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
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.
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.
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