In this feature, Kelly Westphal will show how to set high-speeds, low-speeds and idle bypasses, and explain how they work. The two most common main bypass valves are the brass valve or the quick-disconnect.
Both of these valves serve the same purpose, but what most people like about the quick-disconnect is that on a red flag a crew person can pull back the collar and take one pill out and pop another one in, in a matter of seconds, whereas the brass one will take a little longer due to using a pair of wrenches to loosen up, take apart and put back together.
Both ways are very accurate, however.
The main jet is the most basic adjustment of a constant flow metering system. A smaller main jet makes the engine richer, by allowing less fuel to flow back to the tank and forcing more fuel to the engine.
A larger main jet will make the engine leaner, by allowing more fuel to flow back to the tank, which means less fuel flows to the engines. This valve also acts as a check valve to hold pressure for starting and idling.
Next we’ll move on to the secondary bypass valve.
The flow to the secondary bypass is through a port in the top half of the barrel valve spool. The port is exposed at idle, then is closed off as the throttles are rotated. It is completely closed at approximately 40-degrees of throttle opening (or about half throttle), so it has no affect on wide-open throttle metering.
The secondary bypass normally has a higher pressure setting than the main bypass, so when the engine returns to idle speed the secondary bypass poppet closes and allows the spring and poppet in the main jet can to regulate the idle fuel pressure.
Applications of the secondary bypass valve that pertain to sprint car racing would be:
A) Over-run – at the end of a straightaway when the throttle is snapped closed, the pump is still at a high rpm. The pressure in the system goes high, especially with a small main jet and a jet restricted high-speed bypass (or no high-speed bypass).
The secondary serves as a “dump-off” to reduce the high pressure, thus preventing an over rich condition.
B) Idle – The secondary bypass pressure is usually set higher than the idle pressure so the valve is normally not bypassing any fuel at an idle.
C) Cornering – If you run through the turns at part throttle and the engine loads up, or is sluggish coming out of the turns, install a lighter spring or reduce the shims in the secondary. If it lacks response or backfires out the intake when the throttle is depressed, install a heavier spring or more shims.
Adjusting the secondary bypass valve is achieved by varying the amount of pressure needed to unseal the poppet inside the valve. The wire diameter or stiffness of the spring is the primary adjustment; a stiffer spring gives a higher opening pressure (rpm).
Shims are utilized for fine tuning. If the secondary seems to open at the right RPM but bypasses too much fuel, its flow can be restricted by putting a jet in it.
Lastly in the family of bypasses is the high-speed bypass valve. There are three types of bypass valves here: the brass can; the adjustable high-speed valve commonly known as the adjustable diaphragm valve; and the ultimate high-speed bypass valve, the K-140.
Why do you need a high speed bypass valve? The high-speed bypass is needed to obtain maximum horsepower, because the engine will experience a loss of volumetric efficiency at rpm above the peak torque.
At high rpm, the intake valves are only open for such a short amount of time that the cylinders do not have enough time to completely fill with a fresh mixture.
The high speed also benefits the mid-range performance. If an engine is jetted to obtain maximum horsepower at high rpm without a high-speed bypass, it would be lean around the peak torque rpm.
Unfortunately, jetting rich enough for best mid-range power will cause the engine to run rich and lose power at high rpm. But, by installing a high-speed bypass, separate control of the mid-range and high rpm fuel rates are possible.
The main jet can be run richer for best mid-range performance, while using the high-speed to lean the system for best power at high rpm.
To continue reading, advance to the next page.