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Technical Exhaust Question

Darren_S15

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Ive been trying to find out a bit of info on exhausts but cannot really find anything so thought some of the clever people on here can help.

Obviously everyone goes on about getting 3" bore exhaust pipes for the Spec-R.

But my questions is how much does a reduction in the exhaust pipe reduce bhp?

For example if you have a 3" cat back, but it reduces to say 2" at the muffler will this seriously reduce the total bhp of the car?

All views welcome...

Thanks,
Darren
 

DeanS15

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hey darren, i hadnt noticed until an exhaust maker pointed out to me - most jap systems, despite being 3" bore pipework, reduce to 2.5 in the rear can anyway before exiting via a 4/4.5/5 inch exit....
 

Darren_S15

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Yeah I did wonder if other exhausts reduced in size in the same way to silence the sound.

From what I can find the smaller the bore the less power gains you are likley to see.

If this affects torque I cannot find?

Suprised there is so little about this stuff on the net though.
 

Nicely

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A 2.5" system is usually more than adequate until you get into big figures. The OEM JDM S15 exhaust is actually a hell of an improvement over the Euro S14 system. That one had a horrendous U bend before the backbox and was only about an inch or so in diameter!
 

Darren_S15

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Ok, lets say that I have 3" cat back that reduces to 2" for about 30cm at the rear muffler, would this be enough to stop me putting out the usual 330 bhp people get at stage one?

Or is it possible that because this is only a small section it will have very little effect on the exhausts overall performance?
 
T

tmus

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i don't reckon it will affect so much as it is so far down the pipe, say like the closer to the engine/turbo the greater resistance.

and if it is a cannon type, i would say it is so near to the atmosphere it won't really restrict flow.

OR maybe having the narrow bit at the end helps accelerate the exhaust gas out of the end can into the atmosphere, since the exhaust will have lost some of its heat and flow rate?
 

_Wing_

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The narrow part will affect the flow of the whole system no matter where it's placed, whether it's in the front pipe or on the tip of the exhaust. The longer the length of the reducing pipe the worse it is and it is an injustice to you and your turbo by reducing the diameter, especially for noise reasons.

You're better off keeping it 3" all the way and adding a few "hot dogs" or resonators in the system because this will both achieve good flow/ good power and lower noise :)

And as some already know, some jap systems have the reducer piping snuck in somewhere... usually going into a resonator, the resonator itself or into the muffler/muff itself - these are the systems that are usually "jasma approved".
 
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