Port time area

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danno2

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I'm a little confused. Jennings two stroke tuners manual states "widening the exhaust port increases the engine speed at which maximum power is realized, while reducing low-speed power. And the same pattern is to be observed in increases to transfer port time-area, though in the opposite direction." Please explain how larger transfer ports decreases high rpm power. Does smaller transfer ports increase high rpm power? Thanks.:msp_confused:
 
Widening the ports adds volume to the cylinder. For a given piston velocity, there is a given amount of mixture of fuel and air that will flow. With a wider port, comes a decrease in pressure. At lower piston speeds, this decrease in pressure can decrease the amount of fuel and air that will enter , assuming the width is consistantly enlarged from intake to the cylinder. At higher piston speeds, the volume lends itself to more easily allowing more mixture to flow.

When you add intake timing, you allow more mixture to enter the cylinder earlier in the cycle. This can facilitate more mixture into the cylinder for a given piston speed.

However, there are a LOT of variables. For instance, if you only add intake timing and do not increase the size of the intake port at the carburetor, you will generally increase torque to a point, as you have now increased the rate of flow over time so that the cylinder fills up better(it's also likely add torque at all rpms on most modern saws, actually). If you increase the timing AND increase the size of the intake of the cylinder port at the carb consistently towards the port, you will decrease velocity at lower RPMS and facilitate more flow at higher rpms. Etc.

Unlike automobiles, saws have a very limited means to change their 'gearing' - sprockets, to a small extent - chains, and bars. So how one times and modifies the ports is very important.
 
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Widening the ports adds volume to the cylinder. For a given piston velocity, there is a given amount of mixture of fuel and air that will flow. With a wider port, comes a decrease in pressure. At lower piston speeds, this decrease in pressure can decrease the amount of fuel and air that will enter , assuming the width is consistantly enlarged from intake to the cylinder. At higher piston speeds, the volume lends itself to more easily allowing more mixture to flow.

When you add intake timing, you allow more mixture to enter the cylinder earlier in the cycle. This can facilitate more mixture into the cylinder for a given piston speed.

However, there are a LOT of variables. For instance, if you only add intake timing and do not increase the size of the intake port at the carburetor, you will generally increase torque to a point, as you have now increased the rate of flow over time so that the cylinder fills up better(it's also likely add torque at all rpms on most modern saws, actually). If you increase the timing AND increase the size of the intake of the cylinder port at the carb consistently towards the port, you will decrease velocity at lower RPMS and facilitate more flow at higher rpms. Etc.

Unlike automobiles, saws have a very limited means to change their 'gearing' - sprockets, to a small extent - chains, and bars. So how one times and modifies the ports is very important.

Thanks reeindeer. I understand larger intake and exhaust ports make more power at higher rpms. I assumed larger time area transfer ports would also increase high rpm power, but Jennings said it hurts high rpm power and increases low rpm torque. Was this a misprint ?
 
With smaller transfer ports, the air is forced to move faster as it moves from below the piston into the cylinder. With larger ports the air moves slower and does not scavenge the lower area as well. But as with everything there is cons and pros. The transfers need to be big enough to get all the air possible into the cylinder, but to big and they dont pull enough in.:dizzy:
 
Thanks reeindeer. I understand larger intake and exhaust ports make more power at higher rpms. I assumed larger time area transfer ports would also increase high rpm power, but Jennings said it hurts high rpm power and increases low rpm torque. Was this a misprint ?

Since we are on a chainsaw forum, I assume you are talking about industrial type 2 strokes with box type exhaust. Well, as far as I know, Jennings never worked on, or wrote about this engine type, but rather only piped bike engines. He has not the best reputation among bike tuners, either, so I would not recommend following his guidelines at all when modifying a chainsaw engine.
IMO chainsaw transfers should in general be kept low, and widened as much as possible to gain time/area without opening before cylinder pressure has blown down to below crankcase pressure as box type exhaust offer no suction help for the transfers.
 
Since we are on a chainsaw forum, I assume you are talking about industrial type 2 strokes with box type exhaust. Well, as far as I know, Jennings never worked on, or wrote about this engine type, but rather only piped bike engines. He has not the best reputation among bike tuners, either, so I would not recommend following his guidelines at all when modifying a chainsaw engine.
IMO chainsaw transfers should in general be kept low, and widened as much as possible to gain time/area without opening before cylinder pressure has blown down to below crankcase pressure as box type exhaust offer no suction help for the transfers.

Thanks. I was a little unsure how Jennings book would apply to chainsaws. So will larger transfer ports help or hurt high rpm power? I mean within reasonable size limits. Can you recommend any good publications on chainsaw engines?
 
Larger ports has two meanings. You can increase a ports size by increasing duration. You can also increase a ports area by increasing width. Generally when a person speaks of port area it is width they are speaking of.

Increased port width, to a point, is always a good thing. Locate your ring ends to be sure they will not fall into and snag a opening. On the exhaust side do not go beyond 65-70% of cylinder bore and do not go beyond piston skirt width. Some saws are limited by piston skirt width not percentage. On the intake side stay narrower than the skirt.

Widening ports will increase the saws cutting rpm and give you gains. There will be some gains left on the table though. Modifying port durations will pick up the remaining gains.

Should you decide to increase port durations be sure to research you saw model with in Arboristsite. Some saws have better gains by different durations or compression.
 
Since we are on a chainsaw forum, I assume you are talking about industrial type 2 strokes with box type exhaust. Well, as far as I know, Jennings never worked on, or wrote about this engine type, but rather only piped bike engines. He has not the best reputation among bike tuners, either, so I would not recommend following his guidelines at all when modifying a chainsaw engine.
IMO chainsaw transfers should in general be kept low, and widened as much as possible to gain time/area without opening before cylinder pressure has blown down to below crankcase pressure as box type exhaust offer no suction help for the transfers.

thx GBD. it had to be said.

truest most factual statement made on this topic.

jennings optimum port ranges are much too large for a small BMEP engine (ie. chainsaw).

rep sent.
 
Thanks all for sharing. You guys gave me a lot off good info. Stumpy is definitely an artist with chainsaws. I have a husky 350 2003 EPA saw that needs refreshing. I though I my pep it up a bit while I'm at it. I want to learn all I can. I 'm not a professional,I work in IT. I didn't know anything about 2 cycle engines before I started,but now I think I'm addicted. I just cant learn enough. If anyone knows any good reference material on two cycle engines especially chainsaw engines please share.
 
Thanks all for sharing. You guys gave me a lot off good info. Stumpy is definitely an artist with chainsaws. I have a husky 350 2003 EPA saw that needs refreshing. I though I my pep it up a bit while I'm at it. I want to learn all I can. I 'm not a professional,I work in IT. I didn't know anything about 2 cycle engines before I started,but now I think I'm addicted. I just cant learn enough. If anyone knows any good reference material on two cycle engines especially chainsaw engines please share.

this thread is an interesting topic for someone who is new here with a handful of posts.

anyways... if you like that kind of thing, maybe do some some research on o'hara, gorr, blair, muzzy, and some Japanese guys, who's names i can't remember from yamaha, to name a few. there are some things that were expanded upon after jennings made a chart (from calculations discovered by others) for dirtbikes and GP bikes for the common man.

if ten pennies were dropped on the floor...... and you were trying to find a penny 1985 date on it, it would make sense to do some research to see if 1985 is the right date you were looking for in the first place. ........or take all ten chances to find out that you were wrong, because 1985 is interdependent on many things like port time/areas of exhaust, blow-down, transfer, and inlet.

hope it helps.
-OMB
 
Thanks guys. I did a little more research. Chainsaws aren't motor cycles,modifications on racing two strokes doesn't necessarily apply to saws. Correct me if I'm wrong,the best way for an amateur like me to increase horse power and torque on my saw is with a muffler mod,and maybe widen the exhaust port a little. If clearance permits I can increase compression by leaving the head gasket off. I should leave the transfer ports to the experts until I gain more experience.
 
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IMO chainsaw transfers should in general be kept low, and widened as much as possible to gain time/area without opening before cylinder pressure has blown down to below crankcase pressure as box type exhaust offer no suction help for the transfers.

I added a boatload of intake timing and some exhaust to a 365 special, but did not touch the transfers at all, and went somewhat conservative with the port widths. I removed the gasket(thereby lowering the transfers .016 or so), and opened up the exhaust a lot at the flange and muffler. For a 365special, it had a ton of torque. It easily stumped hardwood trees with a 24" bar.
 
Ive ported many saws, but im no expert by any means. If you plan on running what we call "gasket less" you really need to check your stock squish first. Once you establish that, you need to measure your base gasket and subtract that number from your stock squish number and see where it roughly puts you. It could be to tight and ruin your engine.
 
Your on the right road now. Be conservative if you don't know what's up. The quench or squish is very important because you will need the fuel octane if you get up around 200psi. The damage one never sees and rarely hears is the bearings being beaten out of the lower rod and the mains on the crank.
 

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