zzkamikazezz

So I got bored this evening and started reading up on tuning exhausts to remove drone and think Im at the stage where I know what needs to be done!

Would any one be willing to help with a bit of experimenting involving welding and bending a length of piping to my exhaust to see if this all works?

There are loads of examples showing the calculations I cover below working. I'll explain what I've come up with, please correct me if you spot any silly mistakes. Basically it involves cancelling out the offending frequency. I seem to get the drone at around 3500 dying out towards 4000 rpm so went with 3600rpm for the calculation.

To calculate the frequency causing the drone you do this calc:
f = RPM * pulses/rev * (1/60)
f = 3600 rev/min * 2 pulses/rev * (1/60)
f = 120 pulses/second = 120Hz
Now you need to find the length of the wave at the frequency you just calculated. Wavelength is denoted by lambda (λ), units are meters.

λ = v/f = speed of sound / frequency
λ = (343m/s)/(120Hz)
λ = 2.858 meters

This gives you the length of a full sound wave. The idea here is to reintroduce a sound wave into the exhaust that is 180° out of phase with your drone frequency. To do this, you build your resonator tube at exactly one-fourth the length of the resonant wave. By the time the sound wave enters the resonator tube, bounces off the end and re-enters the exhaust stream, the amplitude is exactly opposite of the drone frequency and will lower or eliminate the volume of the drone.

So in this case the resonator needs to be 71.25cm

So this is the length that needs to be added. Quite long, however it still works if there are bends. I been having a look at undercar shots and I reckon something could be fitted coming out from the exhaust where the oem resonator is and taking up space below the prop shaft area.

So does anyone think they could help try this out on my car?! It'd be great if we got it working for our cars

Here are some threads where it has worked 100%
http://www.performancetrucks.net/for...-drone-489463/
http://forums.corral.net/forums/gene...y-gone-12.html

Here this guy ended up with a really long one but didn't calculate the length so shorter may have been possible?
http://www.northamericanmotoring.com...ust-drone.html

Under car shots of the s2000:

MB

Have you got a decat already? What system do you have?

I think a lot of exhaust turning is a bit hit and miss!

zzkamikazezz

Yep, already a silenced decat. I've the greddy/go tuning se dual. Not too loud but a bit of a droaner.

Seems like quite a few success stories across other forums with this that it makes it worth a try?

MB

Yup i've no doubt a resonator pipe would help.

An exhaust is pretty complex in terms of resonance. Normally you can tap test and find a natural frequency of a system, but with an exhaust you have the added issue of the big rpm range and gas flow it sees. So depending on length of resonator pipe you add, you may just move it further up the rev range, or out of it.

Or you could just use an ASM 70mm single or Mugen, which has zero drone. This is where false economies come in. These systems are expensive because they have been tuned to not drone, which costs money...

zzkamikazezz

I had considered switching exhaust but nice as the mugen is I don't want to go smaller than the 70mm mine is now I'm FI, plus it is so quiet.

The asm isn't made any more and Im still not sold on the single exhaust look. I see go tuning say they sell reconditioned ones how ever, it says contact them so they may have stopped anyhow.

Good point on shifting the frequency. I'm hoping this shouldn't do that as it isn't changing the length or diameter of the exhaust so the frequency shouldn't change. This will just target the problem wavelength by bouncing it back in to the system. Well that's the idea.

Not 100% sure if putting it after the cat or if each branch after the split at the rear needs one though. Honda put theirs after the cat so that would be the first place to try logically I reckon.

MB

GoTuning are coming out with a quiet 70mm Ti system which will be well priced No ASM in stock atm IIRC. They do pop up on here from time to time.

zzkamikazezz

That could be interesting. I was looking / lusting over ti exhausts as well. Most of them seem so damn loud, so if they do a quiet one that'd be cool.

I might give this a go to start with, it shouldn't cost too much to have a play I reckon.

lovegroova

I think you need your ears testing if you think your current exhaust is quiet. I could hear you coming from about a quarter mile away

zzkamikazezz

It is pretty deep. I still don't get how it sound tests at 94db!

Floppy

You think Saz's DMS system is loud as well though and I don't think it is at all. It's all subjective to personal opinion IMO.