Has anyone out there carried out any serious development work on silencer design. We have to comply with strict noise testing to be able to use our cars in UK, and particularly at Goodwood and other tracks. Noise measurement is carried out at 3feet from end of tail pipe at 45degrees, and to reach the required 105db is difficult, particularly when you consider that they are also picking up all of the engines mechanical noise. We have designed a silencer for Robins car, which has also been copied and used on an original race car that requires to be tested at Goodwood regularly, but it is big and too heavy, and I need to make some more silencers now for others cars for this year. I do not believe that I can find an "off the shelf" silencer that will do the job, so I am quite prepared to make them, its just the technology that I am looking for. You know how little room we have to get it in. I also make my own header pipes to copy original patterns (see photos of Robins car on Forum ) Frank

There is a company in California who might
be able to help you out. Powertone Performance advertises a stainless steel
muffler(silencer) that actually increases your horsepower by 8-10% via negative back pressure(vacuum)You would have to contact
them regarding the decibel levels that
their mufflers put out.
Perhaps you could benefit from their
knowledge if not their product. Please
post what you find out. I'd be interested
in 8-10% more horsepower myself if their
advertisement isn't just full of hot air!
Best of luck in your search.



Robert Logan

Defunct Manufactuer - Old RF Company
To comply with Australian Design Rules (ADR's) we have to get down to UNDER 90 db's although as a rear engined car we are given an additional 2 db's .
We also use cats and there are some pics of our ADR exhaust system on our gallery on our web site at .
Please have a look.
I remember well at Castle Combe, Ken Saunders moding cars to pass the noise meters. Oh, we to test for noise with a static test very similar to yours but we also use a drive by test which is a little harder to pass.
Research and Development is not the cost of the parts ON the car but the cost of the parts in the BIN !!!!! I can not remember how many systems we binned and how long it took and I realy do not want to count the cost now.
Good luck, best wishes,
Robert/Bill, thanks for your replies. I too have a big bin of bits, but although I managed to get 105db and an increase in HP over the standard GTD system, I still think that the technology is out there to help us get it better. Several factors apply,including increased noise from higher performance,higher compression engines (low compression/low hp engines no problem)and also from mechanical noise from the engine being added to exhaust noise.I have tried temporary accoustic blankets and baffles to overcome the latter problem. We do not usually have a problem with drive-by noise test, and of course we do not have to use cats yet!I will continue to develope my ideas, and let you know how I get on, but as you also know, the only way to check whether an idea works is to make it and test it, which gets time and money consuming. I would also like to get more input from anyone else who has ideas, even if they have not tried them out yet. Frank


FCatt, Have you looked at the "super trapp"? It's nice because you can add or subtract plates to tune and to adjust silencing, and it's very compact.


Bill, just a note of caution about decreased back pressure and increase of horsepower. It is often the case that this will give gains on the top end at the expense of low end torque. Just make sure that the give and take fits with the power profile you want from your engine. My daily driver is a Dodge Stealth (NA) to which there are precious few mods that can be done. I put on a K&N and the only other real option I had was a new cat back system; with research I found out that these systems would cost me low end torque which for most street use is not the way you want to go. Now if you are tuning for the track where top end is more important this might be a good thing.

Not sure how you ended up with your silencer, whilst it was quite large the silencers that we made for our car were whisper quiet. My build log (RF 117) has some fairly detailed pictures of how we/they acheived this. The workshop have made some tools to enable them to replicate silencers if required.

Hi Frank,

My Findings are as follows prior to recieving noise test in the paddock stuff the silencers with wire wool :laugh:, It is great to see it blowing out on the back straight.

As we will all agree it is unfair how the noise tests in the paddock are carried out, a car passing the pit lane at 120+ mph will not show as great a reading on the db meter as when the car is standing still.

I have demo'd this to many official at a local circuit to here, not in the GT but with an F40 and once the turbo's wer up and spinning slightly less db was measured.

I dont think this will help with muffler design but it is worth the argument :furious: with the officials.


I'm not an engineer, but would something like is used on motorcycle exhaust work on our cars?
It is a db-killer you put in your end pipe and one screw to hold it in place.
So it's loud or quite when you want it to be.

It works on a / my motorcycle, so why not a car?


Mike Pass

Hi Frank,
One of the things which may help is to prevent surfaces flapping or panting especially with the thin grades which are often used in stainless systems. Using a cone or a hemisphere at the ends will cut vibration from flat panels acting like drum skins. Also I noticed that Simpson use swaged ribs on their silencer boxes to reduce drumming of the surfaces.
Good luck with your efforts to reduce what we all know is a beautiful noise.
Last week I went to my motorcycle dealer/tuner and asked him about the thought of a DB-killer for the GT40.
He actually also suggested a removable hemisphere. He offered me to produce one as a prototype. If it works, he is interested in producing more if there is enough interest in them.

So, just lets wait and see if it works.
hi frank im disappointed in you if i can be of any help feel free to ask ive made a pair and found it not that difficult thats what this community is all about kc