Originally Posted by ray_sir_6
Did the formulas from Garrett Turbo 103.
HP = 218 flywheel hp
A/F = 11
BSFC = .5
Therefore WA = 19.98 lb/min
WA = 19.98
R = 639.6
Tm = 106 (datalogged)
VE = .97
N = 7500
Vd = 97.632
You come up with 7233031.728 divided by 355136.
That equals 20.3669, but you have to subract atmosphere, which is 14.7
So that equals 5.6669 psi.
So the math says my dyno is right.
Here's the upper half of my spreadsheet of the same set of equations:
Input Parameter Units Value Notes
H.P. Target 218
Air/Fuel Ratio 11
Brake Specific Fuel Consumption lb/(H.P.*hr) 0.5 Use 0.5 to 0.6 (or higher)
Flow Rate lb/min 20.0
Manifold Pressure Required (Absolute)
Input Flow Rate 19.98333333
Gas Constant - R 639.6
Intake Manifold Temperature Degrees F 106
Volumetric Efficiency 0.97 Use 0.95 to 0.99 for modern four-valve heads
Engine RPM - N 7500
Engine Displacement (Vd) Liters 1.6 Muliplied by 61.02 in final formula to convert to cubic in.
Manifold Pressure (Absolute) PSI 20.4
Boost Pressure Required
Input Manifold Pressure (Absolute) PSI 20.4
Atmospheric Pressure PSI 14.7
Output Boost Pressure Required (gauge) PSI 5.7
So, Ray, you are absolutely right and I do believe your numbers are correct. I therefore retract all of the bad things I said about you; you have my sincere apologies.
On the other hand you've more than completely proved the value of a good and thoughtful analysis for helping spec a turbo charger. And while this might be completely unimportant to you, me, the brain, and everyone else who has completed their first turbo motor build, there are lots of folks who come on line and ask, "which turbo should I pick and how should I pick it".
I think the best answer to this question is study the books, study Garrett's website, run a calculation or two of your own, and then come on over to the site and ask what results others have had with similar goals to yours. Obviously, and as you've just proven, the closer the variables are to real life the closer your results will be to real life.
In my case, and because I didn't want any unreasonable expectations for my own system, I repeatedly chose conservative input parameters. This gave me a conservative view of output that was below what was reasonable for a carefully designed and built motor. For my purposes that was fine; but it did cause me to not fully understand what your motor was capable of.
But hey, it was great fun and I learned a lot from a bunch of folks. Hopefully there are others who learned as well.