Modding and Porting Fundamentals

[Philbert]

I never heard of muffler modifications or cylinder porting before joining this site. I have learned a lot by following several of those threads, and appreciate the detailed descriptions and photos. Still many things I don't understand. Maybe some of this can be explained in this thread to observers/followers like me.

Muffler modifications;
Exhaust port enlarging;
Transfer port modifications;
Timing modification;
Removing base gaskets;
Cutting 'squish' bands;
Machining pop-up pistons;
Others?.

Are some of these modifications specifically to increase crank speed (RPM), or power (HP), or both (or other)? In other words, can each mod be linked to a specific attribute?

Thanks.

Philbert

 

[diggers_dad]

Excellent questions. Speaking ONLY for myself, and including the fact that my experience base is limited, I can tell you this from personal experience. On many newer saws a simple muffler mod done correctly will increase fuel consumption, be louder and allow you to cut faster. I've seen a muffler mod increase speed roughly 10 to 15% on saws with a choked up, restricted muffler.

Making the exhaust port wider in and of itself does not provide any real gains. It has to be combined with the muff mod and re-tuning the carburetor to allow more fuel in. If the carburetor is at its upper limit to begin with, a larger carburetor will be needed to see gains. Again, from personal experience, a larger exhaust opening combined with an enlarged intake and increased fuel flow will again create a faster cutting saw.

Most of the performance gains I've seen come with a price. Usually it's increased fuel consumption. Bumping the timing up a few degrees can increase the RPMs of the saw. However, this increase may or may not equate to an increase in power. You have to include bottom end torque into the equation. Do you want more torque or more speed? What kind of wood are you cutting? Is the price of modification worth the expected gains?

For me, I like to experiment to see what gains I can get out of a saw. It's fun. Removing the base gasket will lead to an increase in cylinder compression. More compression equals more power -> always a good thing.

Squish bands and pop ups I can't help you with. I've not made it that far.

dd

 

[Philbert]

Good point about limits of the carb.

Just to keep the factors clear, faster cutting could be from a more powerful saw. Did you measure speed of the cut (timed from videos) or speed of the chain (tachometer)?

Philbert

 

[Terry Syd]

Muffler mod: drops backpressure in the cylinder from previous exhaust gases trying to exit the exhaust port. Primarily used to increase the power higher in the powerband where there is less time for the exhaust gases to exit. Excessive backpressure will impede the flow of the intake charge from the transfer ports. - Primarily done for more rpm.

Exhaust port enlarging: another way to decrease backpressure. There are two aspects to enlarging the exhaust port, widening and raising. Widening allows a greater area for the exhaust to exit, raising allows more time for the exhaust to exit. This is the concept of time/area that is referred to in modding ports. It is all ways best to try and widen the port before raising the port. Raising the port decreases the trapping efficiency of the cylinder, decreases the length of the power stroke and reduces the compression ratio. - Primarily done for more rpm.

Transfer port modifications: The transfer ports set the 'peak delivery ratio' (max torque) of the engine. Above max torque there is not enough time to complete the flow of intake charge into the cylinder. Below peak torque the transfer ports finish too early and the rising piston creates a low pressure in the crankcase that starts sucking the intake mixture back down into the crankcase. The time/area of the transfers is set for when you want max torque. There are other issues about transfers, such as crankcase compression (crank stuffers) that affect when the peak delivery ratio occurs. The port configurations and angles affect the flow in the cylinder. There is a detailed report on a series of Yamaha DT250 cylinders that all had the same time/area for the transfers, the difference was how the transfer ports were configured. The difference in power between the best and worst ported cylinders was 15%, not just at the top end, but through out the range. - Primarily done on chainsaws to increase the time/area and raise the point of max torque and horsepower. It can also be used to drop the point of peak torque where the engine doesn't have enough 'grunt' down low.

Timing modification: I assume you mean ignition timing. As Blair indicated, the best power occurs when 50% of the mixture has burned by about 10 degrees ATDC. There is a considerable lag for when the ignition bubble starts to propagate, about 10 degrees before anything really begins to happen. If the ignition is not advanced enough, then the burning mixture won't contribute as much as it could to the power stroke. If the ignition is too far advanced, then the engine will be trying to compress a rapidly expanding mixture and the power will drop as the internal resistance increases. - Primarily done to get the most of what you got.

Removing base gaskets: A way to increase the compression ratio. It also slightly decrease the time/area of the exhaust and transfers, while slightly increasing the intake time/area. A higher compression ratio creates a higher pressure in the combustion chamber to push the piston down. - Done to increase torque throughout the powerband.

Cutting squish bands: When a cylinder is dropped down lower by milling the base, the piston will hit the inside of the head. By cutting the squish band the piston can then travel higher without hitting the head. This is done to increase the compression ratio. - Done to increase torque thoughout the power band.

Machining pop-up pistons: Another way to increase compression ratio. The base of the cylinder may not have to be milled if taller piston is used from another model of saw. The top of the piston is turned down so it doesn't hit the head. The pop-up portion of the piston then fits into the combustion chamber and increases the compression ratio. Sometimes a cylinder will have the base milled and then the stock piston can then be made into a pop-up. - Done to increase torque throughout the powerband.

 

[Philbert]

Thanks Terry - that was great, and understandable.

Philbert

 

[firefighterwolf]

I never heard of muffler modifications or cylinder porting before joining this site. I have learned a lot by following several of those threads, and appreciate the detailed descriptions and photos. Still many things I don't understand. Maybe some of this can be explained in this thread to observers/followers like me.

Muffler modifications;
Exhaust port enlarging;
Transfer port modifications;
Timing modification;
Removing base gaskets;
Cutting 'squish' bands;
Machining pop-up pistons;
Others?.

Are some of these modifications specifically to increase crank speed (RPM), or power (HP), or both (or other)? In other words, can each mod be linked to a specific attribute?

Thanks.

Philbert

Finger Ports
Tunded Pipes

More advanced modifcations for racing saws. Not necessarily for work saws.

 

[Chris-PA]

Excellent questions. Speaking ONLY for myself, and including the fact that my experience base is limited, I can tell you this from personal experience. On many newer saws a simple muffler mod done correctly will increase fuel consumption, be louder and allow you to cut faster. I've seen a muffler mod increase speed roughly 10 to 15% on saws with a choked up, restricted muffler.

Making the exhaust port wider in and of itself does not provide any real gains. It has to be combined with the muff mod and re-tuning the carburetor to allow more fuel in. If the carburetor is at its upper limit to begin with, a larger carburetor will be needed to see gains. Again, from personal experience, a larger exhaust opening combined with an enlarged intake and increased fuel flow will again create a faster cutting saw.

Most of the performance gains I've seen come with a price. Usually it's increased fuel consumption. Bumping the timing up a few degrees can increase the RPMs of the saw. However, this increase may or may not equate to an increase in power. You have to include bottom end torque into the equation. Do you want more torque or more speed? What kind of wood are you cutting? Is the price of modification worth the expected gains?

For me, I like to experiment to see what gains I can get out of a saw. It's fun. Removing the base gasket will lead to an increase in cylinder compression. More compression equals more power -> always a good thing.

Squish bands and pop ups I can't help you with. I've not made it that far.

dd

I'm at about the same level. The purpose of the mods is in general to increase the volume of air moving through the pump (engine), and of course add more fuel as well. More fuel that burns is more power. But a 2 stroke is massive compromise, especially when it comes to port timing, and it is difficult to change any one thing with out altering several others as well. A muffler mod may be as close as it gets in that department, depending on how restrictive it was to begin with.

The 2-stroke's biggest compromise is that the exhaust port is open while the incoming charge is entering the cylinder, leading to unburned fuel going out (scavenging losses). Fuel that doesn't burn produces no power. Most of the issues with 2-strokes involve trying to deal with this problem. A lot of the literature of tuning and modifying 2-strokes has to do with resonance tuned exhaust pipes, which have a profound effect because of their interaction with the scavenging losses, but this is irrelevant to work saws.

Attempts are made to direct the fresh charge that moves up the transfers toward the intake side of the cylinder and away from the open exhaust port. Apparently the flow pattern from quad transfers is an improvement, and so quad transfers tend to lose less of the charge.

Strato engines add another path for pure air to enter the engine, and try to delay introduction of the fuel into the cylinder until very late in the cycle, thus reducing the opportunity for it to be lost out the open exhaust port.

All port timing on a 2 stroke is symmetrical about TDC, so if it opens earlier it closes later. Cylinder pressure cannot build until the ports are closed, so if you raise the exhaust port for longer duration by definition you decrease the cylinder pressure.

It is tempting to think of what is happening in the engine in terms of angles relative to TDC and BDC, and whether the piston is moving up or down based on static/low speed position of the piston. But an engine at speed is a dynamic system, and there are delays, inertia and momentum to deal with, so things don't really work that way. For example, fuel/air charge is still moving up the transfers well after BDC while the piston has already started moving back up. So in fact raising the exhaust port may not always reduce cylinder pressure - it may actually raise it at a higher rpm.

 

[Philbert]

Thanks Chris. That was also very clear.

This is a great site when guys share stuff like this.

Philbert

 

[CR888]

+1 on what philbert says. lts quality when guys put effort and thought into their posting.opcorn:opcorn:opcorn:Terry and Chris are alway putting usefull stuff on the table thats helpful and informative.

 

[Moparmyway]

Cutting squish bands: When a cylinder is dropped down lower by milling the base, the piston will hit the inside of the head. By cutting the squish band the piston can then travel higher without hitting the head. This is done to increase the compression ratio. - Done to increase torque thoughout the power band.

GREAT writeup !!
The only thing I would add is that the squish band is also cut to change the shape of the combustion chamber, providing a more efficient flame front to promote better flame propogation. Not necessarily cutting the squish to avoid piston damage from a scrap fight on some models, but a more efficient burn as well - from all models.

 

[WHSH]

Hmmmm, So, what is the ideal, squish band? Hmmmm? Most saws have flat/non angled bands. Some have asymetrical combustom chambers, thus asymetrical squish bands. This propagates the flame front starting at the intake side. Some have a centered dome with the plug on the intake side. Some like the older stihls have domed pistons and bands. This would seem to make the volume of the squish band larger than a flat piston? Just regarding the flame front and skipping the piston smashing part. To small you don't get cooling of the piston crown, two large you get diminished turbulance. Then there the two flame front detonation concern. So given the saws combustion chamber design has any one worked out saw specific squish band mods? Does any one polish the combustion chamber or is this still concidered intake tract, if so were does intake track start and end? Do you polish the piston crown but leave the combustion chamber rough? I would think to knock down the real rough points that concentrate heat and thus act to propagate detonation.

I found this and thought it was good reading.

Source - http://www.ktm950.info/how/Orange Garage/detonation/Detonation_101.html

"It is important to realize the two important functions of reducing the squish band clearance: (a) to enhance turbulence due to rapid ingestion of gas into the combustion chamber, hence increasing the burning rate of the mixture and (b) to reduce the volume of the unburned gas in the boundary layer of cool gas near the piston top and cylinder head surfaces. Typically, gas trapped in the squish area doesn't burn, even if the squish band clearance is relatively large. The cooling effects of the large surface-area-to-volume ratio of this region will prevent any ignition of the fuel-air mix therein, even if the squish band clearance is rather large. Hence any gas caught in the squish band will not be burned near TDC when it does the most good, but later during the combustion process when one cannot extract as much work from the late-burning gases. The amount of gas trapped in the squish band can actually be a substantially greater amount than just the relative volume of the squish band because the pressure wave from the ignition process literally crams a lot of the unburned gas into crevice areas like the squish band. Reducing the squish band clearance will decrease the amount of unburned gas substantially, leading to more complete and faster combustion, lower emissions and improved power. It is one of the few "all gain with no pain" modifications one can carry out on racing or even street motorcycles.
Someone wondered: Is the extra cooling of the squish band less than the added heat?
Basically the mixture in the squish region is in thermal contact with the cylinder wall and piston top and at roughly the same temperature, which is quite lower than the burn temperature. Reducing squish will decrease the amount of the cool gas in the squish region and increase the amount of hot gas in the burn region. A reduced squish clearance will increase temperatures a little even if the compression ratio is held constant. There is no "extra cooling" mechanism if you reduce the squish band clearance. The cooling rate of the gas in the squish zone depends on the thermal conductivity of the gas-metal interface, on the total surface area of this interface and the temperature difference between gas and metal. Note that these factors are all essentially constant at TDC and don't depend on the squish clearance. Hence the cooling rate is the same for large squish clearances and for small squish clearances. Thus there is no "extra cooling" mechanism if you reduce squish band clearance"

 

[mdavlee]

Most people that build 2 piece heads use a 2-3° taper on the squish band. Tighter the squish the better the saw will run. It just won't dissipate heat as good.

Boost ports straight across from the exhaust gets the burnt gases out faster. It's hard to get them to fit on a work saw without external transfers.

 

[blsnelling]

There are some good reads in here. http://www.arboristsite.com/community/threads/chainsaw-faq.148248/

 

[WHSH]

Two piece cylinder and head, and external boost ports are beyond my paygrade and my cutomers willingness to pay. Now, back down to earth, the 2-3 deg taper, I have questions? Is this from edge of bore to end of interior of the squish band? If so, do you start, lets say at .020 squish and end at .024? This would depend on the depth of your band, but for conversation lets say its true. This would be easy to do with the lathe cross slide with set wat 2-3. Have you done this and whats your thoughts, engine heat, hp etc. Very interesting.

 

[Philbert]

There are some good reads in here. http://www.arboristsite.com/community/threads/chainsaw-faq.148248/

Another link (to the same threads?):

http://www.arboristsite.com/community/threads/porting-thread.178713/

Philbert

 

[WHSH]

I am begining to sense some sarcasm, oh great and all knowing ones.

 

[Philbert]

I am begining to sense some sarcasm, oh great and all knowing ones.

???

I think that this thread has stayed on the rails remarkable well. I was following Brad's links and had to dig through one a bit, so I posted another, which may be a quicker route. No sarcasm intended on my part.

Philbert

 

[Chris-PA]

???

I think that this thread has stayed on the rails remarkable well. I was following Brad's links and had to dig through one a bit, so I posted another, which may be a quicker route. No sarcasm intended on my part.

Philbert

Yes, I did not see any sarcasm either.

 

[diggers_dad]

Thanks Terry - that was great, and understandable.

Philbert

+1.

 

[diggers_dad]

Good point about limits of the carb.

Just to keep the factors clear, faster cutting could be from a more powerful saw. Did you measure speed of the cut (timed from videos) or speed of the chain (tachometer)?

Philbert

Yes, sort of, and mostly.

My assumption is a saw that cuts faster has more power. I measured the speed of the cut by making three cuts (down, up, down) and having my boy time me. After whatever mods were completed, I used the same saw, same bar and same chain (freshly sharpened with one or two strokes of the file each time) on the same piece of wood. The only variations that might happen would be from any imperfections in my sharpening technique or different temperatures / weather conditions on the days tested.

I don't always check them with a tach before and after. The last one I did that on was a 41cc Earthquake. I made some extensive (for me) mods including mild port work, muffler mod (desperately needed) and timing advance. The RPMs at full load in the cut went from just under 9,000 to a little over 11,500. Sounds great right?

NOT! The saw actually cut slower. In order to keep the saw from bogging down at "full load" I had to exert less pressure in the cut. The saw sounded wicked but it had no real torque. I had advanced the timing too far. Once I backed it down a couple of degrees it ran much better - about 10,500 - with more torque and a faster cut.

DD