Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
hot-film MAF sensor operates in a similar manner, featuring a heated film. One side of the film maintains a constant reference temperature, while the other side is exposed to intake air. The difference in temperature allows the ECU to determine incoming air quality. The ECU uses this information, in combination with other sensors, to determine fuel injection delivery.
Dirt, oil, spider webs, etc. can contaminate a MAF sensor wire (or film). If the MAF sensor is contaminated by oil, the cause is usually oil vapor from the PCV system in terms of blowby oil that makes its way past the piston rings.
A MAF sensor is located between the air intake/filter and the throttle body, and it measures the flow rate of air that enters an engine. Note the platinum wire in this MAF sensor. Foreign matter easily contaminates these fragile wires. Never assume that a MAF sensor is faulty. Inspect the contact terminals to make sure that the connector is fully seated. That is first thing you can do to make sure it is operating properly.
It’s recommended that you clean the MAF sensor every time you service your air filter. The MAF sensor is located in the engine air duct, between the air box and the throttle body. The sensor is generally secured with two screws. Disconnect the wiring harness connector from the sensor and remove the sensor. Do not touch the platinum wire or film with your fingers. This can leave residue on the sensor wire surface. Clean the sensor air passage and wire using only a MAF sensor cleaner spray. Don’t use a brake cleaner or other type of solvent, as this can damage or further contaminate the sensor. Once the sensor is dry, re-install it. A fouled MAF sensor can cause a rough idle, hesitation, or poor performance at wide-open throttle (WOT).
The formula below that can be used to estimate fuel injector rating, based on injector pounds-per-hour. While not having a direct effect on your exhaust system design, properly matching injector sizes to the engine is one step in optimizing power and efficiency. In an injected system, information obtained from the oxygen and air/fuel sensors in the exhaust stream is used by the ECU to help establish the required air/fuel mixture. If the injectors are not sized properly for the engine, efficiency and power isn’t maximized, regardless of the exhaust system design. One of the factors you consider is brake specific fuel consumption (BSFC).
This is the ratio between the engine’s fuel use and engine power output. At engine idle, the ratio is high due to the throttle being closed. At peak torque, BSFC is low during this point of maximum fuel efficiency. BSFC increases as the engine revs beyond peak torque toward maximum horsepower. BSFC is reduced as engine efficiency increases due to less friction and less drag, such as when the engine features lower tension piston rings, an improved oil scavenging system and/or when drainback coatings are applied to the crankshaft and connecting rods, the use of an electric water pump to reduce crankshaft drag, etc.
Another factor for calculating injector size is injector duty cycle. Most typically, this is about 80 percent.
Injector Rating = (engine HP × BSFC) ÷ (number of injectors × injector duty cycle)
For example, using the formula for an 8-cylinder engine that produces 500 hp and features a turbocharger:
500 × .5 = 250
8 injectors × .8 = 6.4
250 ÷ 6.4 = 39.06 Ibs/hr
In this example, fuel injectors with a rating of about 39 lbs/hr should be adequate and provide a good starting point. If the BSFC is .6, the suggested injector size is around 46 lbs/hr (500 × .6 ÷ 6.4 = 46.875).
Finding BSFC
BSFC can only be accurately determined on an engine dynamometer. However, the following is a rough estimate for illustrative purposes of typical BSFC ratios for various types of four-stroke automotive engines at peak horsepower:
| Engine Type | BSFC |
| Fuel injected and low compression | .4 |
| Fuel injected and high compression | .4 |
| Nitrous injection | .5 to .6 |
| Forced-induction | .5 to .7 |
The intake manifold directs airflow from the carburetor/throttle body to the cylinder head intake ports. The choice of intake manifold style and size of the runners affects the horsepower and torque range, influenced by factors such as cross sectional area and runner length. Generally speaking, a single-plane intake manifold is best for higher RPM power, while a dual-plane design is best suited for lower-RPM and torque at lower RPM.
In terms of the plenum location, again in general terms, taller runners, and higher plenum locations are best suited to accommodate higher engine RPM and power at higher RPM bands.
You always need to be aware; air/fuel that comes in must be able to get out of the engine so you must select your exhaust system according to these factors. Shorter exhaust primaries dump the engine exhaust pulses more quickly to better accommodate higher RPM, while longer primary tubes can improve exhaust scavenging to benefit a lower-RPM powerband. Header primary tube diameter and length affect how the exhaust gases are pulled out of the cylinder head exhaust ports, as scavenging of exhaust can create a vacuum effect, not only pulling exhaust out but aiding in pulling in additional intake charge.
A single-plane intake manifold features a single open plenum that feeds all eight cylinders. Runners are typically shorter than a dual-plane manifold for a more-direct shot at each cylinder, which is generally better for higher-RPM power. Single-plane manifolds also usually feature a bit of an air gap between the runners and block, which aids in reducing the air/fuel temperature. A single-plane manifold is likely a better choice as the engine usually runs above about 2,500 rpm. If you plan to operate the engine primarily at 2,500 rpm or less, a dual-plane manifold may be the better choice.
The configuration of the intake manifold runners directly affects power and where maximum torque occurs. Without discussing a specific engine application in terms of displacement, cylinder head design, and camshaft profile, I address intake manifolds in general terms. In addition to intake manifold runner length, you need to consider runner volume, including length and cross section area. A larger cross-section area and shorter runner length is better suited to higher-RPM ranges and larger-displacement engines, while a smaller cross-section area and longer runner length is better suited to smaller-displacement engines and lower-RPM ranges. Most engine intake manifolds are a compromise by design, a balance between engine power output and low-RPM torque to suite the intended application and driving conditions.