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ALL ABOUT BOOST
Written 6/7/2001
Updated 9/17/2004
Updated 2/7/2006
Updated 5/23/2006
Updated 8/28/2007

I was interested in a forced induction system for my original '01 330i for some time and let me tell you, since then, I did a lot of research and learned quite a bit along the way. This document is constantly being revised, but this is what I have so far:

THE BASICS
An internal combustion engine is essentially a big air pump. It sucks in outside air, mixes it with fuel, burns it, and expels it. On every engine, manufacturers try to maximize this process to produce the most power while maintaining fuel economy, emissions, and safety. Nowadays, the engine control computer (ECU, or DME in our BMWs) is carefully engineered with internal maps that control the fuel and ignition timing to keep this balance in check.

All in all, there is a lot going on under our hoods to make them the most efficient, powerful, and cleanest-burning system possible straight off the dealer lot. We are about to change all thatů with forced induction.

FORCED INDUCTION
Power in an engine is related to the PLAN. P for chamber pressure, L for length of stroke, A for area of the piston/cylinder bore, and N for firing cycles per minute. If we can increase one or more, we make more power. Big-block V8s are a favorite for getting bored and/or stroked thereby increasing the A and L, making more power. Race engines with incredibly high redlines increase the N, making more power. Without opening the engine itself, it leaves us with the P, increasing the chamber pressure. And that means BOOST!

Forced Induction (FI) has been around a very long time. Initially used on aircraft, it can be found everywhere from drag racing to almost every diesel engine on the road today. FI is accomplished by using a mechanical compressor to pressurize the intake air above the ambient atmospheric air pressure. The basis is simple: force more air and fuel into the engine than the engine can breathe on its own. For example, if we can effectively burn twice the air and fuel in the same amount of time, we can make about twice the power. However, controlling this dangerous process safely involves some important factors.

HEAT
The number one enemy is heat. Anytime air is compressed, it heats up. When we put a FI compressor on an engine, the intake air charge is heated and expands, becoming less dense. Less dense air contains less oxygen and will make less power. Also, introduce too much heat and you risk melting parts such as piston crowns and valves. More often, that extra heat can lead to another problem: detonation. Detonation is the unexpected self ignition of the fuel mixture in the cylinder before the spark plug fires. It is bad in every way. Usually referred to as "knocking" or "pinging" because of the clearly audible sound it makes, detonation creates massive pressures and shock waves inside the cylinder and when left unchecked can cause major damage including broken head gaskets, piston lands, connecting rods, valves, and cracked blocks.

The number one defense against heat is intercooling. Intercoolers are metal radiators placed between the compressor outlet and the engine cylinders. They can be air-to-air and cool the intake charge by passing cooler outside ambient air across the radiator. They can also be air-to-water and cool the intake charge by passing water across the radiator. Normally an air-to-air is used, but air-to-water is used is limited space situations. Air-to-air intercoolers require very little maintenance, are inexpensive, and work very well. However, air-to-water intercoolers can be much smaller than air-to-air and water transfers heat 18 times better than air. Air-to-water intercoolers are more expensive, and since they require a pump, can fail. Use of air-to-water intercoolers requires a fail-safe system to ensure that the FI system can run without it. Overall,

Without intercooling, FI systems are limited to very low boost levels on stock engines, usually maxing out about 6psi. And in my opinion, don't consider water injection as a form of intercooling. These systems actually inject water mist into the intake charge and the evaporation process cools the air, but at the cost of displacing that same air and making it less dense. And what happens when the tank runs dry at the worst possible time? Intercoolers make the air more dense, not less. More dense intake air, more oxygen, more power. Skip water injection.

There is also a limit to just putting a huge intercooler on a FI system. Larger intercoolers create more internal drag and induce pressure losses which must be overcome by raising boost pressure. Finding the right balance between flow, thermal efficiency, and drag is the kit manufacturer's goal.

Once we have controlled the compression heat of the FI process, another cause of heat that must be controlled is the actual heat of more fuel burning. This is accomplished by either upgrading the engine cooling system or more likely, by tuning the air/fuel mixture. And heat is not the only thing that the air/fuel mixture can control, we can make power here too.

AIR/FUEL MIXTURE
In a perfect world, using normal pump gasoline, we would want to burn 14.7 parts air to every 1 part fuel. This is considered the stoichiometric ratio (14.7:1) of gasoline. Stoichiometric means a perfect combustion process whereby it is the complete burning of the fuel. All fuels do not have the same stoichiometric ratios, for example, in a top fuel dragster burning nitromethane, the ratio is 1.7:1.

Unfortunately, we don't live in a perfect world, and many other factors affect our fuel burn including air temperature, density, quality, fuel quality, octane rating, additives, engine efficiency (which is inherently low), engine temperature, combustion chamber design; the list goes on and on. This changes the stoichiometric ratio greatly. After much consideration, it is generally found that in an internal combustion engine, ratios between 11.8:1 and 13.0 produce the best power. A higher ratio runs leaner (less fuel but more power and heat) and a lower ratio runs richer (more fuel but less power and heat).

When we add FI, we are forcing more air into the cylinders than the factory engine control systems were designed for. Without controlling the fuel as well, we would run the engine lean. Running lean (as in above 14:1) for long, especially at high RPMs, can easily create the intense heat I mentioned above that can actually burn or melt valve heads, spark plugs, and even blow holes holes through pistons. Very bad idea.

To keep the air/fuel mixture in check, we must either modify the existing fuel delivery systems or create our own. The best way to achieve this is to have the factory ECU reprogrammed with new fuel/ignition maps to handle the higher airflow and install larger fuel injectors. Other options include aftermarket ECUs, piggyback systems, rising rate fuel pressure regulators, bigger fuel pumps, and more. There is no magic FI mixture ratio such as 12:0.1 as is commonly thought. As long as detonation is controlled, then whatever air/fuel mixture makes the most power safely works. The reason behind richer mixtures seen in FI is due to using the fuel to cool the intake charge via evaporation to control detonation.

ENGINE LIMITS
When a FI kit refers to boost pressure such as 6psi or .4 bar, they mean the pressure above atmospheric pressure. Normal air pressure at sea level is 14.7psi or 1 bar (bar is short for barometric). When you add a 6psi FI kit, you are actually making 20.7psi in the manifold. The difference between boost and ambient pressure is called the Pressure Ratio (PR). If we push twice the normal atmospheric pressure through FI, we are making a PR of 2.0 or 29.4psi. This number is used to read the maps that each compressor manufacturer offers (learning to read compressor maps is best left to another page I will have to write).

On pump gas, normal engines like my 330i can take 8-9psi maximum without internal modifications. Higher compression engines like the E46 M3 can take less, more like 6-7psi maximum. To go much beyond that with boost safely, the internal cylinder space must be increased and that means low compression pistons or a thicker head gasket. Specifically designed pistons are the correct choice here. Also, the exhaust path on the E46 is very rough and constricted, cleaning and opening this up would help immensely. At higher pressures, above say 15psi, you are looking at strengthening the entire engine from valvetrain to crankshaft.

There is a key factor to be considered when it comes to how much you can get boosting an engine: Fuel Octane. Most FI kits advertise the power they acheived on standard 91/93 octane pump fuel everyone uses. A few advertise what they are capable on 109+ or higher octane race fuels. The octane rating of a fuel is simply its resistance to detonation. The higher the octane rating, the more power that can be extracted out of it before you reach the detonation threshold. The downside is that race fuel is expensive (up to $14 a gallon), more difficult to find, some contain lead which damages O2 sensors and catalytic converters, and what happens if your engine is tuned for 109 and you accidentally put in 91 or worse. This should be something you ask about.

It is common misconception that engines will blow up from FI alone. The actual difference in cylinder pressures between normal aspiration and forced induction is minimal below about 15psi boost. The same goes for the manual drivetrain. Automatic transmissions, however, are very limited in their power handling. The heat from improper intercooling, bad air/fuel mixtures, and sub-standard gasolines will destroy your engine much faster than forced induction itself. If we keep the temperatures and detonation within safe limits, you can boost like crazy on the right fuel.

OTHER CONSIDERATIONS
Besides fuel delivery, spark and valve timing can have a great or detrimental impact on power, drivability, idle, and fuel economy. This is another reason to go with a FI system that includes full ECU programming. The more advanced the engine controls get, just look at Valvetronic for example, the more time and effort must be put into getting correct ECU programming. Everything from load transitioning, off-idle, partial, and wide open throttle (WOT) must be taken into the algorithms to have perfect software. Anyone can program a car to look great on a dyno chart at WOT, what about partial throttle up a hill, passing after a downshift, etc. There is a lot more to a system than what a WOT dyno run tells you. Look deeper.

Great systems will include a bypass valve that allows pressure in the manifold to be relieved back to the intake of the compressor (or to the atmosphere in MAP systems or turbos) on off-throttle situations such as gear changes.

After a certain point, other parts of the driveline should be upgraded such as tires, clutch, flywheel, bushings, subframes, and so on because wear is increased and better components can last longer.

Adding forced induction to a vehicle under factory warranty has a good chance of voiding any claims against the driveline components. Under the Magneson-Moss Act of 1975, the factory must prove that an aftermarket part caused the failure of the part before they can deny warranty repairs. Racing Use Only products, however, will likely void your entire factory warranty. Luckily most FI systems are not Race Only, and some FI systems even have the option of adding a warranty to cover the drivetrain components at extra cost.

TYPES OF FORCED INDUCTION
Superchargers are compressors of various types that compress intake air via mechanical compression to make boost. Commonly, the belt driven compressors have kept the name while turbochargers took a different path.

TURBOCHARGERS
Turbochargers are just superchargers powered by tapping into the exhaust gasses being pushed out of the engine. A turbine impeller is put into the hot exhaust flow and connected by a shaft to the compressor impeller in the intake flow. As the engine speeds up, more exhaust pushes the turbine faster which turns the compressor faster making boost.

The differences are many:

    Turbos lag a bit from the exhaust flow spooling up the compressor.
    Turbos run hotter than superchargers and require pressurized oil supply.
    Turbos require more air delivery tubing, both intake and exhaust.
    Turbos can be controlled for different boost levels through a wastegate.
    Turbos will generally make more power than superchargers.
    Turbos usually make most of their power in the top end of the RPM band.
    Turbos cost more.

The latest 3 Series model line includes a new N54 Twin Turbo engine straight from the factory in the 335i. It uses two very small MHI turbos to bring the boost on fast and carry it to redline. They are very quick and fuel efficient, but the turbo size has limited the extent that the aftermarket can extract more power. Most current aftermarket tuners are unable to access the DME and are pushing piggyback solutions to market such as the Vishnu ProCede, Split Second Turbo Tuner, Juice Box, and Active Autowerke Xede. They are having mixed results. The next steps will be entire turbo, downpipe, and exhaust assembly upgrades along with actual in-DME software. Trust me, it is coming.

For the E46 M3, Horsepower Freaks has released a $16-20k HPF750 turbo kit. It uses a T67 turbo, cast manifolds, MAP tuning, and completely custom AEM piggyback. The quality of the kit is quite amazing, but it is simply not reaching advertised power levels. To get close to the promised 750hp at the crank, you require extremely high octane leaded race fuel, illegally removing your catalytic converters, and grossly exaggerating the drivetrain loss when you dyno. On standard 91 pump fuel, it makes about the same power as most of the supercharger kits (420whp).

I really think more turbo kits will come out in the coming years for our BMWs. It is just that turbo kits are more expensive to create, tune, install, build, and maintain than supercharger kits are.

SUPERCHARGERS
There are three major types of superchargers, roots, centrifugal, and screw.

Roots
Roots type superchargers are among the oldest style of supercharger. Roots compressors use two lobed rotors inside a casing spinning in opposite directions. Air enters one side, is forced around the outside of the casing, and out the other side. They are very common in professional drag racing. They are noisy and their efficiency falls off at high RPMs, but low and midrange torque is monsterous. Also, Roots do not have a "internal compression ratio" at all which makes them fairly inefficient thermally.

Having an internal compression ratio means that the compressor itself not only pumps air but also compresses it within its housing. When this internally compressed air leaves the compressor housing and enters the manifold, it decompresses and therefore cools making the system more thermally efficient overall. A Roots compressor works by simply pushing more air directly into the manifold than the engine can use creating a high pressure area a.k.a boost, however, it does not actually compress the air itself.

Roots is a "positive displacement" compressor. It creates distinct pockets of air that are then released under pressure into the manifold. This "popping" can be heard and is distinctive. Roots superchargers are available from companies like Whipple, Eaton, and Weiand. They are usually self-lubricated. They work great on low speed large displacement engines, but on newer high revving BMW engines, they have no place. The only BMW application I know of with Roots compressors is Downing-Atlanta with their offering for the 1992-1999 318s.

Centrifugal
The centrifugal supercharger looks just like a turbo with a pulley and drive unit in place of the exhaust turbine. The engine belt drives the compressor which spins the intake compressor turbine making boost. Centrifugals benefit from their small size and relative ease of installation. Since they are basically a fan in a housing, they push air by forcing it out from the center of the impeller. This air is forced through a venturi around the edge of the impeller into the snail-like volute casing and onto the engine. These venturis and the volute create an internal compression ratio inside the supercharger making it more thermally efficient.

Centrifugals are very common in the BMW aftermarket from companies such as Vortech, Rotrex, ASA, and Powerdyne. They are reliable and require little maintenance. About the only drawback to a centrifugal is the fact that since boost pressure is based on the square of RPM, it only makes maximum boost at redline. Air leaks past the impeller at low speeds, so they do not come into boost until the higher RPM band.

Centrifugals require external lubrication. This can be provided by engine oil pressure or a self-contained pump inside the housing. Engine oil fed systems require an oil drain line that empties into the oil pan, preferably above the oil level.

Screw
Screw compressors, or twin-screws as they are usually called, are the best of the roots and the centrifugals combined. They function by using two intermeshing rotors, one male lobed rotor, and one female concave rotor. The twist of the rotors and the design of the rotor interaction creates an internal compression ratio. This makes them more thermally efficient than roots and still create full boost at low RPMs. The torque is immense, but high RPM efficiency is kept high like a centrifugal. Outlet shape changes and drive mechanism changes reduce the "popping" and operating noise you would get from Roots.

Screw compressors can be either pressure fed from the engine or self-lubricated. Their operation creates heat all the time, even at idle, but the benefits far outweigh the minimal added temperatures.

The patents on screw technology is held by only a few Swedish companies such as Autorotor, SRM, and Lysholm. Recently, Opcon acquired them all, but the individual companies still compete in the market. You also find twin-screws on Mercedes AMG vehicles and aftermarket companies such as Eaton license the technology and use it in the aftermarket like on the new Ford GT. ESS Tuning recently acquired exclusive license to produce Twin Screw technology for the entire BMW M52TU/M54 engine plaform (E46 3 Series, Z4, E39/E60 5 Series, etc.) and is now selling kits all the way up to 500hp Stage 3. Other Twin Screw applications from ESS are under development as well.

IN CONCLUSION
Choosing the right forced induction for you car depends on many factors: price, ease of install, desired power, completeness of the system, technical support, reliability, and drivability. To take the full benefits of a turbo system, internal engine modifications are required to strengthen the engine for high pressure levels, but the power created can be insane. A supercharger, on the other hand, will be nicer to your wallet, and easier to install, maintain, and drive.

I personally chose to go with the ESS TUNING supercharger after LONG discussions with them about things like intake/discharge temps (I live in Arizona with 120 degree summers), heat soak, detonation, fuel injectors, etc. They have successfully tested their system in 130 degree heat without detonation by correct ignition mapping and using a colder spark plug. They also include a 1/8" NPT adapter on the bypass connection for a boost gauge. The ESS kit includes all oil feed/drain connections, larger BOSCH injectors, tubing, clamps, bolts, and everything needed to install in my garage. My needs included daily driving ability with occasional track use, reliability, safety, minimal modification from stock, removability, and cost. The ESS supercharger kit fulfills all these needs.

The ESS TX-2 kit I have is based on the ASA centrifugal. I put out 345hp and almost 320ft/lbs torque at the crank. Remember, horsepower is what makes top speed and is simply a function of torque over time. Most of us will never realize our car's top speed safely (I was at 166mph on a long road to Vegas and scared to death). Acceleration is what we all want to feel and that comes from TORQUE. That body-smashed-in-the-seat feeling is what counts around town and at the stoplight. Think about it, how many times do you go top speed versus 0-60? The supercharger increased my torque output by about 100lb/ft and THAT is what I feel and savor.

If you choose to go with a turbo kit, the only one I heard is possibly being developed for the 330i is at Horsepower Freaks.

UPDATE: 12-8-2006
I have since upgraded to the ESS TS2 Twin Screw supercharger on my 330i ZHP. It is AMAZING! Check the DIY and Review in the GARAGE section.

PLEASE NOTE:
I searched all over the Internet for a page containing this information, but there are very few and none of them are as concise. I hope this page helps you understand your options and make an educated decision before buying or installing any forced induction system. This information is based on factual data I have researched and if you feel any of it is incorrect, inaccurate, or missing, please don't hesitate to EMAIL ME.

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