| So I’ve been playing with numbers lately while trying to figure out what turbos would be a good fit for the Z. I know people quote the popular types of turbos commonly used on Z’s, but I remember looking through Garret’s catalog and seeing many different variations of turbos, all of which could potentially be used on the Z. Also, I’ve been playing around with Visual Basic 2005 Express Edition, so I thought it would be neat to make a program to help me in the calculations. Here are a couple of screenshots from the simple program I made: 
If you wanted to play with the program, I have the directory zipped here: [ http://mywebpages.comcast.net/marshall-johnson/Turbo.zip ] (Visual studio wants you to install the .Net framework) Just click on Setup.exe To make the program easy to work with you have to make some assumptions, otherwise the math would get too complicated and the program would lose its convenience. Things like temperature, ambient pressure, humidity, pressure drop, intercooler efficiency and volumetric efficiency are already set to average levels in the program, and as I found out, even when I did fill in all the variables to the best of my knowledge, the results still weren’t completely in tune with reality. So I ended up adding some compensation (arbitrary correction factor) to help make the results come close to what has already been attained in reality. Since the numbers are hard coded, it’s set to deliver reasonably accurate numbers with the conditions that we normally see. That means the efficiency of the turbos will be lower like you’d see when you’re pushing the turbos to their limits. It’s not going to be accurate if you have sport 700’s running 12 lbs of boost, since those turbos won’t even be trying and they’ll be flowing pretty cool air. It’s not going to be 100% accurate, but it will help with browsing the compressor maps on Garrett’s website. Take note of some of these compressor maps from their website. I punched in some numbers to get a line that goes across the map to give me an idea of where the turbo will be performing on the Z. This is the point on the compressor map that the turbo will be flowing at that power level if you’re revving your engine to about 6500 rpm. I added little dots to the maps, the lowest one is the 100 hp point, the next is 200, then 300 and so on. We’ll start with a small T2 turbo (GT20) that’s probably a little smaller than stock: 
As we can see, at 300 hp it’s sufficient but don’t expect it to flow much. The next turbo is in the same class, it’s another T2 size turbo, but with different specs. Even though it’s a T2, as you’ll see a pair of these can flow for more than 500 HP. We all know that a lower A/R ratio on the same turbo (as when you compare the Sport 700's two versions) will give better spoolup, but I’m not really sure how a smaller turbo with a bigger A/R would compare to a larger turbo with a smaller A/R. Hell if I know, I'm not a rocket surgeon or anything. 
Some of the turbos on their site have a compressor map extending high up, making a lot of pressure, but are very narrow. They'd probably be good on a generator or something that likes to keep the RPM constant but needs to make more torque when the load gets heavy. The next is a T22 size turbo, this is one of the higher flowing GT22s out there. 
Here’s a T25 size, but it doesn’t flow that much: 
Here’s the GT2860R: 
And the GT2860RS: 
I’m a little confused as to why Garrett’s website shows the GT28R flowing much more air than the GT28RS. I wonder if they got these maps mixed up? Anyway, post your comments.
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Turbo talk - 
