What kind of muffler do I need?

I'm building a motorcycle powered Locost and just got 'er fired up today!  Holy crap I'm excited, and a little more deaf now.  Granted, it was running just an open header, more or less aimed right at me... Now I'm entering analysis paralysis on muffler designs. 

With this being a track car, it'll need to be quiet enough to not get me booted from events for sound. Some thoughts:

• I'm considering running the exhaust out the back already, so I can install a secondary, transverse muffler out back if need be.
• I definitely don't want straight thru, that seems like a great way to have to fix this again later.  Maybe a baffled design or turbo muffler?
• An oval/racetrack shaped muffler can fit if I turn it on it's side (ugly, but....), so long as it isn't too wide.

Anyone have any experience taming a bike engine or similar with limited space?

I guess the answer could also be a resonator feeding into a muffler now that I think of it.

I vote for a combination:

Motorcycle muffler in the traditional place, down the side of the car.

Route the exhaust to the back.

Have provisions to add a second muffler across the back of the car, if needed for track events or to spare your hearing.

Turbochargers make good mufflers.

ShawnG said:

Turbochargers make good mufflers.

This is THE obvious answer.

Indy - Guy said:

ShawnG said:

Turbochargers make good mufflers.

This is THE obvious answer.

Came here to say the same thing

Straight through mufflers can work well.  Internal volume is key.  You need length and lots of internal volume for packing material.  The rear muffler on my rx-7s is something like 18" long and huge around.  Little bullet mufflers don't do much.

I say rear muffler because the exhaust system has three straight-thru mufflers.  It's not quiet in new-car terms but compared to a rotary with open headers it is silent.

Another thing that helps is a tip restrictor.  The exhaust doesn't need to be 2.5" or whatever all the way back because the gases cool and therefore contract along the way.  On an exhaust sensitive rotary, I found that while a 2.5" pipe off the header made a huge difference in power compared to a 2", a 1 3/4" restrictor in the exhaust tip cut noise significantly and did not affect power to any measurable degree.  Restrictior was just an exhaust pipe adaptor that was a slip fit in the tip, inserted backwards and held in with a self tapping screw.  I made it easily removable but found that it wasn't necessary.

Down turns can also help redirect noise, reducing the perceived volume, even if they don't actually change the sound pressure level at the tailpipe exit. 

try a stock motorcycle muffler.   they are very queit and i'd imagine not very restrictive and probably plentiful.

Is this a situation where a Helmholtz muffler might be beneficial?

In reply to DeadSkunk (Warren) :

Those cut drones at certain speed but not good for noise abatement as a whole.

Not to add complexity, but what about running a cutout before the exhaust exits the engine compartment?

One branch could be the traditional routing and placement for situations where you want to unrefined experience. The other could be a rear exit tucked up tight to minimize visibility while allowing cruising at a comfortable volume. 

Came here to say rotary muffler, but it has already been covered.

In reply to wvumtnbkr :

The "rotary muffler" is another neat concept.  (As opposed to a muffler used on a rotary but it's been used on rotary powered aircraft).

The pipes, usually straight off the engine, enter a barrel at a tangent.  The exhaust gases swirl around inside the barrel and exit at the end from an on center exit.   Most of the noise generating pulses are dulled and abated by the time the gases make it to the exit.

Looks neat, and may partially function like a terminator box if you get the internal volume right, but too big to package inside a car.

I'd imagine that this would have to be made of heavy gauge steel to have any useful lifespan.  Stainless is a horrible material for exhaust systems because of how it doesn't transfer heat as well, so lots of stresses from cold parts pulling on hot parts.

Racing Beat switched their RX-7 muffler supplier from one that made mufflers from 1/8" wall mold steel to one that made them from 16ga stainless.  After I got tired of constantly welding up cracks near where one pipe entered close to the relatively cool shell, I gave up and repacked/replaced the pipes on the 30 year old mild steel unit that it was supposed to replace.  Because the stainless muffler was baffled instead of absorption type, it was also a lot louder than the old one.

Pete. (l33t FS) said:

In reply to wvumtnbkr :

The "rotary muffler" is another neat concept.  (As opposed to a muffler used on a rotary but it's been used on rotary powered aircraft).

The pipes, usually straight off the engine, enter a barrel at a tangent.  The exhaust gases swirl around inside the barrel and exit at the end from an on center exit.   Most of the noise generating pulses are dulled and abated by the time the gases make it to the exit.

Looks neat, and may partially function like a terminator box if you get the internal volume right, but too big to package inside a car.

Exactly, but I was thinking you could add a spinny wheel in it to decrease the energy.  Maybe have a spinny wheel attached to that spinny wheel on a shaft?  I don't know what you could do with the second spinny wheel...  maybe use it to compress a fluid of some type?

In reply to wvumtnbkr :

Late in piston engined aircraft development, they used the spinny thing to help drive the prop in addition to compressing fluids such as air.  They found greater and greater efficiency the more power they took from the turboshaft, ultimately deciding that it was best to just eliminate the piston engine part of the equation altogether...

Thanks for all the feedback. Yes, a turbo is the natural answer, and who knows what the future brings.  

I like the idea of starting with a good bike muffler and adding on if need be.  Time to chase down an OE bike muffler of some sort, preferably the longest I can find.

I want to try a Harley muffler on some 1600cc car. Why? They're cheap ($20-40) on Marketplace and I'm just curious how much sound it would absorb.(very little is my guess)

In reply to DeadSkunk (Warren) :

We put a couple Harley mufflers on our Subie powered Chevette and it worked surprisingly well. Do it!

The cheap Harley mufflers have all had the baffles knocked out with a chunk of rebar. Lol..

In reply to ShawnG :

Around here, all the Harley owners remove the stock pipes 5 minutes after they buy the bike. There are dozens available all the time. 

In reply to Brotus7 :

Now I have to go buy another car to try this. If you know how well they work, why not put one on your Locost ?

In reply to DeadSkunk (Warren) :

I've heard worse excuses to need to buy another car!

In my case, I wasn't sure how well one would work at 12k RPM. If we didn't weld those in, I'd just grab one and see. In any event, I have a muffler coming this week and I'll post up when it's here.