Turbocharge Understanding Guide - How a Turbo Works - Boost Basics

A/R is a geometric characteristic of the compressor or turbine housing. Although it can be applied to both compressor and turbine housings, usually we only concentrate on the A/R of the turbine side, as it can drastically change the spool rate. The A refers to the cross sectional area of the inlet divided by R, the radius from the turbo centerline to the center point of the area A. Since this area is where exhaust gas travels through to spool the turbo, a small area will increase the response and spool rate. At the same time, it will top out the efficiency limit of the turbo rather quickly. A larger A/R will take longer to spool (boost lag) but will be able to flow more air. In short, a small A/R has high response with limited power; a big A/R will result in slow response but with bigger power.

Turbo Breakdown
To help you in your quest of turbocharging your engine, we've gathered some of the most popular turbos together for a jam session. And whether it's streetable power or full-on drag racing you're seeking, we'll hope you'll find these suggestions as a launching point to your goal.

Selecting Your Turbocharger
For all you math geeks out there, HKS has provided us with a set of functions to mathematically determine which turbo size is best suited for your application and how much power you can expect out it. If you only excelled in home ec or PE, just follow the technical illustrations next to the corresponding pictures. Something to keep in mind: these figures do not take into account atmospheric conditions or the effects of intercooling.

1) Calculate the airflow for the engine in its naturally-aspirated form. Use the following formula for standard atmospheric pressure: CFM=(CID x RPM x 0.5 x Ev) / 1728. (CID: cubic inch displacement; EV: volumetric efficiency (0.80-0.90); CFM: air flow rate)

2) Knowing your desired boost level, calculate the airflow rate under boost by multiplying the pressure ratio by airflow rate (na - cfm). Calculate the pressure ratio: (14.7+Boost/14.7) - then calculate the air flow rate under boost by multiplying the pressure ratio by your air flow rate from step one: CFM x pressure ratio (for twin turbo applications divide this number by 2).

3) To covert CFM to lbs/min, use (CFM x 0.076 = lbs/min).

4) Use compressor maps to find the turbo best suited to the air flow rate and pressure ratios you have attained. You can get calculate a rough estimate on Engine horsepower using the following formulas:Lower value: 0.052 x CID x (psi boost + 14.7) = bhpHigher value: 0.077 x CID x (psi boost + 14.7) = bhp

Small Frame Turbos:
Characteristics: High response; good for dual purpose use, such as street and track (drift/circuit; street class drag racing); ideal for smaller 4-cylinder engines; est. 200-400hp

By Phi Phung

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