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Discussion Starter · #1 ·
This is probably a dumb question, but is a bottleneck at the rear of an exhaust just as bad, not as bad, or worse than one much further up?

I figure it is probably the same, but I know that before here I was told that only the header and a little pipe after that is what is really important, the rest after that is just for sound insulation etc. Most likely that doesn't address the bottleneck issue though.
 

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Since the exhaust gasses would be traveling much faster coming out of the engine, and would be much slower farther down the b-piping, it might not be such a bad thing if the restriction was minimal, as it would speed up flow velocity.
 

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to properly speed up the velocity, you would want an hour-glass shape inside. This acts as a venturi-
as the gasses speed up they create a low pressure area in the center, which aids in "sucking" the higher pressure (slower moving) gasses across the restriction. anything else would actually create a seperation bubble and choke off the flow. the faster the gasses cross the restriction, the bigger the bubbles become and the more the flow is choked.

So as MD said, the closer to the engine it gets the more it will restrict flow.
 

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So how much would you want to decrease the diameter to create this hour glass effect? I know it will be different for different setups but if you had to make a "guesstimate" how much would you say would be sufficient on let's say an otherwise stock GSR?
 

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You wouldn't want a venturi in place of bottlenecks. you would want the venturi at the Merge collectors in the header.

(pics borrowed from this article @carcraft.com)
side by side comparison


Inside with venturi:


Inside straight thru:



Another example would be in a stepped header design, the steps would incorporate a venturi.

anywhere else in the system you should eliminate bottlenecks if possible.
 

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Discussion Starter · #8 ·
Thanks guys, some very good information. I am also glad to hear this because my axleback portion is sh*t, pretty far from the engine. I've been trying to replace it for a bit...
 

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How come the ITR (2.25") exhaust has a two 2" bottlenecks? one right after the cat, and one right before the muffler? Don't you think Honda's engineers would have figured this out when they designed the USDM ITR?
 

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ive heard a lot of opinions and seen alot of writeups on things like this, and theres alot of conflicting information going around.
 

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The restriction on the itr exhaust is the muffler not the 2" bottlenecks.

I would think you would want to slow the velocity of the exhaust before it reaches the most restrictive part. The 2" bottlenecks even out the speed in the exhaust system.

It's like everyone running to the exit in a fire drill. lol
 

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kinsinger on Nov/02/08 said:
The restriction on the itr exhaust is the muffler not the 2" bottlenecks.

No, it's the bottlenecks.
 

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Sevendale on Nov/01/08 said:
How come the ITR (2.25") exhaust has a two 2" bottlenecks? one right after the cat, and one right before the muffler? Don't you think Honda's engineers would have figured this out when they designed the USDM ITR?
Maybe because they're a car company like any other car company, and tooling up and using different part numbers constantly in your assembly line increases costs. The ITR is really cobbled together from a few other cars. The head, etc. Spec'ing out, contracting, procuring, and manufacturing specialized parts costs a lot of money.

The ITR was a limited production car, but at the end of the day, it was an Integra. They couldn't do every little thing to squeeze out every little horsepower, or else it would have costed 40 grand, and nobody would have bought it.


Sometimes the simplest answer is the most correct.
 
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