Backpressure- some saws REALLY need it

[Pioneer]

I just put together an Xl-1 Homelite this afternoon, and decided to run it with out the muffler as I didn't have a baffle cap for it. Well the saw started fine, but I could not get it to idle down and the idle mix screw was out twice as far as normal!
My first thought was a bad crank seal, and dreaded the thought of tearing it down all over again. Then I thought just maybe it needed the back pressure from the muffler.
Well, I happen to own a couple of these saws and decided to put the muffler on from the other one.
Bingo! Now it idled right down and I could turn the idle mix in where it should be. I didn't think a simple baffle cap could make that much difference, but there you have it. Some saws are optimized the way they are, and playing around with mods should be done carefully, especially with the higher performance models.

 

[glens]

I think it would be safe to say that only the EPA-generation saws really benefit from muffler mods.

 

[ozflea]

Good morning Glens,
Mate i suppose what you do after you buy your newer type saw is up to you but then as a manfacturer theres rules to build by.
But if ever the saw rules follow car rules and needed an annual checkup to see if the laws were being adhered to then mods would be out, secondly like all things mechanical anything modified possibly would not have the built in longetivity and reliability that most standard saws have.

No i'm not being smart mouthed but manufacturers need to look after their name and reliabity is foremost and cranking up the horsepower doesn't help unless the engine was detuned to start with.

McBob
Is this a fair comment fellas ??

 

[willy]

Might want to check this out, read and see the animation link at the bottom of the page: looksee

Although a saw muffler may not be an optimized tuned exhaust it is a very important part of the engine. Removing or opening it up can be very counterproductive. In some cases MORE backpressure can even be desireable.

The animation shows how the acoustic wave pushes lost fresh charge back into the engine. if this lost charge for example was 10cc, and it gets pushed back into a 60cc engine, the result is like having 70cc of power, that's how tuned exhausts can make more power. If the pipe was removed the 10cc would be lost. If the outlet were opened up too much, pressure in the pipe would be reduced and the acoustic wave wouldn't A. be as strong, B. travel at the correct speed to "arrive on time", thus a power loss. Not trying to give a primer on 2 stroke tuning, just trying to explain that freer flowing is not always the answer in 2 stroke exhaust.

 

[glens]

Hey McBob,

The only thing I've done so far in terms of saw modification was to procure a muffler from an older 036 for my 036 and a front muffler cover for the older 066 for my 066, as well as remove the highly-restrictive baffle within the 066 muffler to match the interior of the older design.  I realize that the port timings and arrangements may be different in the newer cylinders so the older mufflers may not be entirely optimal any longer, but the results were okay.  I realized a 15% reduction in cut time on a 20" (or so) oak log with the 066 merely by replacing the muffler front cover for the one with a second outlet.

I can be found to have said many times here that I believe redesigning the port figures or other saw internals will surely reduce the safety factor / life span of the bottom end.  Engineering blood runs through my veins, so I have great faith in the factory engineers.  If they have to make the saw run quieter they may well change other internals for optimum performance with the new discharge scheme, and they most certainly put as much metal in the crankcases as they deem necessary for the power output.

Glen

 

[max2cam]

That animated 2-stroke .gif in action is really cool! It really shows how a 2-stroke actually operates step by step and esp. the back pressure bouncing back off the end of the expansion chamber! Really good showing how the piston vacuum and pressure operates the whole process! Anything like that showing 4-stroke operation?

 

[willy]

There's many out there, here's one of them: http://www.keveney.com/otto.html

 

[max2cam]

When you compare the 2-stroke and 4-stroke cycles side by side, the 2-stroke seems more advanced and futuristic altho it is nearly as old. The 4-stroke seems antiquated and clunky. In the early years of the 20th century there was a lot of interest in 2-stroke cars (Elmore) and motorcycles but that pretty much fell by the wayside.

 

[Crofter]

The two strokes make a lot of horsepower per pound and are a low parts count engine, but they have a lot of warts. Fuel efficiency is lousy, even the new strato charged ones have a lot of unburned hydrocarbons in the exhaust compared to a four stroke. Longevity sucks because of the rings being unsupported as they travel over the ports, but I dont think we want to trade them in for saws that are twice as heavy for the same output and cost a lot more. I like my modified saws but as far as better mileage or less pollution,..............well, we won't go there!