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The basic problems of filtration of these elements in silencing is down to silencer volume and thermal gradient.

Being as a 600hz note is going to have a wavelength of 22" it's pointless trying to filter it out with anything shorter + quite small made of stainless steel, but of course this is of course what every after market system is doing with straight through silencers.

 

 

To come back to the subject a little, I thought it worthwhile to post the actual photos of what is being offered...

 

I hope this is interesting in the optic of what happens on the Vitesse project above.

 

I didn't need to come back to my scribblings on dirty paper after lying under a car measuring, they happily claim 18" silencers......& are clearly much more interested in the polished tail pipes and satin finished pipes than whether it's unbearably loud or not.....

I suppose they don't expect the engine to make any torque, so it's perhaps rather academic to talk about noise suppression.

 

There is one clear rule hear, moving to a large bore downpipe like this in s/s makes a long piece of pipe with a guarantee to boom.

 

Here are some rough guides to boom frequency at 20C:-

 

speed of sound 343m/s

 

100hz wavelength 3.44m

105hz                    3.28m

110hz                    3.13m

115hz                    3m

120hz                    2.87m

125hz                    2.75m

130hz                    2.65m

 

150hz                    2.29m

 

 

However if we raise temperature to 200C

The speed of sound increases to 436m/s this has the effect of making the wavelength longer.

 

At 400C the speed of sound is 520m/s or some 50% higher.

 

To give this a little twist, the gas temperature is not at all constant down an exhaust system, it's hotter at one end than the other so the temperature gradient plays tricks with any maths, and then the faster the car goes, the more it's force air cooler from the front end to the back, so the back end gets hotter again v the front, but the faster it goes, the more energy is being dissipated to drive it along, so the more energy it has to cool.

It's all about juggling the bit that gets hot v the bit that gets cooler.

 

The clever bit is bringing this all into a stable equilibrium or a "window".

The cooler the gas coming out of the exhaust the quieter it will be, because it will have lost all its energy in the process.

 

However I have a suspicion that somewhere along the line faced with a hurricane force wind at 100mph+ and lowered pressure under the car, thing stabilise pretty much everywhere.

 

This can be called in electronics terms ideally "the impedance match", where the exhaust outlet pressure and temperature comes as close to surrounding air temperature and pressure as is humanely possible to achieve.

 

Let's look at our table again for 400C:-

 

How do we get this?

Pretty simple really = 40 revs per sec for a 4 stroke engine x 60 = per minute (2400rpm) - x 3 for 3cyls per rev, so,-

 

120hz wavelength at 400C 5.20m.

150hz wavelength at 400C 3.47m

 

As you can see, this fits the observed phenomena very well.

If the exhaust is relatively quiet at 20C, when first driving down the road it's for the following reason:-

 

You're not imagining it becoming some 4x louder when thrashed, it really IS going up 6-10dB in SPL.

 

For our 2400rpm "target boom" the tuned length at 20C is 2.86m (probably shorter than what you have on a RWD sports car).

When the temperature rises to 200C this now needs to be 3.64m, which is very likely roughly about the length you actually have.

 

 

tt_1.jpg

 

tt_2.jpg

 

 

At the outlet end the amount of exhaust valve advance determines the final residual gas pressure at peak torque.

The most gas volume usually equates to peak work (or peak horsepower),but at a point where torque and volumetric efficiency have dropped far away below the horizon.

 

Once we're out of "boom" territory therefore the next big barrier is exhaust advance.

This is why long period "rally or race" camshafts are so difficult to silence.

 

Peak cylinder pressure is roughly say 50-60Bar on a decent sports type engine.

If you have a camshaft opening at say 90 degrees, it is effectively reducing the engine working stroke by half, so the engine behaves off cam like a 1L engine instead of 2L.

The peak exhaust port pressure however will be quite easily 10-15bar of the remaining expanding gas.

 

This makes the kind of harsh sawtooth wave spikes you saw in one of the last spectral analysis, once the engine is on cam.

 

Clearly the clever bit is to make the camshaft of such a design that it doesn't show this excessive "cammy" behaviour, and this was the main thinking behind my "hybrid" designs where the "cheap n cheerful", "let's cut the same exhaust profile as the inlet" is the rule not the exception.

 

As you can see, the exhaust is a very important part of engine design.

It's the BIGGEST single factor in maximum horsepower because it affects volumetric efficiency at peak revs more than any other element.

It is also the most visible signature of your car, like it or not.

 

When the neighbours stroll round to complain about "that horribly loud" vehicle, and "when are you going to turn it off", then we already know, we exceeded the limits of what was reasonably tolerable on a rolling road.

 

Now you have to imagine what that means to someone who lives across the street and hears that "infernal din" every day of the week.

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A final note (joke).

There are 2 suppliers I know who have been selling Ti silencers in the UK.

 

Here's one, which was made for Caterham. As we all know, most Caterhams have 4 cylinder engines with 16V engines nowadays which can run 7000-7500rpm.

That's 125revs per second with 2 cyls per cycle so 225-250hz.

catsilencer.jpg

Unsurprisingly they are in the £650-700 a shot and are repackable.

 

Here's what Ammo wrote (but he's also unsurprisingly another one that has left the auto tuning business).

 

"Silencers are available in 2.25â€, 2.5â€. 2.75†and 3†bores with either straight central inlets or offset angled inlets in right and left hand configurations.

As a rule of thumb 2.25† (57mm) is suitable for up to 220 bhp, 2.5†(63.5mm) for 260 bhp, 2.75†(70mm) for 280 bhp and 3†(76mm) for 300 bhp and over (in a single)."

 

It confirms my own "rule of thumb" in the last posts but of course my calcs are for 6 cylinder engines at 6000-6500rpm....(300hz+)

 

So, as you see, the 6 cylinder increases the exhaust filtration loading by 1/3 for a given RPM over a 4 cyl, despite obviously not making the same ultimate gas volume from a 2V engine with a small bore as a 4V engine from a large bore.

 

Unfortunately as you can see, on my spectral graph of the modified TR6 engine which made peak power at 6300rpm the fundamental at 6000rpm is well and truly overshadowed by a first harmonic at 600-650hz of frightening size.

 

This is probably caused by the fact you are forcing 6 cylinders to fire through the same space.

 

It's this harmonic content reinforced by 6-2-1 manifolds that is the undoing of inline 6s and V12s.

 

Here's the sizes;-

"Stock length with 750mm sleeve.....or

Overall length 1200 mm. 2-3dB quieter...."

 

As you see even the 750mm small silencer above is 29" or some 10-11" longer than anything from Toilet tune/Canley etc.

It would be a serious problem to fit this on any TR or GT6 fore-aft, so clearly is far better ACROSS like the original the back of the car.

 

In my opinion a 750mm silencer element is just long enough to silence 6 cylinder content over 5000rpm at 200C, but would be running at temperatures likely to burn out the stuffing at 6000rpm+, if unlike the caterham it's not right in the air stream.

 

The fun thing about 6 cylinder engines making power at 7700-7900rpm is the pitch of the exhaust note at 130-150revs per sec.

This is coming quite close to A=440, however to reach those power levels you have to be running exhaust valve advance of AT LEAST 85-90 degrees, so there's no free lunch on decibel reduction strategies.

 

The Vitesse can have 2 seperate silencers, but that of course immediately would entail modifying the rear tyre well in the boot.

(how about a space saver with half the depth of boot well to make up the extra space?)

 

If you wanted to spend £1300 on silencing doing the job really how it should be, of course then the solution is to instal 2 of the smaller Ti ones across the back of the car with 57mm inlet size.

 

That can be done on the old 2.5pi saloons by fitting the STAG rear chassis leg on the RH, and dragging another pipe through there, then 2 longer silencers somewhere where there is space as they stuck in the STAG.

 

Of course the SINGLE rear silencer which people have been fitting for years on the 2.5pi is WAY to small to filter anything serious at all and has ground clearances problems as well.

 

NB:-

Of course on a Jag, because it was originally engineered properly with 2 seperate rear silencers, the over axle pipes I had modified to 57-60mm anyhow so that suits a power output of 450bhp+ to perfection....

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All silencers use reactive attenuation and or absorption attenuation  they reflect a wave back at the incoming gas flow to cancel the sound wave. The ratio of body length  to pipe diameter should be 10 to 14 and the ratio of body diameter to pipe diameter should be 3. A couple of parameters to help determine size

 

There are several US manufactures who claim the internal design of there silencers do not increase back pressure while lowering noise level.The cut away views look interesting but I have no knowledge of any benefit beyond noise attenuation.

 

The exhaust is a very important part of a system which must all work together. No part operates alone.

 

As for the exhaust loosing it's energy and cooling ? Was this heat radiated to the atmosphere or a drop in pressure due to an increase in volume? Energy gets transferred.

 

Thanks for sharing it was good food for thought

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All silencers use reactive attenuation and or absorption attenuation  they reflect a wave back at the incoming gas flow to cancel the sound wave. The ratio of body length  to pipe diameter should be 10 to 14 and the ratio of body diameter to pipe diameter should be 3.

 

Who says?

It's totally impossible for a silencer to behave in the way you are suggesting.

 

Absorbtion energies behave in exactly the same way as acoustics in a free field.

Eg. If you fill a concert hall with people it behaves in a totally different way to when it is empty.

That is damping in action.

 

The reverberation time drops somewhat, and the damping increases, (excitation time and size drops), so the total SPL drops, and high frequencies are strongly attentuated.

These are well known phenomena, so the extra number of people behaves in a quite similar way to soft furnishings and curtains acting as damping elements in a room.

 

(I was absolutely disgusted in our local opera house when they insisted on re-installing the red carpets everywhere after taking them for cleaning. The acoustic was already too dry, and I assumed they would never return to ruin the already borderline acoustic.Finally they agreed to remove them for good, but then enlarged the orchestra pit, which made things a million times worse! Go figure!)

 

Anyhow, remove all the soft furnishings and you suddenly have a room full of echoes and reflections.

Remove some of them and it already sounds a lot more lively with higher treble energy.

 

This is what happens if like you say you "reflect waves back".

 

Sports silencers.

"straight through" silencers offer damping with low gas pressure resistance because the wire mesh holds a large body of damping material in place,- usually rockwool or the like.

The more you can pack in there, the better the likely effect, but of course it makes it physically BIGGER and fatter.

 

The rockwool acts strongly on the upper frequency range to remove the harsh crackles and harmonics, while the large surface area of the silencer acts to dissipate lots of heat.

It's the temperature gradient that reduces the low frequency content by introducing a large temperature differential between the input and output.

This temperature gradient as we all know "expands the gas" or does work on it much like bicycle pump in reverse.

Again the bigger it is, the greater its ability to dissipate heat.

 

"Smart" silencers also use a technique to make large changes in wire mesh pitch and size internally so that they damp specific frequencies better than others, at different wavelengths in the silencer, while yet another makes wire mesh which twists down the bore to simulate a gun barrel rifling.

This makes the gas lose more heat inside because it has the effect of increasing surface area.

 

I have tested quite a few.

Mike the Pipe, hand made me one, which was nice and quiet, but later on after scratching my head for a lack of top end, it turns out it suffered from substantial back pressure when the 6 were all run into 1 like that.

 

Few I saw, actually rivalled the "diver" type mild steel silencers I used with the barrel swirl and small protruding mesh screen with assymetric internal mesh pitch down the bore. (The only hit being, the Jaguar used to deform and melt the internal screen!)

 

That is actually the type I used for the Vitesse above, with a dual parallel internal cell, but I haven't heard the result yet to confirm how well it works.

The main plan on that, was to get most of the noise reduction from running small twin downpipes (high surface area), so that the silencer itself could be kept small, while inhibiting any boom by the small pipes and making a substantial split to the main lengths half way down the car.

 

On the Jaguar XJS, fitting my special silencer up by the gearbox and removing the centre silencers made a substantial power INCREASE, and BIG drop in high frequency, noise, whereas just removing the centre boxes, makes for a nasty harsh exhaust note with a substantial drop in torque in the mid range.

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It's called science Try Google

Can you post some references for us, Joey? The internet is a mixture of fact, marketing spin, speculation, opinion and gross inaccuracy. I Googled exhaust theory, and I found a mix of old research papers, sites by companies selling their products (eg Burns Stainless) and forum discussions. No doubt this one will be added to the results list in due course.

 

Gareth's experiments will prove useful to several of us here who are building GT6 exhausts and don't want a horrible droning exhaust. I'm starting to think that my Bell muffler might not be adequate, as it's only 400mm long.

Edited by V8 Nick
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Being involved with some prominent audio academics has its advantages:-

 

http://www.theregister.co.uk/2014/07/02/feature_the_future_loudspeaker_design/?page=1

 

http://www.linkwitzlab.com/links.htm

 

Being as we're both actually working on the same engine, with fundamentally different but not totally opposite aims, a lot of time & energy this last 12 months was used up trying to articulate the science of sound suppression in a dynamic (unstable) environment.

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Lots of theorising going on there (universities, people earning Msc etc), interesting as it is.

Precious little real empirical data.

 

There is an excellent contact I have down under in Melbourne who has been doing real hands on stuff for decades.

It's always fascinating to chat with him, and he even has a modified 1966 Mk2 Spitfire.

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here is an Australian test of a group of local silencers tested on a Suzuki ,Toyota and Cobra. At the end is the recorded sound of each silencer. It would have been interesting if they cut open each to see how they differed. http://www.autospeed.com/cms/article.html?&A=0383

 

Good luck with the study of exhaust systems. Should I even mention two stroke exhaust or just let the topic die?

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  • 9 months later...

Well, after two pages about noise and sound let’s get back to the actual topic about getting this car finished and on the road again J

After my garage, terrace and house extensions finally finished, I’ll have time to finish the car … not so sure if there’s still somebody on the forum who believes this or is interested in the progress ….

I managed to fit the engine further back and prepared the radiator for the electric fan. Now I need a longer  tacho drive as it just will not fit.

post-695-0-82560700-1435073772_thumb.jpg

 

More updates should follow before next year this time.

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If you watch this vid as of 5:20, it will be hard not to be interested in a project, that deals with upgrading a Vitesse...

 

 

How much did you place the engine back? Does the Gearbox still fit?

 

Cheers

 

Patrick

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"Upgrading a Vitesse"?!?!?

 

That, Sir, is the Kastner-Brophy Vitesse, the Candy Box, the only racing Vitesse with even a whiff of factory participation, and the only Vitesse that Kas Kastner was ever involved in.

The car was ordered from the factory for the team before Kas gave his notice, and he and Brophy developed it as a personal project

Sad to say, although I've asked Kas for details of what he did to it, at his age he can't remember, but there are a whole heap of pics here, from the time before last when it was sold:

http://www.race-cars.com/carsold/triumph/71vtx1/71vtx1ss.htm

 

Allegedly, 200bhp, and that may be true as it was notorious for using up tyres, but that may have been down to Carl Swanson's driving style.

Anyway, it's now safely in the hands of Butch GIlbert, and all the better for that!

 

John

 

PS.  Yes, I always mount the engine on the back of the turrets, not inside them as OE.    10mms?   If you're lucky, but of course makes all the difference weight balancewise!

Edited by JohnD
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If you sit the mounts on the back of the turrets and slot the engine brackets where the two mount studs go through you can get ~ 12 -15mm.  That takes you pretty much to the bulkhead lip anyway, and gives just enough for an electric fan and trigger wheel.  Gearbox mounts/propshaft sliding spline should cope with this (might need to slot the gearbox mounting plate).

 

Nick

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  • 1 month later...

I also started work on the hardware for the Megajolt ignition and machined the flywheel pully to be able to fit the triggerwheel behind.

Sensor bracket is done, just need to fix the triggerwheel at the 60 degrees.

Tapped 2 vacuum takeoffs at the inlet manifold which I'll fit for now to the vacuum gauge and observe if there is a stable reading.

 

And I'm trying to work out a good working SU-carburettor linkage from the saloon parts I have.

 

post-695-0-67525100-1439983042_thumb.jpg

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What's to go inside the shell?

Have you been following Project Binkie?  https://www.youtube.com/user/badobsessionmsport

 

Chuck the mechanical linkage on the Vitesse, by this time it'll be worn and floppy.   Go for a cable.

 

Timing wheel behind the crank pulley.    Sure the water/alternator pulleys still in line?

I did that with a sheet steel wheel, and the clearance was minimal, so a 5mm plate wheel must push the crank pulley forward.

 

Constructive criticism, I hope!

JOhn

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John,

I'm also a follower of Project Binky and its keeps me motivated to make proffessional modifications on my cars.

BUT I'll keep the trusty A-series engine and concentrate on sorting the chassis and reinforcing the shell at a few weak spots.

 

Regarding the Trigger wheel behind the pully, I machined 3 mm off the pully and I'm able to push the pully against the crankshaft.

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