Properly matched camshafts, valvetrains, and cylinder heads with great flow characteristics don’t perform at their maximum potential unless you’re able to efficiently get sufficient air and the fuel to them in the correct amounts. Not coincidentally, the air intake and fuel systems are responsible for these duties.
This Tech Tip is From the Full Book, FORD MUSTANG 1964 1/2 – 1973: HOW TO BUILD & MODIFY. For a comprehensive guide on this entire subject you can visit this link:
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In this chapter, I discuss the path of the incoming air from where it enters the vehicle, through the throttle(s), and then into the cylinder heads and out the exhaust while also providing guidelines for each vehicle type. I address the fuel system in a similar manner; from the fuel tank to where the fuel enters the intake air. I concentrate mostly on carburetion because that’s what all of these cars originally came equipped with.
I also discuss some cost-effective options for electronic fuel injection (EFI) retrofits. (Completely custom EFI systems are not within the scope of this book and only provide a meaningful incremental benefit in competition/racing use.) The EFI systems I discuss are focused on providing better driveability, starting, and efficiency relative to carburetors while also having similar, if not better, performance for a reasonable cost, and with fairly easy installation.
The stock air intake systems of early Mustangs were, for the most part, not much to write home about. They were made for the masses with little attention to performance potential, except for the relatively rare performance engines not covered here. Most were a closedelement design with heated inlet air to help the engine run better when it was warming up in colder conditions. In warmer climates a common trick was to flip the lid upside down to improve airflow.
An open-element air cleaner was available in some cases, which allowed the engine to breathe better than the closed-element design. It was, however, still forced to ingest underhood air, which is hotter and less dense, therefore offsetting some of the benefit. The noise level also increased but that was usually considered a good thing.
One option in terms of stock setups was a cold-air/ram-air type such as those with dual snorkels, which were routed to the fenders to get colder outside air. Another option was the well-known shaker-style hood scoops, which pulled cold air into an enclosure around the carb(s) and sealed to the bottom of the hood. Retrofitting one of these to a car that didn’t have them, even a mildly modified daily driver, should improve performance noticeably, particularly under certain weather conditions.
Duplicating the OEM setup exactly can be costly but it ensures you’re a lot less likely to have issues with water ingestion, dirt, or other debris getting into the engine. There are many aftermarket variations of these options.
For a high-performance street car or a streetable track-day car, some form of intake upgrade is required to eliminate the restriction of the stock system and to have the higher flow you need for the higher power level. A minimally restrictive cold-air intake system that benefits from at least some ram effect/pressurization as vehicle speed increases allows all the other flow enhancements to realize their greatest potential.
Always use a less-restrictive but still effective permanent air filter (dry or oiled gauze, metal mesh, foam, or some combination of these) to avoid the performance loss that occurs as paper elements become dirty. Be sure to clean the filter when needed and carefully re-oil it if applicable. But remember that too much can cause issues with a mass airflow sensor (MAFS).
It’s generally not very beneficial to insulate the ducting from the cold-air inlet(s) to the engine but an open-mesh screen should always be used to keep any larger debris out of the engine. Make sure all of the joints are sealed but still leave a means for rain/water to escape.
Holley and other carburetors delivered fuel to many Ford small- and big-block engines. These ranged from a simple 1-barrel carb on 6-cylinders to dual 4-barrels as a dealer-installed option on some high-performance models. In most cases, a well-chosen single 4-barrel delivers just as much performance, or more, and it is a lot easier to tune and maintain while also being more driveable.
There are far too many carburetor options to provide a specific recommendation but there are some general things you can look for to help you make a better choice. Of these, the CFM rating (essentially the maximum amount of air it’s capable of flowing) of the carb is the first thing to nail down. Many people tend to think more is better; that putting a big carb on an engine always makes it perform better. Not true. Quite the contrary, actually. An oversized carb doesn’t improve performance but it significantly degrades driveability and fuel mileage while also making the engine harder to start and more prone to fouling.
It’s usually better to err on the side of caution and go with a slightly smaller carb rather than one that may be too big. Power may suffer very slightly but throttle response, driveability, and so forth is markedly better.
Selecting a Carburetor
Some very general guidelines for the CFM rating for a given small-block situation are that less than 600 cfm is almost always more than enough for a mildly modified daily driver. For a street-performance car with mild to moderate mods a 600-cfm carb works in some cases but a 680- to 700-cfm carb is probably called for when there are a few more modifications. By the time you get into upgrading the cylinder heads, cam, and so forth you’re probably ready for a 750-cfm carb. This is also good for most streetable track-day cars of about 600 hp or so. If you have a highly modified street-performance car with larger displacement or a streetable track-day car with more than 600 hp you may want to look at going to a carb with more than 800 cfm.
Other things you must consider are engine size, maximum RPM, level of modification, fuel, power adder usage, vehicle weight, transmission type, and gear ratios. Considering at least these main factors help ensure a wiser choice.
The brand of carburetor is less important than its features. Some are generally only needed for street use (such as a choke) while others are meant solely for the track (extra tunability provisions such as screw-in air bleeds, plus easy assembly and disassembly features, etc.).
A daily-driver carb needs to have all of the provisions in terms of vacuum and PCV connections, a choke, a simple idle circuit, and vacuum secondaries. The choke can be electric (for simplicity) or manual (for greater control). There’s no reason to try to reuse the OEM choke setup because the electric choke works much better and is far easier to adjust.
A high-performance street car likely has an electric choke and adjustable vacuum secondaries at the lower end of the power range and probably migrates to mechanical secondaries and a manual choke at the higher end. In most cases, a more sophisticated “four-corner” idle setup probably provides the extra adjustability needed for a stable idle with the more aggressive camshaft likely being used. The materials used are also better (billet aluminum and higher-quality gaskets, floats, and so forth), as is the design of other components such as the metering blocks, power valves, and boosters.
A streetable track-day car is usually just a larger evolution of what is used for a high-end street performance carb. It has a higher CFM rating, a manual choke (if any) plus all of the adjustability, tuning, and design features needed for optimum performance and durability during track use. It also very likely uses mechanical (instead of vacuum) secondaries and special bowl and float designs.
Some of these features may also be common to some highperformance carbs.
After the air and fuel have been introduced to each other in the carburetor the mixture passes into the intake manifold and makes its way to each cylinder. Although this sounds relatively simple, it is not. First and foremost, you ideally want each cylinder to receive the same amount of air/fuel mixture for each combustion event. This is impossible for a number of reasons so you just try to come as close as you can.
On a relatively well-designed, single, 4-barrel manifold you can get down to about a .5 air/fuel ratio spread across all eight cylinders under peak power conditions. This varies greatly with engine speed and load for no other reason than the differing positions of the throttle plates in the carb(s) affects the path of the mixture. The end result is that you have to tune (jet) for the leanest (weakest) cylinder to prevent detonation, which makes the other cylinders richer than you want. That lowers peak performance and wastes fuel (lower MPG).
Aftermarket manufacturers do their best to design intake manifolds with even flow distribution but they are limited by the location of the carb(s) versus the intake ports on the head as well as the size and shape of the intake runners of the heads. A single-plane manifold may have a straighter shot at each port but that doesn’t always ensure more even distribution. When a dualplane manifold is used to improve low-end performance the increased complexity inherent in the design can also make balanced distribution more difficult to achieve. The higher-flow velocity of the dualplane manifold (due to normally lower volume runners, among other things) can help atomization in general but not necessarily distribution.
Daily drivers and mildly to moderately modified high-performance street cars normally use a dual-plane manifold because this design is best for the 1,500- to 6,500-rpm range in which they are mostly operated. A more heavily modified highperformance street car and a streetable track-day car usually benefits from a single-plane design to get the more even distribution and better performance in a range of about 3,000 to 7,500 rpm.
Pure race manifolds, which may be hard to tolerate in street use, can go beyond this RPM level but are only appropriate for the highest power levels when there is little concern about significant street use.
The conversion from carburetion to fuel injection offers many potential benefits. Among them are better mixture distribution, idle, throttle response, driveability, and gas mileage, plus lower pollution. The knock on fuel injection has been that it was complicated, costly, and didn’t make as much power as carbs. It’s safe to say these concerns have been almost completely eliminated with modern aftermarket EFI systems. (I don’t discuss mechanical fuel injection because there’s little, if any, benefit for street use and the cost is usually high.) EFI systems are now very simple to install and can even be “self learning” to a large degree so the task of tuning is greatly simplified.
Many systems just involve the removal of the carburetor and fuel pump so they can be replaced with a higher-pressure electric pump, a different fuel-pressure regulator, and components such as a throttle body (or multiples) and sensors. The simplest systems look very similar to a regular 4-barrel carb and even use a similar-style air cleaner. Most of the electronics are tucked out of sight so the original underhood appearance changes very little in many cases. At least one oxygen sensor needs to be installed in the exhaust and the fuel line usually needs to be upgraded with a higher pressure rating.
With many systems a laptop or other computer isn’t needed because the systems are self-learning. If you like to tune, the possibility of using a laptop is still generally available. These throttle-body systems have an air/fuel mixture flowing through the intake manifold so they are still subject to less-than-perfect balance between them.
However, the fuel is better atomized by the fuel injectors integrated into the throttle body and the fuel metering is much more precise and adaptive to temperature, barometric pressure, and numerous other parameters that are ignored by a carburetor. This allows the EFI system to continually re-optimize the mixture to compensate for changes in these factors while also continually monitoring the effects of the changes made through the oxygen sensor.
To get the full benefit of EFI requires that you no longer inject fuel into the manifold like a carburetor. Instead, you point a fuel injector at each intake valve and spray the fuel directly into the intake port. This does several good things. First, by only flowing air through the intake manifold the distribution is instantly better because air, being a gas, is naturally inclined to distribute itself more equally. In addition, spraying the fuel right at the valve means there’s no chance of the fuel puddling on the floor of the intake manifold or wetting the walls of the runners to any significant degree. This also helps distribution as well as overall metering. Finally, if you fire the injectors individually (sequentially, as opposed to groups) you can also realize better throttle response and further improvements in overall fuel control.
Modern fuel injectors running at higher pressures do an excellent job of atomizing the fuel into a fog that’s more easily burned. Individual injectors at each port are better than fewer larger ones farther upstream because each one has to deliver less fuel and can thus be more precise and responsive. As the oxygen sensors read the composition of the exhaust gases they can make the necessary corrections to the fuel flow more quickly and more accurately by doing so one cylinder at a time. Combined with the same ability to compensate for other factors such as temperature and pressure, a sequential EFI system provides the potential for extremely accurate, precise, and adaptive fuel control with built-in learning capability and diagnostics for a very reasonable cost.
Engine Control Unit
The amount of fuel to be injected into the engine depends on the amount of air it ingests, just as with a carburetor. With EFI, however, the amount of air is just input data to the engine control unit (ECU), which is used to determine how long each injector stays open when it fires, thus determining how much fuel is injected.
There are two main ways to provide this data to the ECU: Measure it directly or calculate it based on other information. The first approach requires the use of a MAFS, which uses a very ingenious method of measuring air mass directly, based on how much electricity it takes to maintain a constant temperature on a wire that’s been placed in the airflow path. This has the advantage of being a direct measurement instead of a calculation. It also is able to compensate for modifications that improve airflow as you make them.
If you put in that new cam or exhaust system and the engine flows more air as a result the MAFS sees it and the ECU can act accordingly. Similarly, if the engine wears a bit and flows less air it can take care of that too. This ability to continually measure and correct the airflow is one of the main reasons Ford, for one, uses a sequential MAFS-based control system in virtually all of its new US production vehicles (at the time of publication).
The knock against this approach is you must have a MAFS and place it somewhere close to the engine. It also needs to have a certain length of straight ducting immediately after it to ensure an accurate measurement. This need to have/position a MAFS and use a single inlet duct for the intake air can sometimes be problematic. Still, for the majority of high-performance street cars a sequential MAFS-type system provides the best overall performance and driveability for a reasonable cost. Fortunately, the EEC-IV system used by Ford on production V-8 Mustangs from 1989 to 1995 lends itself very well to retrofitting older carbureted Mustangs with EFI.
These systems can very easily be adapted to older vehicles. When supplemented with aftermarket performance parts they produce very high power levels with better performance than most aftermarket systems. Better still, as modifications continue to be made and/or other changes occur the system can readily be retuned as needed by simply swapping in higher flowing fuel injectors and then recalibrating the ECU. Products such as the TwEECer make the latter process much simpler by either interfacing directly with the EEC-IV ECU to selectively modify its input and output signals or by using a special/digital MAFS made by Abaco Performance.
The Abaco MAFS has the ability to be programmed as needed to revise its internal “transfer function,” which is used to provide air mass data to the ECU. This is a better approach than having to get a new MAFS or having the one you have recalibrated by the manufacturer. The Abaco MAFS can be reprogrammed as often as you need for best performance, whereas regular analog MAFSs must have a specific combination.
Racers prefer the second approach because they don’t usually want the restriction of a single-duct MAFS system, however slight it might be. Many aftermarket manufacturers, and even some OEMs, prefer to calculate the air mass using the “speed density” method. You calculate air mass based on measurements of engine RPM and manifold pressure along with a volumetric efficiency correction factor determined during the calibration process. This gets you pretty close (at least during steady running); then you still use the oxygen sensor data to refine your calculation.
Speed density systems don’t have the requirement of using a MAFS or needing straight ducting, etc. This makes them better suited to crowded underhood areas and the use of more complicated intake manifolds (such as multiple carbs) and/or a power adder with positive manifold pressures. A speed density system can make virtually any amount of power, whereas a MAFS system eventually becomes impractical beyond a certain (though still very high) level.
At the highest levels, say 800 hp or more, the speed density system may make a bit more power. This can matter to a racer, but it’s usually not worth the tradeoff in terms of driveability and long-term adaptability, if the car is still driven on the street. Racers are constantly tuning their cars anyway so they are recalibrated on a frequent basis to compensate for changes. For a high-performance street car or a streetable track-day car this level of interaction may not be desirable or practical.
Whether you use a carbureted or fuel-injected approach you still need to pay attention to the details to get maximum performance, especially with a lot of modifications. An incorrect manifold gasket or cheap fasteners, for example, can negate all of the improvements you’re attempting to make, or even result in expensive damage to your newly improved engine. One precaution that can be taken (especially if there are no special features such as sealing beads, etc., on the gasket) is to apply a thin film of RTV or another suitable sealant around the ports and water passages. Be careful not to apply too much; you don’t want any of the excess getting into the wrong places. This better holds the gasket in place during assembly and further reduces the chance for leaks with negligible risk of other problems.
Stainless steel ARP fasteners for the intake manifold and other external engine components can be part of a complete engine kit or can be purchased separately. In addition to not corroding they are stronger, have a more compact head design for easier access, and were manufactured to tighter tolerances than cheaper bolts. Reliably getting the fuel to the carb or EFI system in sufficient quantity under all conditions is also necessary for maximum performance. As power levels increase so does the amount of fuel that must be transferred. Likewise, higher acceleration and cornering forces cause more movement of the fuel within the tank, thus potentially moving the fuel away from the pickup. Under some conditions it can suck vapor instead of liquid, with the expected negative results.
Although you always want to be able to use all of the fuel in your tank, with EFI it’s also critical to preventing damage to the fuel pump and other components. That’s why virtually all modern production vehicles equipped with EFI have fuel tanks with internal sumps and/or baffling to keep fuel around the pickup as much as possible. In a high-performance street car or streetable trackday car it will inevitably be necessary to upgrade the fuel supply system to better cope with the need for higher fuel flow under more-extreme dynamic conditions.
The stock fuel tank and lines are usually sufficient for a daily driver unless it’s been more extensively modified to produce significantly more engine power or generate much higher cornering forces. When an upgrade is required it normally involves installing a new tank with an internal sump and/or baffles such a those made by Tanks, Inc. When much-higher performance levels and/or event rules require, a fuel cell may be needed for safety and better fuel control. In any case, care must be taken to ensure the fuel pickup is also upgraded to reduce restriction to the fuel pump inlet. Not doing so can reduce the fuel flow rate significantly and also cause pump damage or failure.
A higher-output fuel pump (such as those produced in the United States from Aeromotive) is needed along with upgraded fuel lines, filters, and fittings, etc., to reliably provide the needed fuel. An in-tank pump is normally adequate for most street-driven vehicles while a larger capacity external pump is needed at the higher power levels and/or when a power adder is used.
Nitrous Oxide Injection
Once you’ve upgraded the fuel supply system to get more fuel to the engine you may also want to consider adding a simple and relatively inexpensive way to allow the engine to use the extra fuel supply to provide even more power, at least for brief intervals. This can be done very effectively with the installation of a nitrous oxide injection system.
Supercharging and turbocharging are other examples of “power adders” that provide extra air mass, thus allowing more fuel to be burned to create more power. These are not discussed here due to their high cost and frequent need for custom installation procedures. Besides, the injection of pressurized nitrous oxide along with additional fuel can provide tremendous increases in power for short periods of time. This is because nitrous contains much more oxygen per unit mass than does air so injecting it instantly adds additional oxygen into the combustion chamber, which in turn allows more fuel to be consumed. This creates higher cylinder pressures that translate to more power.
Nitrous can be injected into each intake runner or from a plate underneath the throttle body/carburetor. It can be introduced with the additional fuel (a “wet” system) or it can be injected alone with the additional fuel coming from elsewhere, such as the fuel injectors on an EFI engine (a “dry” system). It can also be injected in multiple locations and in multiple stages to achieve different performance levels and benefits based on intended use.
In drag racing, for example, a two-stage system may inject a smaller amount during the beginning of a run to help prevent excessive wheelspin and then add the remaining amount later in the run once more-stable traction has been achieved. The Nitrous Express Gemini Plate System uses a specially designed, billet Spraybarless plate mounted underneath the carb. It provides 50 to 500 additional horsepower for a limited time. This superior design provides exceptionally balanced distribution along with excellent atomization to deliver maximum performance and efficiency. The high-quality components provide the required flow and have exceptional durability.
Regardless of the type of system chosen, the mounting position of the bottle is critical to getting all the nitrous out of it. The valve end must be raised and the bottle lined up along the centerline of the car so liquid nitrous covers the internal pickup tube (at the bottom of the bottle) even under hard acceleration. Never mount the bottle sideways (90 degrees to the centerline) or level.
Water/methanol injection systems allow increased ignition timing and/or compression ratios, thus improving performance. In essence this can compensate to a degree for inadequate fuel quality and/or octane. This is particularly true in certain areas of the country, such as in California, because these systems can be used to compensate for the availability of only 91-octane premium pump gas. The injection of a water and methanol mixture (along with the potential addition of other additives such as nitromethane) primarily has the effect of cooling the intake charge through the process of converting the injected water into steam during combustion. This lowers the peak combustion temperature while also increasing the burn rate due to the increased surface area of the fuel droplets coating the water droplets.
When a combustible liquid such as methanol is included in the mix the additional energy from its combustion is added to that from the combustion of the primary fuel mixture. This results in a greater total release of energy over a longer period of time and the extra performance comes without the higher pressure spikes that lead to detonation.
One of the more sophisticated injection systems is manufactured in the United States by Snow Performance. This Stage-3 system uses true 2D mapping to ensure the precise delivery of the correct amount of liquid under very high pressure so it is more evenly distributed throughout the intake charge and is more quickly and completely ignited.
A unique control unit allows for mapping based on manifold pressure and fuel injector pulse width (in the case of EFI) in any proportion desired by the user. The control unit can also be programmed to operate only under specific conditions and to also provide a warning should the included reservoir run out of liquid. These features allow for extremely accurate metering of the injected liquid, which can reduce the amount of liquid used and/or needed to achieve the desired result. This generally yields better performance in the low- and mid-range load points because they are not receiving more liquid than is really needed, as is common with less-capable systems.
Even after you’ve made all the upgrades to the air intake and fuel systems you can never be totally safe from mishaps that are beyond your control and unexpected. Being able to watch what’s going on with your air/fuel ratio on the road while datalogging it with some other parameters (such as RPM, boost, etc.) for subsequent evaluation can help. The ability to immediately act on it to protect your engine is even better.
AEM Performance Electronics has developed just such a device for any vehicle: the universal Digital Wideband Failsafe Gauge. It provides a highly accurate Bosch wideband oxygen sensor (which never requires any open-air calibration to be performed) and an internal datalogger capable of storing up to three hours of data for subsequent playback using the included software.
The gauge can simultaneously display boost pressure for pressurized cars. Best of all, the gauge can be programmed to provide a warning and an output signal that can be used to retard spark if the air/fuel ratio or manifold pressure/vacuum falls outside of the ranges that you’ve programmed into it. There’s also a trigger function to begin datalogging automatically.
After upgrading your engine to get more air and fuel into it you also need to upgrade your exhaust to get the extra spent gases out of it. For a daily driver this may just mean converting to dual exhausts and/ or putting on a less-restrictive set of mufflers and at most, maybe a set of headers to go with them.
For high-performance street cars and streetable track-day cars, however, a complete makeover is needed. Functionality, performance, and durability under more extreme use are the main priorities. For most street cars a 2.5-inch exhaust should be used.
The streetable track car usually has larger (more than 3 inches) tubing and less concern over the noise level or appearance because the car also probably sits a bit lower and has a lot of undercar reinforcements such as subframe connectors. It may also be necessary to use oval instead of round tubing to maintain sufficient ground clearance. It’s also more likely that the exhaust terminates before the rear axle to not only reduce weight and restriction but also to avoid interference with the axle and suspension.
For example, Doug’s headers are manufactured with a machined sealing bead and a thick flange, and are stitch welded at the port. To minimize exhaust leaks the company includes a set of its own proprietary header flange gaskets so all of that good work doesn’t go to waste if inferior gaskets are used. Doug’s gaskets are super thick and made from a specially formulated material that can withstand temperatures up to 1,100 degrees F as well as up to 3,700 psig/255 bar of exhaust pressure. Each gasket is also precisely matched to the port shape of its respective header.
Written by Frank Bohanan and Posted with Permission of CarTechBooks