Daydreaming about tuned pipes

[bikemike]

I like the silencer in the first vid! No silencer on a pipe is ear-splitting even with earmuffs. All of the pipes I have do a good job of silencing.

I like how the last guy launches the cookie in the second vid.

Yeah that was Philbert running the saw last. he had some fun letting it piss rev it put a smile on my face. Paid 6 bucks for the power head last year and chitty oregon b/c on it with the stupid rolled over rakers. But it still cUT pretty good. much quiter without the silencer than my old poulan I sold this summer

 

[hal9000]

Any one of these motorized bicycle pipes is an improvement on the the 38cc saw by a wide margin. I'll be trying one on a 45cc Stihl at some point. The chrome banana has potential; I just need to cut, turn and tuck it to the side and it'll work for nearly everything except cutting at ground level.

The 034 (56cc) was being held back and needs a bigger pipe. The exhaust port was almost twice as big as the pipe header opening.

Useful info on the 034 there. Do you think the saw was choking because of the small header? That pipe I posted the photos of is broken off at the header - I assume it broke right at the weld. The "modified" inlet size is now about the same-slightly larger than my exhaust port, and the length is about right that once I true it up and bend it back around it should be exactly the right length to sit where I shot the photo. The stinger (such as it is) looks a bit smaller than the exhaust port though. If I do it I'll have to figure out a silencer/spark arrestor setup, but I think that will be relatively straightforward.

 

[bikemike]

Useful info on the 034 there. Do you think the saw was choking because of the small header? That pipe I posted the photos of is broken off at the header - I assume it broke right at the weld. The "modified" inlet size is now about the same-slightly larger than my exhaust port, and the length is about right that once I true it up and bend it back around it should be exactly the right length to sit where I shot the photo. The stinger (such as it is) looks a bit smaller than the exhaust port though. If I do it I'll have to figure out a silencer/spark arrestor setup, but I think that will be relatively straightforward.

I think your pipe looks good once you have the header curved more to fit to the saw then the stinger is the easy part. rule of thumb I always went it for the stinger is to never have it bigger than the bore of your carb. They do need the back pressure to run and help keep the compression in cylinder. the pipe on a 2 stroke works like a supercharger on a car.

 

[hal9000]

Good idea. I think I've seen a ratio for the stinger:exhaust port size somewhere in one of the design docs I looked at too. I wasn't too worried about that, just mentioning it. That stinger is a prior modification anyway. There's a patch mid-pipe that I'd been wondering about, and at this point I'm pretty sure it was originally a center outlet pipe. No idea why somebody would have modified it. There isn't supposed to be a performance difference between the two (Center out might even be a better physical configuration for the silencer in this case)

 

[bikemike]

Good idea. I think I've seen a ratio for the stinger:exhaust port size somewhere in one of the design docs I looked at too. I wasn't too worried about that, just mentioning it. That stinger is a prior modification anyway. There's a patch mid-pipe that I'd been wondering about, and at this point I'm pretty sure it was originally a center outlet pipe. No idea why somebody would have modified it. There isn't supposed to be a performance difference between the two (Center out might even be a better physical configuration for the silencer in this case)

Well I know for the gopeds scooters they do have a few pipes with a exit tube coming out of belly and they are a performance ad on that seem to work especially to keep everything compact. many rc nitro pipes are similar too and they can pull 30,000rpm

 

[B440]

Center bleed pipes are quieter and allow a more-compact design. I've been looking at go-ped pipes for that reason, but so far, I have not seen anything that would be a good match for a saw.

 

[Rx7man]

I gotta chime in here..
I pored over the 2 stroke books for days and days, then looked at other design tools, and ended up making my own simulator (that took weeks). Anyhow, I did finally make a pipe for my husky 61, and tried both the end and center bleed.. yes the end bleed is DEAFENING!!, center bleed is actually tolerable to run and still sounds nice.
Pipes are very specific (if they're done right).. I have a moped pipe (with twin outlets even!) that I thought about using somewhere, but I think it would only be suited for about a 35-40cc saw. The pipe from the moped will change the performance.. Where and how.. well, that's a wild card..

Buddy who raced motocross back in the day had a saying about pipes.. "Long and thin is in, but short and fat is where it's at".. Long, thin pipes are "well behaved", smooth power and lower RPM peak torque.. Short and fat pipes are nasty, violent, peaky power that take skill to use.

Perhaps at some point I'll revisit my simulator and hook up a few thermocouples to my existing pipe so I can get real world feedback.. The temperature of the pipe is the single biggest wildcard when designing the pipe.. if your calculated temperature is too high by 200F (ie, you're overoptimistic about your BMEP), the sound waves in the pipe will travel significantly slower making peak power at a proportionally lower RPM.. the good thing about that is it's usually relatively easy to shorten the header and belly a little to compensate for that.

Here's my saw, yes, I do buck and fall with it, though it's a thirsty beast

 

[hal9000]

...Pipes are very specific (if they're done right)... The pipe from the moped will change the performance.. Where and how.. well, that's a wild card...

Buddy who raced motocross back in the day had a saying about pipes.. "Long and thin is in, but short and fat is where it's at".. Long, thin pipes are "well behaved", smooth power and lower RPM peak torque.. Short and fat pipes are nasty, violent, peaky power that take skill to use.

No argument from me there. I was actually hoping the "long and thin" profile on this pipe would increase the chance that its effective range would overlap with the saw. I'm not worried about max performance... Even a 10-15% boost over the stock box muffler would be enough reward for the effort in my book.

The temperature of the pipe is the single biggest wildcard when designing the pipe.. if your calculated temperature is too high by 200F (ie, you're overoptimistic about your BMEP), the sound waves in the pipe will travel significantly slower making peak power at a proportionally lower RPM.. the good thing about that is it's usually relatively easy to shorten the header and belly a little to compensate for that.

Are you sure about the direction of bias here? I came across a similar comment elsewhere, but they were stating just the opposite (But this is the internet after all... Every "fact" hidden behind a dozen fallacies!). It was in regards to old 2 stroke race bikes that would run a water/alky fogger into the pipe to boost torque, or header wrap the pipes to boost top end. I was actually considering wrapping the pipe (and or shortening the belly) to bias the rpm range up if needed.

 

[Rx7man]

about temperature and direction, we're both saying the same thing... rereading it though it does sound a little vague.
If your calculated temperature is too high, (thus your real temperature is cooler), the sound will travel slower in a cold medium.. you put it together and the pipe that was designed to run hot and provide peak torque at a given point, the peak torque RPM will drop as the actual temperature drops. This is exactly how the water fogger works, colder temps for lower peak torque RPM, and a wrapped pipe for higher peak torque RPM.

 

[hal9000]

Thanks for the clarification. You just saved me from trying to find that article again to see if I'd misinterpreted it.

It's funny you mentioned goped pipes when you did. I've been trying to figure out exactly what that pipe came off of and hadn't found anything even close until last night when I started looking at goped pipes. I still haven't found an exact match, but as a group they match better in configuration than anything else. Especially with the patched center outlet.

I'm sure the pipe is designed for a smaller motor than my 390, but with the truncated headpipe and a new stinger, I'm not sure what impact that will have. Any thoughts?

 

[Rx7man]

throw it on and see! What displacement is the 390? sorry if I missed that earlier.
I found that moped pipe at the dump with nothing attached to it.. They're made cheap, and it's easier to just bend a piece of pipe (uniform diameter) than truly make a header with a taper on it.. that's probably a large part of the reason they have long headers and stubby chambers on them

 

[bikemike]

I gotta chime in here..
I pored over the 2 stroke books for days and days, then looked at other design tools, and ended up making my own simulator (that took weeks). Anyhow, I did finally make a pipe for my husky 61, and tried both the end and center bleed.. yes the end bleed is DEAFENING!!, center bleed is actually tolerable to run and still sounds nice.
Pipes are very specific (if they're done right).. I have a moped pipe (with twin outlets even!) that I thought about using somewhere, but I think it would only be suited for about a 35-40cc saw. The pipe from the moped will change the performance.. Where and how.. well, that's a wild card..

Buddy who raced motocross back in the day had a saying about pipes.. "Long and thin is in, but short and fat is where it's at".. Long, thin pipes are "well behaved", smooth power and lower RPM peak torque.. Short and fat pipes are nasty, violent, peaky power that take skill to use.

Perhaps at some point I'll revisit my simulator and hook up a few thermocouples to my existing pipe so I can get real world feedback.. The temperature of the pipe is the single biggest wildcard when designing the pipe.. if your calculated temperature is too high by 200F (ie, you're overoptimistic about your BMEP), the sound waves in the pipe will travel significantly slower making peak power at a proportionally lower RPM.. the good thing about that is it's usually relatively easy to shorten the header and belly a little to compensate for that.

Here's my saw, yes, I do buck and fall with it, though it's a thirsty beast

This guy is another guy I was thinking of whenergy someone said piped saw are for racing only. that old saw is holding nice rpm in the cut the whole time and bet it didn't come close to getting hot

 

[bikemike]

started as this working it to this 1 cut at a time and ended up here

 

[hal9000]

throw it on and see! ... What displacement is the 390?

Yep, that's the plan. Hoping to start this weekend. It just has to wait until I finish my homework (I'm finishing a mid-career masters degree), re-trim my living and dining rooms, and do the layout for my new kitchen and bathroom cabinets. So in the meantime, I'm just satisfying myself with talking and asking questions about it.

I wish I knew the exact heritage of this pipe. It's definitely a little less "short and stubby" than the goped pipes I've seen. The closest in terms of layout and profile is a pricey stainless one made by flowsystems, but this definitely isn't that.

The ms390 is a 64cc saw by the way.

 

[bikemike]

Any one of these motorized bicycle pipes is an improvement on the the 38cc saw by a wide margin. I'll be trying one on a 45cc Stihl at some point. The chrome banana has potential; I just need to cut, turn and tuck it to the side and it'll work for nearly everything except cutting at ground level.

The 034 (56cc) was being held back and needs a bigger pipe. The exhaust port was almost twice as big as the pipe header opening.

Get rid of most of the header tube on the stihl shorten it up and tune the carb bet it will scream like a ktm 50

 

[Rx7man]

What diameter is the header pipe on it? Unless you're tuning it for grunting, anything less than about a 7/8th" ID isn't going to work well with that displacement.

The one I built isn't pretty, but I didn't have any fancy tools to build it.. car body type sheet metal, tin snips, and a piece of pipe in a vice for bending it around.

One more note about stinger location.. If it works better for you to have it come out of the end of the pipe, you can still get it quieter by sliding the stinger in until it's in the belly area. The sound waves concentrate as the come down the reflector cone, and right at the tip is where you have the most extreme sound levels..

 

[hal9000]

There's no straight headpipe on mine, where it currently ends is around 1" diameter I think, but I'll have to measure to confirm that.... I was planning on having to make a short adapter to convert from the round pipe to the rectangular exhaust port, so I'll have some leeway in terms of adjusting length and size of the pipe.

 

[hal9000]

So I've managed to find a bit of time to fiddle with that pipe over the weekend. Once I make a flange and get things tacked up I'll post a few photos. But I have a question about the shrouding on the saw....

It looks as if taking the box muffler off will change the airflow pattern around the cylinder. Should I worry about making a heat shield or size the flange to mimic the profile of the stock box muffler to avoid overheating part or all of the cylinder?

 

[bikemike]

So I've managed to find a bit of time to fiddle with that pipe over the weekend. Once I make a flange and get things tacked up I'll post a few photos. But I have a question about the shrouding on the saw....

It looks as if taking the box muffler off will change the airflow pattern around the cylinder. Should I worry about making a heat shield or size the flange to mimic the profile of the stock box muffler to avoid overheating part or all of the cylinder?

I don't think youl need to worry about a heat shield on the saw. I have not put one on any of my saws. And I don't just fire up make a few cuts and shut it down il run the piss out of them

 

[hal9000]

Good to know. When I cut my flange, I went ahead and made it approximate the footprint of the muffler for good measure. Other than adding an oz or two, I figured it couldn't hurt.