Anyone doing a little porting really should have knowledge of the port timing of thier perticular engine. As in what degree s do the ports open , close.
Whats the blowdown time, etc.
If you just start expanding the ports with no prior knowlege ,You've a very good chance od messing up the port timing
ANYONE CONSIDERING DOING A LITTLE PORTING SHOULD TAKE HEED
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 2stroke 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 ,it was 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.
Compression
Controlling Factors: changes in bore and/or stroke, compression ratio, changes in squish clearance, changes in exhaust port timing, air density changes
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.
Crankcase (primary) Compression Ratio
Controlling Factors: bore, stroke, type of crank (pork chop, full circle, etc.), crankcase volume.
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.
Crankcase Pressure Time (CPT)
Controlling Factors: intake port duration, transfer port duration, crankcase compression
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.
Compression Ratio (Geometric) and (Trapped)
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),
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.
What this all means is that a bigger exhaust port is not neccesserily going to do you any favors.
Unless you have some spare engines kicking around to mess with, be sure to do your homework first.
Good luck all
Haggard
