Your Exhaust Decibels

It could be useful for people to share their exhaust volume here...? I would find it useful anyway . Now circuits are clamping down of noise levels, it would be great to get an idea of different systems and set-ups - before getting to the scrutineer/sound man.

eg; Will a single 'box (2.5 inch pipework) pass?

Mine:
93DB - mk1 golf, flowed cast manifold, 2.5 pipework, centre and rear box, mild steel.

 

Surely depends on engine tune/modifications?

 

Ankor powered VR6. STD engine honest. 89dB@ 4K rpm today at Bedford. Exhaust system Miltek 2.5 inch

 

Would it?
Yeah, the Red line would affect it, as most sound tests are at ....% rpm from maximum.

 

3/4 of your max RPM, my 1.8T is 103 @ 4500 rpm

 

straight through supersprint backbox mk2 8v modded. 89db @4000

 

MK2 16v Modded runs a Miltek manifold and system. 94db @ 4000rpm

 

Thanks for the replies.

 

This is an area that I find highly frustrating being a sound engineer. Sound pressure levels get banded about in the motorsport world without any real meaning attached and with no apparent standardisation.

A figure of 90dB means absolutely nothing unless the distance and angle from the point source is also given, also there are several different weightings which can be applied to sound pressure levels which determine which frequency ranges are included in the level and to what extent they effect the final figure.

Presumably most of these figures people quote will have been measured "A" weighted but its probably worth checking as many of the people doing the checks probably don't understand either.

Sound pressure level of a point source will theoretically have a spherical wavefront, therefore any doubling of distance means a -6dB change in measured level. From this you can see that if the level is measured 2 meters from the exhaust or 0.5 meters from the exhaust it will have a large impact on measured level. We could also consider whether it is measured on axis with the source (the exhaust usually) or at an angle to the source as this can have a significant effect too.

There could be further things to take into account, for example ground proximity effects or considering the noise to be a line source at greater distances however the simple point source example I have given shows how there is a need for more information in order to understand these quoted levels.

Apologies for the rant, hope it was mildly informative at least to some.

 

Having been a Tour & Poduction manager for over 10 years I understand what you're on about & you're probably right about those measuring not really knowing much except the figure on the little screen on the db meter.

However most sound tests I've had done before racing are done with the meter attached to a 1m steel rod which has a 45 degree angle at the end which is held near the tailpipe.

So the measurment given would be XX dbA @ 1m, 45 degrees off axis.

For the record my Mk2 8v race car with a sraight through 2.25" Pipewerx single box system usually reads 97db @ 4000rpm

 

The noise police dont need to know what its all about, at the end of the day if your car reads over 105db on there little screen at a distance of 1mtr @ an angle of 45deg you wont be going on track unless its a track day then its 103db

 

If I had a car that was over the DB reading, I would be on the case of the marshall and measuring the distance and angle to check

 

I wasn't saying the assembly area marshalls needed to know the ins & outs of SPL measurement, I was merely explaining to Jimmy how some consistency was factored into the measurement by the use of the angled 1m rod.

 

Tested at Brands today.

Flowed cast manifold, 8v. Single 'box, 2.5 inch system - 100 DB @ around 4500 rpm'ish.
Sounds louder in the car!

 

Think mine is 104.3 @ 5500 . 8v with a single Jetex rear box (the biggest S/S one they do cut off with a 30mm rear section) all on a 2.5" system. To be fair I'm running a 320 degree cam and a very modified manifold in to a 2.5" collector so probably as bad as you will generally get so with less cam it should be quieter.

Based on that I would think a single silencer 2.5" would be fine for racing but probably a bit close to the limit for trackdays. Bit close for comfort this year for my racing as well, testing can vary a bit, so might be a bit conservative about the rev limit

 

Thread revival:

The current system on my Mk2 ABF 16v track car is a KR e/m with standard downpipe and a 2.5" Jetex stainless setup. I have a custom Y-pipe in place of the front resonator box, straight through centre section and then the twin rear box setup (one box before rear beam, one after) and twin tailpipes.

At trackdays last year, I was only recording 83-85 db in noise tests at 0.5m and 4500rpm, so there is definitely room for more volume!! I'm also concerned that 2.5" bore is a little on the large side for a 170bhp 2.0 16v, so have now got a quote for a 2.25" custom system with twin (smaller) rear boxes and a single 3" tailpipe.

The thing is, can I in fact get away with dropping to just a single back box and still meet say a 95 db noise limit, or is best to stick with two smaller boxes instead?

 

I have the 2.5" Jetex but without the front "resonator" box and its between 91-94dB. Not sure how much of this is down to the lack of resonator or the difference in outputs between the two 2.0 16Vs

Quiet enough for the UK circuits that I can think of.

@ Smudge, shouldn't 3/4 rev be at 5600 rpm on your car mate?

 

My custom exhaust is 2.5 on my 130bhp 8v. So don't think its overkill.
As for 1 or 2 boxes i was advised to go for 2 on my current setup.
Just because only had 1 on previous exhaust which was 2.25 but after
5 or so track days started to burn the baffles in the rear box and failing
sound tests. 101db must add that the exhaust was quite old so not sure
if it was on its way out anyhow. I think having two boxes helps with the heat.

 

Lol yep looks like I got confused with the lowly 8v engine there! 4k is more like half the rev range!

 

I am a type approval engineer, and for new vehichle and relacment silencer homologation tests to european regulations the static noise test is taken 0.5m from the tail pipe, mic level with it, and at 45deg angle, both inwords and out words to find the worse case. The revs used to always be 3/4 of PEAK power revs, but this has been changed a little so if the peak power is between 5K and 7K iirc it is now taken at 3750rpm, the noise test is done by holding the revs steady, the mic is rest and then the throttle closed, and it is the peak sound level inc any pops on the closed throttle run down,

If the msa book says 1m distance then that's easier to pass.

My 1.8t at bedford with middle box, and back box (big boxes) in 2.5inch with K04 turbo was a piffling 78 db (A) max, its way too quiet!