2 stroke silencers, does volume matter?

[reepicheep]

So I'm getting ready to rebuild a KX-80 that came in a bucket... crank is on the shelf, head is on it's way to Eric Gorr, and I found a cheap FMF fatty pipe on ebay.

That leaves the carb (another thread for another day) and the silencer.

The easy answer is to just get an FMF Q series. $140 or so. No doubt a great choice.

But I'm always looking for an excuse to weld, and can weld aluminum, so I'm wondering about tweaking the stock silencer. This will be a woods bike, so low end power and quietness are my top priorities. I don't mind top end staying the same, or even loosing a bit.

I initially assumed I would be doomed for a DIY two stroke exhaust fabrication. Way too much voodoo math there for a hack like me. But the more I dig, the more the voodoo seems to be concentrated in the header pipe, which I am going off the shelf FMF for. The silencer seems to be an afterthought, and robs power for sure, but nobody seems to talk much about silencer volume (or if it matters)

If I took the stock pipe, and grafted on another baffle at the end, or made the pipe have a higher volume some other way... would it really matter in terms of performance?

I'll probably just buy the FMF Q series pipe, $150 is a bargain compared to the time I would have to put in to make something that would perform as well (and that assumes I have the skill, which I don't). But it's interesting to think about.

 

[_JOE_]

From my understanding flow is the only thing that matters with a silencer on a 2 stroke. And you're probably right about the price of the Q being a bargain when you figure in what your time's worth.

 

[dirt bike dave]

The length and diameter of the stinger will absolutely effect the power characteristics.

Seems to me the silencer is acting like an extension of the stinger.

 

[reepicheep]

That is a good point Joe. Though the part of the pipe I would modify would be after the stinger and even after the exhaust can inlet. So somewhere in the system it must stop mattering much, provided air can still flow freely (which is probably a much tougher thing to achieve than it seems on it's surface).

At the end of the day, I'm sure I would not get it as quiet and as powerful as an FMF Q. And frankly, I'd like to put my money where my mouth is and reward FMF for making a quiet two stroke pipe. Loud bikes cost rights (IMHO).

But I'm kind of itching to try and weld on that aluminum factory KX-80 silencer. :) Maybe I'll try both, and see how they compare.

 

[helio lucas]

reep, i like to think two stroke silencers are just that. they have not the effect a four stroke silencer has, on power. although they can alter the power curve, they do just slightly. the seem to have a more pronouced effect on throttle response than on raw power.

now, volume does matter on silencing ability. basicly, silencer diameter controls the maximum absorption of sound and lenght the dominating frequency and a bit absorption as well. so more volume, more absorption, less sound. at least to my understanding. :nod:

about the inner, perforated pipe, choose a proven inner diameter (stock is good). larger will be noisy and may increase low speed power and response because of reduced back presure. smaller may eventually increase peak horsepower but that involves some serious tuning.

:cool:

 

[Lorin]

I took the spark arrestor off my 98 yz and installed the internals on to the "stock" exhaust silencer of my 03 yz to have a spark arrestor. The overall length of the stock canister remained the same, and the stock silencer has the same shape as my sa did. I cut the perforated pipe shorter to allow the s\a part (turbine core) to fit and then riveted it in place. Works fine, looks stock, and passes testing.

 

[jaguar]

FYI it would be best to increase the flow restriction since you want the power peak to occur at lower rpms. Less volume is output at lower rpms and therefore more restriction is needed to maintain ideal backpressure. This isnt just theory, I have tested it and found it to be true. You could replace the perforated pipe in the silencer with one of lesser diameter.