What the hell is an A/R ratio

[Indecisive]

[span style=\'color:red\']Posted by Magnum[/span]

A/R Ratio (Area/Radius Ratio) - There is an excellent description of A/R in the Corkey Bell book \"Maximum Boost: Design, Testing, and Installing Turbocharger Systems\", since I can\'t think of a better way to define the term I\'ll copy Mr. Bell\'s description:

\" While basic turbine size reflects a measure of the turbine\'s flow capability, the A/R ratio is a method of fine tuning between basic sizes. To easily grasp the idea of an A/R ratio, imagine the turbine housing as nothing more than a cone wrapped around a shaft to look like a snail. Unwrap the cone and cut of the small end a short distance from the tip. The hole in the end of the cone is the discharge area. The area of this hole is the \'A\' of the A/R ratio. The size of the hole is significant, as it determines the velocity with which exhaust gases exit the turbine scroll and enter the turbine blades. For any given rate of flow, a smaller exit will require that the gases flow faster. Thus, the area of the exit is important in controlling the velocity of the gases as they enter the turbine blades. This velocity has much to do with controlling the actual speed of the turbine. It is necessary to keep in mind that the area of the exit is the controlling factor in the bad side-effect of exhaust gas back pressure and, thus, reversion into the combustion chambers.

The \'R\' of the A/R ratio is the distance from the center of the section area in the cone to the center of the turbine shaft. All \'A\'s divided by their respective \'R\'s will give the same dividend.

The \'R\' also has a strong influence in controlling turbine speed. If one imagines the turbine blade tips will travel about as fast as the gas is moving when it enters the tip area, it is easy to see that a smaller \'R\' will impart a higher rotating speed to the turbine.\"

In short, all other things being equal, the smaller the A/R the less lag there will be, but the turbo may not be able to produce power at higher RPMs. A larger A/R will produce more lag but have a lot of top end power.