Custom Exhaust Questions - RB25DET Neo

[LjB123]

Hey guys. So my R34 GTT has a custom 3.5” exhaust (done by previous owner) with a high flowing cat, and then just a Varex cannon muffler on the back. The car does drone alot and is really loud even on highway driving just cruising, as you can imagine due to the Varex muffler basically being a straight through with the valve open. As much as i love a loud RB, it can be a bit much with no other muffling in line and just driving casually. I wanted to lower the volume output on the exhaust a bit, as well as remove the drone (probably a way to do both at once), and was going to do it all myself as I am a fabricator and have done exhaust work in the past, so the fab work isnt my issue, im just not sure on the right product or type of product to use to get the job done, and i doubt an exhaust shop would be wanting to help me as they wont be making any money from helping me, unless they supplied the product? Anyway any help or guidance in best way to approach this would be awesome

 

Thanks in advance

[GTSBoy]

Centre-offset muffler under the floor, same as stock. Just....3.5".

The change from straight pipe there to centre-offset will mean that you need to remake some of the pipe (add a bend or two).

[Kinkstaah]

Go to SP mufflers and get the muffler of size you want and make a nice mid muffler.

It's about that simple! I have 2x 3.5 Magnaflow straight through Mufflers and while it's pretty loud with the Varex open, it's quiet enough with the Varex closed. You can also buy off the shelf Magnaflow straight through mufflers as well.

You also have a turbo to quiet the car down too. Don't overthink it. Buy my extra Magnaflow muffler sitting on a shelf doing nothing (buy it new, ignore this part)

[LjB123]

7 hours ago, Kinkstaah said:

Go to SP mufflers and get the muffler of size you want and make a nice mid muffler.

It's about that simple! I have 2x 3.5 Magnaflow straight through Mufflers and while it's pretty loud with the Varex open, it's quiet enough with the Varex closed. You can also buy off the shelf Magnaflow straight through mufflers as well.

You also have a turbo to quiet the car down too. Don't overthink it. Buy my extra Magnaflow muffler sitting on a shelf doing nothing (buy it new, ignore this part)

Cheers. I more want it quieter with the varex open. I only have the varex closed when i start up and roll out the driveway to keep everyone happy on days i take the car to work at 5 AM or come home late. So just a mod muffler will do tge trick? Something like that would require a retune hey?

[Kinkstaah]

I did say "2x Magnaflows" then referred to a Varex. So to un-confuse that, my rear muffler is hot-swappable with a Varex.

When I had a RB Turbo, I had the 2x Magnaflows, then went LS and though "Oh that's way too loud" and ordered a Varex.

A mid muffler will definitely help a lot, even if straight through. SP Mufflers can make anything as big or as small as possible, which will help even more. Looking at my own exhaust I could make the muffler twice as long if I wanted to. Or you could go crazy and get 2x mufflers from SP and full every remaining space with muffling.

In my experience closing the Varex makes the car run VERY rich (obviously). I'd say adding a muffler *will* affect things, on a technical, physics level, but I don't think you'd need a retune as the actual effect would be the car running a *tiny* bit richer due to more restriction, on a strictly physics level, but you kind of don't have any option if you want the car to make less sound.

[GTSBoy]

Straight thru's have so little restriction that it's almost certainly not worth worrying about effects on tuning. Even a centre offset shouldn't upset things, but would be noticeably more effective (quiet).

[LjB123]

48 minutes ago, Kinkstaah said:

I did say "2x Magnaflows" then referred to a Varex. So to un-confuse that, my rear muffler is hot-swappable with a Varex.

When I had a RB Turbo, I had the 2x Magnaflows, then went LS and though "Oh that's way too loud" and ordered a Varex.

A mid muffler will definitely help a lot, even if straight through. SP Mufflers can make anything as big or as small as possible, which will help even more. Looking at my own exhaust I could make the muffler twice as long if I wanted to. Or you could go crazy and get 2x mufflers from SP and full every remaining space with muffling.

In my experience closing the Varex makes the car run VERY rich (obviously). I'd say adding a muffler *will* affect things, on a technical, physics level, but I don't think you'd need a retune as the actual effect would be the car running a *tiny* bit richer due to more restriction, on a strictly physics level, but you kind of don't have any option if you want the car to make less sound.

Yeah great. Just got off the phone with SP, and will see about getting one of their mufflers to weld in in the next month or two. For me the varex is to get me out of the driveway, down the road and thats it. It stays open 99% of the time driving as it does definitely run rich with it closed as you said, and restricts the boost build up. Thanks for your help

[LjB123]

33 minutes ago, GTSBoy said:

Straight thru's have so little restriction that it's almost certainly not worth worrying about effects on tuning. Even a centre offset shouldn't upset things, but would be noticeably more effective (quiet).

Thanks mate!

[Kip]

I'm struggling with a similar issue on an RB26. Make sure you know whether it's volume or resonance. Mine seemed louder when cruising compared to at idle. I downloaded a decibel meter for my phone. Take my less than scientific results with a couple grains of salt, but the decibels were the same at idle and at cruise. Mine seems to be a resonance problem. At 2500-3500 rpm, the sound is different and "louder" to my ear. I think it's vibration from the pipe, the car body, the floor or something creating tone.

Anyway, consider that it may not be strictly a volume problem.

[LjB123]

1 hour ago, Kip said:

I'm struggling with a similar issue on an RB26. Make sure you know whether it's volume or resonance. Mine seemed louder when cruising compared to at idle. I downloaded a decibel meter for my phone. Take my less than scientific results with a couple grains of salt, but the decibels were the same at idle and at cruise. Mine seems to be a resonance problem. At 2500-3500 rpm, the sound is different and "louder" to my ear. I think it's vibration from the pipe, the car body, the floor or something creating tone.

Anyway, consider that it may not be strictly a volume problem.

Yeah i get what you mean. Mine thought i wouldnt mind being a bit quieter overall as well as have the drone taken out is more what i was trying to say. Im going to start with a mid muffler, and go from there 

[GTSBoy]

It can be worth experimenting with adding a Helmholtz resonator to some systems, particularly if it is one tone that drones excessively. This is a short(ish) length of pipe teed off the side of the main pipe, ending in a dead end. The length tunes the frequency at which it works. You have to experiment to find the right length. There's a recent youtube vid where someone did it, might be worth a look.

[LjB123]

5 minutes ago, GTSBoy said:

It can be worth experimenting with adding a Helmholtz resonator to some systems, particularly if it is one tone that drones excessively. This is a short(ish) length of pipe teed off the side of the main pipe, ending in a dead end. The length tunes the frequency at which it works. You have to experiment to find the right length. There's a recent youtube vid where someone did it, might be worth a look.

Yeah ive looked into those. I saw one guy who made his adjustable so he could fine tune it, but theyre a decent amount of work to get right and they only do one particular range of frequencies. I feel like if youre not worried about losing some overall volume in your exhaust (like me) and want an easy solution, slap a decent muffler in and that should do it, as a muffler should take out more of a frequency range but also you lose some of the volume, which is fine for me

[niZmO_Man]

IMO the "straight pipe" approach sounds crap.

I'd (and have) go down the "two fat mufflers with mandrel bent custom exhaust" route. Not only would it give a nice note, it should be below 92dB or whatever the legal limit is.