On Thu, 16 Dec 2004, Robert Rolf wrote:

> 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?

Same reason as above. The intakes are *also* resonance tuned, and the 
volume of a muffler that would shut it up without power penalty would 
likely be half the size of the entire car.

> How much horsepower is being lost in noise generation?
> How much HP are racers willing to give up to suppress noise
> if the trade off is NO racing?

Look, you can always race bicycles, you know.

> 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).

In theory it should work and I know from hearsay that the exhaust 
systems on some 60's and 70's cars were deliberately built like this 
(not race cars) to obtain some specific exhaust sound, even with small 
engines.

> 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?

Again, the noise comes from an air column resonating in the exhaust and 
intake manifolds. You *want* it to resonate. The noise is an unwanted 
byproduct. 'Impedance' matching in the case of exhaust and intake piping 
means flared apertures at the atmosphere side, and so they are. 
Unfortunately this also maximises noise coupling. Tuned plumbing is 
usually not flared (to give a good reflexion at the 'impedance 
mismatch'.

> And if vehicles are noisy because of the desire to have
> free exhaust flow, doesn't injecting water into the tailpipe
> defeat the point by impeding gas flow?

The water mostly gets there to relax exhaust system requirements, like 
heat and volume (water injection increases exhaust density by lowering 
its temperature - this affects the speed of sound in it and with it the 
required muffler size). If done right it should not impede gas flow, on 
the contrary, the expansion of water into steam can be used to generate 
more suction in the exhaust system (but I have never read about it used 
for this).

It *is* possible to build and run large engines quietly, f.ex. see 
commuter aircraft turboprop engines where the majority of the noise 
comes from the propeller in despite of 3000+hp ratings.

Peter
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