Ok.... here is some turbo math from the June 2003 Sport Compact Car magasine.
I'm going to use the A16DMS (1.6L DOHC) as an example in this one.
Lets use 8 PSI as the boost you want to run in your car.
Absolute turbo outlet pressure = Pco
Pboost = Boost Pressure
Patm= atmospheric pressure = 14.7 PSI
Pint= Pressure drop accross an intercooler (say 1.5 PSI for our example)
Pco = Pboost + Patm + Pint
Pco = 8 PSI + 14.7 PSI + 1.5 PSI
Pco = 24.2 PSI
Next you want your Pressure Ratio = Pco/Patm = 24.2/14.7 = 1.646
Next equation finds the intake air density.
General ballpark figure of air temp after the intercooler = 130 deg F on a day when the outdoor temperature is say 75 to 85 degrees F
Intake Air Density = Di
R= ideal gas law constant = 53.3
Di = ( Pboost + Patm) / (R X 12 X (460 + intake temp )
Di = ( 8 PSI + 14.7 PSI ) / (53.3 X 12 X (460+130)
Di = 0.000060154
Next equation then calculates Mass Flowrate
Mf= Mass flowrate
Veff= Volumetric Efficency which is about 90% for your average 16V DOHC and about 80% for an 8V SOHC
Mf = Di X displacement (cu in) X (redline RPM / 2) X Veff
Mf = 0.000060154 X 97.64 X (6200/2) X 0.90
Mf =16.387 lbs/minute
Next we need a corrected mass air flow number
CMf= corrected mass air flow
Tci = Temperature entering Compressor inlet ( say 85 degrees )
Cip = Corrected compressor inlet pressure which is typically about 13.95 PSI due to the drop in pressure accross your air filter
CMf = ( Mf ) X ( (Tci + 460) / 545 ) / ( Patm/Cip )
CMf = 16.387 X (( 85 + 460)/545) / ( 14.7 / 13.95 )
CMf = 16.387 X (545/545) / ( 1.0538)
CMf = 15.55 lbs/min
Now you have both the required pressure ratio ( 1.646 ) and your true mass air flow ( 15.55 lbs/min )
From here you need to find the compressor map of the turbo you want to use and see if it lands in the most efficent part of the map.
If it lands left of the compressor map your turbo is way too big and you will have major problems with surge..... which can destroy your turbo very quickly.
If it lands right smack in the middle of the map you have picked the right sized turbo.
If it lands to the right of the map the turbo is too small and will require that you change to a slightly larger turbo (or else your turbo will be very inefficent and cause you to loose horsepower)
Go to the following link...
http://www.atpturbo.com
here you will find all kinds of turbos with the GT series being the best.
You can also look at the map to see if your turbo will work.
Go to the top right under "turbo info" and select "compressor maps"
If you pick something like a GT25R the map will come up and you can plot your 1.646 by 15.55 number and see that it is a great turbo for a 1.6L engine running 8 PSI of boost.
The plot point sits just on the left edge of the most efficent part of the compressor map ( edge of the 73% eff section )
Now go to something like a GT32 compressor map (larger turbo) and plot the numbers....... you will see that you are way down on the map and this GT32 won't be as efficent as teh GT25R.
You also have to take into account the fact that ideally you want to be in the high right part of the most efficent part of the map.
You could take a line from the start of the graph ( 1 , 0 ) and draw a line to your plot point ( 1.646, 15.55) and you will see that you are off the map on the GT32 for most of the time (drops off at (1.3,10)) where as the on the GT25R it's on the map down to about (1.2, 5)
So this basicly tells you it will take way more RPM before you build any pressure on a GT32 compared to a GT25R
This is known as Turbo Lag which means your engine will be running like it's naturally aspirated for most of the lower RPM if your turbo is too large.
Turbo Calculations
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PrecisionBoost
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PrecisionBoost
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Now look at the GT20 and you will see that the (1.646,15.55) puts you smack dab in the middle of a 79% efficency area of the map which is even better than the GT25R ( only 73% efficent)
The GT22 is also really good with about 79% efficency but it will take slightly longer to spool up than the GT20 so the GT20 will make your car faster because it makes boost at a lower RPM.
If you go to a GT15 you will start to drop off onto the right side of the map which lowers the efficency to about 73%
So the smaller GT15 has the same efficency as the larger GT25R but the GT15 is a better choice because it will spool up way quicker and give you more horspower (and boost) at lower RPM.
But overall the best choice is a Garrett GT20
The higher the efficency the more power you will make so allways go for the best efficency, and avoid large turbos at all costs.
The GT22 is also really good with about 79% efficency but it will take slightly longer to spool up than the GT20 so the GT20 will make your car faster because it makes boost at a lower RPM.
If you go to a GT15 you will start to drop off onto the right side of the map which lowers the efficency to about 73%
So the smaller GT15 has the same efficency as the larger GT25R but the GT15 is a better choice because it will spool up way quicker and give you more horspower (and boost) at lower RPM.
But overall the best choice is a Garrett GT20
The higher the efficency the more power you will make so allways go for the best efficency, and avoid large turbos at all costs.
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PrecisionBoost
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Next we will try to figure out how much "lag" this turbo will have by plotting a line to a lower speed.
So first thing I do is print out the compressor map and draw a dot at my plot point of 1.646,15.55
Next I'm going to take a ruler and draw a line from that point to the (1,0) graph origin. ( a straight line is not correct but it's just an estimate)
Next I'm going to find the point along the line where it doesn't show any more data which is about ( 1.35, 8.5 )
If you follow the graph line over you can see this intersects a 74% efficency line on the compressor map.
So now we work backwards with the formulas to see if we can get some info about the RPM and Boost at this lower level.
Mf = CMf X 1.0538
Mf = 8.5 X 1.0538
Mf =8.96 lbs/min
Pressure ratio =1.35 = Pco/Patm
Pco = 19.845
So 19.845 = Pboost + 14.7 + 1.5
Pboost = 3.645 PSI
From this we can now see that at this lower RPM the turbo is only putting out about half of the amount of boost it does at Redline.....but 3.6 PSI will still generate a significant amount of power (rough guess 25% more than if it was naturally aspirated)
We now find Di
Di = ( 3.645 + 14.7 ) / (53.3 X 12 X 590 )
Di = 0.000048614
Mf = Di X 97.64 X ( RPM/2 ) X 0.9
8.96 = 0.000048614 X 97.64 X (RPM/2) X0.9
Engine Speed = 4195 RPM
So I'm guessing that at about 4200RPM the turbo will be generating about 3.6 PSI of boost.
It's too bad the maps didn't extend further down, but I'm guessing that this turbo shouldn't have too much lag between shifts.
If it were a larger turbo we might find that it is really inefficent at 4200RPM and the power would drop dramaticly.
When your efficency is lower the turbo doesn't convert the power so in reallity my straight line approach is nothing more than a guess since there are way too many factors involved to figure out the real path the turbo will take ( how many PSI at whatever RPM you want to calculate)
Some guys might say 3.6 PSI is too low but consider a the fact that a larger turbo might not generate any boost at all at this RPM.
I hope that this has helped you guys get a rough idea of how to figure out which turbo to use on your cars, and I must stress that there are way too many factors to generate a perfect model of how the Turbo will react.
Simple things such as intercooler efficency, engine volumetric efficency,outside air temp......(many others) can make these numbers change quite a bit.
So first thing I do is print out the compressor map and draw a dot at my plot point of 1.646,15.55
Next I'm going to take a ruler and draw a line from that point to the (1,0) graph origin. ( a straight line is not correct but it's just an estimate)
Next I'm going to find the point along the line where it doesn't show any more data which is about ( 1.35, 8.5 )
If you follow the graph line over you can see this intersects a 74% efficency line on the compressor map.
So now we work backwards with the formulas to see if we can get some info about the RPM and Boost at this lower level.
Mf = CMf X 1.0538
Mf = 8.5 X 1.0538
Mf =8.96 lbs/min
Pressure ratio =1.35 = Pco/Patm
Pco = 19.845
So 19.845 = Pboost + 14.7 + 1.5
Pboost = 3.645 PSI
From this we can now see that at this lower RPM the turbo is only putting out about half of the amount of boost it does at Redline.....but 3.6 PSI will still generate a significant amount of power (rough guess 25% more than if it was naturally aspirated)
We now find Di
Di = ( 3.645 + 14.7 ) / (53.3 X 12 X 590 )
Di = 0.000048614
Mf = Di X 97.64 X ( RPM/2 ) X 0.9
8.96 = 0.000048614 X 97.64 X (RPM/2) X0.9
Engine Speed = 4195 RPM
So I'm guessing that at about 4200RPM the turbo will be generating about 3.6 PSI of boost.
It's too bad the maps didn't extend further down, but I'm guessing that this turbo shouldn't have too much lag between shifts.
If it were a larger turbo we might find that it is really inefficent at 4200RPM and the power would drop dramaticly.
When your efficency is lower the turbo doesn't convert the power so in reallity my straight line approach is nothing more than a guess since there are way too many factors involved to figure out the real path the turbo will take ( how many PSI at whatever RPM you want to calculate)
Some guys might say 3.6 PSI is too low but consider a the fact that a larger turbo might not generate any boost at all at this RPM.
I hope that this has helped you guys get a rough idea of how to figure out which turbo to use on your cars, and I must stress that there are way too many factors to generate a perfect model of how the Turbo will react.
Simple things such as intercooler efficency, engine volumetric efficency,outside air temp......(many others) can make these numbers change quite a bit.
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PrecisionBoost
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Next I want to do some calculations for the 2.0L 16V Nubira engine.
For the first calculation lets use 12 PSI as the boost you want to run in your car.
Absolute turbo outlet pressure = Pco
Pboost = Boost Pressure
Patm= atmospheric pressure = 14.7 PSI
Pint= Pressure drop accross an intercooler (say 1.5 PSI for our example)
Pco = Pboost + Patm + Pint
Pco = 12 PSI + 14.7 PSI + 1.5 PSI
Pco = 28.2 PSI
Next you want your Pressure Ratio = Pco/Patm = 28.2/14.7 = 1.918
Next equation finds the intake air density.
General ballpark figure of air temp after the intercooler = 130 deg F on a day when the outdoor temperature is say 75 to 85 degrees F
Intake Air Density = Di
R= ideal gas law constant = 53.3
Di = ( Pboost + Patm) / (R X 12 X (460 + intake temp )
Di = ( 12 PSI + 14.7 PSI ) / (53.3 X 12 X (460+130)
Di = 0.000070754
Next equation then calculates Mass Flowrate
Mf= Mass flowrate
Veff= Volumetric Efficency which is about 90% for your average 16V DOHC and about 80% for an 8V SOHC
Mf = Di X displacement (cu in) X (redline RPM / 2) X Veff
Mf = 0.000070754 X 122 X (6500/2) X 0.90
Mf =25.25 lbs/minute
Next we need a corrected mass air flow number
CMf= corrected mass air flow
Tci = Temperature entering Compressor inlet ( say 85 degrees )
Cip = Corrected compressor inlet pressure which is typically about 13.95 PSI due to the drop in pressure accross your air filter
CMf = ( Mf ) X ( (Tci + 460) / 545 ) / ( Patm/Cip )
CMf = 25.25 X (( 85 + 460)/545) / ( 14.7 / 13.95 )
CMf = 25.25 X (545/545) / ( 1.0538)
CMf = 23.96 lbs/min
Now you have both the required pressure ratio ( 1.918 ) and your true mass air flow ( 23.96 lbs/min )
For the first calculation lets use 12 PSI as the boost you want to run in your car.
Absolute turbo outlet pressure = Pco
Pboost = Boost Pressure
Patm= atmospheric pressure = 14.7 PSI
Pint= Pressure drop accross an intercooler (say 1.5 PSI for our example)
Pco = Pboost + Patm + Pint
Pco = 12 PSI + 14.7 PSI + 1.5 PSI
Pco = 28.2 PSI
Next you want your Pressure Ratio = Pco/Patm = 28.2/14.7 = 1.918
Next equation finds the intake air density.
General ballpark figure of air temp after the intercooler = 130 deg F on a day when the outdoor temperature is say 75 to 85 degrees F
Intake Air Density = Di
R= ideal gas law constant = 53.3
Di = ( Pboost + Patm) / (R X 12 X (460 + intake temp )
Di = ( 12 PSI + 14.7 PSI ) / (53.3 X 12 X (460+130)
Di = 0.000070754
Next equation then calculates Mass Flowrate
Mf= Mass flowrate
Veff= Volumetric Efficency which is about 90% for your average 16V DOHC and about 80% for an 8V SOHC
Mf = Di X displacement (cu in) X (redline RPM / 2) X Veff
Mf = 0.000070754 X 122 X (6500/2) X 0.90
Mf =25.25 lbs/minute
Next we need a corrected mass air flow number
CMf= corrected mass air flow
Tci = Temperature entering Compressor inlet ( say 85 degrees )
Cip = Corrected compressor inlet pressure which is typically about 13.95 PSI due to the drop in pressure accross your air filter
CMf = ( Mf ) X ( (Tci + 460) / 545 ) / ( Patm/Cip )
CMf = 25.25 X (( 85 + 460)/545) / ( 14.7 / 13.95 )
CMf = 25.25 X (545/545) / ( 1.0538)
CMf = 23.96 lbs/min
Now you have both the required pressure ratio ( 1.918 ) and your true mass air flow ( 23.96 lbs/min )
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PrecisionBoost
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Now if you took that GT20 compressor map ( http://www.atpturbo.com/root/index.htm ) you would find that the plot point of 1.918,23.96 is way off onto the right of the map which indicates an efficency of about 76%
This is certainly no where near the middle 79% efficency but it's pretty good.
This turbo would spool very quickly and if you took a path back to zero flow you would see that actually gets more efficent as you drop down in RPM so you would probably see great boost levels at lower RPM
The one I like is the GT28RS and if you plot the points you end up dead center in the middle of the most efficent part of the map which is as perfect as you can get.
So the GT20 will spool quicker but the GT28RS is capable of running with better efficency and more top end power.
The GT25 also looks nice with the plot points just touching the top right part of the most efficent part of the map which means it should produce good efficency numbers at lower RPM.
This is certainly no where near the middle 79% efficency but it's pretty good.
This turbo would spool very quickly and if you took a path back to zero flow you would see that actually gets more efficent as you drop down in RPM so you would probably see great boost levels at lower RPM
The one I like is the GT28RS and if you plot the points you end up dead center in the middle of the most efficent part of the map which is as perfect as you can get.
So the GT20 will spool quicker but the GT28RS is capable of running with better efficency and more top end power.
The GT25 also looks nice with the plot points just touching the top right part of the most efficent part of the map which means it should produce good efficency numbers at lower RPM.
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PrecisionBoost
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Next I want to do some calculations for the 2.0L 8V Sunbird engine.
Lets use 18 PSI as the boost you want to run in your car.
Absolute turbo outlet pressure = Pco
Pboost = Boost Pressure
Patm= atmospheric pressure = 14.7 PSI
Pint= Pressure drop accross an intercooler (say 1.5 PSI for our example)
Pco = Pboost + Patm + Pint
Pco = 18 PSI + 14.7 PSI + 1.5 PSI
Pco = 34.2 PSI
Next you want your Pressure Ratio = Pco/Patm = 34.2/14.7 = 2.327
Next equation finds the intake air density.
General ballpark figure of air temp after the intercooler = 130 deg F on a day when the outdoor temperature is say 75 to 85 degrees F
Intake Air Density = Di
R= ideal gas law constant = 53.3
Di = ( Pboost + Patm) / (R X 12 X (460 + intake temp )
Di = ( 18 PSI + 14.7 PSI ) / (53.3 X 12 X (460+130)
Di = 0.000086654
Next equation then calculates Mass Flowrate
Mf= Mass flowrate
Veff= Volumetric Efficency which is about 80% for an 8V SOHC
Mf = Di X displacement (cu in) X (redline RPM / 2) X Veff
Mf = 0.000086654 X 122 X (7000/2) X 0.80
Mf =29.6 lbs/minute
Next we need a corrected mass air flow number
CMf= corrected mass air flow
Tci = Temperature entering Compressor inlet ( say 85 degrees )
Cip = Corrected compressor inlet pressure which is typically about 13.95 PSI due to the drop in pressure accross your air filter
CMf = ( Mf ) X ( (Tci + 460) / 545 ) / ( Patm/Cip )
CMf = 29.6 X (( 85 + 460)/545) / ( 14.7 / 13.95 )
CMf = 29.6 X (545/545) / ( 1.0538)
CMf = 28.09 lbs/min
Now you have both the required pressure ratio ( 2.327 ) and your true mass air flow ( 28.09lbs/min )
Lets use 18 PSI as the boost you want to run in your car.
Absolute turbo outlet pressure = Pco
Pboost = Boost Pressure
Patm= atmospheric pressure = 14.7 PSI
Pint= Pressure drop accross an intercooler (say 1.5 PSI for our example)
Pco = Pboost + Patm + Pint
Pco = 18 PSI + 14.7 PSI + 1.5 PSI
Pco = 34.2 PSI
Next you want your Pressure Ratio = Pco/Patm = 34.2/14.7 = 2.327
Next equation finds the intake air density.
General ballpark figure of air temp after the intercooler = 130 deg F on a day when the outdoor temperature is say 75 to 85 degrees F
Intake Air Density = Di
R= ideal gas law constant = 53.3
Di = ( Pboost + Patm) / (R X 12 X (460 + intake temp )
Di = ( 18 PSI + 14.7 PSI ) / (53.3 X 12 X (460+130)
Di = 0.000086654
Next equation then calculates Mass Flowrate
Mf= Mass flowrate
Veff= Volumetric Efficency which is about 80% for an 8V SOHC
Mf = Di X displacement (cu in) X (redline RPM / 2) X Veff
Mf = 0.000086654 X 122 X (7000/2) X 0.80
Mf =29.6 lbs/minute
Next we need a corrected mass air flow number
CMf= corrected mass air flow
Tci = Temperature entering Compressor inlet ( say 85 degrees )
Cip = Corrected compressor inlet pressure which is typically about 13.95 PSI due to the drop in pressure accross your air filter
CMf = ( Mf ) X ( (Tci + 460) / 545 ) / ( Patm/Cip )
CMf = 29.6 X (( 85 + 460)/545) / ( 14.7 / 13.95 )
CMf = 29.6 X (545/545) / ( 1.0538)
CMf = 28.09 lbs/min
Now you have both the required pressure ratio ( 2.327 ) and your true mass air flow ( 28.09lbs/min )
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PrecisionBoost
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Even for the higher 18psi on the 2.0L turbo sunbird engine I like the GT28Rs as the plot points end up in the center of the top portion of the compressor map with a 77% efficency.
If you try to plot these points on a GT25 you will see that it's way off the map which indicates that it's too much boost and flow for the GT25 to handle.
If you try to plot these points on a GT25 you will see that it's way off the map which indicates that it's too much boost and flow for the GT25 to handle.
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GsiTurbo
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WOW!!! All I have to say. This should certainly go the the Archives. Very good technical post Chris!
I'll play with my PC and will try to make a program that will calculate that for us on the fly. I can use the current compressor maps to have the program plot the efficiency as well.
Stay tuned!
I'll play with my PC and will try to make a program that will calculate that for us on the fly. I can use the current compressor maps to have the program plot the efficiency as well.
Stay tuned!
__________________________
2002 Lanos 1.5 SOHC... stock!!!
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PrecisionBoost
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I was trying to figure out the line which dictates the RPM levels at which full boost is created and I found a site which make my calculations look like grade 9 math.
http://cybrina.mine.nu/MR2_Docs/compres ... w_maps.htm
It's exactly what I was looking for and it is a great source of information.
http://cybrina.mine.nu/MR2_Docs/compres ... w_maps.htm
It's exactly what I was looking for and it is a great source of information.