Many racers have considered using mechanical fuel injection on their cars only to wonder what they should do and how to get started. If you are one of those people, you’ve come to the right place! If you haven’t already read our first article, you should: What You Need To Know About Mechanical Fuel Injection.
Continuous-flow, mechanical fuel injection has many benefits. It is relatively simple to set up with few parts requiring regular maintenance. Those parts are readily available world-wide from multiple suppliers. The different designs and layouts make the engine stand out in the car, as it is the most visible part of a racing engine. Multiple fuel lines, stacks, and large air inlets provide the appearance of stunning power levels. Let’s take a look at the different features and attributes of the different styles of mechanical fuel injection.
STACK-TYPE FUEL INJECTION
Early mechanical fuel injection started out with a velocity stack-type configuration. It features one fuel injection nozzle and one throttle bore per cylinder. Common in drag, circle track, top speed, and other forms of racing, it often represents the ultimate image in power production from a naturally aspirated racing engine. Stack-type fuel injection is considered to be the standard in appearance by many.
The fuel system tuning in a well-developed fuel injection setup like this involves simply changing a main bypass jet and/or a high-speed poppet pressure setting for variations in air density.
For idle-to-full-throttle drag racing, the fuel mixture is simple to control with jetting for maximum power. For circle track racing, where the throttle is modulated–such as in tight turns around quarter-mile ovals–the low end can be set up with a secondary bypass circuit to provide a surge of fuel coming out of the turns for good response.
Stack systems require adjustment of individual throttle plates, which may vary in seating at idle position with engine heat expansion, and that can strain the manifold. Some of the Kinsler stack-type setups go to great lengths to segregate manifold sections and throttle shafts to minimize this problem.
The stack type is reported to provide the quickest e.t.’s over other fuel system alternatives in short course, tight dirt track or pavement circle track racing. Stack-type fuel injection is not considered very streetable however, since there is no fuel modulation with engine load. Although, with an automatic transmission and a high-stall torque converter, some setups work on the street. Stack-type fuel injection setups often appear in car shows such as Autorama and Goodguys events.
Alcohol is the most common fuel used with this and other mechanical fuel injection setups. Gasoline is used by some, but it is a bit harder to tune. It is more susceptible to spark plug fouling, and as a result, it is not as popular, however E85 works well with mechanical fuel injection. Nitroromethane and methanol, in just about any ratio, can be made to fly with mechanical fuel injection.
In drag racing, mechanical fuel injection is common in several drag racing nostalgia and bracket ET classes, and it is the standard in Pro Mod, Top Alcohol, and Top Fuel eliminators. In circle track racing, the stack type is common in most of the 305, 360, and 410 cubic-inch engine classes throughout the world, and is common in outlaw classes with oversize engines as well. It is also a frequent choice in drag, circle, and sprint boat racing.
New systems cost between $5,000 and $8,000, which is a bit more costly than a simple throttle body fuel injection setup or stock-type carb and manifold. However, it is a more-developed standard for quite a few racing classes. Many suppliers maintain inventory of low-cost used parts for sale for this combination to help with costs, along with their inventory of new parts.
THROTTLE BODY FUEL INJECTION
Throttle body fuel injection is a standard worldwide in many racing classes such as bracket drag racing. Both single throttle body and dual throttle body setups are common and have a very functional appearance, similar to that of singe and dual four-barrel carburetors.
Tuning is a matter of matching jetting to the size of the engine. Throttle bodies such as the Rons Terminators and T2’s with their rigid throttle assemblies remote from the engine, are probably easier to set the idle for than the stack-type fuel injection setups with multiple throttle plates.
For high performance sports boating, this type of injection is often chosen instead of the larger-bore stack-type MFI. The latter may require more transition fuel circuit tuning against the load of a propeller in the water during prop recovery with RPM and speed, to get up on plane.
This is not recommended for street use for all the same reasons as stack-type mechanical fuel injection. However, with a simple throttle body, it may be more streetable for those quick trips to and from motorsports events and car shows, where the engine is run mostly at idle or low speeds. A high stall-speed torque converter makes a setup more forgiving on the street.
This type provides high power output, especially when combined with a well-developed manifold such as modern single- or dual-plane intake manifolds common with modern V8s.
Throttle body setups are common in many drag racing, top-speed events, and boat racing classes where fuel injection is specified or allowed.
This type of fuel injection offers the lowest cost of any mechanical fuel injection alternative; a quality system is in the $2,000 range for a completely new system plus the cost of the manifold.
SUPERCHARGED VERTICAL THROTTLE ASSEMBLY
A vertical throttle assembly on a blower is a common combination that started in the 1950s and is carried over today in many nostalgia and professional motorsports events. It is popular due to fast throttle response from the throttles (and FI nozzles) down close to the blower inlet.
These FI throttle assemblies are extremely popular from an appearance standpoint. The tall height usually sticks out of the hood on full-bodied performance cars. It often blocks the drivers view but makes a striking appearance.
Tuning is a matter of matching jetting to the blower size and overdrive. There is a huge amount of power available from a relatively simple blower setup and a large amount of torque from idle on up. This type of fuel injection is easy to set up with a Roots or screw blower.
Frosting at idle from methanol fuels can change the idle speed if nozzles are located anywhere near the throttle plates, especially at humid, cool evening operation.
Mechanical fuel injection on a positive-displacement supercharger is not recommended for the street. However, again, with high-stall torque converters coupled with the broad torque curve from the blower, they are frequently driven on the street. Roots blowers are relatively inefficient, and highway fuel mileage in a high performance engine on the street may be only a couple miles-per-gallon.
These throttle assemblies are allowed in many drag racing modern and nostalgia classes, and in many top-speed event classes around the world. Additionally, they are common in many boat racing classes around the world. They are not common in most circle track racing, due to the inherent naturally aspirated nature of that form of racing.
These supercharged vertical fuel injection setups are one of the highest horsepower-per-dollar combinations of the mechanical fuel injection world. Throttle assemblies with the fuel system for the blower are reasonable in price, only hitting the wallet for a couple thousand dollars.
Roots blowers are reasonably priced and there are many used ones available as well. In addition to new systems, many blower component suppliers inventory a lot of low-cost used parts for sale for this combination.
NATURALLY ASPIRATED HAT-TYPE INJECTION WITH TUNNEL-RAM
Hat-type fuel injection with a tunnel ram, atop naturally aspirated engines is popular, and for good reason: the lower profile of the hat assembly provides more driver vision in many center-steering, front-engine configurations. Show-quality installations can feature well-finished tunnel-ram manifolds to further increase the visual appeal of this setup.
Gene Adams, a well-known fuel injection expert in drag racing, ran a back-to-back test. On a record-setting nostalgia nitro dragster, he compared a stack-type fuel injection setup to a tunnel-ram with a fuel injection hat. “The tunnel-ram and fuel injection hat on nitro made more power at all engine speeds,” according to Adams.
Tuning is similar to other naturally aspirated setups. However, large throttles and rapid vacuum loss in the manifold, when the throttle is slammed open, necessitates solid fuel transition circuit adjustment. A properly set up fuel injection system, idling on the rich side will not bog when the throttle is slammed open.
Lugging an engine with a clutch may be more troublesome with mechanical fuel injection because this setup does not have any fuel modulation with engine loading.
This type of fuel injection is probably less drivable at low-speed and part-throttle than other fuel injection configurations. For drag racing, it is usually coupled with a high-stall-speed torque converter. With full-throttle launches against a transbrake, response is recovered for quick 60-foot times.
The hat type is allowed in most motorsports classes that allow mechanical fuel injection, however in some circle track racing, the stack types may be specified.
When a cast tunnel-ram manifold is used, this is a lower-cost way to achieve a high-performance fuel system, even over premium carburetors. This is especially cost effective and very common for motorsports classes where methanol will be run.
HAT-TYPE INJECTION ON SUPERCHARGED ENGINES
Hat-type injection on a supercharged engine is a very common setup. The low profile of the hat helps with vision-limited vehicle configurations. The forward-facing throttle assembly lends itself to ram-air performance increases. It is also considered very visually appealing. This combination is one of the ultimate to satisfy the ego.
For tuning, jetting is again sized to the blower setup. For drag and top-speed event racing, the most common modes of operation are part-throttle, low-RPM to full-throttle, and high-RPM.
Racing automatics with torque converters work well with fuel injected, blown engines such as this. The torque multiplication from a typical racing torque converter provides a lot of tire spinning power.
Low-end torque is so great that a clutch would work well, although best with alcohol fuels that can be run rich without plug fouling. Imagine pulling a gear with a blown alcohol engine in front of a manual transmission! Our blown alcohol racecar idled at 2,300 rpm for the best drivability. You could not stall our engine at idle, even in gear with the brakes on. It would push through the brakes. Some race teams set up dual brake calipers to hold the car against a torque converter. Others use really sloppy converters with a lot of slip at idle in gear to hold the vehicle.
For boat racing, prop-slip makes this an easy combination to get working really well. Abundance of low-end power makes getting on-plane a breeze, even with a large diameter prop that lugs a marine engine.
Hat injection on a supercharged engine has very high horsepower capability. While it is most often run on methanol, blown fuel injection is sometimes run on gasoline or E85. Nostalgia nitro classes with 6-71 blowers reach 2,600 horsepower. Professional nitro classes with 14-71 blowers and very high volumes of fuel can exceed the 10,000 horsepower mark.
Blown fuel injection is not recommended for street use due to very poor mileage. However, due to high torque, a blown fuel-injected engine is quite streetable behind even a medium-stall-speed converter. We encountered a blown alcohol street machine a while back that averaged 2 miles per gallon. It had a 60 gallon fuel capacity on board – giving the car a bit over a 100-mile effective range.
Blown fuel injection is allowed in many nostalgia drag racing classes, current drag racing bracket classes, Top Alcohol, and Top Fuel eliminators. It is allowed in many top-speed event classes, as well as in many marine and drag racing classes. It does not appear in dirt or asphalt circle track racing.
This configuration generates a very high horsepower-per-dollar ratio, especially in the used parts market. Power levels exceeding 2,000 horsepower are easy to achieve from a blower and fuel injection setup that costs less than $5,000 using used parts. For a setup using all new parts, the costs increase but are still less than $8,000. For power levels approaching 3,000 horsepower, costs go a bit over $10,000 for a blower and fuel injection system on alcohol. For power levels up to about 5,000 horsepower, a moderate-percentage nitro setup with a moderate blower overdrive can be done for a modest price increase.
WRAPPING IT ALL UP
Constant flow, mechanical fuel injection for racing is a classic, natural combination. In the naturally aspirated engine world, cool-looking fuel injection configurations are easy to tune and make a lot of horsepower.
In the supercharged world, stand back. An abundance of torque from idle on up from a blower with a linear fuel injection fuel pump, linear blower air consumption, and linear horsepower curve make possibly the easiest racing configuration to set up and tune, ever.