Inside an OEM Honda muffler / FN2 Civic Type R

Notes: Inlet tube outside diameter 60.5mm (2.36”), outlet tube outside diameters 45mm (1.75”).

[@edwinbath1512]

Thanks for this!!!! It helped me to make an educated guess about the cause of a very annoying rattle inside the muffler - and to fix it with a simple self-tapping screw into the left-most resonator box, into the exit pipe. Rattle gone (y)

[@ksoma228]

Very educational and informative! A detailed explanation of how the FN2 civic Type R stock exhaust system works and what its constructed from. Thanks for your expert knowledge, would love to see more of your vids explaining more of the FN2 Type R parts ^_^

[@brettmaddison442]

Fantastic work really helpful 👏 thanks

[@TB993tt]

Hey Lorenzo awesome work with the vid and your comments are super informative. I am running my built HC NA motor with 4P head through catted (HJS100 cellPE 108X130mm) 4-2-1 Solid Fab manifold into totally stock cat back and am at 265HP which is pretty amazing. My tuners tell me a 3" cat back may yield towards 280HP but I hate the resonances which the K20 can produce in the FN2 (I've tried many other zaust combos). Can you imagine any secret sauce set ups which will keep the exhaust resonance free yet free up some power ? I was contemplating showing a local exhaust fabricator the pics of the muffler internals (courtesy of you) and see if they can build a slightly bigger muffler using 63.5mm inlet sizes but using the same internal baffler layout including the Helmholtz...... do you think this is feesible or prone to errors in the baffling and Helmholtz set up ?

[@TassieLorenzo]

Hi TB993tt, That sounds like an awesome engine, bravo!! I think the gain from going to 63.5mm from 60mm will be minimal and you could consider at least 70mm/2-3/4" (or just do the 76mm). "do you think this is feesible or prone to errors in the baffling and Helmholtz set up ?" Sounds good to me! Helmholtz resonators (or quarter wave tubes) are really sensitive to exhaust temperature. Ideally you'd make it with a slip fit, so you can adjust the resonator length in case it's not quite right. You could try to remake the OE muffler in 70mm or 76mm, with say 2x 60mm or 2.25" tailpipes. I think the way it feeds the exhaust directly into the resonator chamber makes it particularly quiet. I'll try to take some measurements of the resonator chamber for you:

[@TassieLorenzo]

The dimensions of the OE resonator chamber are roughly: INLET PIPE = 2" x 145 mm CHAMBER = 140mm (cross ways) x 150mm (-4.8mm double wall thickness) x 140mm (-4.8mm double wall thickness) CHAMBER = 2700 (cm^3 / mL) Except the inlet takes up part of the volume of the stock chamber, so it's more like: CHAMBER = 2700mL - 72 mL Probably not significant, so don't worry about that. It wouldn't have to be the same layout , it could just be your 63.5-65-70mm exhaust (with some kind of straight through muffler) but with a 145mm-long 2" arm branching off going into a chamber with a volume of 2700 mm^3. If you have one of the single exit exhausts, you could branch the dead-end resonator chamber off to where the right hand exit of the exhaust would normally go maybe? [Stock back box = 150mm tall, 140mm deep, 540mm long]

[@TassieLorenzo]

Here are some pics: Muffler cut open i.imgur.com/rHWXRyn.jpeg i.imgur.com/AMsYAPA.jpeg Modded muffler i.imgur.com/BbLDFuv.jpeg i.imgur.com/mSesowo.jpeg The left solid baffle that forms the resonator is 140mm in from the end like I said. The right hand perforated baffle is 100mm in from the right end, and the second perforated baffle after that is another 120mm in from that. But I got rid of the middle baffle and don't have any drone at all, so I don't think it's important for drone (getting rid of the central baffle only made the exhaust like 20% louder, which is OK for me). Overall dimensions 140mm x 150mm tall x 540mm long like I said. Hope this helps, cheers. I think it's mainly the Helmholtz chamber that's important for eliminating drone/resonance. If you can find a place that has an English wheel to roll truck mufflers they can probably do a project like this.

[@ejtisevn]

What about those small muflers before tips?

[@TassieLorenzo]

They are like small resonators, a perforated tube through the centre with packing around the outside. They are probably there mainly to adjust the tone to get rid of some raspiness. I'm curious how much effect they have on the volume, but it's probably not much.

[@Matty_Champwhite]

Brilliant little video mate well done, For those of us that don't want a stupid level of volume it's great to learn the inside workings of our standard back box. Mine currently has the cat and just a back box as the middle resonator was deleted by the previous owner. I think I'm going to go for a piper manifold on sports cat but also fit a resonated middle section back in to try and deal with some of the added noise. M2 do a middle resonated section that will mate to the OEM back box. What's your thoughts on that, do you think I'll be able to keep the volume down

[@TassieLorenzo]

Sorry about my slow reply. What type of backbox do you have -- stock or M2 (or something similar to M2 like Mugen or Invida)? The latter already use a fibreglass packed design like a resonator. So adding the mid-resonator is like adding a bigger rear muffler of that type of muffler, whereas the stock rear muffler is hollow and so the stock mid-resonator is used more to adjust the tone of the exhaust (remove rasp) than to reduce noise (though it will reduce noise a bit too). "do you think I'll be able to keep the volume down" I'm not really sure. The resonator will make it quieter to offset the manifold, but by how much I dunno. How did you get on if you've already done this? 🙂

[@Matty_Champwhite]

@@TassieLorenzo yeah long since changed now, I’m 3” with 2 resonators and a muffler took the plunge

[@1994FishFreak]

What I want to know is what’s inside those little bulbous bits at the end of the pipes, always wondered if you could increase the volume of the OEM backbox by gutting those end pipes out, the EP3 sounded great stock unfortunately the FN is far to quiet stock, I ran the M2 backbox then fitted the non res M2 mid section and as much as I love it, it drones like nuts, half tempted to go back to just the backbox or mod the OEM, wish I could fab up a valved exhaust as I love having it loud but there are times where it could do with being toned down I.e. got family in the car or start ups in my neighbourhood

[@TassieLorenzo]

"What I want to know is what’s inside those little bulbous bits" I realised I forgot to answer. They're just resonators. They have a perforated tube through the middle (so they are straight-through) and are packed with fibreglass around the outside. Honda are probably just using them to adjust the exhaust tone a bit (their position near the end of the exhaust is probably important, a bit like those resonated exhaust tips you see sometimes) -- probably to make it sound less raspy, and a bit deeper and smoother. If you gutted those it probably wouldn't make much difference to volume (a small difference) but it probably would have a bigger difference on the tone of how it sounds.

[@rimka11]

I can see that there is a lot of room for flow improvement. Without hurting attenuation too much. WTF Honda?

[@TassieLorenzo]

It's designed to be quiet I guess. 😕 On the bright side, the stock exhaust is 60mm up to this point and is a fancy oval shape. I tried removing the middle baffle and welding it back up, i.e., so the exhaust gas dumps into one chamber and then directly out the two tailpipes instead of needing to flow backwards through the perforated screen. (I made sure the left tailpipe still had a little bit of a bellmouth on it, i.e., by welding on the contraction piece that's originally right next to the baffle, as I figured that was important.) For me, this sounds perfect, subtle OEM+. This still has a similar tone as stock as all the resonators are still there, but is about 20% louder (very subtle). It's a warm, pleasant sound IMO, the car essentially sounds like what you'd expect it to sound like (as stock), the typical techy Honda inline-four sound. You can hear the exhaust better inside the car with the rear seats folded down now, although you still can't really hear much difference inside the car with the rear seats folded up (it sounds nice in tunnels and things with windows down, however completely stock was already not too bad for that). It's about the same kind of difference as deleting the resonator on a stock turbo car, if a little less (you don't want do a resonator delete on an NA Honda as it will sound too raspy IMO). It blends nicely with the stock intake sound^ and neither exhaust nor intake dominates. ^ If you don't know, there's a divider in the stock airbox that blocks off about 20% of the air filter area for no apparent reason (maybe better efficiency on part throttle when cruising?), so that can go away! I recommend keeping the turning vanes on the sides though as throttle response seems worse without those (at least without the ones in the top half of the airbox, as I mixed modded bottom half with stock top half). "room for flow improvement" Maybe modding the stock back box gained 0.5-1hp if lucky, lol. 😂 The FK8 has two straight-through mufflers for better flow *but* the exhaust diameter up to that point is still 60mm like the FN2, despite the FK8 having >100 more hp! European turbo cars like the GTI and Megane RS usually have 65mm exhausts, so Honda choosing a smaller size is really weird. The most recent Subaru STi also still had a 60mm exhaust too and Evos always had 60mm stock exhausts -- I guess it's to do with Japanese automakers trying to make their cars polite and quiet? 🤔 For the FN2, straight-through back boxes like an M2, Mugen or Invida will have virtually no flow restriction but will also be much louder than stock (straight-through mufflers usually are kind of loud on NA cars, they aren't as loud on turbo cars though hence why a lot of stock mufflers on turbo cars are already straight-through mufflers of some kind). I think the Milltek FN2 back box however is some kind of baffled back box and I'm not entirely sure how it's configured or how it works.

[@rimka11]

​@@TassieLorenzo I mean you don't really need to go straight-through in you need some flow. Check for example dynomax super turbo muffler. There are some pictures with it open. That is almost perfect design if you want some good attenuation without restriction. Too bad that 9 out of 10 mufflers are designed with completely different goals.

[@TassieLorenzo]

@@rimka11 TBH, the Honda muffler works the same way as a Dynomax Super Turbo one. It just uses the perforated screen instead of a tube to reverse the flow, since the screen uses less space. Subaru & even Porsche mufflers (when the bypass valves aren't open) use a screen instead too. They are all in the class of "reversed flow" mufflers.🙂 Here's a 911 GT3 stock muffler. When the bypass valves are closed the exhaust gas goes out of the inlet tube (red arrows) then needs to flow backwards through *two* perforated screens (blue arrows) before it can get to the outlet tube: cimg6.ibsrv.net/gimg/rennlist.com-vbulletin/1024x768/gt3mufflerexhaustpath1_e82821d317adfc0cfbbbc1425a3a48d4b18251bc.png [The black arrow is the direct path when the valves are open, which bypasses all this.] The EK9 Type R muffler is essentially exactly a Dynomax Super Turbo style muffler, while the DC5 & FD2 Type R mufflers are too -- just with a spring-loaded bypass flap to partially bypass the reversed flow bit when there's enough exhaust pressure. I don't think the FN2 style is necessarily worse than those, it's just different. It would be cool if it had a spring-loaded bypass flap or even an electronic one, but oh well. It shouldn't be underestimated that 180 degree bends, like in the Super Turbo, still cause pressure drop in and of themselves. Even in the Mugen Twin Loop muffler where the 180 degree bend is done with a perfectly smooth bent tube and not just a wall, there is still a loss associated with going around a 180 degree bend (two losses for going around them twice!).