksusa.com/images/?pid=&id=1304

Problem is they're very expensive and I doubt they're street-legal. Also I think they are designed for use with bigger-than-stock turbos.

 

[91TB78]

I originally asked b/c at www.vividracing.com I was just checking out some products and I couldn't really think of much to add to the STi (down the road of course). The WRX has several nice headers out for it, so I am sure the same can be said for the STi with enough R&D.