Noise

Martin Perry · Dec 9, 2004

This has probably been covered before, but...

My car was noise tested at an airfield track day two weeks ago (Hullavington, Wilts) at 105 db. Has been consistent so har this year at 99.8, which has been useful for this track and Castle Combe which have 100 db limits.

System is standard and in good condition externally, but I suspect there may be a baffle working loose to give this higher reading.

Anyone else had a similar experience?

I tried an additional muffler on the tail pipe, on loan from Merlin Motorsports at Castle Combe (cheers gyus) and it only took off 1db!

I don't want to have to replace what is otherwise an OK system ahead of time, but in the mean time will miss these two tracks which are my local ones. Hullavington is also the only one which keeps going late in the year. Also great value, £99 for the day and open pit lane, but I digress. Any ideas?

Thanks.

Guest · Dec 12, 2004

Dude,
The art of noise testing seems to be very un-sientific - operators can stand basically anywhere, at any angle, plus use a different testing rig from the others. The results are usually different. You could dispute the reading and ask for an alternative reading (maybe??)

Melv · Dec 13, 2004

I'd second what Peter says.

Depressing the clutch is worth 1-2db. Those Merlin pipes are 2db max......have only needed to use one where borderline.

Even a cloudy sky and change in atmospheric pressure can make a difference.

Rgds
Mel

Martin Perry · Dec 13, 2004

Thanks guys.

A mate also suggested I turn up late on the basis that often there will be only one marshall available to sound test and you can ease off the revs a bit.

Sounds like a 'get round it' rather than a 'solve it'! I'll give it a go...

Matin

Guest · Dec 13, 2004

Sounds like my car last year - consistently tested at 102db with a std 3.2 exhaust. I had to plead with the Castle Combe guys while pointing out that it was totally std in order to get on track. Closer inspection later in the year showed that the cross over pipe needed replacing. Now it tests at 95/96db like any std 3.2 exhaust ought to test.
The std test is 1 metre from the pipe at 45 degree angle at 4500 (or 3/4 max) revs. The tests have in my experience produced very repeatable results at a number of venues and from my own tests with a borrowed noise meter. My own tests on a 3.8 RSR (incidentally 105/106db with a modified Supercup exhaust) also proved very repeatable and matched the results at a couple of different circuits.

If you are testing over 98db with a std exhaust, I think you have a problem.

Richard

Andy Tims · Dec 13, 2004

Could be something to do with Castle Combe - either their kit or methods.

I got on to Goodwood OK on a 98db day, but @ Combe they tested me @ 100.5. Did get on, but had to use a bit of persuasion.

Standard exhaust on mine, but higher rev limit on the CS.

Andy Tims · Dec 13, 2004

ORIGINAL: Melv
Depressing the clutch is worth 1-2db.

Mel

Probably being thick , but do you mean 1-2db more or less?

Jamie Summers · Dec 13, 2004

ORIGINAL: Martin Perry

Thanks guys.

A mate also suggested I turn up late on the basis that often there will be only one marshall available to sound test and you can ease off the revs a bit.

Sounds like a 'get round it' rather than a 'solve it'! I'll give it a go...

Matin

Will definitely be using this tactic on my next visit to Bedford - I just don't think there's any other way of getting my new 105db 964 past the test !

ps for whatever reason 911s seem to suffer particularly on static tests but are usually ok on driveby tests. As Mel says putting the clutch in removes a bit of transmission rumble which can be worth a couple of db.

pps My old 3.2 had trouble with noise tests (standard exhaust) - never chucked off track but a couple of 'red stickers' on track days. I think as parts of the system get a bit tired they seem to get a bit more noisy.

Guest · Dec 14, 2004

I think you need to change your signature line Jamie!

One easy fix for noise that I have used with good success in the past is to get your local exhaust shop to make you up a piece of bent pipe (technical name, a "downturn"). So long as this is slightly larger than the diameter of the exhaust pipe (after removal of tip/trim/coffee can etc) it can be secured by a large hose clamp and can be used to direct the noise into the ground where it disperses in all directions and effectively fools the noise meter into reading 1/2db lower. For drive by tests you can angle it into the centre or outside of the circuit as appropriate.

Many years ago I helped prepare a 2.9 litre RS with megephones for the Wellington Street Race. Noisy enough to wake the dead, but we got it into the race with downturns fitted. I don't recall db readings but it made a difference. Also, critically for this highly tuned MFI engine, no change in backpressure in the exhaust system. If you don't fancy the bolt on cans that Merlin et al offer, this is another idea to keep in mind. Jamie, you will be severely restricted if you are really pushing 105db. My friend with the RSR had to run under 4500rpm at Bedford in order to run at all. Its a shame we can't run these cars sounding the way they ought to but that's life. I would get a fix before the trackday season for piece of mind.

Richard

Jamie Summers · Dec 14, 2004

Richard,
Thanks, all points noted. The previous owner managed to get it on to Bedford a while back (was a bit 'conservative' with the revs at the test !) and was ok on the drive by apparently. I'm doing a day at Bedford on the 5th Feb, so we'll see how it goes then. Long term I think I'm just going to have to replace the 'cup pipe' with a standard primary silencer - shame ! Mel Spear has done this on his Cup car and now is ok at noise testing time ! A downturn might be a worthwhile short term solution - I'll have to see if I can think of anyone to make one up for me - any ideas ?

Yes, must get round to changing sig etc - just waiting for some sunny weather to take some piccies !

Martin Perry · Dec 14, 2004

Jamie

The guy who looks after my car in Bath would sort a downturn for you OK, and I'm going to aks him to do one for me. Hopefully that and 'clutch in' will enable me to get some more track time out of this system.

Not close to London, but a very pretty place to visit. There's currently a temporary ice rink in the showpiece Georgian splendour of Queen Square too, a la Central Park. Avoid weekends though - too many shoppers.

Cheers

Martin