The Big Pump Myth

By Spud Miller

One very commonly held belief surrounding mechanical fuel injection is that for the purpose of making good pressure in the system, the bigger the pump the better. People very often put a much larger fuel pump than they need on their vehicle, thinking that the fuel will then atomize better and make more power as a result. Good intentions, but this is untrue.

A mechanical injection system employs precision "leaks" into the motor, and these leaks or nozzles are what determine the required system pressure at a particular engine speed. Here's how it works: For a given set of nozzles and particular engine speed, there is ONE pressure that must be achieved in order to get the correct amount of fuel to fall out of those nozzles and into the engine.

To understand this, it helps to look at just a single slice of the fuel graph - just one point on the curve. The correct amount of fuel at that point doesn't change, there's one right answer for that. If the nozzle size is increased, less pressure is required at that point (and the rest of the RPM range too) in order to get that correct amount of fuel to pass through into the motor. As nozzle size decreases, the pressure at that engine speed must increase in order to force that same amount of fuel through those smaller holes. If you put smaller nozzles in your injector, you must also make the bypass pill smaller to raise system pressure and keep the fuel to the motor the same. So, as nozzle size becomes smaller, so does the main pill. Larger nozzles require the main pill to get larger as well.

To illustrate, let's say you have a 500 inch motor that wants to gobble 2.76 GPM at 7500 RPM. For best horsepower, this combo needs precisely that amount of fuel at that engine speed. No more, no less. Let's also say you've put .038" nozzles in this motor, and as a result, 54.9 PSI is required to make that desired 2.76 GPM fall out of them. In order to hit that pressure at 7500 RPM with those nozzles, a .120" main pill is what you are using.

That is pretty dismal fuel pressure, so we replace those .038's with .032" nozzles. Since the holes are smaller, more pressure is required to make that same 2.76 GPM enter the motor - it turns out you now need 111.7 PSI. To get that required higher pressure, you now have to run a .100" main pill. Now you have the same fuel going to the motor, but at over double the pressure.

Notice as we went through the above exercise, we never even mentioned the pump. Since the GPM required hasn't changed, there's no need for a bigger fuel pump. In fact, to show how little the pump has to do with it, lets look at what happens when a person changes the pump and not the nozzles.

Going back to that 500" motor with .038" nozzles...presently, with a 6.2 GPM pump, a .120" main pill is required to make 54.9 PSI and expel 2.76 GPM at 7500 RPM. If we now double the pump output to 12.4 GPM, the required pressure to make 2.76 GPM enter the motor at 7500 has not changed a bit. It still takes 54.9 PSI to make the motor run its best. However, now to achieve that pressure, you'll need to run a .200" pill instead. Nothing is gained here, and in fact when bypassing this much fuel, severe turbulence in the tank and pump inlet can result. Fuel temperature increases quickly as the pump churns through the same fuel over and over. Increasing fuel temperature changes the specific gravity of the fuel and hence the overall tuneup gradually changes. Consistency suffers - not a very good deal.

Since the fuel requirements of the motor are roughly linear with respect to RPM, the required fuel and hence required pressure is directly proportional to engine speed. The line on the graph for required fuel volume and pressure is up and to the right. If you asked a mechanical engineer to design a linear fuel pressure regulator unit that would follow engine speed, it would probably have springs, balls, pulleys, belts and so on. It turns out that a fuel system with all of its "leaks" passes fuel in a very linear way as pressure increases. Just a small piece of brass with a hole in it to adjust the overall pressure works great! So, instead of thinking of the main pill as "returning fuel", it is much more accurate to think of it as bleeding off the right amount of pressure in the system to achieve the right fuel flow into the engine.

Ideally, a person should shoot for changing the main pill area 6 to 8% from one step to another when running methanol. This is enough to see a difference, but not so much as to blow yourself out of the water and miss the sweet spot. The percentage area change when moving from a .205 to a .210" pill is less than 5%. However, the difference between a .050" and .055" main pill is over 17%. HUGE! Moving from a .110 to a .115 pill is a friendly 8%. If you have pills in increments of .005", you'll want to be running in the range of .100" to .130" if possible and using the proper pump for the job will get you there. Running a tiny pump will make your main pill too small to be nicely tunable. Also, huge or tiny bypass pills very often don't flow what they should and are not consistent throughout the pressure/RPM range.

The old adage "bigger is better" applies to a lot of things in racing, but fuel pump capacity isn't one of them.

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