Searched and couldn't find any discussion about this particular question I have on the UK Mod. I've read all about and listened to the 38mm vs 44mm vs 48mm, etc. I personally think the 38mm sounds slightly better, but that got me thinking:

How does the placement location, mod pipe length, angle, etc. affect the sound?

My thinking is that the acoustics must be different as the sound waves would travel a different path, over a different length, or whatever other variables one might consider.

I'm curious if anyone has played with it or done any research into how the sound is affected by any or all of these variables.

If I've missed some good reading material on the subject, do please share if you would. It's a very interesting subject to me. In the meantime, I'll be brushing up on general acoustics to see if I can figure out some theories before I actually do the mod.

 

I don't think any of that matters much. I bought a uk modded exhaust that already had a 38mm uk mod.

It was WAY TOO LOUD. Like obnoxiously loud. I too listened to Every. Single. Video on youtube with uk mod. Using headphones trying to get clues as to how they really sound. What everyone says is true, you can't tell from video, gotta hear in person. Video will tell you what it sounds like, but nothing about how loud it is.

So, I cut slots into the uk pipe using a sawzall, and bought some 1.5" fender washers, and jammed them into the slot. Initially I drilled center hole out to about 3/4". But it was still too loud. A single wall with a hole isn't as much a restriction as a section of pipe with the same size hole.

So then I replaced the washers with less size hole. Eventually I just switched to washers with hole as is, around 3/8".

It was still a bit too loud, so I added a second set of washers to eaxh uk pipe, about 2" away from the first set. That did it. Perfect. So then I used some high temp JB weld to secure the washers from falling out (once hot, the slot the washer had to be hammered and pryed into would expand).

To get these washers to fit flush into pipe, I had to grind half the outer circumference down by about 1/8". Make half the washer 1/8" less in diameter.

Anyway, the point is this abrupt wall with a hole, followed by another wall with hole, has to be waaaay more turbulent and disruptive of resonance and linear flow and reverberation and sound wave acoustics and pipe length and angle of attack than any version of how one might execute a normal uk bypass pipe.

So yeah, research done. None of it changed much other than volume (I can occasionally, at certain rpm and light throttle, detect a very slight whistle. But I have to really, really listen hard to hear it. I suspect its turbulence as flow exists washer holes).

My results lead me to believe my exhaust sounds pretty much the same as a uk modded exhaust with a smaller pipe. Difficult to say what the equivalent pipe diameter would be. Maybe something between 3/4" - 1".

What everyone says about individual taste on exhaust loudness is also very true. Many others would think my exhaust is now way too tame (but guessing much fewer would think its too aggressive, unless they think stock exhaust sounds great.)

If I were to make further changes, it'd be to make it even quieter. Its almost perfect now, just occasionally its still louder than desired from cabin.

I've had someone else drive it so I could also hear what it sounds like to everyone else, as I don't want it to be offensive to neighbors and others encountered in the wild. Its very nice, if anything, still too loud.

If I had to guess, all those bypass pipe variables you cite actually DO affect the sound, but its all so overwhelmed by the raw volume of the flow that goes straight through you can't hear it. Very much like that noise an engine makes that goes away above a certain speed. Its really still there, actually even louder than before, you just can hear it above the din of road noise.

 

That's really strong feedback. Thanks!

I don't want loud either. I just find the stock exhaust a bit too "polite", if you will. Just a little more growl is what I'm after. I'm not trying to build the fastest, wildest S2K on the internet so spending several grand on a J's Racing titanium system is just not of interest. I just want to hear the car a bit more.

So now my question would be whether a proper mod would be to not create an open bypass, but to simply weld a pipe to the OUTSIDE of the piping. No airflow, just vibratory resonance from the main pipe which could amplify some of the noise without actually bypassing the muffler (which is essentially what the UK mod does). That would be a matter of how much vibration would be able to pass through as noise, which may not be much on a system with a large resonator plus a Helmholtz. Not to mention those pipes are already sharing vibration by going into the same muffler assembly anyway.

So I suppose I will do a bit more research and see if I can find a good muffler option, and perhaps just swap mufflers (or bite the bullet and get a full catback). Whatever I do, I may test out my theory of doing a "closed" UK mod and see if it makes a difference at all. I kinda doubt it.

Thanks again for the response. Very helpful.

 

I don't think a closed pipe is going to do anything at all. Vibrations are so overpowered by actual exhaust notes, even stock.

Its easier to make it quieter than louder, as you can do my washer trick to quiet things down. Whereas louder would require uk do over, plus hacking out previous bypass pipe.

So my recommendation based on what I am just guessing would be your desired volume level, is to start with a 1" bypass, on just one side (passenger, muffler with more direct flow, much more heat shielding on this side, demonstrating how much more flow it gets).

Then decide if you want to do the other side same size, bigger, or not at all. You can also nozzle down the 1" woth washer if its already too loud with just the one side.

This way you can go a little bigger, a lot bigger, or smaller, depending on how you like 1" one side.

It'll be difficult to know what size to make side 2, as it won't react the same since it fets less flow. Not to mention complexity of human perception of sound intensity (which it seems you're already familiar with). But at least you'll know if you want to go up, or down from the 1" one side.

Don't worry about balance, its already imbalanced stock as evidenced by heat shielding (or just place your hand at each muffler exit and feel difference in force of exhaust).


Fianl thought. I looked into variable uk mod. Using inline butterfly valves on each side. You could use those dampers meant for exhaust cutouts. They don't make any that allow anything other than full on, full off. But you could use a stepper motor, with a sort of volume knob in cabin, to dial up as much loud as you want.

Cruise control uses a stepper motor that opens a butterfly. Its already hardened for hot, harsh environments. Can use any junk yard plentiful version (need 2). Then arduino or something to control it.

One problem, volume won't be proportional to degrees of butterfly open. A butterfly is not a linear flow control, and sound level not linear with change to flow.

So not sure if this wouldn't be more like on or off as to sound level anyway. Which leads me to second idea:

Just use those aftermarket cutoutd, to achieve 4 volume levels:

Both closed, stock
Both open, full loud
Left open & right closed
Right open & left closed

Those last two would be different, since one muffler gets so much more flow than the other. Not sure how much different, guessing it'd be subtle.

Problem may be finding cutouts small enough. They're typically used with V8 hot rods with giant pipes.

You could also do stepper motor thing, and still use set steps instead of infinitely variable, as maybe these wouldn't be so much on off but off, halfway on, on, as to perceived volume.

Both closed
Both 'halfway' open
Both full open
One halfway open, other closed
One halfway, other open
Etc.