Misconceptions on Turbocharger Inducer Sizes Turbocharger inducer sizes Lately I have seen a lot of turbo inducer sizes quoted on the forums, the size of the compressor inducer will not tell you the performance capabilities of the turbocharger. To do this you will require more information on the compressor wheel and the compressor housing. There are a number of factors in compressor wheel design that can dramatically change the performance. Compressor maps are the best way are the best way to determine a wheels performance capabilities. You must also keep in mind that the map is obtained using a specific compressor housing. Using a different housing will change the map. You should look for a map that best meets your performance goals and also meets the engines surge requirements. A good design will do this and still protect your turbo from off throttle surge or bark that will reduce the life of the turbo, (the BD Super B is a good example) The wheel trim (difference between inlet to outlet size) a larger diameter wheel rotating at the same speed will produce a higher air velocity that converts to higher pressure. Also the length of the inducer veins can be different a larger shaft diameter would require a larger compressor hub diameter so inducer vein length would be shorter. (I have seen examples of 20lbs/min differential with the same size inducer) New casting methods and hardening processes allows manufacturers to design thinner veined and taller compressor wheel designs without sacrificing strength. This also makes extended tip designs possible on high speed compressor wheels. In the past this design was used only on very large slower speed turbochargers. Keep in mind that if you have two wheels with the same inducer size that perform at the same efficiency but one has a lager overall diameter. Borg-Warner’s extended tip is a good example as it increases diameter without changing much of the total mass, at the same rotating speed the larger diameter wheel will produce a higher air velocity and static pressure in the engine inlet plenum. This will result in more air into the engine when the valves are opened. And then there is the turbine. Twin or compounding turbochargers. There is a misconception that bigger is always better when it comes to the turbochargers used for the first stage of compounding turbochargers, this is not always true. The rule of thumb in compounding is that the inducer size of the second stage turbo should be the same size as the first stage outlet. If this is not done you will risk over speeding the first stage. Say the first stage has an outlet size of 3.1 ins as on the S400 and you are using a HX35 as the second stage that has an inducer size of about 2.2 ins, the match will not be very good. Yes you will produce boost in fact the first stage pressure will be high( 35-40lbs) and the temperature will be also be high. The reason this happens is a compressor wheel is an open rotating compressor and not positive like a piston compressor. When the air encounters a smaller diameter it will not compress to that size. Instead the flow becomes restricted, this reduction in flow reduces compressor wheel load and the turbocharger speeds up producing a higher boost at a lower flow. The second stage wastegate now has a problem controlling first stage air velocity and increased boost. Usually this is means the wastegate has to be opened almost fully instead of controlling overall boost levels. If the match was correct the overall boost level could be controlled with the second stage wastegate. The end result is running any turbo with the wastegate open all the time is very inefficient. A wastegate is designed to control the max boost level not to run open all the time. When we built the BD compounding system we built so that it could be controlled this way. The only other factor that will increase boost is too much fueling. We developed our own wastgate that can be adjusted to 65lbs this has worked very well on 98-02 trucks. Our own truck produces 625HP and has run the quarter mile in 12,2 seconds and has been a daily driver for over 2 years. People who have made the change from twins using a larger first stage are surprised at the increased performance. They also thought bigger was better John Todd
