Читать книгу BMW 3-Series (E36) 1992-1999 - Eddie Nakato - Страница 10
ОглавлениеINTAKE MODIFICATIONS AND FORCED INDUCTION
At its most basic level, an internal combustion engine is an air pump. Air is sucked into the front and pushed out the back. In this chapter, I examine how air is sucked in through the intake and air filter to the intake manifold and the combustion chambers. In the case of turbo-charged and supercharged engines, I discuss how air is pulled through the turbocharger, then pushed through the intercooler into the engine.
Performance enthusiasts should never lose sight of the fact that all of the air that the engine is going to use to make horsepower has to come in through the air filter, and has to go out through the tailpipe. You need to remove horsepower-robbing restrictions from the entire path. If you were watering your lawn and realized not much water was coming out, and you looked at the length of hose and realized it was kinked in the middle, you’d unkink it and fix the problem, right? That’s basically what you have to do with the engine’s intake and exhaust systems. You must figure out where the kink in the hose is (where the restriction is happening) and correct it. That’s the key to performance.
Here’s a great example of a cold-air box; air is ducted into this box from outside the car. It has no chance to heat up next to the engine before it is used. This means the air is denser and helps make more power. This can be a real improvement.
The cold-air intake is simply better for filtering air and getting it into the engine. The stock system was designed to be inexpensive to produce, and to keep water and particles out of the engine no matter what you do to the car. The CAI is designed to be efficient. The materials don’t matter to the function, but that carbon fiber sure looks great.
A turbocharger offers a big step up in performance, but it costs a lot of money and you’re way out of the safety zone with any kind of forced induction. Plan on spending a lot of money if you go this route.
Within reasonable limits, all products and upgrades discussed in this chapter are aftermarket solutions applicable to naturally aspirated E36 engines. No North American–market E36 models used forced induction from the factory, so any such modifications depend entirely on aftermarket parts. Those projects are generally too involved for the home mechanic to undertake, so although I discuss at them, I didn’t attempt them myself.
An internal combustion engine is a marvelous piece of technology. For the most part, modern engines run longer, smoother, more reliably, and produce more power per cubic centimeter of displacement than any mass-produced engines ever sold to the public before.
A bewildering array of products claim better flow, more pounds per square inch (PSI) of boost, and a cooler intake charge. Generally, more power is what you’re after. But before you dive into your project engine and install a bunch of parts, you need to develop a build-up plan, because each part affects the function of other parts. The engine operates as a system and its parts are interdependent.
And because an engine is a system based on the flow of gases, the tightest point in the system generally governs the total output. The classic demonstration is to attempt to breathe through a drinking straw. It doesn’t matter that there’s a room full of air, or that your mouth could gulp in a lot more air; the total oxygen available in that case is what you can suck through that straw.
What this means in real terms is that you may see an incremental improvement in power by relieving a restrictive component in the system, but real power gains require thoughtful modification to the entire system for the most efficient flow and greater energy output.
The Science of Combustion
A tablespoon of gasoline and a quart of air have a finite amount of energy potential held within them. We can change that potential into different forms of energy such as heat, motion, and light by putting the fuel and air into an internal combustion engine. You can theoretically create perfectly efficient engines and drivetrains, but you can never get more energy out of that spoonful of gas and bottle of air than the native elements hold. So to make more power in your car, you have to put more of those elements through the system, and make the system as efficient as possible.
Inefficiency can include such flaws as not burning all of the fuel you put into the combustion chamber. These are the “unburned hydrocarbons” that are measured in many emissions tests. This happens when the air/fuel mixture is incorrect. The theoretically perfect mixture is called stoichiometric and is about 14.7 parts air to 1 part gasoline. The reason you care is that if you have an imperfect mixture, you’re not getting all the energy you can out of the fuel and air.
To that end, everything you change is about putting more air and fuel through the system. I discuss replacing the stock air intake and filtration with a more direct and less restrictive product, and how to make that air flow faster into the engine with the use of a more advantageous intake manifold. Finally, I look at aftermarket solutions to change the engine to forced induction, with the caveat that such modifications are typically far more expensive than simply installing a better engine or purchasing a later model 3-series that was turbocharged from the factory.
The easiest step in improving engine performance is to look at your E36’s air intake. You want the least restrictive system you can possibly find that provides clean cold air to the engine. You really do need a filter, though, because all kinds of stuff can be sucked into the engine otherwise. The front end of every car is sprayed with road debris all the time. Nothing good has ever been reported about feeding dirty water and rock chips into a BMW engine.
Here’s what most stock air filters look like after a few years. That slab of paper never flowed very well, and now it’s all clogged up. Time to replace the whole system.
You can gain about 4 hp and 4 ft-lbs of torque here, and maybe a little more. If you do nothing more than purchase an aftermarket high-flow air filter, you will see some benefit. But for just about $100, you can take the next step to a high-performance engine.
Another improvement that goes with an improved air filter is what racers call a cold-air box. If you remove the stock airbox, a bare aftermarket air filter pulls in air that has passed through your car’s radiator or intercooler (or both) and is hanging around the hot engine bay. To bring in truly fresh air requires some ducting and separation of the intake point from the rest of the engine bay; do this right and it sets you apart in both performance and engine bay dress-up.
Here’s another kind of cold-air box. This one relies on the hood being closed to seal the air intake filter off from the rest of the engine bay.
Cold-air intake kits are available from virtually every E36 aftermarket manufacturer.
Mass Airflow Sensor
All E36 models have mass airflow (MAF) sensors in their intake flow. The MAF sensor is a delicate little device that tells the DME how much air is coming in through the air filter. Positioning the sensor is crucial if you want an accurate reading; and you do want an accurate reading or the mixture will be wrong. E36 MAF sensors use a “hot wire” design that measures the amount of air passing through by the “windchill” on the filament. A secondary intake air temperature sensor corrects the MAF output for the temperature of the incoming air. Because the MAF output depends on accurate cooling of the hot wire, these devices are very susceptible to dirt and oil. It is vital that you use a high-quality air filter (and don’t over-wet the oiled varieties!) to keep the MAF clean and functioning correctly.
The MAF sensor measures airflow by the cooling of the hot wire, and the DME calculates how much air it has by assuming that you’re using the stock intake. If you change to an intake that is even slightly smaller or larger, the DME does not make the correct calculation because the DME works based on the stock intake pipe diameter. If you put on a larger diameter pipe, it flows more air than the DME is calibrated for, so the mixture is lean. A smaller pipe flows less air than the DME is calibrated for, and the mixture is rich. That’s why aftermarket intakes tend to end right before the section of pipe that contains the MAF sensor. After the MAF sensor portion of the intake, the air passes into the intake manifold, which I discuss separately.
So, to choose a cold-air intake (CAI), you’re mainly looking at where the intake gets its air and the flow capacity of the filter. Any of the available cold-air intakes on the market flow enough extra air over the stock unit to give you all the benefit you need. I selected the most inexpensive CAI I could find. As with the cat-back exhaust, there’s only an incremental power gain to be found here. Once you have smoothed the intake path, brought cold air to the intake, and installed a clean, new filter, the rest of the differences between individual units are comparatively small.
The mass airflow (MAF) sensor is one of the most important parts of the car. It measures how much air is entering the engine, and that allows the DME to decide how much fuel to offer. If the MAF becomes dirty or the size of the tube in which it sits is changed, it cannot accurately report airflow, and the DME cannot adjust the timing curve or fuel map to optimize performance.
You can confidently select from any of the popular CAI units on the market. Brands such as Dinan, BMP Design, Injen, Active Autowerke, AFE, and Eurosport all make good products that give you about the same benefits.
Project: Upgrading an Air Intake
For this procedure I installed Injen’s cold-air intake for the E36. This kit retails online for $130 to $150 and includes an air filter, a tube that leads to the MAF sensor tube, some installation brackets, and a heat shield. The whole system installs in the same part of the engine bay where the stock airbox goes.
Follow These Steps
Disconnect the battery. This is not to prevent shock so much as to re-initialize the DME. You should not be close to any wiring in this project.
Tip: Cover the engine with a blanket when working with the retainers and screws on the under-hood duct (or any time you’re working in the engine bay). A dropped washer or screw can become wedged among all the parts on the engine and cause a world of trouble.
Remove the air inlet from the air filter box. This flattened tube brings air in from behind the headlights and simply disconnects from the air filter box. This part can be stored or discarded because you won’t use it again.
Then remove the stock airbox. Two spring-clips hold the airbox to the MAF sensor tube. Undo a mounting bolt on the right side of the box as you’re looking at it from the front of the car and the entire box comes out as one piece.
The stock air box and MAF. You can find a few horsepower here by changing to a nice cold-air intake setup, and give yourself a little more space in the engine bay as well.
This snorkel delivers air from just under the hood to the stock air intake box. Remove it, because you have plenty of cold air through the grille.
Here are the parts to the Injen cold-air intake. I bought it on eBay. The smooth metal tube, cone filter, silicone fitting, and heat shield are all good quality parts.
Install the new heat shielding as instructed for the kit. The heat shield mounts to the chassis with some self-tapping screws and uses the existing support studs for the stock airbox. I drilled starter holes for the new screws to ease the installation. In the case of the Injen kit, I found that the edge of the heat shield made contact with the upper radiator hose. To eliminate the risk of abrasion, I trimmed back the heat shield to clear the radiator hose by a few millimeters.
Do not discard the heat shielding; bringing in cool air is one of the most important functions of the intake, as cool air is much denser than hot air. If anything, you may want to fabricate additional pieces to further isolate the intake from any air that has passed through the radiator or had a chance to heat in the engine bay.
The heat shield installs to the chassis rail with these self-tapping metal screws. They have a little drill bit in the point to help them start.
Some of these holes line up with studs that are already on the chassis rail, and some use self-tapping screws to fasten the shield.
Here you can see the heat shield installed with the self-tapping screws and the stud that attached the stock airbox.
I found that the heat shield wanted to rub against the radiator hose, which could lead to a cut hose down the road. So I used the bench grinder to create a little more clearance for the hose.
The heat shield has been bent in a sheet metal brake to fit into its space perfectly.
There’s the hose with the modified heat shield. It could use a little more clearance toward the top.
With the clearance to the hose fixed, the heat shield is installed along with the silicone fitting to the MAF tube.
Place the rubber adapter around the end of the new intake tube and install the two large hose clamps loosely. Install the short ram tube by slipping it over the MAF sensor tube. Turn the new intake tube to orient the air filter end with plenty of space around it, and to match to the support bracket. This bracket mounts on the fender wall where the old airbox also mounted.
Carefully fit the new intake tube snugly against the MAF sensor tube. You know if it is not snug because it is not even all around. When oriented, install the mounting bracket and tighten its nuts and bolts, and tighten the hose clamps.
The silicone connector fitting holds the MAF tube on one side and the cold-air intake tube on the other. Hose clamps keep everything in place.
The cold-air intake tube has a bracket that helps support its weight, and it attaches to the same place the stock unit used for support.
Install the new air filter. Virtually all aftermarket cone filters come pre-oiled from the factory, so you don’t need to add oil. But take a moment to note the maintenance schedule for the filter so you can clean and oil it (or replace it) as needed. Use the hose clamp that came with the air filter to tighten it to the intake tube.
This cone filter offers a lot of surface area, and it’s out in the open so you can see when it becomes dirty. Some cone filters can be washed; read the instructions on your kit for cleaning information.
I found that one extra bolt was needed to hold up the other stock components that relied on the original air intake bracket for support. A used 10-mm bolt, nut, and washer from the metric fasteners bin did the trick.
Clean the engine bay of tools and any loose hardware. If you use a towel or blanket to protect the engine from loose parts, remove this as well. Reconnect the battery and turn the car’s key to the “on” position for 10 seconds to re-initialize the DME. Then start the car and carefully inspect the engine bay for good connections and correct operation.
Tip: Carefully inspect your work before you test-drive the car. With all the manipulation you performed, it’s easy for a connection to come loose. Any disconnected tube can cause a check engine light to come on shortly after you start the car. If this happens, check the intake air path and make sure all tubes are properly connected. If you find and refit a disconnected tube, the light may remain on for a few restarts. If the light does not go off soon, seek professional help.
The mounting bracket is not optional; without it the weight of the filter and vibration of the car would shake the filter and intake tube loose in minutes.
Our cold-air intake looks great installed and brings a bit of color to the engine bay as well. It sits right in the airflow through the grille and is protected from the worst of the engine heat by the heat shield.
Project Results
