KIPS Valves Explained
The KIPS (Kawasaki
Integrated Power Valve System)
The following discussion assumes that the reader has a basic understanding of the two-stroke engine. To fully explain all the affects of the KIPS I would quite literally have to write a novel, so lets keep it simple. If you would like to brush up on your knowledge before reading on, check out: Eric Gorrs Basic Two-Stroke Tuning, Marshall Brains How Stuff Works,, and Eric Murrys How Two-Stroke Expansions Chambers Work. Pay particular attention to discussions on port timing, the importance of scavenging and the effect of ignition timing and compression on engine performance. A basic understanding of these topics will increase your understanding of how and why your KDX works immensely.
The KIPS power valve system and the CDI work together to broaden the range of useful power produced by the engine. They do so by manipulating the exhaust ports dimensions and timing as well as the ignition timing.
By manipulating the exhaust ports dimensions and timing engine designers/ tuners control effective stroke, exhaust-gas velocity and the pressure of the compression wave. To understand how they work together, we need to first understand how they work individually.
The effective stroke is a measure of the distance from Top Dead Center (the highest point in the pistons travel) to the moment the exhaust opening is exposed. Increasing the distance raises the cylinders compression because the piston and rings are able to trap more air. At lower rpm, a higher compression ratio allows the engine to produce the best possible low to mid-range power. At higher rpm, the engine needs a lower compression ratio and therefore a shorter effective stroke to allow the motor to rev higher and produce the best possible power. If the compression was not lowered, the faster rise in pressure at high rpm would cause combustion gases to auto-ignite / predetonate resulting in excessive heat and catastrophic engine damage.
To understand the importance of exhaust-gas pressure and velocity we first need to understand the role of the expansion chamber and its effect on power. After reading How Two Stroke Expansion Chambers Work, we can see how important it is to properly time positive and negative compression waves to maximize engine output. These waves are traveling at ultra-sonic speeds so timing is everything. Negative compression waves help to scavenge burned gasses out of the cylinder and positive compression waves force unburned air/fuel back into the cylinder. The expansion chamber is in fact a super charger that forces more unburned fuel/air mixture into the cylinder than its calculated volume. A four - stroke XR 250 engine for example can combust about 220cc of fuel/ air every 720° of crank-shaft rotation but a two- stroke 200cc KDX engine can combust about 288cc of fuel/ air mixture every 360° of crank-shaft rotation. The two-stroke engine is producing more power per cc of displacement and producing it twice as often. That is why two-strokes are theoretically able to produce twice the power of a four-stroke engine of equal displacement.
So how then does controlling the exhaust-gas pressure and velocity improve engine performance? The speed and pressure at which exhaust-gas exits the cylinder determines the time at which the returning compression wave reaches the open exhaust port. The goal is to time the returning compression wave so that it reaches the exhaust port while it is still fully open forcing the maximum amount of unburned fuel/air into the cylinder as possible. At low RPM the compression wave returns too early. It forces the unburned air/fuel mixture from the header back into the cylinder but escapes again through the exhaust port drawing it back out defeating the purpose. At high rpm the compression wave can return to find the exhaust port partially or fully closed again, defeating the purpose.
How does the KIPS valve improve low and high rpm power?
The KIPS valve system is comprised of a series of geared shafts driven by a centrifugal advancer that rotates two sub-exhaust ports and a main exhaust port slide valve into position. The centrifugal advancer is driven off the crankshaft. As the engines speed reaches about 6000rpm centrifugal force on the weights overcomes the pressure excerpted by the spring and the advancer snaps open. The advancer shaft transfers this motion to the advancer shaft lever, to the main shaft and on to the slide valve and sub-valves causing them to open.
Up to 6000 rpm, the Slide Valve is dropped into the main exhaust port, the sub ports are in the closed position and the Halmhotz resonator is open to the exhaust port. The lowered slide valve increases the effective stroke of the engine, increasing the compression for better low rpm performance. The sub-exhaust ports are closed to retard the escape of exhaust gases, slowing the returning compression wave. The Helmhotz Resonator is open, producing a delayed pressure wave that retards the escape of air/fuel from the cylinder. The CDI advances the ignition from about 6 ° BTDC at idle, to 21° BTDC at 6000rpm, allowing the flame front sufficient time to travel across the chamber, producing the greatest possible pressure rise. The CDI and KIPS Valves work together to produce the best possible low-end performance.
At 6000 rpm the slide valve snaps open, the sub ports snap shut and the CDI retards the ignition back to the base (idle) setting. The elevated slide valve decreases the effective stroke of the engine, lowering the compression preventing auto-detonation and controlling cylinder temperature. Opening the sub-exhaust ports increases the exhaust port area allowing exhaust gasses to escape more quickly, improving top-end power. The Halmhotz Resonator is closed. Retarding the ignition allows the flame front adequate time to ignite the charge of air/fuel, reducing pumping losses and shifts heat from the piston crown to the expansion chamber.
As you can see the KIPS valve is not only an engineering marvel but also a piece of art.
The early KIPS system and the new generation (1995 on) differ only slightly from each other. The older system uses a rotating valve in the main exhaust port and the mechanical drive for the KIPS system is different. The end goal however is the same.
Over time, carbon builds up on the sub port valves and causes them to seize in their bore. When they do, the geared advancer shaft strips the gears form the sub port valves rendering them useless. To prevent the valves from seizing its important to remove and clean the valves anytime the top end is disassembled for service. Proper jetting and the use of a quality two-stroke oil will also increase the life span of the KIPS system by preventing an excessive build-up of carbon on the exhaust valves. A tell tail sign of sub-port valve damage is the loss of bottom end power.