theoretical exhaust noise

[Rusted Pistons]

I'm going to make an exhaust myself as it seems its gonna be £300 at the very least to get someone to do it for me, not to mention getting a car with no MOT and an engine that hasnt been run in yet to a place to do it! My mate who owns a garage can get hold of 3" mild steel pipe in 2m lengths, and I've found a supplier of old school cherry bombs in 4" can and 3" inlet and outlet so I'm going to make it out of 3" pipe right the way to the back of the car with only the cherry bomb at some point in the pipe. Its a CA18DET and I have a nice wrapped tubular manifold and 3"downpipe already. What I'm wondering is in what way do you think the noise would alter if I ran the cherry bomb in the middle of the system and had simple pipe coming out at the rear or if I ran the pipe to the rear and had the cherry bomb as a rear box? I have an ignition cut launch control system so rev limiter will produce lots of big flames and explosions ;D how do you think these would be affected by whereabouts I place the cherry bomb? Ta laas! ;D

[dbizzle]

I have no advice to offer, but just want to register my keenness to see & hear this once you've knocked it up! ;D

[jasonb360]

well, in terms of physics... (uh oh I hear you cry!) - a silencer placed at the end of an exhaust system will be more effective at attenuating lower frequencies than a silencer placed nearer the engine. This is because the speed of sound increases with temperature.

A very basic example is... if you have a silencer that is 0.75m long then for air at 400degrees celcius, the speed of sound will be 520m/s (roughly!!) a crude estimate for the frequency of a single wavelength that fits into the 0.75m silencer is speed/wavelength = frequency.

At 400 degrees, near the manifold, the "lowest" frequency that could be attenuated by the silencer is 520/0.75 = 693Hz. Whereas at the end of the exhaust, if the temp is ~90 degrees for example then the speed of sound is 380 m/s and thus in the same silencer 380/0.75 = 506Hz. Therefore the silencer would improve attenuation for a further 190 Hz frequency band lower than if it were placed near to the manifold.

That probably makes no sense as I just rambled a quick example. In reality the problem is completely different (silencer performance is not based on whole wavelengths, temperature performance changes... silencer volume et. al.)

However I hope that gives you the basic gist...

Summary:
- silencer length is broadly proportional to low frequency performance
- silencer performance at low frequency improves with increasing distance from engine

Hope that helps even a little bit?

Cheers

Jason

[markgenesis]

A 3" system with a 4" Cherrybomb, i predict that'll be damned loud indeed , should'nt really matter where the Cherrybomb is as they are marginal silencers anyway but i'd use it on the rear of the system, big flames and explosions should be no problem

[Rusted Pistons]

Wowsers, I don't even know what attenuated means! ;D ;D

I sort of get the gist though, the further away the silencer, the better it will silence, so If I have a cherry bomb (which doesnt really silence but would make the noise sound 'less like a tank and more like a race car' for want of a better description) is it that the further away the cherry bomb the quieter but boomier, the closer to the engine, the louder and raspier?....! ;D ;D

I guess the pops and crackles it should make on the overrun would be more pronounced if the cherry bomb was somewhere near the middle...

oh and dbizzle - keenness noted! ;D

[jasonb360]

It should be less boomy the further from the engine... however, a cherry bomb is barely a silencer so you'll probably barely notice the effect of moving it around the exhaust system.

I'd go for a backbox though! - more practical

[Rusted Pistons]

markgenesis - cheers, I predict and hope massive loudness too

[Rusted Pistons]

Right, this'll be my setup then;

Engine - tubular manifold - big turbo - 3" elbow and downpipe - 3" pipe - 4" cherry bomb in middle of pipe - pipe sticking up at rude angle under the rear valance -LOUD NOISES!!

Cheers for the help guys

[jasonb360]

hehe, I too am going for some loudness on my mk1 cav - getting a tony law manifold... and going to run it into a single silencer 2.5" system - can't beat a good asbo exhaust on the right car.

Best of luck with it! - and post a vid with some revving when it's all done

[Rusted Pistons]

Excellent! Vids should be posted when the systems are made!

[markgenesis]

Got the ASBO bit right, i've heard the law are clamping down on loud exhausts in some areas, using sound meters at the side of the road , last time i used a Cherrybomb was on a Hillman imp and that was freakin loud, would set car alarms off no problem, had to push it to the end of my street when leaving for work at 6.00 am

[jasonb360]

yeah, my peugeot 106 rally car failed an mot even though it had a genuine peugeot groupN exhaust which was completely "construction and use" and TUV certified.... (all of this information was stamped on a visible shiny plate on the exhaust too!!) - not to mention it is barely loud at all compared to most cars.

However, now I am back living in the sticks again I can use my local garage... who passed the same peugeot with a completely straight through 2" exhaust pipe - that was stupidly ear bleedingly loud.

[spunkymonkey]

Jul 23, 2010 21:17:22 GMT jasonb360 said:

yeah, my peugeot 106 rally car failed an mot even though it had a genuine peugeot groupN exhaust which was completely "construction and use" and TUV certified.... (all of this information was stamped on a visible shiny plate on the exhaust too!!) - not to mention it is barely loud at all compared to most cars.
.

That'd be RFR 7.1.2.b:

b. a silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a standard silencer in average condition.

Note that :

(a) it doesn't matter whether the silencer is TUV marked, or marked in any other way come to that. The comparison is with what would be expected from a standard system.

(b) it's a subjective test, so different testers will have different opinions.

Having an MOT based on the subjective opinion of a particular tester doesn't make any difference if VOSA do a spot noise check, because they use objective measurements plus it may not be the exhaust that was fitted at the time of the test

[uncleterry]

If your sticking it closer to the front, at least you have space to attach a proper silencing muffler at the back should you find it too loud (or get pulled over too often - more than likely!). Thats probably the approach id go for if i didnt fancy pulling the whole lot apart again at a later date.
And yeah, the law will hate you with a single cherry bomb. I carry a decibel meter in the car at all times now (go to maplins, £25 iirc, suprisingly decent piece of kit), pretty sure something like 102db is the usual limit unless they're being arsey. If i take the baffles out of my backbox, mine is well over that - even though it still has a cat and midbox and has tiny pipes compared to the 3" your using lol.
Just a word of warning...

[popuptoaster]

a turbo will silence the exhaust to some degree so worth trying with a cheery bomb, i ran my 24v Cortina with straight through pipes and a stock 2.3 back box on it for stealth purposes, it was quiet until i gave it a bootfull at which point it would echo up and down the road and pop back through the pipe on the over run so easy to escape notice and nice to cruise in but still sounded ok.

[needabeetle]

Personally I'd have 2 cherry bombs in there - one midbox, one as the backbox. It'll still be ridiculously loud and spit flames, so don't worry!

I've had several cherry bombs in my time and love them

[kapri]

As mentioned earlier in the thread

www.the-ace.org.uk/loud-exhausts-cost-cash.html?searched=exhaust&highlight=ajaxSearch_highlight+ajaxSearch_highlight1

[jasonb360]

Jul 24, 2010 8:41:44 GMT spunkymonkey said:

Jul 23, 2010 21:17:22 GMT jasonb360 said:

yeah, my peugeot 106 rally car failed an mot even though it had a genuine peugeot groupN exhaust which was completely "construction and use" and TUV certified.... (all of this information was stamped on a visible shiny plate on the exhaust too!!) - not to mention it is barely loud at all compared to most cars.
.

That'd be RFR 7.1.2.b:

b. a silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a standard silencer in average condition.

Note that :

(a) it doesn't matter whether the silencer is TUV marked, or marked in any other way come to that. The comparison is with what would be expected from a standard system.

(b) it's a subjective test, so different testers will have different opinions.

Having an MOT based on the subjective opinion of a particular tester doesn't make any difference if VOSA do a spot noise check, because they use objective measurements plus it may not be the exhaust that was fitted at the time of the test

Yes, that was my frustration with the tester really, the silencer is in fact much bigger than a standard silencer and the car isn't any louder at normal running revs. Only at 5500rpm and above does the sound get raspier due to the higher flow. However, I understand that the law is the law - apparently if I owned a subaru impreza then that could be loud because thats how they are from the factory.

I don't care too much about a spot check anyway as I'm an acoustic engineer... I'd actually quite like to see what equipment they are using and the manner in which they perform an "objective" test. Noise stuff like this is quite a big arguement amongst road rally guys, as the stationary "unloaded" noise test is fairly ineffective at determining the noise of the car when driving it.

[PhilA]

Having tried this with Peco straight-throughs in several different permutations on the same car, putting the thing at the back of the car gave a snarly note, not as bass but a bit more purposeful sounding. Putting it in front of the rear axle in the stock position (under the rear seat) made it sound more like an angry Morris Minor- deeper but more nasal.

And yes, as many have said, how loud the car is at the MoT station and how many old people phone to complain about it is subjective. My Victor was so loud it'd set car alarms off driving down the street. The MoT man used to sit and blip it on the ramps just to listen to it...

--Phil