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.
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
= mean piston speed
(feet per minute) and S
= piston stroke in millimeters.
700 Polaris example:
= 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
= Brake mean effective pressure in psi
= Piston stroke in feet
= Area of one piston in square inches
= 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
= 0.223 (68/25.4 = 2.677"/12"=0.223 feet)
= 7.988 square inches
= 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
, in your engine to determine how to get the HP you want! Are the numbers you used realistic??