Selecting a Turbo using Compressor Maps.

[Fox Slaughter]

Alright I am going to break down how do you decide what turbo is right for your setup. There will be math involved and lots of comparing maps. So lets jump right into this.

So you wanna build yourself a turbo demon? Well first things first, you need to know what turbo will work well with your setup. I will right another thread sometime on how to pick your AR Ratio and such. This one will be just selecting the turbo.

First you need a simple formula. This formula will get you the CFM of what your engine needs roughly at a desired RPM level.
The Formula is as follows.

Cubic Inches * Revolutions Per Minute * .5 * Volumetric Efficenecy / 1768

So lets use a stock LT1 for example.

350ci * 5700 * .5 * 85% / 1768 = 480 (479.56) CFM

So now we have the rough CFM of what a stock LT1 needs at 5700rpm.
Next step is to figure out how much boost you want to run to get the Pressure Ratio.
The formula to calculate your Pressure Ratio is as follows.

PSI + 14.7 / 14.7

So say we want to run 10psi.

10 + 14.7 / 14.7 = 1.68

Now we have the Pressure Ratio and CFM we can now calculate how much air the engine will flow at 5700rpm and 10psi.
This formula is as flows.

1.68 * 480 = 806 CFM

Thats how much the engine will flow roughly at 5700rpm and 10psi of boost.
Most compressor maps are done in Lbs/min.
Once again we must convert, CFM into Lbs/min.
Formula is...

CFM * .0691

Plug it in.

806 * .0691 = 56 (55.96) Lbs/min

Awesome almost done.
So now we start looking at some compressor maps.
Fig9.jpgt66.jpgt72.jpg
Here are some turbo compressor maps.
They are left to right.
60-1, T66, T72.

Now what we do is take our 1.68 Pressure Ratio and 56lbs/min and plot them on the compressor maps.
This is what we get.

60-1.jpgt66.jpgt72.jpg

As you can see the 60-1 is to small it won't flow enough air to even land in any island.
The T66 is better but only lands in the 60% island.
The T72 is a little better still, but not a super great choice.

Ideally what you want is for the lines to meet in the most efficient island of the turbo.
Once you have that squared away you need to plot a few more points, and connect a line between all the points.
You want the points to be on the right side of the left most line, this is the Surge Limit.
You do not want your turbo to go into the surge limit.

So do the math for different RPMs and different boost levels and make sure you stay right of the surge limit.

I hope this gives you an understanding and gives you some knowledge into what what turbo will work well with your setup.

[masterfab]

good job tj

[Fox Slaughter]

Thanks.

I'd like to get this stickied so people can consult this first. Or least make a thread and sticky it and put this and my other turbo thread under it.

[birdie2000]

Good info. How do you get your VE for that calc? Do you just guess/use a rule of thumb?

[Fox Slaughter]

Good info. How do you get your VE for that calc? Do you just guess/use a rule of thumb?

As a Rule of thumb just for rough calculations most N/A engines will flow about 85% Volumetric Efficiency. The problem with calculating the correct VE is you need to know how much airflow is the engine is suppose to flow, and then the real amount of airflow. Then obviously the difference is the VE amount.

Yes some engines will be more efficent than others, its just a generalization. Thats why when you pick a turbo you want it to be in the Most Efficient island because obviously the calculations aren't perfect. So if you are say in the 80% island that means it will be ~80% possibly in the 70s which is still perfect for a turbo.

By doing a head and cam swap all you are doing is raising the VE of the engine, trying to get it close as 100% as you can.