Choosing a Turbo

[Edub1]

If you’ve come here seeking a straight forward explanation of how to choose a turbo, you’ve come to the right place.

Please understand that we are discussing the KA24 motors. While some of the concepts are universal, the data I will be using in my simplified explanation is based on this motor.

The first thing you need to determine is how much boost you plan on running or how much power you are looking for. For most of our purposes, the Garrett T3 or combinations thereof seem to offer the most bang for the buck. By the way, T3 is a flange size and a general turbo size. The turbo’s A/R numbers denote the interior volume of the housings. It is my understanding that the exhaust should be around 0.63, this will allow for fast spool but not restrict air flow at high RPMs. The compressor A/R is of little importance here. Trim size has to do with how much air the turbo can deliver. Large trims deliver more air but spool slower due to increased mass.

Compressor maps: This shows you how the turbo will act on your car.

Suppose you are after a modest 225 HP and plan on running about 7Lb of boost. The question is what turbo is going to work efficiently in this range. This is determined by looking at a turbocharger’s map. The map is simply a graph that plots the amount of boost you are running against the amount of air your motor will consume at various RPMs under said boost. For an excellent explanation of how to read these maps check out http://www.turbosaturns.net

If you are a little rusty with your algebra, go to http://www.turbocalculator.com, download a map for a Garrett T3 trim 60, print it out and keep reading.

When looking at a turbo map, you will see oddly shaped concentric areas. The innermost area represents the turbo’s highest efficiency, with efficiency decreasing as you move outward. The key here is to keep the turbo operating above 60% - look at the rings.

The numbers on the left hand side are the “pressure ratio.” This is simply how many times the normal atmospheric pressure your motor will see in total. Normal atmospheric pressure at sea level is 14.7PSI (1ATM). So add your planned boost, (say 7lb) to 14.7 and divide by 14.7 7 + 14.7 = 21.7 21.7/14.7 = 1.48 “pressure ratio.” Simple isn’t it.

You can also think of it this way. 7.35lb is ½ ATM. So, you have 1ATM to start, plus the 1/2ATM from boost. This gives a total or “pressure ratio” of 1.5 - See how close that is to 1.48? What if you boost at 14.7PSI – think you’d have a PR of 2.0? If you said yes, give yourself a gold star.

So, now you know what the numbers on the left mean. This is also what is meant by “bar” of pressure. 1 bar = 1ATM = 14.7PSI

Now find your PR on the left (1.4 and draw a horizontal line through the map.

The numbers along the bottom relate to how much air your motor will consume at a given RPM range under a certain pressure. To calculate this, you need to use the formula below. Or, you can just skip that part as these are provided for our motor. The numbers are given in the chart below.

Formula: (CID X RPM) / 3456 = CFM CFM X .069 X .85 = lbs/min 85% is an average efficiency (VE) for modern motors.

Here is what this all comes out too. Dyno corrected.

3000 rpm = 7 lb/min 3500 rpm = 9 lb/min 4000 rpm = 11 lb/min 4500 rpm = 12.5 lb/min 5000 rpm = 13.5 lb/min 5500 rpm = 14.5 lb/ min 6000 rpm = 13.5 lb/ min 6500 rpm = 12.5 lb/min

Multiply the numbers above by your PR which again was 1.48 for 7lbs. You don’t need them all as we are concerned with the RPM range between 3500 – 6000 RPMs. This gives 13.3, 16.3, 18.5, 20, 20.7, 20 respectively. Now, find these numbers along the bottom of the map and draw a line upward until it meets the PR line.

Now you are done. Where these lines intersect is how efficient your turbo will be at a given level of boost and a given RPM.

I’ve chosen the Garrett T3 60 trim. For 7-12lb boost applications, you’ll find that your turbo is efficient at low RPMs for real world driving and continues right through the sweet spot until about 275HP at 12lbs and 6000RPM. By the way, if you take your air flow along the bottom of the map and move the decimal over one place to the right, you will have your approximate horse power. Look at the data chart showing 14.5 at 5500 rpm. Advertised HP for the SOHC KA is 140. And we all know where to find it.

Hope this helpes!

[Chezedik]

The numbers on the left hand side are the “pressure ratio.” This is simply how many times the normal atmospheric pressure your motor will see in total. Normal atmospheric pressure at sea level is 14.7PSI (1ATM). So add your planned boost, (say 7lb) to 14.7 and divide by 14.7 7 + 14.7 = 21.7 21.7/14.7 = 1.48 “pressure ratio.” Simple isn’t it.

See, it turns out we were saying the same thing, you were just thinking in terms of total manifold pressure, where as I was referring to actual boost.