Compressor map calculations

[justin]

So, in my boredom tonight I decided to run some calculations for my new engine to determine what might be the best turbo to run for a given horsepower goal. I already have a t04e 60 trim compressor on a t3/.63 turbine. Disclaimer, I am a novice at these calculations so bear w/ me :p

I'll post my calcs for a 300hp situation. I think my pressure ratio is somehow incorrect (higher then I expected), but I can't figure out what I did wrong. My data is pointing me to turbo's like a gt30r or gt3071r type but the pressure ratio is just barely staying inside the surge line (on the compressor map). My calcs are based on the tech info from the garrett website, and that is what I based a lot of my unknowns on too.
http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html

Anyways, here are my calculations from my notes. I know there is probably a lot more too it then this, but this is what I have so far. Any idea where I went wrong, or did I? Does gas constant change for e85? I have no idea what that value means :o

300hp calculations:

BSFC=0.55
AFR=12
per min = /60

actual airflow:
Wa = 300*12*0.55 /60 = 33lb/min

My specs:
Wa: 33lb/min
R: 639.6 (gas constant)
Tm: 130F (IM temp)
VE: 97% (volumetric efficiency)
N: 7200rpm (max rpms)
Vd: 1.6L * 61.02 = 98CI (displacement)

MAP = 33 * 639.6 * (460+130) 12453012
---------------------- -------------- = MAP 36.4psia (36.4 12.4 = 24psi of boost)
.97*7200/2 * 98 342216

P2c = 36.4+2
= 38.2psia

P1c = 12.4-1
= 11.4psia

Pratio = 38.2 / 11.4
= 3.3

300hp plot 3.3 pressure ratio and 33lb/min air flow


Leads me to gt30r, gt3071r
http://www.rbracing-rsr.com/turbo/TurboMaps/gt30rcompress.gif
http://www.rbracing-rsr.com/turbo/TurboMaps/gt3071rcompress.gif

Way out of range of my t04e 60trim :( :
http://www.rbracing-rsr.com/turbo/TurboMaps/t04e-60.gif

I've got to be doing something wrong :smashfrea

Justin

[HONDA GHANDI]

That pressure ratio does seem extremely high for some reason. My gut tells me it should be around 1.5-1.8.

[Skaterkid]

You need to subtract atmospheric pressure from your MAP pressure to move from absolute to relative pressure. That will put your PR in the proper spot.

[STIBungy]

Justin, if you're interested in a GT3040R, hit me up.

[justin]

You need to subtract atmospheric pressure from your MAP pressure to move from absolute to relative pressure. That will put your PR in the proper spot.

Yea, I did do that in the MAP equation. So, in the P2c calculation I need to use 24psi instead of the absolute of 36.4psia? The examples use the absolute pressure.

Well, anyways, if I do it that way I get:


P2c = 26 + 2
= 28

P1c = 12.4-1
= 11.4psia

Pratio = 28 / 11.4
= 2.45


So, a pressure ratio of 2.45 plots out a little better, although I'm not sure that's right still?

Justin

[Skaterkid]

Yea, I did do that in the MAP equation. So, in the P2c calculation I need to use 24psi instead of the absolute of 36.4psia? The examples use the absolute pressure.

Well, anyways, if I do it that way I get:


P2c = 26 + 2
= 28

P1c = 12.4-1
= 11.4psia

Pratio = 28 / 11.4
= 2.45


So, a pressure ratio of 2.45 plots out a little better, although I'm not sure that's right still?

Justin

Nope, you're correct, I was wrong. Sorry about that. Anyways, your gas constant is constant and does not change for any type of fuel. One thing I have noticed is that you are assuming max power is at 7200. You are using a B16 correct? Usually, a car's NA peak power is a good place to use for where your turbocharger's peak power will be. I think a B16 peaks near 8000 so that'll pull your PR down slightly. Otherwise, everything looks pretty good.

[justin]

Nope, you're correct, I was wrong. Sorry about that. Anyways, your gas constant is constant and does not change for any type of fuel. One thing I have noticed is that you are assuming max power is at 7200. You are using a B16 correct? Usually, a car's NA peak power is a good place to use for where your turbocharger's peak power will be. I think a B16 peaks near 8000 so that'll pull your PR down slightly. Otherwise, everything looks pretty good.

Thanks for the gas constant info.

Engine is a d16, stock head, so 7200rpm max for now. The only values I guessed on based on the garrett notes, are bsfc, VE, and Tm. Everything else should be fairly accurate.

Justin

[justin]

Well, I found a different way to calculate pressure ratio, basically just do it in the opposite direction that I did before.

Pressure ratio = atmopsheric pressure + boost pressure / atmo pressure


So for 2 different examples, of the boost I think my engine can support easily:

14psi of boost:
Pratio = 12.4 + 14 / 12.4
= 2.1

20psi of boost:
Pratio = 12.4 + 20 / 12.4
= 2.6


Using this, along with the lb/min values I got from previous calculations I can more easily tell which turbo's will work for my setup. In fact, both of my Pratio's (2.1, and 2.6) plot pretty nicely on my t04e 60 trim compressor map, using 33lb/min and 38.4lb/min respectively for the air flow.

Now all I need is a car that drives on it's own power :D

Justin