Intake Silencer Design

[MattPearson]

I would like to design an intake silencer for my 2si 690. How would I go about taking this task on? Thanks

[Vance]

Hi Matt, I would look at water craft. Most of the 2 strokes have very sofistacted inlet silencers that still flow a lot of air. Some of the sport bikes have nice inlet silencers also. Big is better and it needs to be better than an exhaust muffler because it sucks.

There is a good book called The Scientific Design of Inlet and Exhaust Systems by Smith, but it mostley deals with four strokes. Most of the principles apply.

Thank You, Vance

[Chuck Irby]

Matt, you are aware of the intake silencer design by Rotax, I'm sure. I have one on my 618 and it works rather well. Just wondering if maybe you couldn't use that basic design and make your own to fit over your three carbs. What are the filter i.d.'s where they slide over the carbs and the dimensions between them?

[MattPearson]

I was thinking of doing just that with the Rotax silencer, but I didn't know if it was that easy or if there was more to it. I may check out the book you were talking about, Vance.
I need to get my hands on a Rotax unit and see if it uses baffles inside or if it is just a canister.

[GyroRon]

Matt just make a big aluminum box to replace your airfilters with and put one big airfilter on the end of it. Same thing as what Rotax uses except not plastic.

[Aussie_Paul]

The intake silencers certainly do decrease the 2 stroke noise. I had one on my 618.

When I changed to it, the "airport rats" came and asked what I had done, as it sounded nicer and quieter.

You can't beat the "airport rats" to let you know both good and bad, and they have no vested interest.

Aussie Paul.

[jdalexa]

I've been wondering about the Rotax intake silencer. I'm thinking of putting one on my 582 on my Dominator. Stealth is good. Although many of the Bonanzas and twin-Beeches that fly out of my airport are much louder, I'm the one that's remembered since I'm unusual.

[MattPearson]

Chuck is sending me a dual rotax intake silencer. I will see if I can't come up with something from it. The racket is definitely louder on the intake side of my engine than on the exhaust side.

[Chuck Irby]

Matt, those three carbs really suck, huh?

[PW_Plack]

John,

You're not remembered because you're unusual. You're remembered because the Bonanzas and Beeches are noisy, but up out of earshot much sooner!

[Brent_Brown]

Matt be ready to rejet that puppy when using a in box. as all rotax motor have to be too.

[rwr]

The Rotax silencer is a good place to start, but it might restrict the flow a bit since your motor has more HP and more displacement.

The Rotax silencer is a composite molding with all surfaces rounded in compound curves, and the air inlet is through two tuned tubes (diam and length of the tubes matter). It appears that lots of accoustic study and design went into the thing to make it match the requirements of the Rotax twin-cyl motors pretty closely.

You could make one from aluminum, but unless you are an expert with an English wheel, it will be hard to build one with compound curved surfaces. If you just build a box of aluminum, the sides will resonate. I suppose you might try a large aluminum tube,.. at least the single-curved sides would resonate less.

One other way to build it might be from fiberglass. Buy some plain old household styrofoam from Home Depot. Glue the pieces together in the approximate shape needed, carve and sand to sculpt it into the exact shape you want, then cover it with a few layers of resin and fiberglass. After it sets, soak the styrofoam with gas and it will dissolve, leaving you with your silencer ready for final fitting.

Use a large K&N filter; larger than the cone filter on the Rotax unit.

Designing the mounting system will be another challenge. Allowing it to move some but not too much, and not get fatigue cracking here and there, will take some trial and error. Let's hope the error part leaves your prop in one piece.

Send some pics when you are done, eh?

[rehler]

Look at the standard Rotax 912S set up. It is a simple round tube of aluminum (about 4" diameter) with the two carbs attached near each end and the air filter in the middle (bottom) - slight bend in tube, but no compound curves, just simple construction (tube with ends closed).

[MattPearson]

It is really nice to have sounding boards to bounce ideas off of.
Looking at the volume of air going through the 582 vs my 690 (assuming that each cylinder fills completely with air) is less than 10 cubic feet difference.
The other thing is all of the "magic" is done by the third cylinder and the port timing thereof. There is no tuning in the exhaust and no rotary valves. That being the case, a simple air box may work quite effectively. I will just have to play with it. I am planning on using foam/fiberglass construction.

[rwr]

Ken, The challenge for silencing the 912 is much different that of a twin cylinder two-stroke.

Matt, you are right, the flow difference is not much,... sometimes, though, slight differences in resonance frequencies, manifold lengths and widths can change the result a lot.

Rest assured, there is tuning in the exhaust system. It may not be a huge tuned chamber like RD Aerosports, but it is a purposefully designed set of compromises for size, noise, weight, cost, and performance. Just try to alter it in some way and see what happens.

Also, the rotary valves tend to cause less intake noise, not more. Your intake tuning has more noise, than one with reed or rotary valves. The difference isn't much, but it does point out more of a need for a silencer.

It would seem that having three cylinders instead of two would be a help to you, since the primary frequency is 50% higher than for an equivalent twin cylinder motor, thereby reducing the size of the silencer required. As the frequency goes up, the size of the box needed to stifle the resonances goes down.