short story: 2009 X sedan. stock cat went loose inside, Nissan dealer wanted $1300 for one, ha!

I bought Fast Intention high flow cats (non resonated) from a fellow enthusiast here for a third of that and had them installed. all good.
Shop that installed them noted my mid pipe had a leak on the top of it, they wanted like $800 to replace, ha!

I bought a stainless Megan Racing mid pipe for a few hundred and got it put on. Looks very nice under there.

Since that install tho...I've had a nasty noise at certain rpm's when you get on it. It sounds exactly like a loose heat shield.
I took it back to the shop that did the Y pipe and he could find nothing. He thinks that a muffler is loose internally, I don't agree.


Thoughts? The FI cats do lose the brace that attaches to the transmission, (I did a lot of research here and that does cause a bit of vibration at idle)

Is there a hidden heat shield we can't find?
With the loss of bracing is it setting off some sympathetic vibration?
Or is it some magic resonance with different exhaust sizing?

I really sound like what we used to call a ricer. (is that still even used?)

 

Yeah, similar issue with the megan pipe. The outlet of the megan pipe is 2.75" or something, and I'm assuming you are still going into the stock Y pipe which is a smaller diameter. I think that's the cause.

 

ah, thanks iCrap, I will listen to your wisdom (read some great info from you when I researched exhaust).

Hmmm, is there a better replacement for the Megan Y pipe, get a bigger mid-pipe, or find a stocker (although they seem to rust out pretty quickly).

How did you solve yours iCrap?

 

There's really no solution. I have high flow cats on my engine stand on a stock down pipe and it has just a slightly resonance however it still there.

It's not until you take a good look at the OEM cats that youy understand why they're made the way they are.

A high flow cat will have one less honeycomb many times, smaller cell count, much thinner piping. You add that to a smaller, thicker piped Y pipe with heatshields and rubber external resonators and it tames it quite a bit.
The issue with this is that you still have a mid pipe and muffler that interact.

Perfect example is how horrible muffler deletes sound, if just a muffler delete introduces drone, rasp and resonance then you can agree that anything else you try and do will yield similar results.

Now where some find responses in are with the mix and matches of resonators properly placed. Cats need resonators and the Y pipe as well.

Unfortunately it will always end up in the change of the setup.

 

Thanks Bull, that is useful info.

I do recall now that the stock Y pipe has two, what look like exhaust hooks with rubber mounts that hang there- that must be the rubber external resonators/dampers?

So I either start playing mix and match with other pipes/resonators, or I go back to a stock Y pipe. The sound with just the FI cats was actually fine with me, it made a touch of rasp maybe, but I liked it. I'm too old for loud ugly exhaust
Or I could try to get something welded up to replace the big stock brace on the transmission, Rochester did that, but that's a lot of custom fab and it might not help.


Damned, I realize my shop threw my stock Y pipe away; maybe I could have gotten it welded. Anyone got a stock Y pipe they want to sell?

 

There has been documented info that these rubbers isolators help however I'm a firm believer that with thinner and bigger piping more of these will be needed in the Y pipe for it to actually help with the extra resonance.
A mix of these and a few resonators in the system will lower this resonance without hurting performance like it would with the stock Y pipe

 

So searching some more and there are several posts that talk about going from an aftermarket 3" Y pipe to a smaller 2.5" creates turbulence and that's where a lot of the baddy noises come from. I can see that. Would also create some back pressure and poor flow I'd gather.

Z1 has some stock Y pipes for $158. I assume from doing upgrades at their shop. Seeing as they're in GA i'd guess most of the pipes should be in a low rust condition.
I called and the guy went back and looked, said they only had Y pipes in their 370Z area. Searching around, lotsa posts that say the Y pipe is the same for Z's, coupes and sedans. Latest post I found was 5 years ago, still the case?
(one claim was that the Z pipe was slightly bigger, but unverified; one person had checked and Nissan and Infiniti part # were the same)

 

Possible that the Z Y pipe was a Nismo.

 

Hey, so I replaced my rusted out y pipe with a Megan racing one 2 years ago and it's been fine. Recently I started popping a p0430 code and was considering replacing the cats with a high flow unit like kenetix. Wondering if this is just gonna be a recipe for bad noises and if there's a reasonable solution to keep things relatively quiet.

Has anyone tried replacing the junction between the y and midpipe with a resonated x crossover?

 

do the kenetix use the big factory brace mounted to the transmission? You will be much better off I imagine, or good to go, it's been done here many times, but no guarantees right?

I can't recommend the FI high flow cats at all, it doesn't use the big brace, they just hang there. I hate aftermarket *hit that doesn't a) improve things as they claim and B) keep it close to stock with low to no side effects
- especially if it's $$$ like Fast Intentions who present themselves as at the top of quality aftermarket. I found a stock Y pipe and went back to it; took care of 80+% if the resonance issue, but I still get a car vibration at idle and a bit of rattle/resonance.

 

Hate all you want, but FI has engineered some really top-shelf aftermarket exhaust parts.

As for the bracket from the cats to the underside of the transmission... see a problem, fix a problem.

 

haters gotta hate right? Probably the only thing we disagree on.

No, FI does make some top shelf stuff, I heard one of their full systems and it was great, and beautiful. Welds were lovely. They just missed on the cats. Most of the other aftermarket cats use the factory brace. In my younger years I might have enjoyed the challenge, but too much to do with kids and work and other family. It's just a PITA and I don't expect to have to take more time and money to fix their engineering mistakes. IIRC, you had to find a really good guy who could custom weld that. I know a few race shops but they will not be cheap and it will take hours out of my days, don't have it. I took the time to go find another factory Y pipe from the south, no rot and get it on there, more time and hours. Mostly sorted out now.



Anyway,

 

A big issue is how many folks underestimate the factory exhaust due to how "small and restrictive" they are. Data shows that increased flow and size in the exhaust really helps these engines breath but at a huge cost which is sound.
Most of the "tactics" used in the exhaust world don't seem to help with noise however understanding these on the VQ will show you that MANY folks out there are raspy and with drone and it's not so much of an issue due to less rasp and drone than a VQ.

Though the cat brace helps in some vibration absorbtion, it's main function is to provide support to the cats with the transmission to prevent or extend their lives. This brace is not the source of this noise nor will it fix anything auditory.

OEM is a thick steel body in J form with two higher count membranes VS High flow cats that are mostly thin steel or stainless tha are a single, lower count membrane. HFC alone will change sound characteristics, enough to the point were it will require resonators as close to the cats as possible to compensate for the sound reduction of the second core.

You add a thin and bigger diameter pipe and you are now moving this resonance to the center of the car. The idea is to keep this sound by the cats or close to the engine by using the right resonators. You should focus first on cats and Y pipe first then leave the mid and muffler for last. It does require mix and matching of parts which can be more work than buying a cat back or full exhaust but then the prices of those is what made folks go the other way.

If you want it ALL in the VQ exhaust world expect to pay good money for a setup or some decent time making your own.

 

I think they just avoided the brace to move from added costs and welds, they should have it imo.



This does not stop the vibration (6MT), Still working on a bolt on solution, basically need to tie it into the trans with rubber mounts.

 

Bull, as always you have great knowledge and it's appreciated. Yes, I need to realize the bracing is for vibration, which I would like to get rid of; actually if that was gone, touch of rattle too that my mechanic doesn't see why, I'd probably be happy. I actually do like the slightly higher amount of exhaust noise that the HFC's put out, it's very nice when it's cold. Yes, I should have gone with resonated cats, what I would recommend for those that care.
I do recall several mentions of the FI cat's have cracking issues from not being braced, it's a long run there from the heads back to the hangers without the bracing. I know another local shop that works on a lot of P cars, used to do SCCA autos with one of their guys, maybe they would do a brace for me.

The place everyone keeps telling me to go to is on the rougher side of town, and far from me. I've called and they will not give any idea if they can do it or price or anything. No appointments. You show up at 8am and there is usually a line of 5-10 guys already, you get your name in and wait your turn, then they tell you if they can, and the price. In my younger years I'd have loved it. Now I'd be late taking the kids to school