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Porting 2-Stroke Engines
Crankcase and Cylinder
Crankcase and Cylinder porting is probably the most controversial subject among Performance 2-Stroke engine builders. Part of the problem is understanding the "pulse" or "charge" nature of the airflow in a 2-stroke engine. The other is how "Air Density" (barometric pressure, temp, humidity), and Elevation Change, affects horsepower. The laws of physics have not changed and the mathematical formulas are readily available. To obtain maximum torque and horsepower requires; applying the laws, and, the mathematical formulas, to the pulse/charge effect of the airflow. The computer is one of the tools we use to compare engines and determine the changes required to improve performance.
Port Work
The exhaust port designed by the computer is mapped out on a plastic template. Other cylinder mods are incorporated into the template. The template provides consistency between cylinders in one engine, and, other engines. The template insures a perfectly symmetrical exhaust port. The template also provides a reference if port work needs to be matched later. The cylinder is marked, and the cuts are made to the cylinder. The exhaust port is smoothed, sanded, then polished, and the exhaust flange matched. Port work involves matching all the mating surfaces and gaskets from the carb boot thru to the exhaust flange. Increasing the airflow thru the engine provides the most useable gains in Torque and HP. Port work is not Black Magic! Port work IS; tedious, precision work, to perfect air flow!! We will work with your engine to achieve your goals. Help you decide on your priorities: of reliability, RPM, & type of fuel. Then design specific engine mods for your riding, racing, and elevation.
Before You Start!
A squish test is required before the engine is disassembled. A compression test is recommended as a reference for comparison. With the engine apart, the parts can be measured, to determine the work required. Accurate and precise measurements of widths, heights, and corner radius, are required for the software inputs. Precise inputs in the computer give reliable outputs.
You Can Check Here
Crankcase Reed: Remove the reed cage and see if cylinder sleeve blocks airflow? Does the reed stopper cover the Boost Port? (Needs Reed Spacer) Is there a smooth transition from the reeds to the base of transfer ports? Cylinder Reed: Check for reed stopper covering Boost Port? Can the air move freely under the piston and around it to the sides? Piston Port: Is the intake tract open and smooth for maximum flow?
Choices to Consider
Consider piston speed before you decide what rpm the engine will run at. What RPM pipe(s) are available? Will your RPM choice require a custom built pipe (Expensive)? or modify an existing pipe(s)? Check the piston speed for the stroke in your engine. Some engines can be run much faster, others, are running near maximum piston speed stock. Suggested maximum piston speeds: Trail sleds / dirt bikes = 3700 feet per minute, Hill Climb/Race = 4000 feet per minute. You can run faster speeds, but you risk reliability and longevity. Note: Watercraft run lower piston speeds Due to extended WOT operation.
Piston speed in feet per minute = stroke in mm / 25.4 x 2 x rpm / 12
Example #1: 700 Polaris twin engine has a stroke of 68 mm and runs at 8300 rpm, what is the mean piston speed?
68/25.4 = 2.677" x 2 = 5.354" x 8300 = 44,440.944" / 12" = 3703 feet per minute
Find RPM @ 4000 fpm: 4000' x 12" = 48000" / 5.354" = 8965 rpm (700 Polaris)
Example #2: 700 SX Yamaha triple engine has a stroke of 59.6 mm and runs at 8500 rpm, what is the mean piston speed?
59.6/25.4 = 2.346" x 2 = 4.6929" x 8500 = 39,889.76" / 12" = 3,324 feet per minute
Find RPM @ 3700 fpm: 3700' x 12" = 44,400" / 4.6929 = 9,461 rpm
Another way to find mean (average) piston speed:
Use this formula: MPS = S x 0.1666 x rpm / 25.4
MPS = mean piston speed (feet per minute) and S = piston stroke in millimeters.
700 Polaris example:
MPS = 68.0 x 0.1666 x 8300 / 25.4 = 3702 feet per minute
Do you want horsepower or torque? Both, right!! Remember This: Speed ='s HP & Acceleration ='s Torque. High Horsepower numbers can be very misleading due to high RPM. The true test of a properly modified engine is Torque. The engine must produce high torque over a wide rpm range, or hillclimb and race track times will be slower due to poor acceleration. Off and on the throttle tests the engines ability to perform at all rpms.
Brake Mean(average) Effective Pressure is the test / comparison of the engines efficiency, regardless of its displacement, or, its operating rpm. To achieve high BMEP numbers, all parts* of the 2-stroke cycles must be optimized.
* Parts: Intake / Reeds -- Head Design / Squish Clearance -- Transfer Port Timing & Aiming -- Design & RPM of Tuned Pipe.
BMEP = HP x 6500 / L x RPM (2 - Stroke)
L = engine size in liters.(700cc = .7 L) HP = horsepower
See BMEP page w/ examples & 4-stroke BMEP formulae
I use the TSR computer software to calculate/predict the horsepower.
You can work (change variables) with the BHP (brake horsepower) formulae:
BHP = PLAN / 33000
P = Brake mean effective pressure in psi
L = Piston stroke in feet
A = Area of one piston in square inches
N = Number of power stokes per minute
Example: Polaris 700 twin cylinder which will run at 8,000 rpm, deliver an average pressure of 135 psi, has a bore of 81 mm and a stroke of 68 mm. What is the predicted BHP?
P = 135
L = 0.223 (68/25.4 = 2.677"/12"=0.223 feet)
A = 7.988 square inches
N = 16,000 (8,000 rpm x 2 cylinders)
BHP = 135x0.223x7.988x16,000 / 33,000 = 116.59 BHP
You can play with the variables, P=psi & N=rpm, in your engine to determine how to get the HP you want! Are the numbers you used realistic??
Crankcase and Cylinder
Crankcase and Cylinder porting is probably the most controversial subject among Performance 2-Stroke engine builders. Part of the problem is understanding the "pulse" or "charge" nature of the airflow in a 2-stroke engine. The other is how "Air Density" (barometric pressure, temp, humidity), and Elevation Change, affects horsepower. The laws of physics have not changed and the mathematical formulas are readily available. To obtain maximum torque and horsepower requires; applying the laws, and, the mathematical formulas, to the pulse/charge effect of the airflow. The computer is one of the tools we use to compare engines and determine the changes required to improve performance.
Port Work
The exhaust port designed by the computer is mapped out on a plastic template. Other cylinder mods are incorporated into the template. The template provides consistency between cylinders in one engine, and, other engines. The template insures a perfectly symmetrical exhaust port. The template also provides a reference if port work needs to be matched later. The cylinder is marked, and the cuts are made to the cylinder. The exhaust port is smoothed, sanded, then polished, and the exhaust flange matched. Port work involves matching all the mating surfaces and gaskets from the carb boot thru to the exhaust flange. Increasing the airflow thru the engine provides the most useable gains in Torque and HP. Port work is not Black Magic! Port work IS; tedious, precision work, to perfect air flow!! We will work with your engine to achieve your goals. Help you decide on your priorities: of reliability, RPM, & type of fuel. Then design specific engine mods for your riding, racing, and elevation.
Before You Start!
A squish test is required before the engine is disassembled. A compression test is recommended as a reference for comparison. With the engine apart, the parts can be measured, to determine the work required. Accurate and precise measurements of widths, heights, and corner radius, are required for the software inputs. Precise inputs in the computer give reliable outputs.
You Can Check Here
Crankcase Reed: Remove the reed cage and see if cylinder sleeve blocks airflow? Does the reed stopper cover the Boost Port? (Needs Reed Spacer) Is there a smooth transition from the reeds to the base of transfer ports? Cylinder Reed: Check for reed stopper covering Boost Port? Can the air move freely under the piston and around it to the sides? Piston Port: Is the intake tract open and smooth for maximum flow?
Choices to Consider
Consider piston speed before you decide what rpm the engine will run at. What RPM pipe(s) are available? Will your RPM choice require a custom built pipe (Expensive)? or modify an existing pipe(s)? Check the piston speed for the stroke in your engine. Some engines can be run much faster, others, are running near maximum piston speed stock. Suggested maximum piston speeds: Trail sleds / dirt bikes = 3700 feet per minute, Hill Climb/Race = 4000 feet per minute. You can run faster speeds, but you risk reliability and longevity. Note: Watercraft run lower piston speeds Due to extended WOT operation.
Piston speed in feet per minute = stroke in mm / 25.4 x 2 x rpm / 12
Example #1: 700 Polaris twin engine has a stroke of 68 mm and runs at 8300 rpm, what is the mean piston speed?
68/25.4 = 2.677" x 2 = 5.354" x 8300 = 44,440.944" / 12" = 3703 feet per minute
Find RPM @ 4000 fpm: 4000' x 12" = 48000" / 5.354" = 8965 rpm (700 Polaris)
Example #2: 700 SX Yamaha triple engine has a stroke of 59.6 mm and runs at 8500 rpm, what is the mean piston speed?
59.6/25.4 = 2.346" x 2 = 4.6929" x 8500 = 39,889.76" / 12" = 3,324 feet per minute
Find RPM @ 3700 fpm: 3700' x 12" = 44,400" / 4.6929 = 9,461 rpm
Another way to find mean (average) piston speed:
Use this formula: MPS = S x 0.1666 x rpm / 25.4
MPS = mean piston speed (feet per minute) and S = piston stroke in millimeters.
700 Polaris example:
MPS = 68.0 x 0.1666 x 8300 / 25.4 = 3702 feet per minute
Do you want horsepower or torque? Both, right!! Remember This: Speed ='s HP & Acceleration ='s Torque. High Horsepower numbers can be very misleading due to high RPM. The true test of a properly modified engine is Torque. The engine must produce high torque over a wide rpm range, or hillclimb and race track times will be slower due to poor acceleration. Off and on the throttle tests the engines ability to perform at all rpms.
Brake Mean(average) Effective Pressure is the test / comparison of the engines efficiency, regardless of its displacement, or, its operating rpm. To achieve high BMEP numbers, all parts* of the 2-stroke cycles must be optimized.
* Parts: Intake / Reeds -- Head Design / Squish Clearance -- Transfer Port Timing & Aiming -- Design & RPM of Tuned Pipe.
BMEP = HP x 6500 / L x RPM (2 - Stroke)
L = engine size in liters.(700cc = .7 L) HP = horsepower
See BMEP page w/ examples & 4-stroke BMEP formulae
I use the TSR computer software to calculate/predict the horsepower.
You can work (change variables) with the BHP (brake horsepower) formulae:
BHP = PLAN / 33000
P = Brake mean effective pressure in psi
L = Piston stroke in feet
A = Area of one piston in square inches
N = Number of power stokes per minute
Example: Polaris 700 twin cylinder which will run at 8,000 rpm, deliver an average pressure of 135 psi, has a bore of 81 mm and a stroke of 68 mm. What is the predicted BHP?
P = 135
L = 0.223 (68/25.4 = 2.677"/12"=0.223 feet)
A = 7.988 square inches
N = 16,000 (8,000 rpm x 2 cylinders)
BHP = 135x0.223x7.988x16,000 / 33,000 = 116.59 BHP
You can play with the variables, P=psi & N=rpm, in your engine to determine how to get the HP you want! Are the numbers you used realistic??
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There appears to be a lot of confusion over quite simple matters to do with OKO/Keihin carburetion. Removal of the float bowl, and fuel leaks from the float vent tube, seemingly causing particular concern. This being the case we hope this page may be of help to anyone who has had any problems in these areas.
We were contacted by a customer recently who asked about the modification pictured above. This involves blanking off the float chamber vent on an OKO carb.......possibly to prevent excess fuel flowing from the vent tube? Altering any carb in this way, can in some circumstances mean that if the bike to which the carb is fitted is parked at an acute angle with the fuel switched on, and the carb needle valve is not sealing properly, that it is quite possible that the crankcases will fill with petrol.
However the Keihin/OKO do indeed sometimes suffer from excess fuel flowing from the float chamber overflow, so we can see the reasoning behind this type of modification, but would certainly not recommend carrying out alterations such as this, as engine damage can result in some circumstance! If there is a problem with lack of clearance for float vent outlet tube, then it is possible to carefully modify the outlet for more clearance, but it must not be blocked off entirely though!
The first thing to check in relation to any carb which has fuel flowing from float bowl vents when it shouldn't be, is the condition of the needle valve, and its seating. You will need a magnifying glass to properly examine these 2 parts, as very often damage will be so slight its going to be very difficult to see with the naked eye alone. If there is any sign of damage or wear, the affected parts will need to be replaced. Classictrial provides full spares and technical back up to any customers who have purchased carbs from ourselves.
Another not so immediately obvious cause of excess fuel running from float bowl vents, is if the float bowl has been removed forcefully by someone who is not familiar with the OKO/Keihin carbs. In most cases this results in the float tangs being distorted/damaged, and that's something that might not be noticed, without actually looking for this particular problem specifically.
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HOME LATEST >SERVICES LINKS CONTACTTECHNICAL >HONDA >FANTIC >PRODUCTS >
There appears to be a lot of confusion over quite simple matters to do with OKO/Keihin carburetion. Removal of the float bowl, and fuel leaks from the float vent tube, seemingly causing particular concern. This being the case we hope this page may be of help to anyone who has had any problems in these areas.
We were contacted by a customer recently who asked about the modification pictured above. This involves blanking off the float chamber vent on an OKO carb.......possibly to prevent excess fuel flowing from the vent tube? Altering any carb in this way, can in some circumstances mean that if the bike to which the carb is fitted is parked at an acute angle with the fuel switched on, and the carb needle valve is not sealing properly, that it is quite possible that the crankcases will fill with petrol.
However the Keihin/OKO do indeed sometimes suffer from excess fuel flowing from the float chamber overflow, so we can see the reasoning behind this type of modification, but would certainly not recommend carrying out alterations such as this, as engine damage can result in some circumstance! If there is a problem with lack of clearance for float vent outlet tube, then it is possible to carefully modify the outlet for more clearance, but it must not be blocked off entirely though!
The first thing to check in relation to any carb which has fuel flowing from float bowl vents when it shouldn't be, is the condition of the needle valve, and its seating. You will need a magnifying glass to properly examine these 2 parts, as very often damage will be so slight its going to be very difficult to see with the naked eye alone. If there is any sign of damage or wear, the affected parts will need to be replaced. Classictrial provides full spares and technical back up to any customers who have purchased carbs from ourselves.
Another not so immediately obvious cause of excess fuel running from float bowl vents, is if the float bowl has been removed forcefully by someone who is not familiar with the OKO/Keihin carbs. In most cases this results in the float tangs being distorted/damaged, and that's something that might not be noticed, without actually looking for this particular problem specifically.
The Yamaha Blaster is a 200cc air-cooled single cylinder two-stroke all-terrain vehicle produced as an entry-level machine manufactured in Japan and sold in the United States from 1988 to 2006. Because of the Blaster's initial low price tag, it sold in large numbers for many years[citation needed]. Its two stroke engine is easily modified by enthusiasts and a large aftermarket now exists for the quad. A wide range of add-ons are readily available from simple bolt on exhausts and suspension parts to complete aftermarket frames and larger displacement engines.
The heavily finned, air-cooled Blaster engine has roots from a water cooled machine, as evidenced by the plugged water pump casting on the right side of the engine. The history of the engine in its water cooled form can be traced back to the Canadian market Dt200 and the Australian market WR200. Theoretically, it would be possible to use parts from either of these bikes and build an all-Yamaha water cooled Blaster engine, or to simply swap the engines since the engine mounts are nearly identical.
For the 2003 model year the Blaster was updated with a re-styled front nose, and the headlight assembly was moved down from the handlebars to the nose,and a all around lighter machine for greater performance.. The problematic mechanical rear and front drum brakes were replaced by dual hydraulic disc brakes in both the front and rear.
Because of U.S. government emissions requirements, the Blaster was discontinued for 2007 and was replaced by the entry-level Yamaha Raptor 250, which uses a cleaner-burning, less powerful four-stroke engine.
The heavily finned, air-cooled Blaster engine has roots from a water cooled machine, as evidenced by the plugged water pump casting on the right side of the engine. The history of the engine in its water cooled form can be traced back to the Canadian market Dt200 and the Australian market WR200. Theoretically, it would be possible to use parts from either of these bikes and build an all-Yamaha water cooled Blaster engine, or to simply swap the engines since the engine mounts are nearly identical.
For the 2003 model year the Blaster was updated with a re-styled front nose, and the headlight assembly was moved down from the handlebars to the nose,and a all around lighter machine for greater performance.. The problematic mechanical rear and front drum brakes were replaced by dual hydraulic disc brakes in both the front and rear.
Because of U.S. government emissions requirements, the Blaster was discontinued for 2007 and was replaced by the entry-level Yamaha Raptor 250, which uses a cleaner-burning, less powerful four-stroke engine.
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i already have this stuff so im not concerned with winning/ breaking the 1hr rule but i dont agree there, no disrespect or anything but there are some members that cant become supporting members for whatever reasons but are still here everyday and contribute in thier own ways. i mean it would be kinda sh*tty for a non supporter with less than 100 posts win and then dissapear dont get me wrong.
