Internal combustion engines are air pumps and how much power they make is determined mostly by how much air they can get into the cylinders during each intake stroke. How much air the cylinders can intake is determined in large part by the intake design and BMW intake systems have historically featured some very advanced technology, in this article I will go into detail on what sets BMW intakes apart and the evolution of BMW intake systems from the vintage period to today, but with a focus on the systems from 2001 and newer
I want to try to make this information approachable to all so we will start with a basic review of how an engine operates, below is labelled cutaway diagram of an engine
The piston moves up and down in the sealed cylinder chamber and there are four strokes to a typical automotive combustion engine
Intake Stroke - the intake valve opens as the piston travels down which creates a vacuum which draws air (which is mixed with fuel in port injection or carburated engines) into the cylinder chamber
Compression Stroke - the intake valve closes and the piston moves up, compressing the air (or air/fuel mixture) until the piston reaches TDC (at this point, in a direct injection engine, the fuel injector will fire)
Power Stroke - a spark plug ignites the compressed air fuel mixture, combusting it. It is a controlled explosion - the force of the expansion from the combustion force the piston down which rotates the crankshaft (not pictured) which is how the engine makes power
Exhaust stroke - the exhaust valve opens after the engine reaches TDC, and as the piston moves upwards it forces the exhaust gas out of the cylinder
This process looks like the below .gif
This article will be focusing on the intake side of things as the exhaust is not as technologically advanced as the intake systems. Exhausts are fairly limited in what they can do, with the most important consideration being scavenging - which essentially means tuning the exhaust pipe length and diameter in order to allow the high velocity gas from one cylinder's exhaust pulse to draw a vacuum in another cylinder's exhaust pipe so that as that next cylinder opens the exhaust valve, there is a vacuum in it's exhaust pipe, helping draw the exhaust air out of the cylinder quickly. The less exhaust that remains in the cylinder, the cooler the intake charge will be (thus the more it can be compressed before ignition for greater power) and more of the cylinder's volume becomes available for fresh air/fuel mixture instead of remnant exhaust gas. Other than that, you want an exhaust to be as free flowing as possible. Backpressure, despite popular forum folklore, is not a good thing. I believe some people conflate back pressure with scavenging effectiveness, however they are distinctly different principles. Scavenging is good, backpressure is bad.
Now, now that we understand how an engine operates lets take a step back...
Up until the early '80s, most BMW's were carbureted. This meant that a carburetor supplied an air/fuel mixture to the engine at a set rate depending on throttle position. The intake air (blue) mixes with the fuel (yellow) and the air/fuel mixture (green) reaches the cylinders
Carbureted intake designs are not ideal for a few reasons - the main reason is that it is difficult to control the exact amount of air/fuel mixture going into each cylinder. There is also perceptible throttle lag with these setups, as there is a delay between pressing the throttle and the outside air passing the throttle plate and then making it to the cylinder
Both those issues can be somewhat alleviated with the use of more carburetors. The more carbs you have, the shorter the distance to the cylinder from the throttle and the more control you have over the specific amount of air and thus fuel making it to each cylinder
Most of the technology around carbureted engines went into combustion chamber design, with the M30 in particular being a triple hemispheric combustion chamber design. The goal of introducing a hemispheric design into the head is so that as the engine is compressing the air/fuel mixture, the air has 3 hemispheric pockets to help swirl and mix the air, as a more homogenous air/fuel mixture burns more efficiently than a less well mixed air/fuel charge.
This setup is still not ideal though as the amount of fuel entering the cylinders is directly proportional to the throttle position. Adding more carbs also introduces more expense and complexity. The automotive industry as a whole therefore moved from carbureters to fuel injection. With fuel injection systems, the fuel is injected much closer to the engine. This allows for increased performance and efficiency as only the fuel required for combustion is injected into the engine. Fuel injection setups also allow for variable amounts of fuel for a given RPM, allowing engines to achieve good power figures while still getting reasonable fuel efficiency when cruising. Below we see a traditional throttle body setup, with the throttle body mounted at the entry point of the intake manifold. This provides relatively poor throttle response due to the distance from throttle plate to the cylinders and because the manifold will remain under vacuum until the throttle is opened, meaning the pressure takes a moment to equalize before the manifold is filled with air
An age-old solution to this problem is similar to what we looked at above with the additional carbs and that is individual throttle bodies or ITB's. Engines with ITB's have the throttle bodies placed directly in front of each cylinder. This allows for very fast throttle response as the air in the manifold will already be at an equalized pressure and because the throttle plate is located very close to the cylinder, meaning it is very quick to fill the cylinder from the time the pedal is pressed. ITB's were a staple of naturally aspirated BMW M series engines from the late '90s up until they went turbo. However, despite not using ITB's the modern M turbo engines did not sacrifice much in the way of throttle response due to another technology I will talk about later in this article which was used on every M turbo engine except the first generation S63 V8
ITB's were cost prohibitive for non-M engines, so BMW continued to develop the traditional style intake manifold, creating the DISA manifold. The DISA intake manifold features two sets of intake runners for each cylinder and uses an electronically controlled DISA valve - which is essentially a second throttle body - to control which path the air follows. The first use of this technology on a BMW engine was the M44, before being used on the M52TU and then the M54. This technology allows for the benefits of a short intake runner manifold with the benefits of a longer intake runner manifold. The runner lengths and manifold volume are optimized for each specific engine. A later evolution of this technology featured a three-stage DISA, with three different length intake runners, as featured on the N52. I will talk about that intake manifold a little bit later as due to the very large intake manifold volume, it would seem as though it would introduce a lot of throttle lag, however due to another BMW technology utilized on the N52, this ends up not being the case
Above we see the DISA valve closed, forcing air to travel through the long intake runner. Once the DISA valve opens, as seen below, the intake has a shorter path it can follow, which helps cylinder filling at high rpm, increasing top end power
The N62 is the last naturally aspirated V8 that BMW produced. It was also in my opinion one of the best naturally aspirated V8 engines ever made and I will explain why in more detail in a future article. For now though I will focus on the intake manifold and what I think is one of the most underappreciated pieces of technology of all time - the DIVA intake or Differentiated Variable Air Intake. This is the only continuously variable length intake manifold ever made and even BMW themselves didn't use it for the N62's entire production run, switching to a cheaper to produce and easier to package two-stage intake manifold design for the later year engines
In the DIVA intake the air enters in the middle of the intake and there are intake runners mounted around the inside perimeter of the circular intake. A motor at the rear of the intake spins a shaft which rotates the intake runners, allowing the length to vary infinitely between a wide operating range
Above the DIVA intake is in it's fully "closed" state, with the longest length runners possible. The actual operating range is not shown proportionately in these images. Below is the intake in it's shortest intake length "open" position
The turbocharged engines