EFI Introduction 101 - Engine Builder Magazine

EFI Introduction 101

EFI has become the tool that has unlocked the next level of engine performance. Nowhere is it more evident than in drag racing, where EFI has taken over where the rules permit. Take Joe Dunn’s Pat Musi-powered ’68 Camaro that just rocked the competition with a 6.38 @ 218 mph pass. Its 755 cid big-block features a well-tuned EFI/nitrous setup that repeats run-after-run and doesn’t kill parts in the process. Musi has become the go-to guy when it comes to integrating EFI and nitrous.

From an engine builder’s perspective it doesn’t get any more extreme than Pro Mod racing and the only two Pro Mod cars now running over 250 mph are EFI- equipped and run Wilson Manifold EFI setups. But it’s not only these mega-engine cars that are depending on EFI. Virtually any engine combination can benefit from EFI as well. The move to EFI in racing is strongest in the Mustang NMRA and also “street car” racing sanctioning bodies like the NMCA. EFI is the perfect complement to high horsepower power adders like nitrous, turbos and superchargers.

Carburetors will never be totally overshadowed by EFI, because they will be the mainstay of sportsman racing for the foreseeable future, and remain the backbone of hot rodding. However, more of your customers will be looking for performance engines with a total EFI package. It’s a great opportunity for you to up-sell EFI components or sell a complete EFI system.

The question facing today’s engine builders is – do you spec out the EFI induction system and include it with the engine purchase or leave that up to your customer? If you are leaving it to your customer, two things may take place. First, there’s a good chance that the owner will get it wrong and blame you for its lack-luster performance. And second, you will be leaving a lot of money on the table for someone else, and today that can mean the difference between making a profit and taking a loss.

We will briefly touch on all areas of EFI induction systems. Each is so complicated that it deserves its own stand-alone technical analysis. Fortunately, most of the information you need is available on manufacturer’s Web sites and numerous forums. Like anything else, understanding EFI is a learning process, one that will reward those engine builders who take the time and make the effort to figure it all out.

Where to Begin

The experts we spoke to including Keith Wilson (Wilson Manifolds), Kenny Duttweiler (Duttweiler Performance), Pat Musi (Pat Musi Engines) and several component manufacturers. All stated that the first thing you need to determine is the target horsepower for the engine you are building. This holds true from the most basic street engine to the most radical race engine. Here’s why: once you know the horsepower you can precisely calculate the amount of fuel and the amount of air the engine needs. Once you know the fuel required you can determine what fuel pump, lines, pressure regulator, fuel rails and injectors the engine will need. Likewise, once you know the airflow requirements, you can determine throttle body size and manifold configuration.

Fuel Flow

Determining fuel flow for an EFI engine can be tricky because it really depends on the Brake Specific Fuel Consumption (BFSC) of the engine you are trying to fuel. Basically, the rule of thumb is to use a .5 BFSC for normally aspirated engines and .6 BFSC for engines with power adders. Using these figures (600hp X .5 = 300lbs/hr.) for a hypothetical 400 cid, 600 hp engine we get the following results.

The folks from Aeromotive caution that a small change in BFSC can greatly alter the fuel requirements, so if you are not sure of your engine’s BFSC, err slightly towards bigger injectors  (more on injector sizing later).

Air Flow

Determining an engine’s airflow requirements is fairly easy if you know its volumetric efficiency (VE). A good running 2-valve pushrod V8 is probably in the 85-95% VE range. A highly tuned race motor may exceed 100% VE. To simplify the formula most people use this formula that solves for 100% VE and then adjust the final flow number up or down.

CFM = CID (displacement) X RPM


For example our 600 hp hypothetical engine has 400 cid and revs to 7,000 rpm. When these numbers are plugged into the formula we come up with an airflow requirement of 810 cfm. If you believe your engine has an 85% VE you multiply 810 X .85 = 788 cfm. That is why a 750 cfm carb works so well on this extremely popular engine combination.

Okay, we now know the two things we need to coordinate an EFI system for our hypothetical engine: 300 lbs/hr of fuel delivery and 810 cfm of airflow. Ken Farrell, one of the designers of Professional Products’ bolt-on PowerJection, says this is where many engine builders fail. “They end up with a bunch of mismatched components that don’t play well together.” So now is the time to select the right fuel pump to go with the right injectors to go with best intake and top things off with the best tuning solution.

Fuel Pumps

Cars with existing EFI systems have fuel pumps that are closely sized to the O.E. power demand. If only a slight power gain is required you can usually get more effective fuel delivery out of the stock EFI system by adding an adjustable fuel pressure regulator, and bumping the pressure up slightly. But for anything more aggressive you will require an aftermarket pump with high-flow filters and a bypass fuel regulator. The Aeromotive Fuel Systems Web site has some excellent tech bulletins on how to properly set up a fuel delivery system for EFI. Fuel pump selection, in-tank pickup tubes, line routing and fuel regulator plumbing are all very critical to how the system will operate and how durable it will be.

One of the biggest problems with EFI is overheating the fuel (which can boil at 150° F in some circumstances), leading to fuel pump cavitation and vapor lock. This situation very quickly leads to fuel pump failure.

There is so much more to understanding your fuel pump requirements. You need to know how much fuel a pump can deliver at a given pressure, and how fuel system pressure affects pump volume. It’s always best to go slightly larger than needed, but too large a pump can lead to the fuel overheating problem mentioned earlier. You also need to know exactly how much voltage the fuel pump is getting under operating conditions. Voltage has a huge effect on pump output.

Where EFI differs from a carburetor in the real world is its lack of forgiveness. Ken Farrell claims that most of the problems he ends up troubleshooting are things like bad ground connections, or not knowing how much voltage is going to the pump and shoddy wiring.

Injector Sizing

Injector sizes are fairly easy to determine once you know the fuel flow requirements of the engine. You simply take the fuel requirement in lbs./hr and divide by the number of injectors you are using, divided by .8 that represents the size injector needed for an 80% duty cycle. An 80% duty cycle is the accepted standard for injectors. For 600 hp on an eight-cylinder engine that would be 300÷8=37.5÷.8=46.8lb/hr injectors at 80% duty cycle.

An important point to bring out here is that a smaller injector can be made to deliver more fuel by upping the fuel pressure. However, it is much better to properly size the fuel injectors to reduce demands on the fuel delivery system, especially in extremely high-output engines.

Fuel Rails

There was a little disagreement amongst our panel on when and if larger fuel rails are needed. The consensus was that any time injector size was increased 60% over stock that the larger fuel rails were a good idea. Another rule of thumb was when the 1,000 hp level was reached it was time. Keith Wilson said that injector distribution was enhanced with the larger fuel rails. He also mentioned that Wilson Manifold’s fuel rails had a unique patent where the fuel entry to the injector is radiused for better flow.


The consensus on manifold design is to forget whether the engine will be carbureted or injected, select the intake manifold that will make the most power and design the EFI system around it. In some cases the best choice would be a state-of-the-art single plane 4V manifold. If rules allow, a taller fabricated sheetmetal intake might be required.

There are some interesting manifold choices for the more popular engines. FAST has several new manifolds for the GM LS engines the LSX and LSXr are designed with larger runners and accept larger (up to 102mm) throttle bodies. These manifolds are made out of a polymer and extremely lightweight. They also unbolt and open up for easy modifications.

Professional Products offers manifold upgrades for GM LS, Ford 2V Modular and Acura engines. They have taken a different approach with cast aluminum manifolds that are available in satin or with a show-type polished finish. They are effective, too, with the Ford 2V adding an additional 21 hp to the rear wheels. Most of these manifolds are fashioned after the O.E. intake manifolds that may or may not pass a visual smog inspection.

The next few manifolds we are going to list probably wouldn’t pass a visual smog check, but have wide application. Edelbrock, Accel DFI and Holley have several “ram” style manifolds that are low enough to fit under most hoods and have enough runner length to make a broad torque curve. Edelbrock also has one of the largest selections of single 4-barrel EFI intakes for a variety of engines.

At the top of the heap for aftermarket EFI intake manifolds is Wilson Manifold’s GM LS Billet Bank intake manifold. As the name implies, much of the manifold is whittled on a 5-axis CNC out of a piece of billet aluminum. Yes it works, and yes it’s expensive.

For unlimited race applications it’s hard to beat a dedicated sheetmetal intake. They are expensive, but give you many options for throttle body and injector placement. Wilson claims that the higher you move the injectors in the runners, the more power the engine will make. The limitations are the physical space to plumb the injectors and some safety concerns. Fuel stand-off from high injector placement can be a fire hazard.

Throttle Bodies

If you have done an airflow computation on the engine you are tuning you will know how much airflow is required and how large a throttle body you need to reach that number. As Keith Wilson put it, “At wide open throttle it’s tough to be too big – you don’t even need a throttle body. The engine will just pull in the air it needs. It’s for part-throttle driving that you can get the throttle body too big. It simply uncovers too much area too fast, causing the car to lunge when you touch the throttle – not what you want when parallel parking!” Wilson Manifolds, Edelbrock, Professional Products and several other companies offer throttle bodies in incremental plus sizes. If your engine calls for an 82 mm throttle body, going to an 85 mm won’t adversely affect performance, while a 96 mm might just be too much and cause those driveability issues that Keith talked about.

The four-hole throttle bodies like those used on single four-barrel and sheet metal intake manifolds have fairly decent driveability characteristics because they have smaller air valves and most can be set up with progressive linkage. Another advantage is that they are low profile and fit better under a hood than most carburetors.

Aftermarket EFI Systems

After years of development, there’s a lot to choose from out there in complete aftermarket port-type systems. Edelbrock has 10 Pro-Flo2 systems for AMC, Chevy, Chrysler, Ford and Pontiac. Professional Products’ ProJection II systems are available for Chevy, Ford, Pontiac and Oldsmobile. Accel DFI and Holley also offer several systems as well do a few specialty companies like Street & Performance. However, most of these kits are limited to 500 hp and lower applications. They are very complete and therefore rather pricey. They were intended more for the end-user than the professional engine builder. If you need port-type EFI it makes much more sense to spec out a system from components to fit your exact engine combination and needs.

The only exception to that statement is for mild (less than 550 hp) carbureted engines whose owners want to transition over to EFI in the easiest way possible. For those applications system such as Professional Products’ ProJection III or FAST’s EZ-EFI, for example, bolt in place of a four-barrel carburetor, they are self tuning and very easy to install. Systems  such as these offer excellent performance and all of the advantages of EFI at a reasonable price. The only limitation is that they can only support about 600 hp.

Engine Management Systems

Here’s where EFI gets difficult, but with the proper planning anyone can get through it. There are two very different scenarios with a couple of very different solutions. The first is for those applications working with an O.E. ECU. Let’s say your customer’s engine was going back into a car that already had EFI. That solves a lot of problems because you know he has the right ECU, wiring harness and sensors. If the vehicle is GM for instance you can use aftermarket software like HP Tuner to do the tuning.

If the engine is a stand alone going into a racecar, street rod or earlier musclecar then a stand-alone engine management system (EMS) is probably the way to go. The experts’ advice is to select one EMS system, become familiar and comfortable with it and stick with it. There are systems ranging from very expensive Bosch and Motec EMS systems used on race cars like the Daytona Prototypes, to very affordable and easy to use systems from FAST, Accel DFI, BigStuff3, Professional Products and many others.

Just beware of potential component compatibility issues such as drive-by-wire (electronic throttle bodies) with mechanical throttle body computers. Incorrect wire harnesses and incompatible sensors. Take nothing for granted with EFI.

Some of the best advice we heard is to align your company with a full service tuning shop, and rely on them to spec out the EMS system and components. This is a very specialized skill set that is extremely crucial to how the finished product performs.

Cold-Air Intakes

One of the first modifications to get the most out of a new EFI system is to add a cold-air intake. There’s a good deal of power available without much work or money. How much power? At the top of the scale is K&N’s intake system for the Shelby GT500 Mustang. K&N claims a 50 horsepower gain with no other changes (10-20 hp is typical for most V8s).

How can the factory get it so wrong? They don’t. The carmakers just have different standards for noise, vibration and harshness, as well as considerations for how fast the factory airbox can be installed as the car comes down the assembly line. The aftermarket has no such worries. Aftermarket intakes improve power through lower restriction filters, isolating underhood air temperatures from the intake, and smoothing the flow path all of the way to the throttle body.

There are several intake manufacturers to choose from and most use the oiled cotton gauze steel mesh sandwich pioneered by K&N. Some new entries into the performance aftermarket, such as those from R2C Performance, are featuring a synthetic filtering media that, like cotton gauze filters, can be cleaned and reused. That’s where the similarity ends, because you simply blow the polyester high density fibers clean from the inside out. R2C’s technology was spun off from the military. They first entered the performance market in dirt car racing with a traditional round filter. They claim that the lofted gradient density media is excellent at filtering out dust, and that performance is as good at the end of the race as the start.

R2C’s intake systems for today’s production vehicles are easily spotted because they use 3-inch square intake tubes instead of traditional round tubes. They claim 17% more cross-section area with square versus round tube.


A couple of points struck me after researching this article.

• You no longer need to build an engine specifically for EFI. No more camshaft or manifold vacuum limitations – just build the best engine you can.

• With EFI you can’t overlook any of the details such as the vehicle’s charging system or its fuel delivery system. These little gremlins are extremely hard to trace unless you know what to look for.

• The successful engine builders of the future will have the ability to design, install and tune EFI systems.

For information on determining fuel flow through the engine using the Brake Specific Fuel Consumption (BFSC) you can visit www.aeromotiveinc.com or rceng.com. joe dunn
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