Corvette Fuel Systems - Fueling The Fire

Flash back over three and a half decades. It was a gorgeous late spring day. Temperatures were in the mid to high eighties. The t-tops were off. The big block was running at full song. Seemingly in an instant, the gas pedal went spongy. The rat motor stumbled a wee bit, and then simply quit. We popped the hood in an effort to diagnose the problem. As it turned out the combination of a tight engine compartment, big compression (the factory LS6 was fitted with 12.25:1 pistons and converted to LS7 internal specifications) and big tube headers was simply too much for the fuel delivery system - big stock fuel pump and all. The fuel was percolating in the fuel bowls as well as in the fuel lines. The recurring dilemma was eventually overcome with almost every band-aid fix known to man. But it was never wholly resolved. Aside from boiling the fuel, there likely wasn't sufficient volume to satisfy a thirsty, big horsepower big block.

When it comes to fuel delivery systems, there's no question that big is pretty much the operative word. Proof is the honking fuel pump you'll typically find nestled beside the fuel cell in a NHRA Pro Stock racecar. The trouble is if you have a modified street-driven Corvette--fuel injected or carbureted--then you'll find yourself at a fuel delivery crossroad. You see, with a big cubic inch engine on board, or one with a serious power adder or two (blower, turbo, nitrous or even a combination of the three), there's a definite need for plenty of fuel. On the other hand, those big monster pumps that feed a Pro Stock drag car don't particularly like to run for extended periods of time such as in a street driven car. They weren't designed for that purpose. And heat becomes an issue (this is really critical if the pump configuration doesn't incorporate some form of return or by-pass system - deadheading pumps build heat in a big hurry). The foremost reason is there is nothing to cool the big electric motors used in these pumps.

So what electric pumps are designed for the purpose? You just have to take a look at a modern fuel injected Corvette for some insight. In those applications, the pumps are engineered so that the fuel runs through them. As a result, the fuel supply actually cools the pump as it operates. That means the pump in question can run pretty much wide open for as long as necessary (and as long as there is fuel in the tank).

Does this mean you're stuck with one of those little itsy-bitsy in-tank pumps that are found on most late model Corvettes? Not at all. Several companies offer big in-line fuel pumps that are engineered so that fuel runs through them (for extended operation). Case-in-point is the MagnaFuel 625 pump shown in the accompanying photos. This particular pump offers a number of features you should look for when shopping for a pump:

Aside from the continuous duty capability, this pump is self-priming. That means you don't have to fret about pump mounting. In fact, it is possible to install it vertically or horizontally (mount bracket included). Another bonus is the pump size. It's physically rather small, measuring just under 7.50-inches in length (not counting the fittings or the by-pass). Taking the mount bracket into consideration, the pump measures just under 3.75-inches in height while the body diameter is 3.00-inches.

The pump is also quiet. Now, that's not a big deal for a Corvette with a large displacement, barely muffled engine on board, it's probably not a big consideration, but if you have something a wee bit tamer, there's a good chance the drone factor could drive you nuts. Internally, this particular pump incorporates double support bearings and uses polymer wear plates (which obviously contribute to the low noise and smooth operation). The actual layout and construction of the pump eliminates the need for shaft seals. Without shaft seals, then there's nothing to leak (aside from an O-ring, which as most know are regularly reliable).

One thing we mandated for our application (a home-brewed 572 cubic inch combination) was a pump that didn't require an electrical step down device to reduce current draw. This particular fuel pump is based around a high-torque custom motor with very low current draw (12 amps at 45 PSI). This also means you don't have to wire in a relay for the pump (although you can). The other piece of the puzzle as far as we were concerned was the capability to rebuild it if necessary. The MagnaFuel pump is completely rebuildable with a simple O-ring and wear plate kit.

Sharp readers will note this piece is actually an electronic fuel injection pump. How can you use it with a carburetor? Simple. Remember that by-pass arrangement mentioned above? MagnaFuel offers a by-pass package for these pumps and it is much like the system used in the big Pro Stock fuel delivery systems. It simply threads on the outlet of the pump. In operation, you set the final outlet pressure (from the pump to the regulator) you desire by swapping out by-pass springs. The balance of the fuel (excess) is by-passed and sent back to the tank. That process eliminates the "dead-heading" dilemma we alluded to earlier.

So how do you regulate the fuel? Even though unnecessary fuel is bypassed back to the tank, some form of device to control fuel pressure is essential (that's no secret). Typically, two different types of fuel control devices are available. One group is engineered for carburetors while the other is designed for fuel injected engines. Electronically fuel injected engines mandate operating pressures ranging from 35-85 PSI. Meanwhile, carbureted engines typically see operating pressures in the 4-12 PSI range. You can get two or four port (outlet) regulators, and you can usually obtain them with or without boost reference capability (meaning the regulator is designed to work in conjunction with a blower of some sort). In both families (EFI or carburetor), you'll find regulators grouped into the horsepower level (engine) they are designed to sustain. For a serious performance application, an adjustable regulator is an absolute necessity.

Remember when we mentioned the capability of field-servicing an electric fuel pump? The same applies to the regulator. You should look for something like the MagnaFuel regulator design where a removable cartridge is employed. Here, the basic internals of the regulator can be inspected or replaced without removing the regulator body from the respective mounts or without removing the hoses.

Hang on...we're not quite done yet. Fuel is dirty stuff. Pump gas is likely worse than race gas. And that's why fuel filters are especially important in a modified car. Use a high capacity aftermarket in-line filter, or even a pair of them - one before the pump (pre-filter) and one after. We're not talking about the easy-to-get jobs that you can find at your local dealership or auto parts store. Most of these filters are just too small and cannot handle the volume of fuel required for a healthy modified engine. They can constitute a restriction in the fuel delivery system -- even if they aren't plugged.

What about disposable plastic filters? They're a complete waste of time and dollars. Cheap plastic filters create a considerable fuel system restriction, can easily be plugged and almost all examples we've seen have far too small an inlet/outlet port for real high performance use. There's more too: Virtually all disposable filters out there are designed so that you use worm gear clamps to hold them in place. With the high-pressure electric pumps currently used in modified applications, these filters can easily be collapsed (internally) by the force of the fuel pump or if they aren't secured properly, they can be blown right off the fuel line (not good).

Here's what you can do about the fuel filter situation: Several aftermarket companies offer high quality in-line filters that are up to the rigors of performance use. These filters are positively huge and are designed to handle high pressures along with plenty of volume. By adding a pre-filter along with an after-filter you won't choke the fuel delivery, and at the same time, it can protect your engine from debris and junk in the fuel. These filters aren't small and they aren't cheap. But then again, performance never was inexpensive.

In the end, can the average little guy quick-street Corvette combination pick up performance with the latest high volume fuel delivery systems? There's no question they can and today's fuel combinations will most definitely improve the performance level. You'll also find that your Corvette will become much more consistent and reliable (and no vapor-lock, heat soaked issues like we regularly experienced decades ago). But in some cases, be prepared to find a completely new tuning combination. The reason is a good fuel delivery system will pump more fuel and less air and vapor to the carburetor or the injector nozzle.

Finally, if you have a car with small fuel bowl capacity, there's a very good chance you'll find it needs more fuel pump capacity than a 1250+ HP Pro Stock drag racer. Don't forget, at the launch, a Pro racecar might have over 2.5 G's of force applied to it (which the fuel has to overcome). But there are four needle and seat assemblies, and four rather healthy fuel bowls available to accept this load. On something like a 1X4 BBL street-strip Corvette you might be dealing with 1.1 G's on the launch. Unfortunately, the fuel only has a pair of needle and seat assemblies and two fuel bowls to spread the load over (and something with a Quadrajet or Carter carb is worse with only a single needle and seat along with a rather small bowl). It's something to think about.

In the meantime, check out the accompanying photos for a closer look at the basic hardware we're using to feed a high horsepower, big cubic inch Chevy engine. You might want to copy it.

Meanwhile, the outlet side of the pump has an optional bypass assembly along with wire connections (the wiring is dead simple: red-plus, black-negative). More on the by-pass in the next photo.

Our engine combination is old school carbureted. It needs a bypass. The bypass we're using is a spring-type arrangement (shown here "blown apart" - it resembles a giant carburetor needle and seat assembly). It is not physically adjustable, but you can change the spring to change the specific pressure at which the pump returns fuel to the tank. MagnaFuel can supply you with various springs for this combination, although they'll likely nail down the right spring with your order.

A fuel pressure regulator is an essential piece of the fuel delivery puzzle (that's no secret). As pointed out in the text, when selecting a regulator, you first have to choose if your car is carbureted or fuel injected. And there's also a special series of regulators for Corvettes with blowers or turbos (they're boost referenced). For certain EFI applications, there are some companies who build fixed EFI regulators, but for a modified Corvette application, an adjustable regulator is absolutely necessary. Because of the design, this particular adjustable regulator doesn't pressure creep (this is something that's critical in any engine--fuel injected or carbureted). Just like the pump, the regulator body is machined from 6061-T6 aircraft-grade aluminum alloy. Fuel pressure is set using the adjusting screw/jam nut system found on the topside.

Once the application is determined (EFI or carbureted), you then have to figure out how many ports the regulator will require. The 572 we're building is set up with a single four barrel, hence the two port arrangement. If you look closely, you'll see the serviceable, stainless steel cartridge mentioned in the text.

Also note the port for fuel pressure gauge. MagnaFuel recommends installing a pressure gauge right on the regulator for adjustment purposes then they advise you should remove it (most of the little fuel pressure gauges out there eventually leak).

For a serious street driven Corvette application, it's a very good idea to use two filters - a pre-filter mounted between the fuel tank and the pump followed by an after-filter, mounted between the fuel pump and the fuel regulator. Note that these in-line filters are engineered so that the fuel flows from the outside of the element to the inside. The filter bodies (also built from 6061-T6 aircraft-grade aluminum) are marked with the direction of flow. They're also color coded: Blue end caps on the filter signifies a 74 micron filter, black end caps are used for a 25 micron filter.