Tuned Pipes for Chainsaws

My buddy Joe and I have been doing some experiments with 'tuned pipe' (not expansion chambers) on chainsaws. The idea being to keep the saw between max-torque and max-HP rev range. If lets say the engine develops max torque at 6000 and max HP at 10K you want the saw to stay in its' power band. After reving to 10K going to 11 or 12 gets you nothing, saw is already past its' max range.

A tuned pipe will keep the engine in the max power range. There is a whole pile of measuring and math that goes along with this. If you look closely at the muffler of this saw which has a tuned pipe, try to find the 3.75 inch pipe.

http://firewoodhoardersclub.com/for...ne-last-one-especially-for-member-jchin.4634/

Most of the length of the pipe is hidden inside the muffler! Tuned pipes ONLY work if they are the only exit, so the factory outlet must be blocked off, I do this by fabricating a copper plate that you install in place of the spark screen, which effectively blocks the factory outlet but is removable if you want all the 'manly' noise back!

 

Removing overly restrictive (EPA) components from inside your muffler will improve the ability of the saw to flow, boring a hole in the side of your muffler will also improve performance, if it allows some of the exhaust gases to bypass the restrictive internal components. The average box muffler on a chainsaw is between 5 and 10 times the engine displacement. A 372 muffler is approximately 3x3x3 or 27CI. 27x16.38=442cc. Divide this by engine displacement and you get 5.78. So, it takes 5 power strokes of the engine before the muffler is full and starts to push exhaust out the outlet. Each exhaust pulse bounces off the front of the muffler and returns toward the back helping the engine to scavenge. The tuned pipe can help control and manage this effect to maximize perfromance.

 

So, the pipe installed in the muffler on the saw above is 3.75 inches long with an ID of .5 inches , it would make things easier if you could stick it out the side, but a 4 inch long pipe hanging out the side of your saw would be a huge PITA and make the saw fairly useless! That is why you hide most of it inside the muffler. However, the pipe cannot come within its' diameter of any wall, corner or other obstruction within the muffler. That fact dictates some very creative bending of the pipe and the associated challenge of how to get that pretzel you just created inside the muffler!

 

Could you elaborate on this scavenging? I'm not too clear on what it means.

 

This may give you a basic understanding

 

We start with the OEM specs then we adjust by putting the engine on a small engine dyno owned by a kart builder/racer friend of mine to see how much fluff there is in the OEM claims.

Surprisingly the OEMs no matter which, 'as a rule' do not inflate too much but they all 'round' in their own best interests.

 

Klr do you have any pics of these pipes installed in a muffler and how you determine the way to do it? I have played around building and changing pipes on bikes and quads and been thinking of doing it to chainsaws. Thanks

 

Something like this?

 

It all depends on the measurements of the saw in question. An example would be a HVA 372 between 71.4&76.4cc, the calculation for 'first order harmonic' is 0.500ID pipe between 3.75&4.25 inches. Most of the pipe is buried inside the muffler for obvious usability reasons. There are two things you must keep in mind; 1) the pipe cannot come any closer to any wall, corner or any other internal obstruction than its' ID, and 2) the tuned pipe must be the ONLY exit to the muffler. Remember a tuned pipe is not a DP muffler. The object of a tuned pipe is to keep the engine within its' max torque and max power RPM range.

 

The saw in this ad has a tuned pipe. It looks like a DP but it is not.
http://firewoodhoardersclub.com/for...ne-last-one-especially-for-member-jchin.4634/

 

CWN877, most of the tuned pipes I make start out as 0.500 ID mild steel conduit. I find that 0.500-0.800 make the best pipes but as said above it depends on the saw and a big gob of math. The standard box mufflers found on saws have enough room in them to make the addition of a tuned pipe quite easy, just a gentle curve in the pipe to make sure that the exhaust does not melt the plastic. However, to build and install a pipe on some saws requires that you make the pipe a pretzel!

 

OK thought that I would let you guys know that I had a tuned pipe fail in the field the other day. No damage to the saw. The pipe (4.25 inches) came loose from the body of the muffler, I believe that the silver solder failed due to vibration or a combination of a weak solder joint and vibration. Most of the length of the pipe is inside the muffler un supported because it cannot be close enough to anything internal to the muffler because of tuned pipe requirements to be attached to anything.

So even though tuned pipes do work, I cannot reccommend doing one on your own saw until I come up with a way to get some support inside the muffler for the pipe. I am toying with the idea of a flange installed on the pipe installed inside the muffler, we will see.

 

Ill be watching this closely !

 

Owner says it was a cold (bad) silver solder joint, my bad! However the viabration thing still worrys me have to come up with a better way.

 

When it comes in the mail tomorrow I will have a look and post some pics. However MM I think it was my problem since I have a bunch in the field on 390, 394 and 395 saws and this is the first to fail. Additionally this is the first one I used .5 inch conduit on not R-531 or 4130 thin wall.

 

If you were using EMT, it would have been hard to get all the plating off. This could be your problem.

When I grind off the galvanized layer on steel prior to welding it, even though I can't see it, there is still something imbedded in the metal. I can tell by the extra splattering I wouldn't normally have.

 

I would wonder if copper would be a better medium for the tubing? Possibly an issue with the temps? pliability being the biggest factor
Thin wall swagelock SS tubing with a bulkhead fitting would be optimal it tends to deform with tight radii though - I might could(southern slang) bend one for you if the specs were forwarded or send the muffler and receive a tuned port muffler.
The requirements of .500 clearance to exterior walls or the tubing thickness is the formula? can you go smaller/larger tubing or is this a general rule of thumb from experience?