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Old 01-13-2012, 06:12 PM   #1 (permalink)
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Default Twin Turbo vs. Single Turbo V6: A Dissertation

Lately I've noticed quite a few threads about single turbo setups on the 370Z. I've been wanting to share some insight with the community regarding the mechanics of single turbos as compared to twin turbos as it is related to the 370Z specifically.

This is some pretty heavy reading and goes into quite a bit of detail, so be warned.

There are several aspects of turbochargers that define how it operates. Before we get into that discussion, we need to define a few terms. Those of you with a physics or engineering background can skim/skip this part.
  • Adiabatic Efficiency: air temperature increases when compressed. Adiabatic Efficiency tells you how much more the air temperature increases when compared to the ideal gas law. Nothing is 100% efficient. The more efficient a compressor is, the less extra heat it generates when compressing the air.
  • Exhaust Enthalpy: Enthalpy is the total energy of a substance. Enthalpy includes heat, pressure, and velocity. Enthalpy is what drives the turbine of the turbocharger. The more enthalpy the turbine gets and the larger the difference between the enthalpy on the inlet side and the enthalpy on the outlet side, the more mechanical energy the turbine extracts from the exhaust gasses. This is extremely important as this affects boost threshold, response and top-end power.
  • Polar Moment of Inertia: this describes the effect of mass in a spinning object. Simply weighing an object that will be spinning doesn’t tell you the whole story. If you have a 10lb mass that is 1ft in diameter (evenly distributed mass) and compare its polar moment of inertia to a 10lb mass that is 2ft in diameter (also evenly distributed), you will find that the smaller object is easier to spin as it has a lower polar moment of inertia. Also, if you have the mass concentrated near the axis of rotation, it will have a lower polar moment of inertia than an object with the same mass and diameter with its mass evenly distributed.
  • Heat: not to be confused with temperature. Heat is energy and the ability to do work. Temperature only tells you how fast the atoms/molecules of matter are moving (root mean square of the velocity). Heat (expressed in BTU's or Joules) can be calculated from the temperature, mass, and specific heat of a substance. Since exhaust gas is mostly hot air with a little bit of water vapor, it has a pretty low specific heat and therefore, a small change in heat makes a pretty significant change in temperature.
  • Heat Transfer: basically, nature abhors a vacuum and something that is hot wants to be the same temperature as its surroundings. Therefore, it will transfer heat to the surroundings. The bigger the difference in temperature, the faster heat will transfer. The transfer medium also affects the transfer rate. Some materials transfer heat more effortlessly than others. Aluminum transfers better than steel and steel transfers better than ceramic. Also, the thicker the transfer medium is, the higher the resistance value is (commonly called “R-value”…home insulation is rated R-19, R-8 etc.). The higher the R value is, the more resistance there is to heat transfer. A thick iron casting is going to have a higher R value than thin stainless steel piping.
  • Thermal Mass: pretty self explanatory. The more mass you have, the more heat will be stored.
  • Boost Pressure: often (mistakenly) thought to be the cause of horsepower in forced induction. This is simply a measure of resistance to airflow. The higher the boost pressure is, the harder the turbo is working to get more air into the cylinders. Also, as boost pressure increases, air temperature also increases dramatically.
  • Air Flow: This is what makes horsepower. In naturally aspirated engines, it is usually measured in cubic feet per minute (CFM) because air density doesn’t change much in atmospheric engines. In forced induction, however, it is better to measure it in terms of mass flow or pounds per minute (lb/min). A rough guide to converting lb/min to horsepower is 9.5 – 10.5 horsepower for every lb/min. On the VQ37, it’s closer to 10.5hp for every lb/min thanks to VVEL and high stock compression.
  • Turbo Lag: Often misused terminology. I will define two other terms to differentiate two different aspects of turbo performance.
  • Boost Threshold: (what people incorrectly tend to use the term “Turbo Lag” to describe) this is the engine rpm that the turbo makes positive boost pressure at in WOT operation. This is a number like 3800rpm.
  • Boost Response: (what Turbo Lag is supposed to be). This refers to the time it takes for boost to build once the engine is above the boost threshold and WOT is applied. To illustrate, imagine that you are going through a corner and are at part throttle through the corner. As you approach the exit, you floor it and there’s a brief moment where nothing seems to happen, then, very quickly, boost comes on like a freight train. That brief moment of nothing is the true definition of turbo lag. For this article, we’ll refer to it by its inverse: boost response. The more responsive a turbo system is, the less “laggy” it is.
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