Big junkyard out there full of turbos, trying to do some calculations in Excel to pick a good turbo (or two).

I started by picking a fairly low VE for the stock 78 SBF (.75) and calculating the CFM from 1500 to 5500 rpm.

I then used the formula: n(lbs of air)=(psi-a * CFM * 29)/(10.73 * T(R)) I used that data to build a table accross my RPM range, with a column for 5, 10 and 15psi boost.

I figured that gave me a set of data points that I could look up on compressor maps to get an idea of where the turbos "happy spot" is in the boost/rpm range.

So am I doing this right? A "better" turbo is one where more of my expected boost/rpm points fall inside the compressor map, correct? If so, all the singles that are efficient in the 3000rpm 'sweet spot' fall off the right side of the map towards redline. Twins on the other hand, fit the range all across the table. Mitsubishi TH04s look just about right, and have the bonus of coming from a Volvo (SBF+240 brick combo). Saab GT17s would be tapped out if I push farther, but sit above 70% efficiency for much of the rpm band at 10+ psi. If the math lines up, that's somewhere in the neighborhood of 350rwhp and I am good with that.

I'm unfamiliar with these calculations, so any feedback is appreciated. Should I trust these figures enough to install the little Saab turbos?

Thanks!

 

This excel file is probably as good as it gets for the specific Boost Guy.

www.gnttype.org/techarea/turbo/turbocalcs.xls‎

If you put a space before each cell calculation you can copy that formula and see how it works.
You can then remove the space and the formula will return the normal answer.

Simply but with a lot of the solid math for picking a turbo.IMO

Tom Vaught

 

I'm getting a bad link, but that is the site I pulled the formulas from. I made my Excel sheet after reading this: http://www.gnttype.org/techarea/turbo/turboflow.html

I'm not sure if I can attach my worksheet to a post.

 

If you used the formulas (from John E) it is about as good as it gets with a basic excel program.

Tom Vaught

 

www.gnttype.org/techarea/turbo/turbocalcs.xls

 

Excellent, thanks!