With all of the talk about turbo EFI, I thought it might be helpful if I share some information on selecting the right turbo for your car. They tell us, "The best way to learn something is to teach it." While I'm not an expert on turbo systems, nor do I claim to be, I'd like to share what I've learned. I'm sure some of you are also contemplating the move to turbo, so hopefully you'll find some benefit to this info.
Turbo manufacturers create a map for you to use to see if the particular turbo in question is a good fit for your engine. It is much cheaper to look at turbo maps and do some math calculations than it is to buy and swap out turbos until you find the one that works well with your engine.
A turbo map looks like this: (from Honeywell Garrett)
What the heck is all that mess?
Well Garrett did a good job of explaining the different areas of the map, but you need to have some data (pressure ratio (Y axis) and corrected air flow (X axis)) from your engine before this thing will do you any good. Let's take a look at the engine that I'm going to build. I want to have a streetable 2276 that I can crank up the boost on, and make some passes at the strip. I figure I'll run 5-8 lbs of boost around town and 15 or so at the track with race fuel. Let's figure out the pressure ratio for my engine first.
To figure out the pressure ratio we need to use the following formula:
Pressure Ratio = (Boost PSI + Atmospheric Pressure*) / 14.7
*Atmospheric Pressure at sea level = 14.7
Now if I substitute my numbers in there...
Pressure Ratio = (15boost + 14.7) / 14.7 = 29.7/14.7 = 2.02
That would work if I was at sea level, but I live in Las Vegas, Nevada which is at 2165 feet above see level. To adjust for your elevation, use this chart...
Altitude (ft)
| Pressure (psi)
|
Sea Level
| 14.7
|
1000
| 14.2
|
2000
| 13.7
|
3000
| 13.2
|
4000
| 12.7
|
5000
| 12.2
|
6000
| 11.8
|
7000
| 11.3
|
So I'm going to use the 13.7 PSI from the chart for my atmospheric pressure number.
Pressure Ratio = (15boost + 13.7) / 14.7 = 28.7/14.7 = 1.95
The next thing I need to find is my engine's airflow needs. On the X axis on the 2nd graph we see air flow given in pounds per minute. Some compressor maps give it in Cubic Feet per Minute (CFM). To convert pounds per minute into CFM, you need to take the temperature of the air into consideration (the ideal gas law tells us that as gas heats up, it expands, which means that the hotter the gas, the less it weighs per cubic feet, which is why a hot air balloon rises). Fortunately, most compressor maps are taken at 85F (you can usually tell by looking at the formula written on the map which has a temperature number like 545 and subtracting 460 from that number to convert it to Fahrenheit). One cubic foot of air at 85F weighs 0.07282 pounds. So, at 85F, convert pounds per minute to CFM by multiplying by 13.73.
To figure out the CFM requirements of my engine in naturally aspirated mode, we need to use this formula...
CFM = Displacement in Cubic Inches / 3456 x RPM x VE
To convert from cc's to cubic inches = CC's X 0.06102 = CI's (My 2276 x 0.06102 = 138.88)
The RPM is the Max you want to buzz your engine up to and the Volumetric Efficiency of a turbocharged VW engine is roughly 80%
So in my case...
CFM=138.88(CI's)/3456 x 5800(RPM) x 80%(VE) = 186.46 CFMBUT since this is a boosted engine, I need to multiply my
CFM x
by my Pressure Ratio186.46 x 1.95 = 363.60 CFM
No, I have one last simple calculation to do to in order to start looking at the turbo maps. If you look the X axis on the maps are in lbs of air per minute. To do this I'll take my CFM # and multiply it by .070.
363.60 (CFM) x .070 = 25.45 lbs/minute
So now with those 2 pieces of data about my engine, I can then decide if the turbo listed above would be a good choice for my engine. I take my Pressure Ratio # and find it on my Y axis. I make an imaginary horizontal line across the map. I do the same thing with lbs/minute # and make an imaginary vertical line up the map. I can then read the map based upon where the lines intersect. Take a look at the attached file. I don't know which turbo this map is for, but it's just a map that I swiped off of Garrett's website.
As you can see the I am within the turbos efficiency rings, but it could be better. I could try and re-run through the math and use a higher max RPM. It may move the "crosshairs" a little further toward the center of the "islands."
I hope this information helps you. It's pretty straight forward. If I've had a typo or two, or made a mistake, I apologize. It's late and I've got school in the morning.
Scott Faivre
