Silencer Advice

[Stephen Tonks]

Hi. I had a sound meter as a christmas present. I have just got round to doing a test today, and am a little concerned. I have put my car forward for the Cadwell Park do on the 17th March 2010. Having done a test at tick over, the meter was reading 104 dB. Apparently there is a maximum of 105 dB on the circuit at Cadwell, so I dread to think how loud my car would be when measured at 4 or 5000 rpm The car is fitted with a stainless steel exhaust system, but appears to be a straight through with little or no packing. Has anyone got any words of wisdom as to what I can do to resolve the problem.

Stephen

[Curtis]

I may have a very simple, and inexpensive fix for you.
Check all so how they check the level's.
(distance, and angle)
Going to out in the shop later today.
I'll see if I can make a quick mock up,
to take a couple of pictures for you.
A picture is worth a thousand words !
I'll send them to you this evening.

Cheer's

[Martin]

Stephen,
Mick Sollis at Southern GT does some inserts that just fit in the back of your tail pipes. Maybe worth giving him a call.
Martin

[Curtis]

Here ya go !
Please forgive my cobbled up mock up's.
But I think you'll get the idea.
I used the single on a car I was driving on the street that had straight pipes.
And got by the cop's with out getting a ticket.
You don't have to drill the mounting holes till you test them.
Just use some small vice grip's, or c clamp's to test.
Just a thought, and Cheap to do !

Cheer's !

[Ian Anderson]

Steven

Where was your car when you did the test?

Were there walls / fences etc on the side or anything behind the car for say 30 feet?

If so you could be getting a fair amount of echo / bounce back making your level higher than it is.

But that said I'm looking at getting something to quiten mine too


Ian

[Chuck]

Curtis:

Your concept looks similar to the Vortex insert. The effectiveness of this device has been debated on other forums, but might be of interest.

Dynatech 72-32520 - Dynatech Vortex Insert Cones - Overview - SummitRacing.com

[Curtis]

Same basic concept for sure.
I went for the V rather than the screen type because,
I think it would deflect the sound waves much better,
while still allowing a good flow of exhaust
And with it only taking a few min. to build you can make a few of a diff. size.
And be able to tune to your needs, or likes.
Just a little bit of scrap, and a hour or two and your down the road.

Cheer's !

[Malcolm]

Top man Curtis, instead of just saying you had an idea to quieten an exhaust down, you showed a picture. That is very refreshing! Such a simple idea and if someone can verify it works it will help loads of guys. Genuine thanks.

[Steve H]

Steve - does your sound meter have a 'weighting' switch? If so, check which type they are using at the circuit - it's usually 'C' - which seems not to be so onerous. My meter has weighting A and C. On A the car registers very high dB, but switched onto C it is less so.

[Stephen Tonks]

Hi Guys.

Wow !! Thanks for all the advice. I shall definitely have a go with the design submitted by Curtis. I would also like to respond to some of the other comments. There is a fence at the side of the car but there is nothing behind the car. There way be some echo, so I will try the test again where hopefully nothing will interfere. My sound meter does have a weighting switch but unfortunately it is already set to "C". I am conducting the test based on information I got off one of the circuits' web sites. They say the reading should be taken at 50 cm away from the exhaust at 45 degrees.
I have seen some silencers sold by Merlin Motorsport. An additional silencer fitted to the end of an exhaust tail pipe. Used on track days where the existing system fails the noise test. The Merlin Muffler can take from 0 to 9 Db off the noise reading. They do a try before you buy service, so it might be worth a look.

Stephen

[Jason Lindsey]

Hi Stephen

I am looking at those slip on silencers from merlin too. I have a GTD with the standard GTD exhaust, but it is very loud. I like it very loud, so need something so i can get it on a track day. I have also looked at the Supertrapp silencers that look like discs on the end of the exhaust. Not sure how effective either of them are? I'm probably asking too much from a slip on type silencer to quieten my car down enough.

Jas

[Malcolm]

Jason

With either of the GTD systems (cross over and non cross over) if you repack them, you should be able to go on a 105 Db track day. A pair of bolt on extras may be worth while in your tool kit but I have always run a standard GTD silencer and never had issues if it were properly packed. And to make a non repackable system in to a repackable system is not rocket science.

[The Baron]

Originally Posted by Bigjas

Hi Stephen

I am looking at those slip on silencers from merlin too. I have a GTD with the standard GTD exhaust, but it is very loud. I like it very loud, so need something so i can get it on a track day. I have also looked at the Supertrapp silencers that look like discs on the end of the exhaust. Not sure how effective either of them are? I'm probably asking too much from a slip on type silencer to quieten my car down enough.

Jas

I used Supertraps on a 550 hp SBC with no silencers at all. With these devices I passed sound tests at Donnington, Goodwood, Castle Combe and Lydden Hill. Mind you, this was a few years ago and am not sure if they have changed the rules since. I also adopted a much lower "peak revs" figure for the percentage test, but one has to be careful with that because if the track also has a drive by on the main straight, then you will see the meatball flag for sure.

To be honest, if I was to do it today, I would prefer a properly designed system which successfully balances noise/power for all applications. There's quite enough to fiddle with without burning your hands on hot pipes...

BTW I would add that I firmly believe (but don't have the science to back it up) that Supertrapps are far more efficient in scavenging mode when placed at right angles to the direction of travel and sited in the airflow. This of course, would not suit a '40 asnd like I say, I have no data to back that up, but it certainly 'felt' better.

[Randy V]

Originally Posted by stephentonks

Hi Guys.

Wow !! Thanks for all the advice. I shall definitely have a go with the design submitted by Curtis. I would also like to respond to some of the other comments. There is a fence at the side of the car but there is nothing behind the car. There way be some echo, so I will try the test again where hopefully nothing will interfere. My sound meter does have a weighting switch but unfortunately it is already set to "C". I am conducting the test based on information I got off one of the circuits' web sites. They say the reading should be taken at 50 cm away from the exhaust at 45 degrees.
I have seen some silencers sold by Merlin Motorsport. An additional silencer fitted to the end of an exhaust tail pipe. Used on track days where the existing system fails the noise test. The Merlin Muffler can take from 0 to 9 Db off the noise reading. They do a try before you buy service, so it might be worth a look.

Stephen

50 CM is Awfully close... (like 20 inches?) Racetrack DBA meter readings here in the states is generally 100 feet from the track. Are you certain that their spec isn't 50 m (as in Meters?) - which would be more like it...

One thing I've learned over the years is to NOT point the exhaust down (at least not in close proximity) to the ground. It "Cracks" much louder and the sound pressure reading is much higher.

I've used the Vortex cones that Chuck has shown here. On one of my cars (310 Cubic Inch) - the engine would no longer make it to over 6200 RPM - shift points were normally at 6800 RPM. While it did knock the pressure reading down by 5 dB, it was quite literally like stuffing a potato in the tailpipe as the engine was very lazy afterward..

[Curtis]

I don't have any experience with the inverted cone system.

How ever I used super traps on my old sprint car.
The exhaust comes down and hits a dead end, and is forced out the sides,
between spaced plates.
On these I used extra spacers and plates, along with leaving the bolts lose.
Yes I used something like lock tight as not to lose them.
They worked, and yes they rattled, but didn't make any more noise than the gear drive in the eng. ( at least not from the driver seat ;-) )
They never checked the dB level, you just had to have them !

Now if you take the area of one of your exhaust ports and deduct that from the area of your pipe coming out the other end.
What's left is the size of the V deflector you should be able to use with out adding any back pressure. (limiting rpm or flow)
And if using the 2 V deflector style it may add a bit more pressure, but don't think to any effect.
Should just deflect the sound waves a bit more, while still providing good flow.

I did it, and Never burned my fingers a bit !

Cheer's !

[Damian]

This might not be what you are after but why not call Burns Stainless and have them make you something? I had them make me an ultralite version for my M3 and under idle you could barely hear the exhaust. Burns Stainless - Home

[Stephen Tonks]

Hi.

I have been doing a bit of work on my silencers in order to address my sound problem. Before I go down the route of undertaking the work that Curtis outlined, I thought I would have a look at the exhaust system, and try and address what I think might be the root cause of the problem. With that in mind, I proceeded to get my grinder out and cut a rectangular section out of my wonderful stainless steel silencers. These have a bit of a claim to fame, as they came off a Ray Mallock GT40 and I have a clip of a review that Tif Nidell did whilst at Top Gear, on the Ray Mallock GT40, with this exhaust system on.

After cutting the rectangular sections out, I then found that they was very little packing left in the silencers. I have included a photograph of what came out of both(!!!) silencers. The paper I have put the bits on is A4, so hopefully this gives you some idea of the small amount that was left in. I have purchased some rectangular pieces of stainless steel and have had them bent and drilled, at a local machine shop. Once the silencers have been re-packed, I can pop rivet the stainless steel back on. I have purchased some exhaust packing material from Merlin Motorsport, price £13.50, and plan to do this job over the weekend.

I may well have to go down the route of adding something in the exhaust, but clearly the lack of any significant packing in the silencer, is a bit of a problem. I have also included a photo of one of the silencers.

Hopefully this will improve the situation, but I shall report back my findings in due course.

Stephen

[Trevor Booth]

Stephen,
it is probable that the track sound level meters will be on A weighted frequency, A weighted is more universally used than C weighted. The A and The C refer to frequency filters. You should also have your SLM set on fast response.

The track noise limit specified should include the weighting used i.e. 105dB(a)

if you are getting a reading of 104 dB(a) with vehicle idling I would suggest your SLM needs calibrating. The normal speaking voice is about 60 dB(a) when no background noise is present. < 40dB(a)-50 dB(a)

[Jac Mac]

D.I.Y.

For some reason the drawing 'lost' a line in the upload along the top or what would be the RHS of the muffler case.... however this should get the basics of what needs to be done... works on most stuff I have built them for, cant say whether it will work with some of the distances/ angles I have seen posted here...

[Charlie M]

I bought a dual inlet/outlet Flowmaster muffler, I think it was this one:
Flowmaster 9525454 - Flowmaster Super 40 Series Delta Flow Mufflers - Overview - SummitRacing.com

I've only run the engine on a test stand but it is fairly quiet (a LOT quieter than my Cobra).

Charlie