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Turbocharging Basics 101: Session 3:Pumped Up
An article from Import Racer! By Dino G. Tadokawa

Introduction
SESSION 1
SESSION 2
SESSION 3
SESSION 4



RACING APPLICATIONS

Now, for our next lesson we will be doing a matching analysis for a full-race (drag racing) turbo engine application. For this application we will base this study off of one of Gary Kubo’s world record setting Honda four-cylinder B-18A VTEC race engines. In this case the priorities/assumptions are quite different from the previous street driven turbocharging matching analysis.

For drag racing turbocharger matching applications some of the basic assumptions are:
  • Peak power is very important, response is not so important as long as you are within your intended power band.
  • Inter-cooler efficiency is very high because of a liquid-iced water-cooled inter-cooler, so higher boost levels can be used without risking detonation.
  • Engine rpms are higher (hence, higher volume flow).
  • Engine is more highly tuned (intake, exhaust, valve train, advanced engine management...etc.) so volumetric efficiency and specific power (hp/pound of airflow) are higher than normal.




For this example, we will be basing this matching analysis on one of Kubo Racing’s previous year’s record setting Honda engines. (And you thought we’d give out his current program secrets...right!). For this particular race engine, Kubo had a redline target of 9,000 rpm with the goal of making over 600 hp at approximately 8,500 rpm. This was very achievable.

Assuming an inter-cooler effectiveness of 90-percent, average compressor efficiency of 75-percent, engine volumetric efficiency of 97.5-percent, we can determine from utilizing our quick calculation method, which was taught in the previous lesson, that the GT45 compressor flow and pressure ratio capabilities look quite promising.



In the most basic form a turbocharger can be viewed as an exhaust driven air compressor—or simply, an air pump. A highly sophisticated air pump of course! In general, the more air you can cram into an engine, the more power your engine can create; this is accomplished with the right proportion of fuel and correct spark timing. Engine design integrity as well as engine swept volume, maximum rpm and volumetric efficiency plays a big part in this assumption. In any case, with a turbocharger onboard it makes your small displacement engine think it's really a large cubic inch displacement engine. Your little turbocharged 1.8 VTEC can hang with the domestic big boys and in many cases they can blow the Detroit iron into oblivion. Let’s revisit the turbo gurus (all with the help of our lovely teaching aide) to figure out what an engine can do with a bunch of compressed air.

Comparing this operating line to the match obtained with the street T28 from the previous lesson, we can observe the following differences:

  • We are flowing well over 60 lb./min of air, this should result in power ratings over 600 hp on a highly tuned race engine.
  • The rpm at which the turbo will begin to make significant amounts of boost is significantly higher than for the T28 (5,000 rpm vs. 3,000 rpm for pressure ratio = 1.8). The larger turbine stage on this turbo causes this.
  • The peak pressure ratios that are used are much higher than those for the T28. This results in the higher levels of boost required to move the amount of air through the engine required for 600-plus hp. If we had tried to run these levels of boost with the T28, the air flow rate would have been too high for the compressor. So for this application we required a compressor with the ability to flow more air than a T28, but we also have to increase the amount of boost to get this amount of airflow. This is one of the key points you should have learned in Session 2.
  • The efficiencies along the operating line are not as good as for the T28. Unfortunately there are very few, if any, compressor wheels available with good efficiencies at pressure ratios above 3:1, so there is not much that can be done. This wheel had the highest levels of efficiency along the operating line that we could find.




So, here we have a good example of the type of turbo you would select for a pure drag racing application, and are able to compare it with the more streetable turbo from the previous lesson. You can see the differences that will result (response, airflow, boost required) as well as what is needed to meet the different requirements.

Some of you may have noticed that the turbine side has been largely ignored in these lessons. For the calculation of the final operating lines, the turbine power balance equation and actual performance data was used. However, to include these details in the lessons would have increased the difficulty of this study material significantly, so we decided to leave them out. If you are interested and have access to turbine performance maps, exploring these details makes a great independent study project. For those of you who are not interested, the general turbine matching trends discussed in Session 2 should provide guidance as to a good baseline turbine selection.

Finally, there are some points to consider when doing dyno testing. Like many other areas of life, the more information you have the better equipped you are to make a decision. In this case, for developing your engine/turbo system, you should try to measure the following parameters (bold items are very important):

  • Compressor Discharge Pressure and Temperature (before inter-cooler or throttle)
  • Turbine Inlet Pressure and Temperature
  • Intake System Air Mass Flow Rate
  • Intake Manifold Pressure and Temperature (probably already being measured)
  • Compressor Inlet Pressure and Temperature
  • Turbine Discharge Pressure and Temperature



Turbine Gas Flow vs. Pressure Ratio Map


With the information in BOLD you can make the following five comparisons:

  1. If you know compressor discharge pressure and air flow rate you can plot your compressor operating line on various compressor maps and potentially find a better compressor for your application. This can be through better efficiency, lower surge flow rate or higher choke flow rate depending on your requirements.
  2. If you have compressor discharge temperature, you can tell which turbocharger compressor is operating at higher efficiency for a given boost and air flow rate level (lower discharge temps = higher efficiency)
  3. If you see relatively high compressor discharge temperatures at low rpm, you may be operating in surge (marked on GT45 map, characterized by flow instability, noise and very low efficiency). In this case you may want to change to a smaller flow compressor.
  4. If you see a drop in boost at very high rpm (even with the wastegate closed) then you are probably operating in choke (characterized by a fixed flow limit, marked on GT45 map) and should consider changing to a higher flow rate compressor. You may also be running into a fixed flow limit of your intake system, so check this before changing turbos.
  5. If you see turbine inlet pressure (back-pressure) increase rapidly at high engine rpm then you may consider a larger flow turbine to improve peak power by reducing back-pressure, or you will now understand why you have good response.




For street applications, it is important to also test your engine management system (primarily fuel settings) at part throttle settings when adding/modifying the turbo system.

Unfortunately there is a lot of hype out there regarding turbos, and we recommend that anyone involved in building turbocharged engines should seek out as much qualified turbo machinery knowledge as possible to help make sense out of it all. Remember, it’s like going to the doctor, a second opinion never hurts!

Well, I hope today’s lesson satisfies everyone’s craving for knowledge and perhaps we’ll see some very happy people who used the information in these lessons to make some better turbo choices.

In the Final session we will take a good look at and compare standard production turbochargers to that of real “factory werks” racing turbochargers... class is dismissed.

No Really, Teacher, the dog ate my homework...

In the last lesson (Basic Turbocharging 101, Session 2), we accidentally left out a couple of items that need to be addressed to complete your basic understanding of turbocharger/engine matching.

The legend on the chart “Flow Rate vs. Pressure Ratio” was missing. The different lines represent different levels of sophistication in turbocharger system design, to help you use the most reasonable “quick” match for your application you should know what they represent. The lines were:



On the T28 compressor map (page 22), there are two operating lines plotted. The line made of squares represented the results of our quick match using the “Flow Rate vs. Pressure Ratio” chart, and the dashed circles line represented the operating line after a complete turbo/engine match was done, including turbine performance. Of particular interest is the difference between the two lines at 3,000 rpm. The detailed analysis shows that the compressor will not be operating at the same pressure ratio as the quick analysis did. This is because the turbine is not able to provide enough energy to the compressor at this low flow rate, even with the wastegate completely closed. In real life this is experienced as an inability to make the desired level of boost at low engine speeds, it is often incorrectly referred to as “turbo lag”. Higher levels of boost could be achieved (at 3,000 rpm) if the turbine were made smaller (smaller A/R and or wheel) but then some performance at high engine rpm would be lost due to excessive back-pressure. For this application it was decided that this was acceptable because there was still some positive boost at 3,000 rpm.

This leads to the following simplified points to complete the discussion of turbo/engine matching in addition to the points already discussed in previous lessons:

  • The compressor/engine must be matched in terms of pressure ratio and airflow rate (this was Session 2).
  • The turbine must be able to provide the compressor with the power needed to operate at the desired pressure ratio/air flow rates (from compressor/engine operating line) at the proper corrected speeds with airflow equal/less (wastegated) than that flowing through the engine.
  • Power is a function of flow rate, pressure ratio and temperature change, increasing any of these will increase the power involved. Specifically, the equation for power (compressor or turbine) is: Power = WC p³T where W is mass flow rate, Cp is a thermodynamic constant and ³T is the temperature change across the compressor or turbine (this can be calculated from the equations in Session 2).