The Eaton M-90 blower (supercharger) is a roots type positive displacement pump which traces its roots back to the 1800's with the Roots brothers.   The supercharger is matched to the engine by its displacement and belt ratio; by pumping more air into the intake than the engine can use, air pressure and density are increased in the intake manifold, resulting in positive manifold pressure (boost).  By stuffing more air (and of course, the right amount of fuel with this extra air) into the combustion chambers, more power is produced.

A normally aspirated (NA) (non-supercharged) engine's maximum manifold pressure will never exceed the outside ambient barometric pressure for the altitude.  For example, at sea level this will be 100 Kpa (Kilopascals) or zero psi of "boost" at wide open throttle (WOT).  Thus, if you drive a NA car at high altitude, you'll experience decreased performance since the air is thinner and less dense.  This is reflected by your manifold absolute pressure (MAP), which will indicate less than 100 Kpa or "negative" boost (vacuum) at WOT.   Additionally, if you drive below sea level in Death Valley for instance, you'll experience increased performance because of the denser air.  Of course, that's if you don't detonate your engine to death from the intense dry heat, but that's another subject!   You'll actually see the MAP read above 100 Kpa, and show some "boost" due to the increased air density - a natural "supercharging" effect.

Roots type pumps were actually not originally intended for use as air compressors (our understanding is they were designed, of all things, as blowers for ventilation!)  so some of this increased manifold pressure is recycled back into the blower case, reducing efficiency and creating additional heat.  Anytime a fluid is compressed (yes air is a fluid), heat is produced - this is a fundamental law of physics called the ideal gas law - so even at 100% adiabatic efficiency, some heat will be produced when air is compressed simply due to this law.  However, we all know that there must be heat created in compressing this air due to friction, so this lowers the adiabatic efficiency.  Roots type blowers are actually quite inefficient since they create a great deal of heat under boost.  Adiabatic efficiencies of typical roots blowers are only around 50%, but fortunately for us, the Eaton unit is actually an improved roots design due to their twisted helix rotor.  Each rotor is twisted 60 degrees to form this helix, improving efficiency and reducing pressure fluctuations.

A common misconception is that more boost is always better.   This is true, but only to a certain extent, because superchargers operate most efficiently at a certain speed.  To create more boost, blower speed must be increased, more heat is generated, and efficiency suffers.  One must understand that high density is what's important, not necessarily high pressure (boost).   Compressing air does increase both pressure and density, but the resulting heat generated simultaneously reduces density as well.  To achieve optimum performance from a blower, we want to create the most pressure (boost) while adding the least amount of heat, thus achieving the highest air density possible in the intake manifold.  As blower speed is increased, at some point the heat created decreases the air density to where addidtional boost is just offset by the increased heat, resulting in no density (and therefore power) gains; further increases in boost may actually even decrease power.

The function of an intercooler is to remove heat from the inlet air charge.  As explained above, removing heat increases the density (more molecules per unit area) of the inlet charge allowing more fuel to combust with the end result being increased HP.  Another benefit is greater resistance to detonation.

Definition of terms

T1 = blower inlet temperature
T2 = blower outlet temperature
T3 = intercooler exit temp
T4 = intercooler coolant temp

P1 = absolute ambient pressure (approx. 14.7 psi)
P2 = absolute pressure at blower outlet (boost + P1)

° R = ° F + 460

According to the Ideal Gas Law (PV = nRT) anytime a gas is compressed, its temperature rises.  In a perfect world, the temperature rise purely from compressing air (assuming 100% adiabatic efficiency, neglecting frictional heat) is given by:

Example: T1 = 85° , P1 = 14.7, P2 = 25.7

Unfortunately, because the Roots style blower is far from an ideal adiabatic compressor, it also adds heat to the charge.

Working through this equation with an efficiency of 60% (remember the Eaton twisted helix roots design is more efficient than conventional roots blowers at 50%), we get an actual T2 of ~241° F, and this is very close to what we've actually measured from our thermocoupled engineering development vehicle.

As stated above, the more dense the inlet air charge, the more fuel we can burn, therefore resulting in more power.  The density ratio of the inlet air charge is an indicator of the power gain.

The density ratio in the above example works out to:

Therefore, the blower is helping us make 36% more power than a normally aspirated engine.

One easy way to reduce heat in the intake charge is to reduce the temperature of the air being drawn into the blower itself.  This can be accomplished with the addition of a Thrasher Cold Air Kit which reduces the intake air charge by up to 70° F.  (NOTE: The preceding example already makes the assumption the engine is getting air at the ambient temperature.)

Another way is to add a Nitrous kit!  N2O evaporates at -127° F!  Using a N2O kit on a turbo or blown application artificially super cools the intake charge.  It works great because you get the extra HP of the N2O kit AND it delivers an intercooling effect on the intake charge.  Big fun, but the drawback is having to refill that blue bottle!

Adding a heat exchanger (intercooler) into the intake flow can also remove heat.  The greater the efficiency of the intercooler, the more effective it will be.

A good intercooler core design will typically have an efficiency of 0.6 to 0.8.  The best way to improve efficiency of the intercooler is to use a core which has a large surface area.  Using a core which is too small does not allow the cooling medium to adequately conduct heat out of the intake air charge.   This presented quite a challenge since our Eaton M90 blower outputs through an extremely small area as shown on our blower disassembly page!  We calculated the minimum amount of surface area required, and immediately became so disillusioned that we almost dropped this project!  With the available hood clearance, at first we saw no possible way to squeeze this much area under there as the core would needed to be much too thick - and anything less would have been a complete waste of time and money because the calculated temperature drop would have been insufficient.  But after studying the situation more we developed  a solution and had a core custom manufactured to our specifications to achieve this surface area without excessive pressure drop.  Our design incorporates the largest possible core for the best efficiency possible without interfering with airflow into the intake manifold, and still (barely - it's close!) allows the use of the stock hood! 

Let's be conservative and use an intercooler efficiency of 0.6 and a coolant temp of 110° F, T3 is calculated to be:

Assuming 0.5psi drop across the core and T3 = 162° F, let's revisit the density ratio equation.

The power gain over the non-intercooled engine is conservatively:

  You can see now why a core with less surface area is simply unacceptable!

Although the above figures are for illustration purposes, they outline some the steps involved with designing blown/intercooled engines.   Once again, we can not overstress the use of  a core with adequate sizing! Turbocharged and centrifugally supercharged engines accomplish this by routing the outlet of the compressor as needed to a space large enough to accommodate the proper size core.   Unfortunately, most roots style blower's (such as our Eaton M90) couple their output directly to the intake manifold, so we do not have the luxury of locating the intercooler elsewhere.  Therefore, we must sandwich the intercooler between the blower and intake manifold, so on a street car core size must be compromised to avoid hood clearance problems. Future updates to this page will show our approach to solving these and other issues, and of course, power gains.