how to damp a racing exhaust already fitted with a DB killer ?

[jihem]

Dear all,

 

 

 

I have booked a trackday with my Ghezzi, but the circuit (Mettet) has strict noise levels rules: your engine can't exceed 95 db at 5000 rpm (FIM rules, with sonomoter on at 45° right hand side of the bike). Even with some DB killers fitted into my Tubi racing exhausts, I'm doing 99 db...

 

Now, Bruno from Ghezzi-Brian hasn't got any Supertwin OEM homologated exhausts left and all I can do now, I think, is playing around with the DB killers I have. Can i add some parts to damp some more the sound ? I don't care much about loosing some HP as long as I don't burn a valve or have a bike that just cough cough every time I open the throttle.

 

I know usually people want to make more noise while I want to make less noise

[luhbo]

...can't exceed 95 db at 5000 rpm (FIM rules, with sonomoter on at 45° right hand side of the bike). ...

 

Do you have the possibility to alter the spark map? When have you repacked your cans the last time? Do you think you could cheat them if you add some exhaust nozzles pointing to the left side or straight upwards?

 

Hubert

[jihem]

Altering the spark map would mean putting the bike thru a testbench and fiddling with the PCIII >>> a bit expensive for my taste and I don't think I can win 4 or 5 Db just by doing that.

The cans have never been refurbished (50.000 kms) but anyhow, they are nearly open cans (racing exhausts).

Changing the pointing of the nozzles wouldn't alter dramatically the sound level, and i don't want to touch the Tubi anyways...but thanx !

I was thinking more in the terms of welding something at one side of the Db killers but I don't know how hazardous that can be...

[GuzziMoto]

In the USA, we have something called a db snorkel. It is made mainly for two-stroke dirt bikes. It does a good job of quieting them down. It is basically what it sounds like, a snorkel that attaches over the end of the exhaust which runs back down the length of the can and then back up the can with a 90 degree outlet on the end. The increase in length/volume is I believe what does the quieting down. It does not hurt the performance of the motor. Whatever you end up with, increasing the volume/length of the system would be my goal if I were you.

[luhbo]

Altering the spark map would mean putting the bike thru a testbench and fiddling with the PCIII >>> .....

 

No, no fiddling or testbench necessary. And a PCIII wouldn't help you anyway.

 

Hubert

[jihem]

how would you do that ?

[jihem]

yes, I see what you mean but i really want to work with what I have. I don't see a tube going down the side of the Supertwin

[Guzzirider]

What revs do they measure the sound at? If they say rev to 5000 I rev to 4000- nobody has ever noticed cos they are normally too busy looking at the meter!

[luhbo]

how would you do that ?

 

I'd try retarding or advancing this single cell at 5000 and a probably very small opening. I guess you wouldn't notice this "weak" cell under normal use.

 

BTW, English/Scots, does "wee" come from "weak"?

 

Hubert

[raz]

It is so utterly weird the cans formerly known as "for racetrack use only" are now more like "for street use only". Soon they'll want to impose 70 km/h speed limits on all tracks in Europe too.

 

I really loved Guy's advice though

[jihem]

Could you explain some more ? (Sorry, got two left hands and not one brain to operate them )

[jihem]

For a 1100 twin, they mesure it at 5000 rpms, but i will not gamble my trackday on whether the guy has an assistant looking at the dashboard or not at that moment...

 

I'm sure this may sound crude for some, but what about if I was to filled the DB killer (it has no holes on the side) with mineral wool ? Would that muffle the noise (I'm at 99Db with the DB killers now), lowering it down for +/- 5 Db, without endangering the engine and still give me an engine that breathes allright ? I said I know that may sound crude, didn't I ?

[GuzziMoto]

Anything you do to restrict the exhaust flow more then it is already restricted now would likely reduce flow and thus hurt the way the bike runs (less power). I tried with a spark arrestor (on a SuperMoto) to reduce the sound level by wrapping the screen of the spark arrestor with steel wool (mineral wool?). It was a miserable failure. It did not quiet down the sound but it did mess up the way the bike ran.

I will be impressed if you are able to get the bike through without some creative thinking (cheating). 99 to 95 db is a big reduction. One thing you could try, along the lines of what was previously posted, is to "adjust" the tach so it reads 1,000 or so rpms low.

Road racers on the east coast used to have a issue with this if they raced at Bridgehampton. They had a low noise limit. Most of the racers there learned that increasing the volume of the muffling system and not restricting it was the way to go if you still wanted power. They would add extensions onto their mufflers that would add volume and usually also disperse the sound in different directions (towards the ground seemed to work well).

Good luck

[jcbooghs]

Hi Jihem,

same problems here!

 

It's not only the exhaust that makes db on our aircooled pushrod twins. Is your GB breathing free? Try to temper that sound first. Put some Unifilter or Pipercross filters (or alternatives) over the rampipes. Try to minimize that intake sound first.

You say you haven't repacked your cans in 5 years. You should do it now then. Get some rvs (inox) metalwhool. Brico has them (nr2 is better than nr0). Don't use normal steelwhool! Unless you want to put the place on fire. Secondly, go to a shop where they sell fireplaces, kitchenovens, woodburners, ... (hope u understand what I mean 'cause I don't know the exact names for these things) and ask for some thermal (ceramic) whool (the same blankets welders use) Wrap your perforated tube with the rvs whool first and then the thermal whool. Hold the stuff together with some paper (painters)tape to fix it in the outer exhaust tube.

And then there is something i've seen on racing cars, even F1 exhausts.

You have to stop the sound waves and want them to reflect into the silencer whool. Fix a simple metal wire or a small piece rvs in the diameter at the end of the perforated tube. I haven't got the opportunity to test this with a db meter, so I can't say for sure if this really works.

But don't restrict the exhaust too much with db killers.

 

By the way, I don't think you got the way of measuring right.

They will use the new FIM method. db-meter is 2m away from motorcycle and you will have to rev it up from idle 'till red and back to idle! That is what the organisator says.

If they do it the way you assume there will be a tolerance of 2,9 db.

 

And with all those Guzzi's there I don't think you (we) will be the only ones!

 

Do you know how silly this sounds! The classic races in Mettet (there have been held races there since 1925) are not allowed on the track because they run open exhaust and megaphones. They will still have to race in the streets of Mettet as they has done over the last 85 years!!

[Skeeve]

Do you know how silly this sounds! The classic races in Mettet (there have been held races there since 1925) are not allowed on the track because they run open exhaust and megaphones. They will still have to race in the streets of Mettet as they has done over the last 85 years!!

 

We're running into the same sort of problems stateside. Over here, the original owner can place a restriction on the title which remains forever more when the property is sold. Hence, the only solution for these nuisance "excessive noise" complaints & lawsuits is for a developer to not only buy the land for a racetrack, shooting range, airport, whatever, but to buy all the surrounding land, ad a codicil to the title that if any noise complaints directed at the "hub" property under development arise in the future from the parcel to be resold, title will revert to the developer/hub (noise-producing) property.

 

Obviously, this requires a heap of forethought, planning & deep pockets at the outset, which is why these sorts of properties become subject to encroaching suburban sprawl, whose residents then whine about the noise coming from the property [which was why they were able to buy so cheaply in the 1st place!]

 

Best of luck w/ getting your Guzzi quiet again!