In a previous article about mechanical fuel injection, I touched on the use of a high-speed bypass. Let's take a look at this widely misunderstood gadget in greater detail this time.
A high-speed bypass (also known as a "high-speed lean-out") is simply another check valve in a mechanical injection system. It opens and bleeds off fuel to the return circuit at the top end of the RPM range, thereby reducing fuel entering the engine and leaning the air/fuel ratio (AFR). The most common type is the spring loaded "poppet" style that opens when a certain system pressure is achieved. Usually, the spring and shims inside can be changed so that the bypass opens at the desired pressure. Other types of bypasses can be actuated electronically or pneumatically and can be triggered directly by the driver, timers, RPM, etc. While most of us might employ a single, simple high-speed bypass, multiples can be used to achieve just the right fuel curve. The pros may use a half a dozen or more lean-out and enrichment "events" during a pass.
Not everyone needs a high-speed bypass. Every pump is different, and how each one responds to back pressure will vary. Once in a while, a person comes across a "magic" combination that works good with no extra help from a high-speed bypass, but it is pretty danged rare.
Why would a person want to run one? The most practical reason to use a high-speed bypass is because at the top end of the RPM range, induction efficiency usually drops off sharply. Things are happening fast and at some point, the ability of the intake tract to pass air starts to diminish. The cylinders are not getting as full of air as they were in the lower RPM range in a normally aspirated engine. Of course, the point at which this occurs varies depending on head porting, camshaft, runner length, etc. Even superchargers are not equally efficient throughout a wide RPM range, and output drops off at high rotor speed.
Because the mechanical injection system, with its positive displacement fuel pump, continues shooting fuel into the motor in proportion to RPM, it has no way to know that things are getting richer as induction efficiency falls off. The properly setup high-speed bypass should do this for you - it will open up and pull some of that excess fuel away to correct for this. Its job is to help get things back where they belong and keep the motor pulling hard all the way to the end of the pass by avoiding an overly rich condition.
Another reason to run the high-speed bypass is to change the air/fuel ratio altogether. On methanol, the best AFR for torque over the first 2/3 of the track (about 5:1) is not necessarily the best for horsepower at the top end. Generally, this AFR is around 5.4:1. The high-speed can be setup to not only compensate for a drop in induction efficiency but also to shift the AFR to one that will produce the best MPH at the stripe.
Quite a number of people don't run a high-speed because they consider it dangerous or mysterious. It can indeed be dangerous to engine parts if used improperly. You could have your favorite fuel injection expert flow test your system and set it all up for you, removing nearly all of the uncertainty. Or, if you have some test laps to burn, you could find the sweet spot yourself. There are a couple of things you should do to make sure that your high-speed bypass never causes any harm.
Mainly, always run a pill or jet in the bypass to restrict and limit the amount of fuel that can be bypassed. Without a pill in it, a -6 check valve will dump a lot of fuel -- guaranteed too much if opened at the wrong time! Keep in mind, you aren't tuning the motor with the pill necessarily. Mostly, you just want to make sure it doesn't ever bypass too much. You can almost never go wrong by starting with a .040 or .050" jet in the high-speed. A pill that size in the check valve will be big enough in most cases to remove the fuel you need without letting it take too much. How could it take too much? Well, what happens if you over-rev your motor during the burnout? The RPM and pressure increase past the point you normally see on the track, and the AFR will get dangerously lean. What if several nozzles become partially plugged with debris? The system pressure will then be higher than normal, and the bypass will start removing fuel much sooner than it should. Without a pill in it to limit things, you could be in store for melted pistons, blown head gaskets, etc.
Knowing your system fuel pressure at the top end of a pass can really help you get a handle on where to set your high-speed bypass initially and keep you out of trouble. You don't really need expensive data acquisition; just install a pressure gauge with a memory kit on it to hold the highest pressure achieved for viewing after the pass. Be sure to put this on the nozzle side of the barrel valve. Otherwise, when you close the throttle at the top end, the gauge will just record the pressure spike, and it'll always be lying to you. Most barrel valves have a port on the nozzle side of the spool that can be used for this purpose. A tee for the gauge could also be added right before one of the nozzles. Once you know the top end system pressure, you can set your high-speed to pop open just a few pounds shy of this. Connect the bypass to your air hose and slowly crank up your regulator. It'll start hissing and leaking. Keep going....at some point, it will really open up. This is what you're interested in. Just a little hissing or leaking isn't going to effect things too much. It isn't really necessary for the high-speed check valve to stay completely sealed and then pop open suddenly and violently at a certain RPM. Each brand and type of bypass behaves differently. Some will begin opening gradually at a certain point, creating a smooth transition from one AFR to another. Others let go all at once. Mainly, you are looking for consistency. Whatever the check valve does, it should do the same thing every time.
Once you have the initial pressure figured out, it's time for some track testing. Play with the cracking-pressure a few pounds at a time and watch the time slips. Eventually, you'll start to see all the MPH you've gained disappear when it starts taking fuel away too soon. Back up to the best MPH, and you're in "The Zone"! Now that you've found the optimal opening pressure, increase the pill size a little and see what you get. Just work in small increments, and you'll be ok.
Throughout your dabbling with a high-speed bypass, keep in mind that as you change the main pill/jet, the system pressure also changes and your high-speed will now need tweaking in order to open at the same RPM as before. A larger main jet will reduce the pressure in your system at any given RPM as compared to a smaller main jet.
Interestingly, a high-speed bypass doesn't always just help the top end. If you've been running without a high-speed for a while and have a handle on your jetting, you are probably running in a bit of an averaged situation. If your system isn't popping and missing from too much fuel at the top end, it means you have compensated for this with the main jet, and the AFR is probably a touch too lean for best torque on the first half of the track. If you try richening up your main jet one step and then set up the high-speed to get rid of the excess later down the track, you'll see gains at both ends, resulting in better ET and MPH!