Turbo Matching for Noobs
01-14-2005, 02:56 PM
#1
0.5 BAR
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Join Date: Jan 2005
Posts: 192
Turbo Matching for Noobs
This is a great article out of SCC about matching turbos for certain applications. It uses a Volkswagen 1.8T for the example but it applies to all cars.
A correctly matched turbo can add more power to your engine than any other single modification, but the wrong turbocharger can make your car a high-strung mess, a choked torque matchine, or even contribute to serious damage.
Before we get into finding a the ideal match for your engine, we should explain some basics of turbo matching. Choosing the right compressor (thats the intake side of the turbo that compresses the intake air) is probably the most critical step in selecting which turbo you should use. Compressor matching to the engine is also where most beginner mistakes are made. By using the formulas we will soon discuss, and looking at various compressor maps, you'll be able to closely estimate which compressor will be best for your intended use.
The math involved in choosing the correct turbocharger can be reduced to simple algebra while still retaining enough accuracy to make good matching choices. Matching also requires that the manufacturer publishes some important data, the most cirtical being the compressor map. Compressor maps are available from the Turbonetics catalog, the Innovated Turbo Systems catalog and from the Garret Engine Boosting Systems Web site, www.egarret.com.
The compressor map is a graph of the compressor's efficiency plotted against boost (expressed as pressure ratioon the Y-axis) and the mass airlow (expressed as pounds of air per minute on the X-axis). The compressor map is two-dimensional and reads like a topagraphical map, but with the islands representing compressor efficiency rather than altitude. Bisecting the islands moving from left to right across the map are the speed lines. These show the speed of the compressor/turbine in rpm.
But what is compressor efficiency? When air is compressed, it gets hotter. Thats just physics, and there's nothing you can do about it. But ideally, it only has to get as hot as the ideal gas law (PV=nRT) says. When we speak about compressor efficiency, we're referring to adiabatic efficiency or how much hotter air gets than it has to by law. The thermodynamically impossible nirvana of 100- percent efficiency would mean the compressor discharge temperature was perfectly predicted by the ideal gas law. Less than 100- percent means the compressed air is hotter.
The compressor map also has an important landmark, the surge line. The area of compressor surge is a line bordering the islands on the far left side of the map. Surge is when the air pressure after the compressor is higher than the compressor can generate. This causes the airflow in the compressor wheel to back up and stall. This, in turn, causes the pressure to drop, allowing flow to resume until pressure builds up and it stalls again. In severe surge, this can become a violent oscillation that destroys the thrust bearing of the turbo and can even cause mechanical failure to the wheel. Any compressor match should avoid crossing over the surge line.
You want to pick a compressor in which the engine spends most of the its operating range within the highest efficiency islands of the compressor map. You also want to pick the compressor with the highest efficiency. A good compressor means lower intake temperatures, which means higher air density and reduced load on the intercooler. A more efficient compressor requires less shaft power, reducing the energy that the turbine must try to extract from the exhaust. This quickens the turbo spool time and reduces exhaust backpressure, which improves volumetric efficiency.
With less exhaust gas being belched backward into the cylinders along with drops in combustion temperature, the engine's detonation resistance is increased, allowing tuning to be less compromised. Leaner air/fuel ratios, camshafts with more overlap and more ignition advance are all possible, meaning more power and better fuel economy.
To determine if the compressor is a good match for an engine, determine the engine's airflow over the rpm range and plot it on the compressor map. Ideally, the plot will fall over the map's best efficiency island and stay out of the surge range. The calculations are somewhat involved, but if you understand high school algebra, they're not too bad. The more computer literate of you may find this a useful tool and can put these equations into an Excel ------sheet to make it user-friendly.
Using a Volkwagen/Audi 1.8T as an example we'll use a simple, one-dimensional estimation method that will allow you to see if a turbo you're looking at is even in the ballpark. If your potential compressor choice is way off, you'll find your plot completely off the map. This method is also accurate enough to make a choice between two decent compressors.
This method will not allow you to predict lag, boost fall-off, or allow you to design your own turbo. What it will do, however, is allow you to look at a turbo that someone suggests for your car, and determine if they know what they're talking about.
Lets get straight to the math, then we'll explain what the math tells us. Bust out the calculator and sharpen your pencil.
Step 1
Figure out the maximum level of boost you plan to run. Most stock import engines with proper fueling, etc., on 91-cotane pump gas can handle at least 7 to 10 psi. Some very strong stock engines like our propsed VW 1.8T can take up to a 20 psi with proper tuning and race gas.
Basic Compressor match for 20psi on a VW 1.8T
First you have to make a few assumptions:
1. Pick a proposed boost level
2. Pressure drop across intercooler, assume 1.5 psi in most cases.
3. Atmospheric pressure: For sea level, that's 14.7 psi.
From these assumptions you can calculate absolute pressure out of the compressor (Pco):
Pco = Boost + Atmospheric Pressure + Intercooler Pressure drop
Which, in our case, means:
Pco = 20psi + 14.7psi + 1.5psi = 36.2psi
And now it's a simple matter to find the pressure ratio (Pr):
Pco 36.2psi
Pr = ------------------------------- = ------------- = 2.46
Atmospheric Pressure 14.7psi
Calculating approximate density of the air, after the intercooler (This is called Di)
To do this, we first have to guess what the post-intercooler temperature might be. One hundred thirty degrees Farenheit is a good starting point, and its normally what we see on turbocharged cars with a fairly good aftermarket intercooler.
Boost pressure + Atmospheric pressure
Di = --------------------------------------------------------
Rx12x(460+Post-intercooler temp)
In this case, R = 53.3(this is a constant, and what happens to be the same R from the ideal gas law, PV = nRT). The number 12 is there to preserve the inch units in the equation, and 460 is to convert degrees Farenheit to degrees Rankin (absolute temperature).
20psi + 14.7psi
Di = -------------------------------------- = 9.19 x 10 -5 pounds per cubic inch
53.3 x 12 x (460 + 130)
From this we can calculate the mass flow rate (Mf) of the engine at the rpm where we want to do the math:
Di x Displacement in cubic inches x RPM
Mf= --------------------------------------------------------
2 x Volumetric Efficiency
If you dont know your displacement in cubic inches, divide displacement in cc x 16.387.
For the volumetric Efficiency, we can assume 90 percent, which should be typical for this modern five-valve DOHC engine. Of course, the actual volumetric efficiency will vary from one engine to another and from one rpm point to another, and even depending on which turbo you use, but that's what makes this a simple model.
Also, note this formula is good for only one rpm point. You'll have to do this particular calculation over and over, plugging in different values of engine rpm to generate a useful picture of the turbo's performace across the rpm band. This is where a ------sheet comes in handy.
Good luck, I know it does require some math skills and a bit of time but it is a great article and WILL help decide what turbo you want depending on what kind of power you are looking for. Hope this helps a few people.
A correctly matched turbo can add more power to your engine than any other single modification, but the wrong turbocharger can make your car a high-strung mess, a choked torque matchine, or even contribute to serious damage.
Before we get into finding a the ideal match for your engine, we should explain some basics of turbo matching. Choosing the right compressor (thats the intake side of the turbo that compresses the intake air) is probably the most critical step in selecting which turbo you should use. Compressor matching to the engine is also where most beginner mistakes are made. By using the formulas we will soon discuss, and looking at various compressor maps, you'll be able to closely estimate which compressor will be best for your intended use.
The math involved in choosing the correct turbocharger can be reduced to simple algebra while still retaining enough accuracy to make good matching choices. Matching also requires that the manufacturer publishes some important data, the most cirtical being the compressor map. Compressor maps are available from the Turbonetics catalog, the Innovated Turbo Systems catalog and from the Garret Engine Boosting Systems Web site, www.egarret.com.
The compressor map is a graph of the compressor's efficiency plotted against boost (expressed as pressure ratioon the Y-axis) and the mass airlow (expressed as pounds of air per minute on the X-axis). The compressor map is two-dimensional and reads like a topagraphical map, but with the islands representing compressor efficiency rather than altitude. Bisecting the islands moving from left to right across the map are the speed lines. These show the speed of the compressor/turbine in rpm.
But what is compressor efficiency? When air is compressed, it gets hotter. Thats just physics, and there's nothing you can do about it. But ideally, it only has to get as hot as the ideal gas law (PV=nRT) says. When we speak about compressor efficiency, we're referring to adiabatic efficiency or how much hotter air gets than it has to by law. The thermodynamically impossible nirvana of 100- percent efficiency would mean the compressor discharge temperature was perfectly predicted by the ideal gas law. Less than 100- percent means the compressed air is hotter.
The compressor map also has an important landmark, the surge line. The area of compressor surge is a line bordering the islands on the far left side of the map. Surge is when the air pressure after the compressor is higher than the compressor can generate. This causes the airflow in the compressor wheel to back up and stall. This, in turn, causes the pressure to drop, allowing flow to resume until pressure builds up and it stalls again. In severe surge, this can become a violent oscillation that destroys the thrust bearing of the turbo and can even cause mechanical failure to the wheel. Any compressor match should avoid crossing over the surge line.
You want to pick a compressor in which the engine spends most of the its operating range within the highest efficiency islands of the compressor map. You also want to pick the compressor with the highest efficiency. A good compressor means lower intake temperatures, which means higher air density and reduced load on the intercooler. A more efficient compressor requires less shaft power, reducing the energy that the turbine must try to extract from the exhaust. This quickens the turbo spool time and reduces exhaust backpressure, which improves volumetric efficiency.
With less exhaust gas being belched backward into the cylinders along with drops in combustion temperature, the engine's detonation resistance is increased, allowing tuning to be less compromised. Leaner air/fuel ratios, camshafts with more overlap and more ignition advance are all possible, meaning more power and better fuel economy.
To determine if the compressor is a good match for an engine, determine the engine's airflow over the rpm range and plot it on the compressor map. Ideally, the plot will fall over the map's best efficiency island and stay out of the surge range. The calculations are somewhat involved, but if you understand high school algebra, they're not too bad. The more computer literate of you may find this a useful tool and can put these equations into an Excel ------sheet to make it user-friendly.
Using a Volkwagen/Audi 1.8T as an example we'll use a simple, one-dimensional estimation method that will allow you to see if a turbo you're looking at is even in the ballpark. If your potential compressor choice is way off, you'll find your plot completely off the map. This method is also accurate enough to make a choice between two decent compressors.
This method will not allow you to predict lag, boost fall-off, or allow you to design your own turbo. What it will do, however, is allow you to look at a turbo that someone suggests for your car, and determine if they know what they're talking about.
Lets get straight to the math, then we'll explain what the math tells us. Bust out the calculator and sharpen your pencil.
Step 1
Figure out the maximum level of boost you plan to run. Most stock import engines with proper fueling, etc., on 91-cotane pump gas can handle at least 7 to 10 psi. Some very strong stock engines like our propsed VW 1.8T can take up to a 20 psi with proper tuning and race gas.
Basic Compressor match for 20psi on a VW 1.8T
First you have to make a few assumptions:
1. Pick a proposed boost level
2. Pressure drop across intercooler, assume 1.5 psi in most cases.
3. Atmospheric pressure: For sea level, that's 14.7 psi.
From these assumptions you can calculate absolute pressure out of the compressor (Pco):
Pco = Boost + Atmospheric Pressure + Intercooler Pressure drop
Which, in our case, means:
Pco = 20psi + 14.7psi + 1.5psi = 36.2psi
And now it's a simple matter to find the pressure ratio (Pr):
Pco 36.2psi
Pr = ------------------------------- = ------------- = 2.46
Atmospheric Pressure 14.7psi
Calculating approximate density of the air, after the intercooler (This is called Di)
To do this, we first have to guess what the post-intercooler temperature might be. One hundred thirty degrees Farenheit is a good starting point, and its normally what we see on turbocharged cars with a fairly good aftermarket intercooler.
Boost pressure + Atmospheric pressure
Di = --------------------------------------------------------
Rx12x(460+Post-intercooler temp)
In this case, R = 53.3(this is a constant, and what happens to be the same R from the ideal gas law, PV = nRT). The number 12 is there to preserve the inch units in the equation, and 460 is to convert degrees Farenheit to degrees Rankin (absolute temperature).
20psi + 14.7psi
Di = -------------------------------------- = 9.19 x 10 -5 pounds per cubic inch
53.3 x 12 x (460 + 130)
From this we can calculate the mass flow rate (Mf) of the engine at the rpm where we want to do the math:
Di x Displacement in cubic inches x RPM
Mf= --------------------------------------------------------
2 x Volumetric Efficiency
If you dont know your displacement in cubic inches, divide displacement in cc x 16.387.
For the volumetric Efficiency, we can assume 90 percent, which should be typical for this modern five-valve DOHC engine. Of course, the actual volumetric efficiency will vary from one engine to another and from one rpm point to another, and even depending on which turbo you use, but that's what makes this a simple model.
Also, note this formula is good for only one rpm point. You'll have to do this particular calculation over and over, plugging in different values of engine rpm to generate a useful picture of the turbo's performace across the rpm band. This is where a ------sheet comes in handy.
Good luck, I know it does require some math skills and a bit of time but it is a great article and WILL help decide what turbo you want depending on what kind of power you are looking for. Hope this helps a few people.
01-15-2005, 07:38 AM
#4
0.5 BAR
Thread Starter
Join Date: Jan 2005
Posts: 192
Re:Turbo Matching for Noobs
No you dont. Trust me once I finish it you will understand it a lot more, the best thing to do is to go into that other sticky note about the turbo compressor maps and do out these equations for a few different turbos, its pretty neat to see. Definitely a worth while task. I will finish on monday so check back then.
P.S YES I typed all of that! You cant get that copy of the magazine anymore its quite old but regardless a great article.
P.S YES I typed all of that! You cant get that copy of the magazine anymore its quite old but regardless a great article.
01-17-2005, 01:41 PM
#6
Administrator
Join Date: Dec 2002
Posts: 13,991
Re:Turbo Matching for Noobs
Check out this formula
T3 60/63 minor Shaft play 45 bucks
X1 = -------------------------------------- = T3 for $45
14b for 100 bucks, mint
You see HMT's forumla is... whatever is the cheapest and works
T3 60/63 minor Shaft play 45 bucks
X1 = -------------------------------------- = T3 for $45
14b for 100 bucks, mint
You see HMT's forumla is... whatever is the cheapest and works
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