Fuel Systems 101

Constant flow fuel injection systems have been used on about every type of engine ever made whether it runs on nitro, alcohol or gasoline for over 50 years. It is by far the most simple of all fuel systems in its basic design, but one of the hardest to tune. The reason it is so hard to tune is that there is no automatic compensation for atmospheric conditions, engine load, fuel temperature, blower boost, cylinder-to-cylinder distribution or airflow variations due to camshafts, cylinder heads, manifolds, blowers or headers.

All constant flow systems use positive displacement fuel pumps of various designs normally driven at 50% of engine RPMs, either direct off of the camshaft or through a Gilmer style cog belt off of the crankshaft. The key to a winning fuel system is matching the curve to the application, which are all different. A Top Fuel dragster requires a different curve than a fuel Funny Car. A Pro Modified is different than an alcohol funny car and so on.

The fuel pump is the heart of the fuel curve. The curve is governed mainly by the basic design of the pump itself. With so many designs and variations of designs by different manufacturers, most people are unaware of the differences. Vane pumps and external gear-to-gear pumps are very sensitive to tip speed and internal leakage and internal gear-to-gear pumps are more sensitive to clearance than tip speed. Of course, all positive displacement pumps are sized to the application, but size has little effect on the curve within the same design.

All drag racing applications that use a transmission and a supercharger have the most radical fuel curve to try and match. The engine leaves the starting line anywhere from an idle to 6500 RPMs depending on combination and accelerates to 8000 - 10,000 RPMs and then the shift brings it down 1000-2000 RPMs in a couple of hundreds of a second before having to accelerate up again to the next shift point and so on. While all this is happening, the supercharger is pumping air at different and inconsistent rates depending on design, and on top of all this, the most important aspect is the changing loads on the engine due to transmission ratios, clutch slippage, tire and track conditions and atmospheric conditions.

The curve can be adjusted somewhat by the use of normally open jets, normally closed jets, high-speed valves and constant flow compensating valves. Top Fuel dragsters and Funny Cars use up to 15 of these valves activated by air timers, while most alcohol powered cars use only a couple. Jets and nozzle design also have an effect on the curve and more of an effect on the raw horsepower. Different jet designs have individual flow characteristics at different pressures = engine RPMs.

Nozzle spray patterns and location in the ports and manifold have an effect on raw horsepower. Of course this is unique to each engine, cylinder head, manifold, camshaft and blower design.

One might read this and wonder how anyone could make all of this work. As stated earlier in this text, a constant flow fuel injection system is basically very simple. What we've explained are the intricacies and some things a person has to pay attention to if they want to be a contender or a knowledgeable fan.

My company, Tom Anderson Enterprises, designs a totally computerized fuel system, with a flow bench that samples at 100 samples per second on a sweep curve, so we can design and test any fuel system or components that we sell or service.

To do this article justice, we had to separate it in two parts. The first part was to be on the fuel system itself, and the second had to deal with the supercharger.

A roots type supercharger is technically not part of the fuel system, but in reality it is. That is due to the fact that the blower as we will call it distributes the air and fuel differently, depending on design. Roots type blowers have been around forever and anyone who has knowledge of a Detroit Diesel engine has seen one.

The basic design has not changed, but everything else has. Rotor design, boring of the cases, location of sealing strips, inlet and outlet designs are constantly being changed and modified in an attempt to build a better blower. Currently there are only four companies able to produce competitive roots type blower for nitro and alcohol classes in drag racing and it is very difficult to build blowers that perform identical. The blower efficiency is also constantly changing as the sealing strips wear, changing horsepower output and requiring adjustments to the fuel system and race car set up.

In classes without an overdrive ratio restriction, a lot of tuners attempt to compensate for wear by increasing the overdrive percentage. In my years of experience with fuel Funny Cars trying to crutch a weak blower with overdrive percentage was rarely successful. This is due in part to the fact that it takes a tremendous amount of horsepower just to turn the blower. And because it is a positive displacement air pump when it turns faster it makes more boost, but it also takes more power to make this boost and it generates more heat, which robs horsepower and promotes detonation.

I spoke a lot about fuel curves earlier and blowers also have different boost curves depending on design and condition. When a blower wears it has a dramatic effect on the boost curve, that's why it is always best to re-strip the blower rather than speed it up because speed will not restore the curve. The life of the sealing strips is dependent on several factors. If the strip clearances are too tight when the blower is built, it will wear the strips the first time it is run. If the mating surfaces of the blower and manifold are not flat within .001 of an inch, this will also ruin a good blower immediately.

The other major factors are rotor flex, rotor twist and the location and amount of fuel injected to the inlet side of the blower. This varies with blower design and speed. I feel that fuel systems and superchargers are the two most important aspects of a drag racing engine combination, whether it runs on nitro or alcohol. So, in addition to our computerized fuel flow bench, T.A.E. is building a state of the art engine and supercharger test facility in the Rochester N.Y. area. At the heart of our facility is a modified Dynamic Test System engine dynamometer capable of testing engines that produce over 3000 horsepower with accuracy and repeatability of 1 horsepower.

With multiple fuel flows, boost pressures in each manifold runner and nine pitot tube air inlet sensors, this is the only facility in the North East designed specifically for testing superchargers and supercharged fuel injected alcohol race engines.

Now I know that some of the readers will come away from this article having learned a little more than they knew going into the deal, and for that I am grateful. That was my goal in writing it. I hope you all enjoyed the insight on fuel systems and superchargers as much as I did writing it for the best E-zine on the planet.