Port Timing Figures

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dbabcock

Hi Tech Redneck
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I've become interested in 2 stroke port timing lately and I'm curious what some of the saw builders here use for their figures. It seems that there are some pretty standard "cookbook" numbers that most people start out with based on application and what other things have been done to the motor, like higher compression, pipes, reeds, etc.

While I had the flywheel off the 3120, I put a degree wheel on it and measured about 170 degrees of exhaust port timing, but I have no idea about the intake and transfer port figures. I think Ken once told me that the 170 degree number can go upwards of 185 degrees if a head is machined for the saw to up the compression. I think the more exhaust duration that is put in, the lower the effective compression ratio, so this is why stock barrels rarely have more than 170.

I put the wheeel on my 266 and measured something like 145 degrees, but I can't remember the exact number. My 2149 was a real eye opener: 224 degrees! Man, that's more than high-end motorsicle two strokes run. No wonder the little thing motors!

These are the secrets of the masters, I guess!
 
I don't try to hot rod any of these things but what I'd like to know is why none of todays builders never talk about doing something with the crankcase. When I was racing karts back in the 70's with the Mac kart engines, the first place builders attacked was the crankcase.

In a 2 cycle this is the equivelent of a supercharger on a 4 cycle engine and as much or more can be gained from optimizing this function as from anything else. Really makes me wonder about how good todays builders are when they ignore this area of the engine.
 
Why don't you try it....?

"Really makes me wonder about how good todays builders are when they ignore this area of the engine."

Why not mod one yourself and find out how good these guys really are. Be interested to hear how good you are.
 
In the 70's "stuffing" the crank case was all the rage. It lost favor when people relised that high primary compression ratios tended to make the powerbands razor thin and didnt do what they expected for HP. I fyou look at most high perfomance two strokes today such as snowmobile engines you will notice they have huge cases. this provides for very wide torquey powercurves.

Hey Doug, I wondered the same things about the timeing that Ken, Dennis and others put in to saw motors. I even came up with some numbers close to what I think the might use. I will post them tonight when I get home.BTW your observations aboutthe effect of exhaust timing on compression ratio is correct. Higher exhaust ports provide for more exhaust duration but lower compression if the head or base in the case of a saw is not altered to compensate.

BTW on the high primary compression ratio thing. The japs abandon the practice in the sixties. At that time there where 125cc four cylinder roadrace engines the made 55hp and had 18 speed tranies. Talk about thin powerbands.
 
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WRW, There are not many secrets left when it comes to two stroke tuning. With the simulation software available tday any person with mechanical and computer knowledge can come up with a good set of port specs. In my mind the art of it is getting the diegrinder to work properly. I tried my hand at porting on a few scrap cylinders and could really not get the hang of it.
 
When I was a young'n, some friends and I appraised ourselves to be pretty bright engine tuners. We managed to get theoretical BMEP's up over 110 psi on some sno-mo engines, but it was pretty pointless. The "real" tuners of the day stopped short of that, as bottom ends couldn't hold up to it, or the feeble cooling systems on most of those old fan-cooled engines couldn't draw enough heat off. (There are those who propose that the fuel charge handles up to 85% of the heat, but it's the remainder that kills you!).

I remember stuffing a few cases, but the only engine that really responded in any meaningful way was the old Rotax 370 cc opposed twin, with a cavernous void inside. Unfortunately, that engine also routinely grenaded rather violently when forced past about 7000 rpm.

The important point to remember is it may be extremely difficult to justify the expense of welding and machining a chainsaw case for a relatively small gain
 
The important point to remember is it may be extremely difficult to justify the expense of welding and machining a chainsaw case for a relatively small gain
Now days they just use devcon epoxy to stuff cases. This stuff also works great for adding material to transfer ports.
 
A little knowledge is a dangerous thing. I believe that we as human beings have a thirst for knowledge but how that is put into practice is what makes a big difference. Now that this subject has been broached I am sure Ken Dunn will get many more inquiries into his "secret" figures. Having built many a race saw and continuing to build and learn every day, I can tell you this; Don't think that you can get the figures and then build a saw comparable or better than Ken does. Knowing these figures is only a small part of the picture.
So many times in my life I have seen some job or another that an individual has accomplished. Having watched how it was done, and being an incorrigible tinkerer, thought "I can do that and probably a little better, faster or whatever. Most of the time I was wrong. Are you willing to put in the years of practice that it takes to get this right? Well when it comes to two cycle theory, Ken Dunn is the guy that has been there, done that, got the t-shirt and now uses it for a grease rag! If you want a little more power in your saw BE CONSERVATIVE!!! If you want a lot more power in your saw--Send it to Ken Dunn. Mike
ps. If you think any of the above is wrong just ask Dennis, Sawracr, Art, or anyone that has paid Ken to build them a saw.
 
Quote Rupedoggy:

"I believe that we as human beings have a thirst for knowledge but how that is put into practice is what makes a big difference."

I agree with Mike, with the thought that most people never do put knowledge into practice.

Joe
 
Mike, I agree 100%...every saw I build I learn more and more, and I realize that I will probably never know as much as Ken has forgotten. Port timing figures are an interesting bunch of numbers as I have learned..each model of saw likes a different set of numbers. I have been fortunate enough, to learn from Ken, and skip a bunch of destroyed cylinders and a bunch of years of tinkering.

Doug, you may want to recheck the 2149 timing.
 
Mike, My post was not ment in any way to slight the work of Ken, You or Dennis. You are right in that engine work requires alot more than numbers(testing etc). I was only suggesting that the engineering behind porting is not "secret" as some people think. Its simply forward math and computer skills for the most part.
 
What ,, Doug if you 2149 has 224 degrees.. I'm sending mine back to Dennis........
 
"Be wary of a simple solution to a complex problem"

I think that without the experience to judge where the bottlenecks are in a particular engines breathing, you would very often take metal off where you shouldn't. It is a bit difficult to put back!! I think you won't find too many oops! marks filled with silly putty, on a saw you get back from Ken or Dennis.

Frank
 
I don't recall blown seals being a problem. Biggest problems was keeping the carb adjusted lean enough to gain power without getting so lean you stuck the piston.

The class I ran in was basically stock, porting wasn't allowed but since the Mac had a removable stuffer plate we were allowed to modify it cause it could be replaced and not be a permament alteration from stock.

Since we ran road course circuits we were looking for power gains throughout the rpm range and working over the stuffer did work. Those minor mods to the stuffer were done by a friend who better understood what was needed, I've never had the time to really get involved with doing that sort of thing.

Maybe the gains aren't what I'm thinking they would be but I just can't help but think they would be. Like I said earlier, the crankcase in a 2 cycle is the same as a supercharger on a four cycle engine and they produce huge gains throughout the rpm range. I'd just like to hear from one of the builders of modified saws and explain why this area is ignored. If one of them has a good logical reason I'd be happy to drop the subject.
 
Hi Doug, it is great to see you back, stiring up disention amoungst the ranks. If shoes were clues, you'd be running barefoot.
I liked the way you put those ficticional figures in there, designed to fool others, but you so adeptly forgot to state the degree to which the timing must be advanced in order make the whole thing congruent, and I thank you for that.
Regardless, Ken threw a wrench into my day when I recieved in the mail a 385 and an 066 piston and barrel with all the neat figures I dont need to understand. These saws bring a whole new dimension to the word chainsaur.
Anyway, irregardless of whether the elevator goes to the top or not, wisdom is just knowledge in action.
The Chainsaw Phylosofer
 
Spambert,
As usual, you're using my good-natured fun as a vehicle to kiss everyone's a$$ at my expense. Somehow, you and anything technical mixes like Kaopectate and a Fleet enema, so I'll consider your reply for what it's worth: A year's supply of Preparation H and a Whoopie Cushion.

Eric,
Only kidding about the 2149; just wanted to see who was paying attention. I have no idea what Dennis put in there but rest assured, the thing boogies. As far as the 266 goes, it's stock at 154 (I mixed up the last two numbers), but it won't be for long.
 
I like to tinker too. If it weren't for this site I would never have attempted any muffler mods. I took a Shindaiwa 360 that I bought used and opened up the muffler and eliminated the cylinder base gasket ( I sealed it with a gasket eliminator sealer ) to raise the compression. This saw lost no hi-rpm and gained a bunch of low-end torque. I told Ken Dunn about my project and he generously filled me in on the port timing specs. I think I will leave well enough alone.



The one thing I can't figure out is how engine tuners get their specs for expansion chambers. I basically understand how they work. Is it all trial and error?
 
Doug,
I figured as much, I don't think their is any way to get that much.
I know all about the 2149 I have one my favorite firewood saw.
I know what mine is I needed it to build the pipe.......
 
Rich,

The length of the divergent section of the pipe involves only three basic numbers: exhaust port duration, RPM and the speed of sound. Beyond that, I'll leave it to one of the real saw gurus to give you the equation as it would prolly be considered inappropriate for me to do so.
 
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