The Old One - Energy Dynamics : Commentary - Blowers and How-To
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About roots type supercharger, its history, and modifications

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

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About | Archive | Articles | Supercharger | Components | BBS | Contact Us

About roots type supercharger, its history, and modifications