Do I need an over pipe?
 

I'm currently running Tomei UEL Headers, HKS Front Pipe and Perrin 2.5 Resonated CBE along with OFT Stage 2 e85 and I was wondering if it was necessary for me to get overpipe alone with the rest of my set up? And if it is necessary how come? Sorry I'm kinda still learning about some of this stuff. Thanks Ahead of Time!

I've been doing some digging on this too.

The biggest factor in extracting more power in the exhaust is obviously getting rid of the restriction. This would be at the cat, which you got rid of both at the header and the his dual resonated front pipe.

Secondary to this, inner diameter, the smoothness of the path, etc would play a contributing factor too. You wouldn't want a rough surface, or very sharp bends causing excessive pressure drop across the path.

That being said, the stock over pipe is a smaller diameter than what you have before and after, which are both 2.5 inches. There would be a sharp reduction in the pipe diameter. Thinking in extremes, it wouldn't be a good thing if you replaced the over pipe with something as thin as , say, a straw. Simple fluid dynamics in the end.

That's all in theory, in actuality, the penalty in power, especially for a naturally aspirated engine, that you pay for having the stock overpipe vs. the cost, I think would be negligible. The overall volume of air flow for a 2 liter engine is something to consider as part of the equation.

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If your exhaust goes from header to muffler then you should be good. I would be concerned with pipe size, but I think it's 2.5" anyhow so you should be good. Is the op the one with the cat? If yes, you can get an after market op to remove or upgrade the cat.

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Awesome I really appreciate this indepth explanation! Yeah I was just wondering because a lot of people say that if you don't have the over pipe that it chokes up all the air that the Header is bringing in to the rest of the exhaust system. Thats why I thought I'd ask and see what peoples opinions about this were. Thank you I really appreciate it.

You're welcome!

I would say change it out when you eventually go forced induction? Or if you could get a good deal on something nice used? If budget isn't an issue, you wouldn't be hurting anything with getting it right now rather than later right?

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You'll feel a difference with a header even if you don't replace the rest of the exhaust.

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