Degree wheel

[Al Smith]

What are all those numbers and what do they mean.Well,here goes my attempt to explain it,bear with me .

As we all know or should, a two cycle intacts fuel/air mixture,transfers same to the cylider,fires a power stroke and exhausts in one complete revolution .
So what's the deal with "degrees' as read on the whell.Reference,mainly .

It all has to do with the period a practicular port is open during it's cycle,at what point in the revolution it's open and how long.Commonly refered to as time/area .

A gas under pressure such as the exhaust,the charge within the crankcase or the incomming air at atmospheric pressure requires a certain amount of time to get moving,so to speak .Obviously a slower moving piston would require less area because it has more time and vica versa .

This time /area is actually a ratio based on the engine displacement and the rpm range .You can get the actual point in rotation that the ports open and close during the cycle by using a degree wheel but this must also take into consideration the operating rpms an area of the port .

So that said,lets take this step at a time in several posts .

 

[gf beranek]

Go, Al, go!!!

 

[Al Smith]

Setting up the wheel .

Well,first you have to have one to set up . A larger wheel such as a 12" model used on autos is more accurate but a beech to mount and use a pointer on .So I use a 6 inch myself .You can buty them or make them.Either way works just fine.If you want to make one just copy the pic at the end of this thread .

You have to have a method to mount it .Usually and adaptor of sorts that screws into the flywheel side of the crankshaft works well . Various methods have been used,I make my own adaptors but then to I have a small machine shop .

Once you get it mounted ,make a ponter out of something.I use a piece of wire mounted to a screw in the saw case.Anywhere is fine .


That done you have to center the whell on top dead center.It's very important that it be exactly on top dead center or it throws the measurements off .

One method,roll it up to top center,zero the wheel .Looking through the ex port,move the flywheel until you detect the slightest movement and take notice of where the pointer is .Roll the engine back to zero on the pointer and reverse rotate it until you again notice movement.Tack not where the pointer is.Exact xero should be dead center of this fore and aft movement .Adjust accordingly .Recheck until it comes out exact.This is important

 

[Al Smith]

How to read it .

Starting at top dead center roll the engine over while looking through the ex port .When you just detect the port opening,note the degrees.Keep on roll ing and note again the degrees when it just closes.This would be your port duration for the ex .Do the same for the intack and jot down .

The transfers will require an extremely bright light to see through the sparkplug hole.However it works the same .


Now that you have all those figures,write them down.

It helps me to plot them on a circle just to get perspective.Easier for my pickled brain to figure out .

 

[gf beranek]

I following you, Al.

 

[Al Smith]

What it means .

Using a stock Stihlk 038 Mag for reference it goes like this starting from top .
The ex opens at 99 degrees after center.Meaning that for slightly over one quarter turn the piston is in it's power stroke .

As the piston continus down,now with the export opened,it exhaust's and does so for exactly 20 degrees of rotation before the transfer ports open .This 20 degrees is called the blow down.A time in which most of the gasses are expelled.

Remember at the same time the piston is going down it is compressing the charge in the crankcase .

At 119 degrees the transfers open and admits a fresh charge,which follows the trailing edge of the exhaust and actually helps get rid of it . This is a raising pressure charge from the crankshaft following a decreasing pressure charge from the exhaust.Remember,differential pressure is what makes things happen on these things .

Moving still along ,because the transfers lag the ex on the down stroke they will lead on the up stroke,by the same amount,20 degrees .Sealing they prevent any further transfer .Obviously 20 degrees later the ex seals and compression begines .

So what about some that get by the ex lag before compression ? It's just gone . A pressure can muffler can put some of it back as can a tuned pipe but still you can't contain it all .Just the nature of a two cycle .

Intake next

 

[Al Smith]

The intake

Where all the fuel air gets in .

In theory as soon as the piston starts up from bottom center it could accomodate an intake charge .it could be assumed it gets sucked in.Nothing could be further from the truth .It gets pushed in by atmospheric pressure .

As the piston rises there is more room for the charge .In the case of a reed engine this intake could start at 180 degrees of rotation.In a piston port,because of it's nature it actually starts later .

The 038 mag intakes at 287 degrees after zero and closes at 67 after .After it hits top center though actually not much gets in .

 

[Al Smith]

What to change.

If any.A muffler port job wil get you the most gains for the least work and it only takes a few minutes to do with minimal tools . This improvement satisfies 85 percent of most people looking to enhance a saws performance .

Then we have the other 15 percent .

So,just what do yoiu want to do with it.Outrun a 066 with an 044,not likely to happen .Cut mid sized stuff with an 046 at nearly the speed of an 066,possible up to a point .

The first thing that has to happen on a two cycle to gain power is to work on the exhaust port,after the muffler . Holding the power stroke longer will usually get you more torque but maybe a tad slower .The port can be widdened but not raised to gain area .This will evacuate the ex faster with no sacrifice of torque .Raising it will cut some of the power off but raise the rpms a tad . Too high and it becomes a gutless wonder .

More later on the subject .

 

[Al Smith]

Simple easy

Shall we say you just want to get a tad more out of ye olde saw.You aren't going to race it could care lesss about being a cookie cutter .Just widden the exhaust a bit and clean up the casting flaws within the transfer ports,intake etc that might be a hiderance to air flow .

This,combined with a muffler job will usually satisfy 9 out of 10 people because of the increased power level which would be quite noticable .

You might widden the intake just a bit but be spairing .Easy does it .

As I said in another thread ,this is just a simple mod .A more intense one which yields even more power is more complex and I really feel a person should have a more knowledgable understanding of gas movements under pressure,the time/area involved and other things before ever attempting same .Unless of course it's on a junker any way then you don't have a thing to lose .

I do pretty good for a grease monkey but I don't feel I have enough knowledge to pull figures out of my head to relate same to others .That said,the real hot rod hop ups I don't think I'll delve into .

 

[gf beranek]

That's very well explained, Al. Sounds like it wouldn't be all that difficult to do. I wonder why the factory doesn't do it. Warranty protection I guess.

 

[RIVERRAT]

Jerry, the factories dont do this as a practice because of cost. Though the Stihl Magnum versions do have a diff exhaust than their counterpart.

On another note, take John Browning's 45 auto. Known by some as the Colt 45. The most worked over handgun by gunsmiths out there.
One can now buy a slide & frame that needs little to no hand fitting.This level of fit coming straight from the mill.
The barrel lugs & link remain the only thing that needs hand fitting. This in most cases is only necessary when some one is wanting 2" or better groups at 50 yards.
You can walk into a gun store today & buy a massed produced custom 45 that is the equal of what some smiths turned out 20 yrs. ago.....I have wondered at times why this custom level of performance has not yet to be seen in other mass produced stuff. I have always thought its got to be money or what you said.

 

[Al Smith]

Chainsaws are just a tool made for an application .Compare the saw engine with a production ran auto engine for example concerning the cam shaft and valve train .

What is on the average auto is a best of all cam grind when you figure areas of starting,smooth idle and good midrange power for the normal driving with a reasonable milage per gallon figure .

If want extremely good torque ,mid range for say pulling heavy loads you use big valves and a long valve duration . It gets good power but kills the milage.Little valves and a short duration produces a more economical engine but at the sacrifice of power .

The recent usage of variabley adjustable valve trains have addressed this problem on modern auto engines .Some years back it was almost unheard of to get 1 HP per cubic inch.Now it's quite common .


The saw deal and speaking strictly for Stihl .The saws right out of the box have ample power for what they are intended for.Used properly they will give good service for years .

There is some untapped power reserves though for those who desire same .A little tweek really doesn't hurt the saw as far as longgevity of service .However going overboard will shorten the life of them ,a factor to consider . You might with luck get an 046 size to almost hang with an 066 but doing so it certainly will not outlast the 66 for sure .

In my own little life of tweeking I only attempt to jump them about one class or slightly less .044 size to 046 size or equivelant .

Now as far as offering factory enhancements about the only thing I've heard of regarding Stihl is factory duel port mufflers which will crank out more power . As far as that goes though it's real easy to do your own .

 

[Al Smith]

Wheel

Oh dumb me got to running off in the head and forgot to post a picture of the wheel. If you want, just copy it and make your own.Easy enough to do .

 

[Ax-Man]

I am with you also AL. good thread.

I understand what your doing but what I have a hard time with is interrperting what the numbers actually mean and what they are telling you about the engine.


For example you said the 38 opened it's exhaust port at 99 degrees. Then we go another 20 degrees to get the blow down time. Are these good numbers ?? Do the numbers tell you the saw is fast , slow or just mediocore. Dothe numbers mean anything in relationship to the amount of torque the engine will put have .

Maybe I am making this more complicated that it should be as the degree wheel is a just a way to make a map or blueprint of the engine cycle.

Maybe what I should be asking is are there any bench mark numbers out there for the average saw. I know this is going to vary from one model to another but there has to be some kind of norm that is pretty much common to all saws.

 

[Al Smith]

I really can't tell you much about a universal bench mark because I've only taken the readings on Stihls and one little Poulan I had ported . Those numbers I rattled off are pretty close to most Stihl's I' ve done .

Actually the only thing it[degree wheel ] could tell you is how far down on the cylinder the thing is ported .Generally anything longer than 100 degrees after would be most likely a slower running high torque engine,Conversely anything closer to 90 most likely would have more scoot less grunt , in my way of thinking .

If you are just doing a widdening of the ex and a little to the intake with no milling of the cylinder base to raise the comp,the numbers really mean nothing ,because in reality you haven't changed them .

When I did the 038 Mag ,which is a tad more than basic,I purposely left an additional 2 to 3 degrees longer before the ex opened to attempt to increase the torque . It seems to have worked as far as I can tell .

By increasing the port wideth rather than raise it back to stock ,in reality the time/area or time the port is opened long enough to get a get exhaust at a given rpm are about the same,perhaps a tad quicker .

This link is one of the best references I have found.http://edj.net/2stroke/jennings/ I have it downloaed to my puter .Jennings was more of a motorcycle tuner but the same applies to chainsaws as a general rule .

 

[High Scale]

Thanks Al, good stuff.

 

[Ax-Man]

[QUOTE=Al Smith;220317]
Actually the only thing it[degree wheel ] could tell you is how far down on the cylinder the thing is ported .Generally anything longer than 100 degrees after would be most likely a slower running high torque engine,Conversely anything closer to 90 most likely would have more scoot less grunt , in my way of thinking .

This is very helpful Al, it gives me a better idea of what is going on. I like your no nonsense way of explaining things.

When I did the 038 Mag ,which is a tad more than basic,I purposely left an additional 2 to 3 degrees longer before the ex opened to attempt to increase the torque . It seems to have worked as far as I can tell .

How exactly did you do this. Thin or no base gasket???

This link is one of the best references I have found.http://edj.net/2stroke/jennings/ I have it downloaed to my puter .Jennings was more of a motorcycle tuner but the same applies to chainsaws as a general rule .[/QUOTE]

The actual book is kind of hard to come by. I have been looking for it on and off when the mood strikes me. Our local library doesn't have it. Closest library that does have it is 30 miles away. The book has long been out of print and this makes it a collectors item. The prices for Jennings book will give you sticker shock. Beat up ones may go for $ 50. The ones in good to excellent condition go for anywhere between $150 to $200. These are internet online prices. Your version I added to my favorites along the other ones I have downloaded. Your version is a little better to read or what little I have been able to read on the parts that interest me. Thanks

Ha, It worked, Sorry Al, your my first try or should I say victim with chopping a quote into pieces and playing with the cut and paste. I got tired of trying to find the right shade of grey for the quotes and just used red.

 

[Al Smith]

[QUOTE=Al Smith;220317]
Actually the only thing it[degree wheel ] could tell you is how far down on the cylinder the thing is ported .Generally anything longer than 100 degrees after would be most likely a slower running high torque engine,Conversely anything closer to 90 most likely would have more scoot less grunt , in my way of thinking .

This is very helpful Al, it gives me a better idea of what is going on. I like your no nonsense way of explaining things.

When I did the 038 Mag ,which is a tad more than basic,I purposely left an additional 2 to 3 degrees longer before the ex opened to attempt to increase the torque . It seems to have worked as far as I can tell .

How exactly did you do this. Thin or no base gasket???

On a removable cylinder saw,like a Stihl,if you say cut off 20 thou off the base it automaticaly does two things .First it lowers you transfers and exhaust by that much and in turn raises the intake by that much .

That done many people raise back up the intake and transfers back up by that much,more or less to achieve basically stock .You have to remember though this amount of degrees is happening at a different place in the reworked cylinder and increased compression would move the gasses faster.How much,good question.Some people can figure it out, I only guess .

In my case I had cut the piston by about 30 thou leaving a dome which, had I cut the correct amount off the base it would have put me right back at stock.How-some -ever I had a dumb a$$ attack and cut too much off thus I had to install a thicker gasket or else the piston would hit the head .It worked out the material I used came out to about 2 to 3 degrees over in rotation from stock which gave me that much more burn or power stroke .


I could have raised the exhaust back up the two,which would have given me about 18 degrees of blow down.This would have been fine.I opted to see what the additional long exhaust would do and so it looks good power and rpm wise .I'll know more after the rings are broken in as it has only about 10 minutes or less of running time at this post .