I have this exhaust situation and I need to know if it's worthwhile to get it fixed.

My mods are:

2.75" AAM CAI
2.5" AAM RHFC
2.25" Invidia Q300 CBE
Coming soon: 64mm~ ported tb's, ported upper and lower intake manifold, finalized with an ecutek tune.

A few months ago, I installed some 2.5" AAM RHFC and it's mated to a 2.25" Invidia Q300 CBE. Where the flanges meet, there's a stepdown in piping diameter 2.5" > 2.25". I didn't get a chance to get a picture, but there should be a lip that is causing airflow to be restricted at that flange where the cats connect to the cbe, correct? My question is, how much is this going to impact performance? Is this something that's worth fixing with custom exhaust work?

In my mind, I can picture this causing a fair amount of turbulence if it's hitting a solid .25" lip all around the edge. This would affect exhaust velocity and scavenging. So how much power would you think this is robbing me of?

I will include a few pics of my exhaust and also a couple links to some adapters that I think could be welded in. Any thoughts or opinions would be appreciated if this is even worth the cost / effort to do. I know that the 2.25" Invidia CBE is already a fairly restrictive exhaust as is, but I'm definitely not changing it because I love the sound. But I'm going to be getting a pretty good flowing intake system soon, and I want my exhaust to flow as good as possible.

ADAPTER OPTION 1:
ADAPTER OPTION 2: https://www.summitracing.com/parts/w...CABEgJHlPD_BwE

 

Here's an amazing MS Paint depiction of what's going on. lol

 

Difficult to imagine how that really matters. But I'm curious what kind of responses you get.

Mine is actually the reverse of that... 2.25" piping in the RHFC feeding a 2.5" catback. And I have zero interest in doing anything about it.

Awesome MS Paint skills.

 

I think it's generally better to have it open up bigger like your setup, if theres gonna be a mismatch.

 

Interested in what you did/concluded. Sub'd.

 

Any plans on going FI? If not, keep it as is. I asked two different tuners this exact questions and they both said the difference is negligible on an NA set-up.

 

There are calculators for the amount of HP that 2.25" piping is suitable for before it starts becoming too restrictive and its inaccurate for you to say the difference is negligible unless going FI.

With my NA setup making in the range of 390 hp, it would help push me close to 400 if I were to upgrade to a 2.5" catback. Its my main restriction but I like the way it sounds and don't want it any louder, so I'm keeping it as is.

As for what I did, I got a grinder and rounded off the flange a bit where the restriction is, to help smooth the flow at least a little bit.

I'll post before and after (the exhaust wasn't lined up very good the first time).






Did it help? Yeah. A little, somewhat because the exhaust wasnt lined up too well before though. I did notice a slight difference though, and the sound changed a bit. Actually got quieter because there was a leak before.

 

Come on, man. I'm not into chasing HP numbers, but seriously, how do you get to 390 with just I/E and a tune? What is that... 325 at the wheels?

Did you dyno at 325? That would be impressive, and more than a little fun.

 

Feel like you'd need more than just that to make 325 whp. Test pipes and some IM work. Plus Upgraded TBs and the intakes better be the 3'' Admin or similar.

 

I have more than just "basic" i/e...

I have 2.75" intakes with upgraded bellmouth + velocity stack in the end (z1's upgraded filters dyno'd 5whp i think? just as an example)
ported tb's, upper, AND lower manifold
GTR 550cc injectors
Rhfc and catback
ecutek tuned and higher octane fuel
RWD and 6MT

I thought in my original post I mentioned my mods but sorry if I wasn't clear on that

 

Even though I'm not running e85, my injector duty cycle was hitting 95% and I even saw 97-98% a few times in the upper rpm's so I really think I'm putting down some good numbers to need that much fueling.. most people say that with those mods, injectors aren't needed, but I needed them!

And Eugene, my tuner, said from looking at some of the data that im definitely north of 320whp but of course, I haven't dyno'd it yet.

 

Nice! Those AEM intakes are pretty interesting looking. Did you consider cam upgrades while all in on porting the manifolds?

So... dyno tuned at 325 WHP or however you're converting to 390 BHP?

Whoops, you beat me to it by a few minutes. That WHP number is an educated guess. Well OK then. You know, in full disclosure when asked how much HP my car is putting out, I assume 328 from the factory, then add some because of the I/E mods and claim 340 BHP pulled completely out of my bum. I've been known to say "south of 350"... which isn't necessarily a falsehood. I think we all do it.

 

Nope didn't want to mess with cams.. or even headers. I'm extremely pleased with the power I'm putting down now and now ill be focusing on lightening up the drivetrain by lighter wheels, rotors, flywheel, pulleys maybe, and a lightweight battery.

 

Battery, pulleys... meh. But I definitely relate to the rotors and flywheel ambitions. You should have a build thread. I'd like to follow your progress.

 

Lol thats hilarious! How tempted do you think I am to say "just under 400"? Sounds so good, haha