Building a turbocharger system can be as complex or as simple as you want, but building a successful combination that suits your needs and works without issue certainly takes a fair amount of work. In this chapter of our Turbocharging 101 series, we are going to focus on the theory and fabrication of a turbocharger hot side, which basically means any part of the system that deals with moving exhaust gas, including the exhaust manifolds, the manifold to turbocharger piping, the turbine housing, the wastegate, and the downpipe. If it gets hot, and we’re talking melt things in the vicinity and shooting fire hot, it belongs to the turbocharger’s “hot side” and needs to be carefully constructed to work properly.
Choosing Your Components:
Before beginning any turbocharger build, it is critical to clearly define your goals and then select components based on your specific requirements. This means what works for one person may not work for another and you can’t expect to build a hodge-podge system with mismatched components and be happy with the results. As exciting as it is to jump in and order parts, the first couple of days (or weeks or months) of your build should focus on research and some simple mathematics to help get you in the ballpark.
If you haven’t caught on by now, we are planning to build a serious turbo car, from the ground up, to show you how this process comes together. After months of planning and dreaming, we have laid out plans to build a 200-plus mile per hour, standing mile racer out of a 2000 Trans Am WS6. Over the course of the next year, we will delve into the engine build, wiring, drivetrain and tuning, but we are beginning everything with the turbo system. To reach 200 mph and really push the limits of our fabrication and turbo knowledge, we have chosen the following goals for our build.
&bull 1,000 hp at 6,500 rpm
&bull 370-cid engine
&bull Single turbocharger
&bull Air-to-water intercooler
&bull E-85 (112 octane) fuel
Hot Side Components:
Turbonetics Y2K 88mm Turbocharger, .96 A/R
A 1,000hp engine requires roughly 100 lbs/min of mass flow, which means our build requires a fairly large turbocharger with good, efficient flow. Knowing that we needed to make a lot of power reliably, we teamed up with Turbonetics, a company you are no doubt familiar with, and sat down with the engineering team to find a turbo that would work within our parameters. After doing the math (see sidebar, Compressor Maps 101) we chose the famous Y2K Series 88mm Turbocharger, which Turbonetics bills as a “a durable mid-sized frame turbocharger capable of supporting up to 1,200 horsepower from a single turbocharger application.” With massive airflow and ceramic ball bearing technology, the 88mm will allow us to make great power for miles and miles, which is key for a build of this caliber.
Turbonetics NewGen HP Wastegate
Boost control, or the ability to regulate boost to a safe and useable level, is the job of the wastegate. Without a ‘gate, a turbo system would have no way to regulate turbine wheel speed, which would cause the turbocharger to produce an excessive amount of boost and, most likely, destroy engine parts in the process. A wastegate then acts as a controlled diversion for some exhaust gas, allowing it to bypass the turbine housing and thus control boost. Because a wastegate is critical to a turbo system, it is one area you don’t want to skimp on. For our build, Turbonetics recommended a NewGen HP wastegate, which uses an innovative swing-valve (instead of a traditional valve) to control exhaust gas flow and can move a ton of exhaust quickly to help regulate boost easily.
Vibrant Performance Stainless Steel Tubing
OK, so we have a turbocharger and a wastegate...now what? Well, we have to physically connect everything together, using tubing to connect the exhaust manifolds to the turbocharger and wastegate, and then give the spent exhaust gasses a place to go. Up until recently, finding quality, reliable sources for this stuff was difficult, but thanks to companies like Vibrant Performance, those days are long gone. If you don’t already use Vibrant, they are a great source of fabrication material, providing everything from aluminum charge piping to intercoolers and couplers to stainless steel tubing and everything in between. Regardless of your goals, we recommend Stainless Steel piping for everything on the hot side, even if it stretches the budget. Stainless Steel is an excellent material to use in a hot side due to its crack resistance, low thermal conductivity and its long-term corrosion resistance, among other factors. Don’t get us wrong, you can use Mild Steel in a pinch, but Stainless will last a lifetime and offer many advantages over Mild, although as we said, it can be used.
With our fabrication and turbocharger components in hand, the only thing left to do was to break out the TIG welder, band saw, and Sharpie and get to work. For that, we turned to the master LS builders and fabricators at Vengeance Racing, where Ron Mowen and Jay Healy set aside a bay and were more than happy to walk us through the fabrication from start to finish. As you will see, these guys are serious professionals with years of experience and have worked on some major turbo cars in the past. This knowledge, combined with your own ingenuity, can lead to some great builds, if you have the time, money, and inclination to build a turbo car of your own. Without further ado, we present the hot side system on our standing mile project.
Compressor Maps 101
To find the optimal turbocharger for any engine, you must understand how to properly read and plot a compressor map. While these maps may look incredibly complex at first, they are actually quite simple to read with just a couple of easy mathematic equations and a little thought. In basic terms, a compressor map will allow you to see the airflow of a turbocharger at varying pressure ratios and the exact efficiency of the turbocharger at those points. To find the right turbo for the job, you are looking for an efficient compressor (meaning, at least 65-percent efficiency), which can flow enough air to support the power you plan on making. It is that simple!
Think of a compressor map as a dyno graph for a turbocharger, which rates how well “the compressor wheel can pump air without heating it more than the thermodynamic laws say it should.” That is to say that, compressing air produces heat—we know this—and the rise of temperature should always follow the mathematic formula given in the laws of thermodynamics. In the real world, aka the one we live in, we will never see a 100-percent efficient compressor and as such, we can derive an efficiency number for any given turbo by dividing the actual temperature of the air at a given pressure ratio and airflow rate by the ideal temperature. If that’s too much information, don’t worry about it, just understand that you want a turbocharger with at least 65-percent efficiency, but the closer you can get to 70, 75 or 80-percent, the happier your entire combination will be.
Airflow rate = (cid x rpm x 0.5 x VE) / 1728
cid = the engine’s displacement in cubic inches
rpm = maximum rpm the engine plans to run
0.5 = 4-stroke engine, filling on half of the revolutions per minute
VE = estimated Volumetric Efficiency
Example with our 370 cubic-inch LQ9 engine at 6,500 rpm and an 85% volumetric efficiency
Airflow rate = (370 x 6500 x 0.5 x 0.85) / 1728 = 591.5 cfm
Thanks to our calculation, we now know that our 370 cubic-inch LS would require 591.5 cubic-feet per minute of air to make maximum power at 6500 rpm. But, we’re not going to run without boost, so the next step is to factor that into our equation, to determine exactly how much additional airflow we need to make the power we desire.
Pressure Ratio is defined as the total absolute pressure produced by the turbo divided by atmospheric pressure.
Pressure Ratio = (absolute pressure + boost pressure) / absolute pressure
Absolute pressure = 14.7 psi at sea level
Boost pressure = desired maximum boost level
Example for 20 psi, our target maximum boost level
(14.7 + 20) / 14.7 = 2.36
Turbocharged Airflow Rate:
Turbocharged airflow rate = (airflow rate cfm x pressure ratio)
Example using 20 psi on a 370 cubic-inch LQ9 engine at 6,500 rpm and 85% volumetric efficiency
(591.5 x 2.36) = 1395.94 cfm
Converting cfm to lbs/min:
Now that we have the cubic-feet per minute (cfm) we need to convert it to lbs/min, since most modern compressor maps deal with airflow in Mass Flow, since heated and compressed air has slightly different properties than regular atmospheric air. Again, this isn’t anything to concern yourself with, just take cfm and multiply it by .076 to get a corrected lbs/min figure.
Corrected Airflow = (cfm x .076) = lbs/min
Example using our turbocharger airflow rate of 1395.94 cfm
(1395.94 x .076) = 106.09 lbs/min
Now, finally, we can plot this information on our compressor map. Based on some quick research and some rough estimates, we thought the Turbonetics Y2K 88mm would work well for our application, so we started (and ended) with that compressor map. Along the X-axis, we found our pressure ratio of 2.36 and drew a horizontal line. Along the Y-axis, we found our corrected airflow of 106.09 and drew a vertical line. The intersection of those two lines represents the efficiency of the 88mm turbo at our desired operating range, which happens to be a very nice 76-percent. Remember, we are shooting for as close to perfect as possible, which is 80-percent on for this turbo, so 76 is really good and will be almost perfect for our power goals. Of note, you can quickly estimate overall engine power by multiplying a turbocharger’s corrected airflow (lbs/min) by 10, since an average engine requires 1-lb mass of air for every 10 horsepower. In our case, 106.09 multiplied by 10 gives us roughly 1060.91 available horsepower, which is exactly what we are looking for.
Horsepower Potential = (corrected airflow x 10)
Example using our 370 cubic-inch engine with a Turbonetics Y2K 88mm turbocharger at 20psi, intercooled.
(106.09 x 10) = 1060.91 hp
Did you make it all the way through this information? Congratulations! Most people give up after the first equation and just buy turbochargers with cool names. Since you made it all the way, we’re going to let you in on a little secret...you don’t have to do any of this math if you don’t want to! Turbonetics, our partner for this project, is willing to do all of this for you and spec out the perfect turbocharger for any build, from a stock motor street car to a dedicated drag or standing mile racer. Just give them a call or hit up the website for more information.
Cambridge, MA.: Bentley Publishers, 1997
North Branch, MN.: Cartech Books, 2008