That is true to some extent - if nothing else, gas traveling at the outside of the pipe is restrained/contained by the pipe wall and has to flow straight, and gas more towards the middle can be turbulent. It's not like there's dead air in the middle though, under pressure (IE, being an exhaust) there is air flowing in the entire pipe until you get so large that you lose velocity.

But yes, in almost all circumstances smoothing transitions increases flow. How much? That's a CFD analysis question. Maybe 1%? 5%?

 

Wouldn't the flow increase be something closer to the difference between pipe sizes? Like around 11%? Especially because the air that's hitting the lip is getting disrupted and forced into the center area?

 

There's still a flow decrease from one size to another - no getting around that. But the decrease could be minimized by not having a hard lip causing flow reversion.

 

Hey guys, long time lurker first time poster. Just put on 370z wheels on my '12 X Sedan AWD with a staggered setup. I probably have to put a 20mm spacer in the front to make it more flush, any recommendations on spacers? Thanks.

 

Dam. son. You need to get the wheel well lips rolled or you're going to eat the tires...

Edited to add: looks great, btw. One of the options I was considering too.

 

You definitely right about getting the lips rolled, I heard some rubbing pulling out of the shop (I had all 4 winter tires in the back so maybe its because of the weight?). Thank you! was super excited to put these on today.

 

Gloss black grill and custom headlights

 

Fitment was a bit off at first but got it to work. Sounds and looks great

 

I had my exhaust fixed up yesterday. My RHFCs are 2.5" but my catback is 2.25" so there was a lip that was obstructing airflow where the two flanges met. I had the old flanges cut off:



Then we replaced the flanges with 2.5" ones and a pipe reducer from 2.5" to 2.25" so the transition to the catback was smoother. Also, the flange doesn't leak anymore because it wasn't able to seal properly before we fixed it.






I know this "mod" seems small, but consider the fact that this is in an area where exhaust scavenging is still very important and eliminating a restriction on both sides of the exhaust can make a difference. I feel like my butt dyno actually can detect a small change. The idle was a little bit off too, maybe it had to relearn with the difference in airflow. I do know that my exhaust is 100% my bottleneck at this point, so a small change like this can have a decent effect.

The exhaust is a little quieter now too. There was always a certain RPM around 2,000 - 2,500 where I could hear a distinct noise, and that's eliminated. It feels like a good $130 spent for the materials and install.

 

Glad that worked out for you!

 

Backman, I'm impressed with your solution. I don't really think there was a problem to be solved here in any real-world sense, but totally respect what it means to establish peace of mind.

I think I mentioned once or twice how my setup is the opposite... 2.25 outlet from the RHFC to a 2.5" catback.

BTW, great pictures.

 

I thought RHFC from FI were 2.5"?

 

AFAIK that's the case today. But back in 2014 there was an option to go either way, and I picked the smaller diameter outlet to match my OEM catback.

I did my cats first, and back then I wasn't 100% sure I'd be replacing the catback too, so I matched them up.

 

Thank you! Lol, I guess it wasn't really a "problem", but there was actually a small exhaust leak there, so it fixed that. And if I could gain even 3-5hp from this change, it's worth it to me. That would be something like $25-45 per HP. Its not the most inefficient use of money compared to some other mods.

 

So, some quick math... what you're saying is you spent $200 on this project, which feels about right. Could have been more, and probably not much less.

And I really do get it. After installing FI cats long ago, I started freaking out worried about the absence of the OEM support bar. So much so that I had my old mechanic come up with a solution. Was it worth the 2-3 hours he spent on it? Yes. Yes it was.