>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 16 December 2004 18:58
>To: Microcontroller discussion list - Public.
>Subject: Re: [OT] Acoustic damping of vehicle exhaust noise
>
>
>Peter L. Peres wrote:
>> On Wed, 15 Dec 2004, Robert Rolf wrote:
>> 
>>> Is there some reason that these racers just don't use a 'PROPER' 
>>> muffler to solve their noise problem? I have heard big luxury cars 
>>> under full power acceleration and they are very quiet compared to 
>>> small 'sport' cars which have less power.
>...
>> Mufflers eat horsepower by gas friction/back pressure, and they 
>> prevent
>> the operation of tuned (resonating) exhaust stacks, which 
>costs more hp. 
>> They are also very bulky to be efficient on race engines and 
>there is no 
>> place to put them on the car. I have read somewhere that a properly 
>> designed exhaust system for a car has 1/4 to 1/2 of the 
>volume of the 
>> engine (total volume not cylinder capacity). Also on a race 
>engine the 
>> air intake manifold can make more noise than the exhaust 
>under certain 
>> conditions.
>
>OK.
>So why can they not run baffles or ducts that would suppress 
>the noise of the turbocharger(?). Weight?
>

Turbochargers make little noise, and in fact work as very efficient
silencers (mufflers) themselves as they slow down the pulses on the gas
and give a more even flow.  However, most track racing series don't have
turbocharged engines.

>How much horsepower is being lost in noise generation?

Very little, the noise generation is part of the mechanism that is used
to produce MORE horsepower.  Cams with very steep opening and closing
ramps are used to give the maximum possible valve curtain area, and also
produce very strong pressure waves that are utilised with tuned length
exhausts to provide improved cylinder scavenging.  Standard silencers
that use baffles and restrictions to reduce noise effectively stop this
mechanism from working and also increase pumping losses due to the
increased back pressure.  Absorbtive type silencers don't introduce
significant back pressure, but also are not as effective.

>How much HP are racers willing to give up to suppress noise
>if the trade off is NO racing?
>
>Can some of the exhaust gas be diverted down a 1/4 wave longer 
>pipe to achieve phase cancellation as it comes out the end of 
>the exhaust pipe? (given changing RPM this would be a tricky problem).
>
>Being naive on the subject, it just seems that if one draws
>an electrical analogy, one needs impedance transformers to 
>allow for high impedance exhausts to be dumped into low pass 
>filters with minimal loading. Or is part of the 'experience' 
>of racing the gut pounding noise?

It is a big part of the racing experience, but obviously it needs to be
regulated to reasonable levels.  The problem is, who defines what level
is reasonable, and also there is a practical issue.  The track in
question is for speedway, the cars are small and there is limited room
to install an effective silencing system.

Speedway in the UK is motorcycle racing around a dirt oval, the
reference to cars had me confused for a while!

Regards

Mike

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