Supertrapp silencers, power robbers?

[Notso Swift]

Just so I can understand
Your tests are not done drive by?
what are the conditions that they are done under.
... and is this why there are not mant rotories running in the UK

[greenamex2]

Generally you sit in the entrance to the collecting area for practice and have to rev your cold engine to 3/4 maximum revs whilst someone waves a sound meter in the general direction of your exhaust.

Depending on where you are there can be half a dozen cars/buildings/trees/earth banks etc reflecting the sound god knows where.

Some circuits also have a drive by check.

[Al Weyman]

It was explained to me (baffled me to be honest because noise is noise at the end of the day) that driveby checks are not accurate enough as different cars at different times will be using different revs so it would not be possible to get a level playing field. All a bit odd to me. What makes the mothod of testing laid out by Dennis even more bizzare is I really confuse them with my older car as it has side exiting exhausts (each side) so they can only check one side at a time and it reads less than 100 decibles so work it out, all a bit daft if you ask me.

As far as Rotories go, there are two Mazda's in the club i race in and both a very quick. Stacy Vickers has a later model coupe in Pre- 83 Group one and Dave Nixon has an old RX in Post Historics, both go very well indeed.

[greenamex2]

I have got a single side exit exhaust. Half the time the scrutineer still checks at the rear of the car.

Not that I am complaining of course

[Notso Swift]

Originally Posted by Denis Bassom

Generally you sit in the entrance to the collecting area for practice and have to rev your cold engine to 3/4 maximum revs whilst someone waves a sound meter in the general direction of your exhaust.

Depending on where you are there can be half a dozen cars/buildings/trees/earth banks etc reflecting the sound god knows where.

Some circuits also have a drive by check.

That is just insanity!

I have your soultion Al, all you need is one of those exhaust choke valves fitted with a spring so that it is normally open, then when you are being tested pull a cable that closes it off a bit.

We work on 95 dB drive by, and they move around with the noise meter, so they get you at max revs on the straight as well as accelerating through the gears. This means you can't do the sort of cheating that I have suggested above. Even if you know where they are testing you, and you don't normally know or have enough time to pick out the individual tester, and you will be killing your own race any way.

The thing with rotories and noise is that they are such heat pigs and they rev at such a high band that the noise they make is really "pitchy" so it carries and is a more annoying sound than the deep, grunty, V8 sound, for example. We have heaps, indeed they are the car of choice in Improved Production (my catagory) because they are really fast and really cheap (relatively) and light enough (300hp/ton) to make the most out of our control tyre (Yoko A048 or A032) where the top V8's fry them (500hp/ton)

[Al Weyman]

Yeah I thought about one of those devices as I know it would pass a drive by anyhow. Only problem is I am so damned stuck for space under there I recon it will be branch out to two silencers after the axle.

You are right of course about screaming rotarys vs V8's, its the same as the Pug 205 which will also often beat me its just that I race the v8's because I love em and not just to win. The amount of people that come to see my cars in the paddoc compared with a boring (looking) Pug or Rotary has to be seen to be believed, it seems everone loves a big yank and one of my main reasons I can justify the expense of this crazy sport is to publise my little company so that works a treat. If I really wanted to win I would race a rotary or a Pug, I think the equivalancy factor of the Rotaries is prehaps not quite right either born out by their performance. Oh one other thing I weigh in at 18.5 stone so I need a meaty old car to carry me around:-)

[greenamex2]

I tried my version of the supertrapp device at Oulton after someone FINALLY made me get my car back down to the noise limit.

It is an inverted cone with holes stamped in it that is inserted into the back box.

The result?

Noise dropped from 108dB to 104dB after what should have been less than 5 minutes work (long story).

Hurrah, I get to race

Err, then we go out on the parade lap.

The car was totally gutless and won't rev much past about 6000RPM. The cars normally makes about 70BHP at that point and it felt like even less! A totally standard kwik fit exhaust would have been a lot faster!

I actually had trouble keeping up with the cars on the green flag lap and got totally toasted by a bod standard pretty scruffy BMW 318 driven by a total novice!

[Al Weyman]

Ouch I feel for you Dennis, if I had'nt done the engine at Snett I would have had to get the Supertrapps out again just to race as it was I put one of the Moroso Spiral mufflers in line further up from the dual absorbsion mufflers and before the exhaust branches back out to twin (cant run a twin thoughout as too tight in the chassis). Anyhow I almost got there with this tweek so may be of interest to you, it dropped down from 111 decibles to 105.5 although I did cure a manifold blow that may or may not been blowing at Rockingham but I dont think it was) but still not enough (its getting ridiculous this sound thing but it proved the Spirals work and in stock at Real Steel so may be worth trying on your car, not too dear about £40 inc vat and take up very little space, looks a bit like a cherry bomb. I have another Spiral Muffler and if I ever get the engine rebuilt I will try that on one side after the exhaust splits into two as I only need to find .5 decible now. Oh the car definitely felt much better than with the SuperTrapps fitted.

[greenamex2]

Was the only difference between 111dB and 105.5dB fitting the Morosa spiral muffler (apart from the possible exhaust leak)?

What absorbsion muflers are you using?

I feel a possible trip to real steel one lunchtime coming on, just a shame the smallest diameter is 2.5"!

[Al Weyman]

No they are up to 3 inch thats what I have. #

Joe Ellis Mufflers fitted as well

Yes that was the only difference and it was noticibly quieter, I have a new one here off the car if you want to have a look, you can even try it if you like as I won't need it for a while and if any good replace it if not return it.

Sorry read that wrong you want smaller than 2.5 they also do an Edelbrock version in stainless steel bit more expensive, can't you just sleeve it up if they dont do one in your size. It will only act like an expansion box i would think.

[greenamex2]

Cheers for that Al.

The science behind the 'twister' type 'mufflers' is pretty sound. At certain RPM they will do nothing, at others the low and high pressure pulses will go some way to cancelling themselves out.

Got to be worth a try although it may need be a bit of tuning for my silly RPM's.

Pro-flo do one with 2 1/4 pipes, guess I'll have to brush up on my American!

[TVRfan]

we tried supertrapps on our mosler for the 24hr race, they were absolutely useless. the car was so strangled there was no point racing, in the end we put rolled up chicken wire inside the perf in the boxes, bizarre but it worked we got down from 112 to 96,(we had already put two extra silencers on as well) and no change in engine perfomance that we could see on the motec or that the drivers could notice. was quite funny after an hour of free practice to see the chicken wire trying to escape out of the end of the exhaust.

[greenamex2]

I spoke to Tech-craft, described my system to them and they reckoned my silencers were too small in diameter. They wouldn't even sell 4" OD silencers for a 2 1/4" system.

Doing the math going to a 5" OD silencer would give more than a 60% increase in silencer volume.

Live and learn.

Never heard of the chicken wire method before! Not sure were you would find some at Oulton though!

[TVRfan]

always carry it in the truck now just in case

[Al Weyman]

Alright as long as it don't spit out and cause a punture to a following car. I thought such temporary fixes were specifically not allowed.

[knighty]

I dont know if someone has suggested this already.......phone Jetex and order a "decibel insert" for each of your tail pipes..

http://www.jetex.co.uk/index.php

I got pulled up for excessive noise at Donnington with my blackbird powered F4 single seater - which admittedley was loud!......I fitted the insert and it still pulled like a train and it passed the noise requirements........was also speaking to someone recently who races a FF2000 at coombe, he fitted the decibel insert, it reduced the noise by about 5db, with no adverse effects to his lap times.

they are dirt cheap - about £10 I think, and come in a range of sizes, 2", 2 1/2" and 3" I think........they just add a little back pressure and force the exhaust gas a little more into the silencer wadding.

I think the problem with the supertrap items is they are way too restrictive to exhaust flow

[greenamex2]

Originally Posted by knighty

I dont know if someone has suggested this already.......phone Jetex and order a "decibel insert" for each of your tail pipes..

http://www.jetex.co.uk/index.php

I got pulled up for excessive noise at Donnington with my blackbird powered F4 single seater - which admittedley was loud!......I fitted the insert and it still pulled like a train and it passed the noise requirements........was also speaking to someone recently who races a FF2000 at coombe, he fitted the decibel insert, it reduced the noise by about 5db, with no adverse effects to his lap times.

they are dirt cheap - about £10 I think, and come in a range of sizes, 2", 2 1/2" and 3" I think........they just add a little back pressure and force the exhaust gas a little more into the silencer wadding.

I think the problem with the supertrap items is they are way too restrictive to exhaust flow

Good idea, unfortunately my insert WAS from Jetex!

I have got a really touchy engine and the slightly mapping discrepancy causes totally disproportianate drops in power, especially round the mid range.

[Chris Y]

Bloody hell Denis! Did you fit it in sideways? I'm sure I saw a hedgehog overtake you up Clay Hill

[knighty]

Dennis - sounds like your on the right lines - 100mm OD for a silencer - even on a 1000cc bike is considered bleedin small......you need more volume - like you have already concluded........but keep the decibel insert with you when at the track, as it will get you out of trouble for sure.......be sure its a good fit inside the tail pipe - dont be afraid to flare out the decibed OD in order to get a good seal on the ID of your tail pipe

[greenamex2]

Originally Posted by Chris Y

Bloody hell Denis! Did you fit it in sideways? I'm sure I saw a hedgehog overtake you up Clay Hill

To be fair, it was a VERY fit hedgehog and, living at the circuit, it did know it's way round better than me!

[greenamex2]

Originally Posted by knighty

Dennis - sounds like your on the right lines - 100mm OD for a silencer - even on a 1000cc bike is considered bleedin small......you need more volume - like you have already concluded........but keep the decibel insert with you when at the track, as it will get you out of trouble for sure.......be sure its a good fit inside the tail pipe - dont be afraid to flare out the decibed OD in order to get a good seal on the ID of your tail pipe

If I had only had to loose a couple of dB, or had tested the insert on a rolling road before hand, yeah I could probably have seriously reduced the impact of it to the cars performance.

The whole episode has been chalked up to experience.

With sound level meters so cheap now (30 quid or so from Maplins) I think I'll probably buy one.

And when I keep getting told my car is over the limit, I won't keep ignoring it.

And the next time I am at Oulton I am going to run that hedgehog over in practice so he can't beat me in the race!

[greenamex2]

I spoke to somebody (who shall remain nameless) who had detailed knowledge of the Supertrapp silencers (for reasons I can't go in to) and he confirmed that they were not a good thing power wise.

On one V8 it cost over 1000RPM and a lot of horsepower.