Blowers and How To
First we need to understand that a "Roots" supercharger is not a compressor,
regardless of what Mercedes says. They "stack" air upon air and are
appropriately named "blowers".
One of the problems that we run into with any supercharger is that of efficiency.
Efficiency is the ability of the unit to deliver air at both a positive pressure (relative
to atmosphere) and reasonable temperatures. The act of mechanically moving air produces
heat and as the temperature goes up, performance goes down with a greater tendency to
cause detonation in the cylinder. The reason that you are accustomed to seeing the large
intercoolers is not necessarily due to the high temperatures seen in driving a turbo
supercharger, but the amount of boost and this "type of" supercharger cause the
increased charged temperatures. Both turbochargers and centrifugal superchargers are the
same in design with the exception of the method of drive, exhaust vs. mechanical, and both
are big heat producers.
The modern day Roots blower has been refined over the years to be a very efficient
power-producing package. The units are so efficient that turbocharging is rapidly becoming
something of the past. This is evidenced by Garrett's remarks about the market and the
fact that their new hydrachargers are the only thing that will keep them alive in this
business. The ability to package an efficient blower into a friendly underhood
environment, combined with the "drivability" factor are only a few of the
reasons that EATON and a few others are seeing such success in the marketplace.
The OE blowers are in many ways like "stock" cylinder heads on engines and,
similarly, there are considerable gains that can be obtained by reshaping certain areas to
be more conducive to higher flow efficiency. On the blower modifications, greater flow
gains result in less "pumping work" necessary to obtain the same flow output and
the temperatures are also reduced for the same amount of "work". The primary
reason that blowers aren't optimized by their manufacturers is that the tooling necessary
would push the cost into an area which would make the units prohibitive to the OE
marketplace, which is "the" big consumer. It does deserve mentioning that EATON
has listened well to Magnusen and others who have actively modified the blowers and they
have responded by incorporating many of the mods in their tooling updates so what you buy
today is much better than the "same" piece three years ago.
Most of the modifications to the EATON blowers revolve around timing and air flow
improvement. The rotor to case timing is ideally 120-degrees (due to the fact there are
three lobes to the rotors) and the factory is a little conservative in that department. We
begin by modifying the blower cases to provide precisely 120-degree timing. Each case is a
little different, so each is each is treated individually.
The inlet (rear) of the blower case is widened to the edges of the rotors' lobes, and
material is also removed to provide less interference to the incoming airflow. All sharp
edges are also carefully radiused to help reduce turbulence on the inlet side as well.
The charge exit area is also timed to correspond to the inlet side, and there's
considerable material removal (especially on early units) that's necessary to allow the
blower to pump the charge out as efficiently as possible. The exit is increased in area
directly in proportion to the inlet side area increases.
The actual intake manifold is adequate for most applications, but the runners were not
designed with the same areas feeding all cylinders. This was done with the reasoning that
the charge had to travel greater distance to some runners and distribution was a concern
to all. The distribution was not as critical as originally thought, as the blower is still
delivering air to the engine even in NA applications. We open the runners to precisely the
same sectional areas and the only other modification to the manifold is to make sure that
the blower and manifold are not trying to occupy the same space by the bypass which can
damage the "O" ring seal.
The bypass housing is smoked over to make its areas match the intake side of the blower
case and the entry is the exit for the "S" tube, so there's considerable
material removal in that area as well. The area leading to the bypass valve is radiused
somewhat, but that's not an area of great sensitivity when dealing with the flow at any
throttle opening or load.
The infamous "S" tube is a challenge to say the least. The initial problem is
that the runner cross section is constantly changing so the flow is either accelerating or
decelerating as it moves, which uses a tremendous amount of energy. The first operation is
to open the entire diameter (including the down leg) to a slightly greater area than the
throttle body. The next area of concern is the turns, which are really a challenge for the
fast moving air. As air typically will look for the shortest path through the tube, the
majority of the flow tries to hug the inside radius of both the upper and lower turns. A
long used solution to providing efficiency in the turns is to shape the inside radiuses so
the cross section is flat on the area where the flow is concentrated. This is the infamous
"D" port that's common to all cylinder head porters. The flat portion of the
"D" simply provides greater area in the area for the bulk flow and the rest of
the turn area is not so large that velocity is effected and the flow tends to be delivered
in a more laminar state to the blower. The smoother the flow, the less pumping work and
again the blower efficiency grows.
One area of manipulation is again in the "S" tube. If you're familiar with
fluid dynamics, you know that as velocity increases, pressure decreases. The inverse is
found if you lower velocity with the pressure building. This has always been a little hard
for folks with too much "common sense" (myself included) because, if you stick
your hand out the window of a slow moving car, the air "pressure" on your hand
is relatively low, but at higher vehicle speeds the "pressure" pushes more. This
analogy has been incorrect in use since day one, so forget that I mentioned it because it
really simply deals with air speed and forces instead of pressures.
We use pressure manipulation to "cause" the air to move through the
"S" tubes more efficiently. We will slightly expand the cross sectional areas
ahead of a turn to slow the air flow to prevent separation during the turn while exerting
additional pressure on the column of air down stream from that turn. This technique is
carried throughout the "blower system" and it greatly decreases pumping losses
while decreasing exit temperatures at the same time.
That about sums up the modifications involved, but the fact that the pumping losses are
so much less, allows us to use a smaller pulley on the blower for greater boost without
the temperature killing the engine or the blower itself.
The modifications are available in a variety of stages with the "aero"
package being the most time consuming and elaborate. The pictures that accompany this
article are of the most common mods, which were originally developed by Jerry Magnusen. We
have added several twists and the modified "S" tube is a real asset to the
combination.
Most of these modifications can be done at home if you're good with a rat tail file, a
Dremel tool, and you'll also need someone with small hands to reach the middle of the
"S" tube.
Pricing on the various stages and the components that are included will be available
shortly. As for other components necessary, count on buying 310 - 320 cc injectors and a
higher output in-tank fuel pump.
- The Old One, April 1999
