3" Overpipe in a mostly stock exhaust...
 

Before you start throwing full wine bottles at your computer screens, yelling at me as to why I shouldn't be putting a 3" overpipe in my mostly stock exhaust, let me explain my reasoning.

I can't/won't do headers, or a front pipe. I need my car to pass inspection with zero doubt or worry (I'm in NY and they plug the cars in. If you don't pass a plug-in, they don't offer a sniff; you fail), as well as be good at my dealer if I need warranty work. Therefore, the stock cats stay in place.

However, I believe there is a gain to be had in the overpipe, for someone in my situation. Air moves from high pressure to low pressure, right? That being the case, wouldn't increasing the volume in the overpipe (By a ridiculous amount with a 3" overpipe) expedite flow through the first cat, as well as into the second cat?

It just seems like an obvious place to make a decent gain in an otherwise stock exhaust. I can't imagine it reducing power since the piping everywhere else is stock (Save for my trackpipe).

The Cats will still restrict the same amount. There is no going around it other tan replacing them... Not worth it.

I sort of knew I was going to get that answer :/


Thing is, a 3" pipe is nearly 150% of the cross-sectional area of 2.5". That's a pretty substantial difference to accomplish absolutely nothing. That, and there are apparently no dynos proving one way or the other.

Why do you think that increasing the diameter of a pipe between the two biggest restrictions in the exhaust would accomplish anything? If your state only does ODB2 testing (no visual/sniffer) then just get a catless header and a tune and you'll pass just fine.

The only thing needed is to know how much does a catback makes by itself. You can ask any tuner out there or person with experience with the car and they will tell you catback is only for sound enhancement purposes, power comes from replacing/removing stock Cats and even more when coupled with bigger pipes. But the removal/replacement of stock cats, is required.

This. AZ only does the OBDII check as well unless something comes up wierd. Going catless and a tune with a quiet exhaust will keep them from ever suspecting anything.

I don't think a 3" over pipe will even physically bolt up to the stock header, at least not the flanges I use.

Because it's a very large change in volume with the 3" pipe. As for going catless and tuned, I need to pass visual and a tune will void my warranty without question; can't do either.


Not for nothing, I'm pretty sure I've seen several catback dynos around here that show a power gain on this car. Whether or not those gains are deemed significant is one thing, but that doesn't mean they're not there.


That's entirely possible and I have no clue. I was looking at the 3" JDL pipe, but I have no idea if it will physically work.

There's some dynos that have shown 5-8 whp. It has been debated to death if those are actual gains, changes because of resetting the ECU or what.

Still the matter of the fact is, ou increase the volume of a very small pipe, but keep all the restrictions in place. Doesn't seem like it is worth it still... Not trying to be negative, but real.

Increasing the volume between two choke points does nothing but increase the time it takes to fill the area. Think of it this way. You've got a garden hose and you decide you want water to come out of the hose faster. You decide to replace a section in the middle of the hose with a 5" diameter section that's a foot long. Do you think you are going to get more water out of the hose just because you replaced a piece of hose in the middle of the two restrictive ends?

Even if it is five, when you've got very few options, and aren't looking to do a serious NA build, five is plenty ;)

I appreciate that :thumbsup:


I can respect that completely.


I'm thinking about this in regards to a pressure differential. There is high pressure before the first cat, in-between the cats, and after the cats. If you lower the pressure behind the first cat with a massive overpipe, shouldn't airflow through the first cat theoretically speed up?

Maybe that would be true if there wasn't a second restriction right after the increase in volume, hence the garden hose analogy.

Attachment 77480 just to help with visuals, look at this choke point, and this at this mid pipe.

Sent from my Nexus 5 using Tapatalk

It does.


To clarify, my Nameless 3inch op/dp combo bolted up to the stock header without issue.