UK Circuits noise regs - 98Dba @ 4500 rpm

[Julian West]

Gentlemen,

Has anyone on the forum managed to get a Ford 302 engined GT40 down to 98Dba @ 4500 rpm? If so, would you be prepared to share any tips, advice or opinions on how to do so?

Thanks in advance.

Kind Regards

Julian West

[Malcolm]

Mick Sollis exhausts have been noted as quiet and not the size of a suitcase. Suggest you give him a call. He makes it all in house now I believe.

[sibbat2]

Julian, the only circuits where I can take my CAV GT40 are Silverstone and Rockingham where noise is less of an issue. I think the drive by and static noise limits are 102db and 105db respectively. Because I live in Bristol which is not very far from Castle Coombe, last year I thought that I would try a track day there. So I had my muffler repacked, put in decibel inserts and still failed to pass the static limit of 100db at 4500 rpm. My Ford V8 is pretty standard with 400hp.
I think the only way to get below 100db is to put in a bigger muffler. I think Frank Catt does them, although not aesthetically pleasing they seem to work.

[Tony Hunt]

Julian,
Rejoin the Enthusiasts Club.
We organise noise-friendly track days at a number of venues each year, as well as going to Fay de Bretagne near Nantes in August, where there is no noise limit.
Forget Castle Combe. The only person to get below their limits had an extra, full length silencer bolted to the back of the car, supported on a cradle. It looked ridiculous.

[Ron Earp]

Guys, do you know what sort of frequency weighting is used for your sound tests? It is extremely important to have this information so that you may plan a proper strategy and construction for your muffler.

[Brett James-McCall]

Originally Posted by sibbat2

Julian, the only circuits where I can take my CAV GT40 are Silverstone and Rockingham where noise is less of an issue. I think the drive by and static noise limits are 102db and 105db respectively. Because I live in Bristol which is not very far from Castle Coombe, last year I thought that I would try a track day there. So I had my muffler repacked, put in decibel inserts and still failed to pass the static limit of 100db at 4500 rpm. My Ford V8 is pretty standard with 400hp.
I think the only way to get below 100db is to put in a bigger muffler. I think Frank Catt does them, although not aesthetically pleasing they seem to work.

Is your exhaust a CAV stainless one? I heard, and maybe wrongly, that stainless is noisier than mild steel?

[sibbat2]

Yes, it is CAV supplied SS exhaust.

[SteveB]

Julian

I have been reworking my system that was in stainless with twin silencers, headers were 1 3/4", with inserts it passed SVA but over time became louder repacking helped and was passed just, with inserts for the Octane magazine Goodwood trackday.

The reworked system is in 1 3/4" stainless 1.5 wall thickness, the collectors have rotating pulses and the primary pipes made as close to equal length as possible. The first chamber has been made in an antireversion chamber with perf tube and absorbant packing, this then feeds the through type absorption silencer section. The two through pipes are in one box that is about as large as the previous twin oval silencers together.

This new system has smoothed out the exhaust note and removed a lot of crackels pops and bangs and has come in generaly quieter when running up to 4000+rpm for a sound test. After several track outings the Cadwell Park sound check ( repeated 3 times to be sure) gave reading between 96 98 db. The day was a 105db event as was the previous day at Blyton Park. The level could easily be reduced with inserts that are being made.

Steve

[Tom]

Julian,
Two large Supertrapps (18 baffles) stuffed with #2 steel wool was the only way to quiet a street port 13b Mazda rotary on the track. That was with 4 steel pack pre-silencers behind it, but the silencers didn't help much. Only the steel wool made the difference.

If you go this route let me know and I'll offer some tips on how to roll the steel wool 'inserts' and stuff them into the pipe.

[Julian West]

Gentlemen,

Thank you for all the informative replies.

Originally Posted by SteveBarker

Julian


The reworked system is in 1 3/4" stainless 1.5 wall thickness, the collectors have rotating pulses and the primary pipes made as close to equal length as possible. The first chamber has been made in an antireversion chamber with perf tube and absorbant packing, this then feeds the through type absorption silencer section. The two through pipes are in one box that is about as large as the previous twin oval silencers together.

Steve

Steve, your system sounds very interesting. I'd never heard of an anti-reversion chamber. I've googled it & found some interesting info. I think your system is very clever & the anti-reversion chamber looks fairly simple to make and fit inside my existing silencer. I found this image, amongst others showing how they work. I presume there must be a 'magic' formula regarding the dimensions of the chamber. Do you have any pictures of the inside of your silencer?

Cobrajet,

I did consider welding on a couple of Supertrapps in addition to my existing silencing, but I have read varying reports on their noise reducing capability, are they not more suitable for controlling backpressure by reducing/adding more discs?

Kind Regards,

Julian

[Tom]

Originally Posted by Julian West

Gentlemen,

Thank you for all the informative replies.

Cobrajet,

I did consider welding on a couple of Supertrapps in addition to my existing silencing, but I have read varying reports on their noise reducing capability, are they not more suitable for controlling backpressure by reducing/adding more discs?

Kind Regards,

Julian

Julian, that's true for the most part. They're also effective as flame arrestors.

That is where the steel wool comes in. Since there is a back plate, the 14 or so inches of rolled steel wool stuffed into the id of the pipe has nowhere to go and simply starts to compact itself into the baffle area. There is still plenty of exhaust flow with this mod. By adding more baffles you can balance the additional back pressure created by the steel wool insert. The modification can last a few hours on the track with a flame spewing rotary, or a few thousand miles of street/highway driving. When the car got loud, I'd start the process all over again.

Back in the '80's we used this same trick on 2 stroke dirt bikes when riding in noise sensitive areas and where spark arrestor mufflers were also required.

Tom

[SteveB]

Julian

Here are a couple of pics of the inside of the silencer. The inlet pipe projects about 60mm into the chamber produced by the larger bore perf tube, the reversion pulse is scattered by the cone opposite the entry and the projecting inlet pipe.
The sizes are based on the available space and some info on cars that use the idea; most of the idea features are there. If it workes it was a plus if not it still acts as an absorption silencer anyway.
The chamber is also the first lowered pressure zone and the tailpipe effect starts from here to give the longest 'tailpipe' I could get into the system without doing 2x 180degree bends.


Steve

[Ian Anderson]

I thought I read Russ from down under made a silencer yo Jac Mac's design and raced successfully with it.perhaps an e mail to hin or Jac would confirm the db ratings

Ian

[Ian Anderson]

See Silencer Advice

Post number19

Ian

[Mick Ridley]

Here is a pic of Russes silencer. I cant remeber what db level he had with it, but he did say it was very quiet



It works on the design, of a collector box, at the end of the secondary pipes, that fool the engine into believing the exhaust gasses have reached the end of the secondary pipe. ie atmospheric pressure...and any baffles after that will have no effect on the performance of the engine, but it will reduce noice level. You can add as many baffles as you want, given the space available, to make your engine as quite as you want

Its a design that has been around for many years, I have never tried it, but in theory it should work......

[GTRene]

funny, I just came across this, same sort mufflers also without padding to burn out :-) but uses round spindle type sound traps...



they also had those types for strange exhausts, look at this site>>

very interesting me thinks? never seen some things :-)

SpinTech Performance Mufflers - Mufflers - SpinTech Performance Mufflers

[SteveB]

Southern GT have a system available that uses this baffle type of silencer.

Steve

[Keith Baker]

Southern Gt silencers are pretty quiet, ask Mick about there dBa levels.
For my engine and it's output I think it's smooth sounding without loosing its bite.

[Julian West]

To Update,

I was sprinting at Castle Coombe today. I'm using an absorption type silencer that I fabricated myself, newly stuffed with the correct wadding & the scrutineer measured me at 101 Dba @ 4500 rpm.

The noise limit at Coombe for sprints is 105 Dba @ 3/4 max rpm.

So I guess I'm not a million miles away.

Regards,