Case Compression, Let's Talk About It

[Mastermind]

I'm guessing a reed valve engine is more efficient less loses over a piston port ultimately.

Theres a reason powerful two stroke engines on motorcycles use reed valves I think.

 

[rogue60]

Anyone ever run a saw below sea level like down the bottom of one the big open cut pits? might blow the cylinder off lol

 

[rogue60]

While there is a little bit of compression under the piston, it’s more a vacuum to pull the air and fuel through the carb into the bottom end.

A well tuned muffler or pipe will help pull the charge through the transfer ports better thus creating a little more vacuum than a stock saw can.

That extra vacuum created will pull more air and fuel when the intake port opens.

Then the cycle repeats.

Technically speaking air isn't sucked/pulled in it's pushed in on all naturally aspirated engines.

 

[Mastermind]

Technically speaking air isn't sucked in it's pushed in on all naturally aspirated engines.

True. The weight of atmospheric pressure pushes it into any area that has a pressure of less than 14.7psi at sea level.

Takes me back to carburetion classes in the early 80s.

 

[Red97]

True. The weight of atmospheric pressure pushes it into any area that has a pressure of less than 14.7psi at sea level.

Takes me back to carburetion classes in the early 80s.

What are these early 80's you speak of?

 

[Mastermind]

What are these early 80's you speak of?

Back when you were still just the gleam in your father's eye....

 

[Al Smith]

Conversely talking about a pressure can muffler .Once the pressure in the can equals to pressure in the cylinder no more transfer can take place . One might think eliminating the muffler would be the cure .Then because of how small a chainsaw carb is it would over run it's capacity to supply fuel .Bigger is not always better either ,giant carb on a small displacement engine if it would run will load it up .If it would idle it would do so blowing black smoke and cough and wheeze on a spool up .You have to keep things in balance so to speak else it just will not run right .

 

[farminkarman]

Back when you were still just the gleam in your father's eye....

Lol, that's some funny chit right there.

 

[Wanab]

Conversely talking about a pressure can muffler .Once the pressure in the can equals to pressure in the cylinder no more transfer can take place . One might think eliminating the muffler would be the cure .Then because of how small a chainsaw carb is it would over run it's capacity to supply fuel .Bigger is not always better either ,giant carb on a small displacement engine if it would run will load it up .If it would idle it would do so blowing black smoke and cough and wheeze on a spool up .You have to keep things in balance so to speak else it just will not run right .

At what point do you exceed the capacity of your muffler outlet.

 

[Al Smith]

Good question ! It's been suggested about 80 percent of the area of the exhaust port .Some say more .

 

[Wanab]

The way I see it there is no correct percent. That may get you close but what engines really like is when you balance size with VE. Engines near or over 100% require larger outlets than conventional thinking will allow.

 

[Al Smith]

All true but now another story that goes back to the AS days .A guy near Cincinnati became the proud owner of a brand new 880 Stihl .He got carried away with a die grinder and punched the muffler outlet to about 150 percent and leaned the carb high speed jet .Wound up like a model air plane engine .Myself and others caution him on that .Might have made it a week before he cooked it .Not too long after that on flea bay,low hours Stihl 880 $400 on a $1300 saw .Morel of that story is of you want to experiment use a cheaper saw not the most expensive .

 

[Mastermind]

The way I see it there is no correct percent. That may get you close but what engines really like is when you balance size with VE. Engines near or over 100% require larger outlets than conventional thinking will allow.

I wonder if it is actually possible to achieve over 100% volumetric efficiency without a tuned pipe, or some type of boost?

 

[Bjorn]

Is this big enough?

 

[Stump Shot]

I wonder if it is actually possible to achieve over 100% volumetric efficiency without a tuned pipe, or some type of boost?

Absolutely, it's the saw that runs like a mfr and uses fuel like a drunken sailor. To where some of it s actually being lost out the exhaust port, this is 100% +.
Our job is to get as close to 100% as possible without going over At least that's my findings that formed my general opinion of the thing.

 

[Al Smith]

Those megaphones go back to the early days of 2 cycle karting .Some were huge like a tuba .You have to remember how large the carbs were and often had two or more . If you were in the back of the pack they blew so much raw fuel with caster oil in it you'd look like an oil field worker after a race .

 

[Al Smith]

When I built the slide pipe for the mini mac I experimented with it I tried just running the cone with out the slide .Talk about loud but it over ran the little carb at about 2/3 throttle .

 

[Mastermind]

Is this big enough? View attachment 321819

Looks like a Habada saw.....

@spencerpaving

 

[Bjorn]

Yes, it is a spencerpaving saw. I just like the style of the (silencer) and I am sure that a big crankcase is good, for a 2-stroke engine, If you use a calculated pipe or like in the picture a megafon. But a regular Muffler dont help the engine suck fuel from the crankcase so much as a pipe do.

 

[Mastermind]

Yes, it is a spencerpaving saw. I just like the style of the (silencer) and I am sure that a big crankcase is good, for a 2-stroke engine, If you use a calculated pipe or like in the picture a megafon. But a regular Muffler dont help the engine suck fuel from the crankcase so much as a pipe do.

I've watched that saw run.....about made my ears bleed.