porting and compression basics

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[SIZE=+2]Blow Down Time(BDT)[/SIZE]

[SIZE=+1]Controlling Factors: exhaust port opening, transfer port opening, power stroke, crankcase compression and type of exhaust system.[/SIZE] [SIZE=+1]
[/SIZE]Blow Down Time has some control over what kind of power the cylinder will produce. Lower BDT's will make more torque and have a lower RPM potential. Greater BDT's will produce more peak horsepower and have a greater RPM potential.

Blow Down Time is the distance in degrees between the opening of the exhaust port to the opening of the transfer ports. BDT (in degrees) controls the time that a cylinder has to empty itself of exhaust before the transfer ports open and allow the fresh fuel-air mixture into the cylinder. In order for the fuel air mixture to move into the cylinder, the pressure inside the cylinder must drop below that of the crankcase. For example, a RC engine may have peak cylinder pressure of approximately 750 psi and that pressure reduces as the piston travels downward. At 15,000 RPM's the engines BDTiming has only .000244 seconds to empty the remaining cylinder pressure to less then approx. 20psi .in order for the transfers to start flowing fresh air mixture into the cylinder when they ports open. As RPM's increase, shorter BDT's reduce the possibility that the exhaust will have enough time to leave the cylinder before the transfer ports open and increase the possibility of the exhaust and fresh charge mixing in the cylinder and decreasing the engines power potential. Increases in BDT extends an engines RPM potential by allowing more time for the exhaust and pressure to leave the cylinder before the incoming fresh charge enters the cylinder.

Stock Blow Down Times on a GoPed vary from 12' on the GSR 40 to 22' on the G23RC engine. BDT greatly influences the performance of tuned pipes. Engines with low BDT's, like the GEO, GSR and LH all have their transfer ports open when the return wave from the tuned pipe arrives at the exhaust port. When a return wave comes back too soon it may cause a back-flow or delay through the transfer port or increase the mixing of the fresh fuel air mixture with exhaust. Engines with more blow down time experience the same problem, usually at 1/2 the engines peak horsepower RPM. The return wave comes back early at BDC and causes the a dip in the power at that time.

Knowing this you would think that increasing the BDT would be the easiest way to increase your power. The new power comes at a cost though. Increases in BDT by raising the exhaust port, decrease the power stroke. During dyno testing of the GEO engine I found that raising the BDT from 18' to 22' (a mere .030") was the difference from decent power to just mediocre. That particular engine needed that .030" of power stroke for decent power. The power stroke is the distance from (peak pressure) TDC to EO (exhaust port open). The power stroke is the pressure from expanding combustion, pushing down on the piston, creating the torque transmitted by the crankshaft to a GoPed's spindle.

Reducing the power stroke by small amounts at first may help the power. On engines that produce very little power to begin with small changes to BDT can have big repercussions. Reducing the BDT a little more and you may loose bottom end power. Raising the exhaust port even more may decrease the power through out the RPM range.

Our small engines have their own range of BDT & power stroke #'s that work, BDT & PS #'s for larger 2-stroke engines generally do not. The secret is to find that perfect balance between BDT and PS and many other factors to produce the best power.

[SIZE=+2]Compression[/SIZE]

[SIZE=+1]Controlling Factors: changes in bore and/or stroke, compression ratio, changes in squish clearance, changes in exhaust port timing, air density changes[/SIZE]

Compression is the product of the compression ratio and the current atmospheric pressure. Standard air pressure at sea level is 14.7 psi @ 65 degrees Fahrenheit. Even though that is the standard, the actual pressure may be a little higher or lower then that at any given day. Increases in altitude, temperature and humidity change the atmospheric pressure and is calculated to give a corrected altitude. The point of all this is to show that the compression measured along the coastline at sea level will be always be more than it is in the mountains. The deserts on a hot day will be the same as being in the mountains, even if it is at sea level or below.

Compression is a good tool for gauging engine wear and octane requirements and should be checked with a high quality "Snap-On" compression gauge. Many other popular brands will read low, often by 20 to 30 psi. If you don't know what your compression should be, you can get a rough idea by multiplying the engines geometric compression ratio x 15. Compression is checked with the throttle held wide open and strong pulls on the starter rope.

[SIZE=+2]Crankcase (primary) Compression Ratio[/SIZE][SIZE=+2]
[/SIZE][SIZE=+1]Controlling Factors: bore, stroke, type of crank (pork chop, full circle, etc.), crankcase volume.[/SIZE]

Crank case compression ratio is measured in a similar fashion to the secondary CR. All the area under the piston crown at TDC / the area under the piston crown at BDC. The primary compression is responsible for pushing the fresh fuel air mixture up through the transfer ports when they open to approximately BDC. Depending on the compression ratio, blow down time and exhaust port area, the cylinder pressure may be greater than the crankcase pressure. In these circumstances, it will cause a delay in the scavenging charge entering the cylinder. An increase in crankcase compression ratio, exhaust port area and /or blow down timing would help this. Too much crankcase compression can hurt the intake ports flow of fresh fuel - air mixture from the carb, when there is too much intake port timing and /or insufficient port velocity to overcome the back flow from the crankcase.


[SIZE=+2]Crankcase Pressure Time (CPT)[/SIZE]

[SIZE=+1]Controlling Factors: intake port duration, transfer port duration, crankcase compression[/SIZE]

CPT is the amount of time in degrees, that the engine has to build up enough pressure to send the fresh fuel-air mixture through the transfer ports and into the cylinder when the ports open. CPT is measured from the "closing of the intake port" (IC), to the "opening of the transfer ports"(TO). Depending on how the engine is ported and what type of stroke is being used, an RC engine may have as little as 30 degrees and a stock GEO can have up to 65 degrees to build up the pressure to force the scavenging flow out the transfer ports when they open. Since our piston port engines have symmetrical timing on the transfer port and intake port. Any increases of either intake or transfer port timing, will decrease CPT. Examples of this would be, cutting the intake skirt or installing a stroker crank will decrease CPT. Normal ranges for CPT is 38' to 48', lower degrees can give more peak power and the opposite will produce more torque.


[SIZE=+2]Compression Ratio (Geometric) and (Trapped)[/SIZE]

[SIZE=+1]Controlling Factors: bore, stroke, combustion chamber volume, squish clearance or deck height, exhaust port height, type or # of piston ring(s), type of piston crown (radius or flat),[/SIZE]

There are 2 types of compression ratios: Geometric and Trapped. Geometric compression is measured from bottom dead center to top dead center. Trapped compression starts when the exhaust port closes to top dead center. The geometric compression ratio can be measured calculating the volume of the cylinder + the volume of the combustion chamber at TDC / combustion chamber at TDC. The trapped compression ratio can be measured in the same way, except you need to calculate from where the exhaust port closes to TDC + combustion chamber \ combustion chamber.

As listed in C.F.'s increases in bore size or stroke will increase the compression ratio. Decreases in the engines exiting squish clearance or deck height will increase the compression ratio and increase the compression @ approximate. 1 psi for every .001" removed. Combustion chamber volume is normally measured in cc's, but on GoPed's they have been simplified to differences in geometric compression ratios (12:1, 14:1, etc.) Decreases in cc volume will increase compression and increases in cc's will lower compression usually at approximately 7psi / cc. When machining a combustion chamber to increase compression, you can use the approximation of 1 psi for every .002" removed.

Raising the exhaust port will also lower the compression ratio and psi. Because the pressure inside the cylinder rises exponentially small changes in exhaust port height may only change the compression slightly. Piston rings don't actually change the compression ratio, but there can be up to a 30 psi difference in compression between thick dual rings and a thin single ring.

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Degrees after top dead center and Degrees before bottom dead center

Both methods are used to relate port openings and durations in degrees of crankshaft rotation. The latter is the simpler version. Ports that are measured before BDC convert easily to durations by multiplying them by 2. Degrees ATDC though takes in to account that many of the big events (openings) take place "after top dead center". It isn't as easy to convert to durations, but it is easy to relate combinations as openings instead. The method of tracking "degrees after top dead center" seems to be preferred by engineers and in SAE papers and books.



Exhaust Port Duration

Controlling Factors: port opening, blow down time.

The exhaust port duration is another major factor in the power potential of the engine. Like the transfer ports, the exhaust port needs enough area to allow pressure from the power stroke to escape before the transfer ports open. For example an average stock RC exhaust port @ 15,000 rpms will open and close in a mere .0017 seconds.

Exhaust Port Shape and Short Circuiting



Port shape will control the noise level output of the engine. Increase or reduce short circuiting from the transfer ports and have some control on the power delivery of the engine. When the port opens quickly it will produce higher noise levels and a power band that hits a little harder. The opposite is true when the port opens gradually. Exhaust ports that are very wide or have been lowered to the piston crown @ BDC will have a greater tendency to short circuit the fresh fuel-air flows from the transfer ports, out the exhaust port.



Friction

Controlling factors: All moving parts



Friction robes your engine of horsepower and the amount it takes away increases exponentially with RPM. Main sources of friction come from your rings (most of it near the ring end gap) cylinder wall (its not always straight), main bearings and seals. Many times, I have found engines that produced less than normal power and found main bearings in poor condition as the cause. Other sources of friction can come from sealed bearings, leaking main seals(ingested dirt), third bearings, axle bearings, low tire pressure and too much spindle pressure against the tire.






Intake port duration

Controlling factors: crankcase compression, carb size and type, intake manifold, air cleaner



The intake port consumes the fresh fuel air mixture the engine uses to produce its power. The port duration determines the power and RPM potential of the engine. The problem with piston port intake systems is the timing is symmetrical. The sooner you open the port, the later it closes.

An example of this is an old piston port 125cc road racer that may have a port opening around 90' BTDC (the piston is on the up stroke) and has had plenty of time to build up vacuum to suck in the fuel air mixture. The problem is on the down stroke. The engine has had 90' of time to build up pressure inside the crankcase and it wants out. With the intake port still open, a back flow through the intake port at low and possibly the mid RPM range. The back flow will stop when the incoming fuel air mixture velocity is greater than the back flow. Any engine with that much intake timing would have terrible bottom end power, some engines may not even be able to start running.



The later the engine closes the intake port, the shorter the time (in degrees) the engine has to build up pressure to send the fuel through the transfer ports. This is "Crankcase Pressure Time". Increases in intake port timing decreases the CPT. Some engines have such low intake port timing and high CPT that small changes with intake port timing can make a significant difference. The intake port must be a balance of the need for power @ a given RPM vs. torque required for a satisfactory low & mid range power.


As a general rule: timings with openings less than 68' BTDC make good torque, Openings from 68' to 75' BTDC have good overall power and openings from 76' to 82' BTDC can have substantially better top end with a corresponding loss of bottom end.



Port chamfer



A port chamfer is the rounded edge all ports must have to prevent the ring or piston from catching the port. Chamfering the ports is necessary when any of the ports have been cut to change the shape or timing. There are many techniques used by porting technicians to chamfer a cylinder, many depend on how mild or extreme a cylinder has been ported. Many race organizations have written limits to chamfering in to their rule books for classes that run stock cylinders, because exaggerated port chamfers can change a ports opening and duration.



Piston overlap



Piston overlap isn't a common term, but it is a measurement that is built into every engine. It refers to the amount of overlap the pistons bottom edge has over the exhaust port @ TDC. Pistons skirts are designed to be long enough to prevent the exhaust port from opening at TDC. Typical overlaps are in the 1mm or .040" range. Normally piston overlap isn't an issue, until you install a 2 mm stroker crank. Small exhaust port openings @ TDS are acceptable in the .005" to .010" range, but excessive openings may lead to detonation.



Piston pin offset



Piston pin offset is designed into a piston to prevent excessive wear to the exhaust port side of the cylinder and piston of high output engines. Offset normally falls within the range of .5 to 2mm, towards the intake side of the piston. Engines that are designed to have reversible cylinders like the GSR 40 and GEO engines have no piston pin offset and are typically low output style engines.






Power Stroke

Controlling factors: exhaust port opening, blow down time, deck height, squish clearance, stroke and ignition timing.



Refers to the duration of the pressure applied to the piston by combustion, in degrees from peak pressure to the exhaust port opening. With our small engines power stroke is an important factor to consider. Increases in exhaust port duration and blow down time may significantly reduce the power stroke, while stroker cranks and decreases in deck clearances can increase the power stroke. The point here is that port combinations that may work on a larger engine can have devastating effects on the power output capabilities of our small engines.



For example: during a dyno test on the GEO engine I started with the same BDTas a G2D, thenraised the exhaust port (reduced the power stroke) .030" to have the same BDTas an RC cylinder. That .030" (1/32 of an inch) killed the power.



Rod length



Rod length is used as a variable in a trigonometric equation to determine a ports opening in degrees from TDS. It is also used to change or compliment how an engine delivers its power. The shorter the connecting rod is, the less time the piston spends at TDC. It tends to build primary and secondary compression faster, will effect the port timing and will accelerate faster off a corner. A long rod will produce a more "tractable" power band, the piston will spend more time at TDC and BDC and will affect the port timing






Schnurle porting (loop porting)



Schnuele porting refers to a type of cylinder port arrangement that aims the scavenging flows to "loop" through the cylinder and combustion chamber to help push the spent gasses (exhaust) out the exhaust port. There are 2 major types of 2 stroke engines, schnurle and cross flow. Cross flow has been used by older outboard style engines, chain saws and model airplane engines. Many cross flow engine have approximately 4 to 5 small transfer ports on the intake side of the cylinder and 4 to 5 small exhaust ports on the opposite side. The cross flow engine uses a piston with a deflector that aims the scavenging flows upward to help chase the spent gases out the many exhaust ports. But the pistons were heavy and the engines were very inefficient.



Short circuiting

Controlling factors: exhaust port shape and width, transfer port angles



Refers to the fresh scavenging flow that enters the cylinder and the possibility that a portion of that fresh fuel air mixture may by-pass its normal looping flow and exit out the exhaust port. Fresh fuel air mixture that has short circuited and remains unburned is power lost. Many engines like the LH, GEO and GSR have exhaust ports that make an effort to prevent short circuiting. High output engines like the RC uses transfer port angles to prevent short circuiting. When modifying an engine, it is wise to be aware of the possibility.



Squish Clearances



Decreasing squish clearances is normally a quick and easy way to pick up more power. By decreasing the clearance, it promotes better combustion and reduces the possibility of detonation. On Goped engines .020" clearance works well (on larger engines .030" to .040" is better). Clearance of some kind is required because as engine components heat up, they grow and the connecting rod stretches with RPM. Without the clearance, at high RPM the piston will hit the head.



One way to measure the clearance is with a piece of rosin core solder (try a small diameter piece first, like 1/32" , then 1/16" if it doesn't touch) bent to an "L" shape and inserted through the spark plug hole, directly above the piston pin and against the cylinder wall. Then turn the engine over, pull out the solder and measure the thinnest part and that will be your clearance.

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Transfer Port Duration (TPD)





Transfer port duration is another of the main controlling factors for type of power and RPM potential of an engine. As engine RPM and power increases, so does the need for fuel to supply that need.The time that the transfer ports can deliver the fuel decreases with RPM.



As an example, an engine with 120' of transfer port duration has only .004 seconds to deliver it's fresh fuel air charge @ 5,000 RPM. At 10,000 RPM, naturally it's 1/2 that time @ .002 seconds and @ 15,000 RPM's it's only .00133 seconds. In that time the port is only wide open for approx.. .0008 sec @10,000RPM. It is hard to believe that anything can be effective at that speed. Since time is the problem, transfer ports a certain amount of "Effective Area" or "Time-Area" to obtain a given horsepower and RPM potential. Increases in area, # of ports and duration are part of the answer.



The G23RC has a reasonable amount of duration or "time area" and what you do with it will be determined by the kind of power that you are trying to achieve. If you want all out speed, you need to extend your torque and RPM range (you may need more time area) or if you want acceleration off the corners, you will need to build more torque sooner at a possible loss of peak RPM (you may need less time area).







Transfer Port Angles







The transfer port angles have a relationship with the kind of power an engine will produce. Transfer port angles that are close to right angles of the exhaust port, produce more mid to top end power. Ports that are angled back towards the intake side of the cylinder will produce a broader range of power.