Turbo/airflow/HP figure math

[neilb]

Hey everyone, I was just reading an article in this month's GM hightech performance (Turbocharging 102: hot side, July 2011). At the end of the article they provided the math to figure out the estimated horsepower of your engine, taking into consideration turbo boost and such. I tried calculating the numbers and here's what I got:

Air flow rate:

=(cid x rpm x 0.5 x V.E) / 1728
=(231 x 6500 x 0.5 x 0.75) / 1728
= 325.8 cfm

Pressure ratio:

= (absolute pressure + boost pressure) / absolute pressure
= (14.7 + 20) / 14.7
=2.36

Turbocharged airflow rate:

=(airflow rate cfm x pressure ratio)
=(325.8 x 2.36)
= 768.9 cfm

cfm to lbs/min:

=(turbocharged airflow rate x 0.076)
=(768.9 x 0.076)
= 58.4 lbs/min

Overall engine power ESTIMATE:

(Average engine uses 1 lbs of air for every 10hp)

=(Turbocharged airflow rate in lbs/min x 10)
=(58.4 x 10)
=584hp

Therefore, my engine assuming a 75% V.E, running 20lbs of boost at 6500 rpm should make approx 584hp.

The article used an assumed V.E. of 85%, so based on that, my engine would produce 662.4 hp with 20 lbs at 6500 rpm.

Pretty Cool!!

I used this math on the compressor map for my Turbo I found on the Turbonetics website, and found that to reach these goals, my turbo falls within it's 70% efficiency. It's top efficiency is 78%, so I'm not far off. Looks like I made a good choice. (My turbo is the Cpte60)

I'm not really sure how to know what the V.E. of your engine would be. I guess the more free flowing (Heads, Cam, Intake etc.) the higher the V.E. I've heard most engines are close to the 75% mark.

 

[87GN_70GS]

Depending on resonant tuning/heimholtz (sp?) effects, VE can be over 100% at certain operating conditions (ex., ram intake).

 

[GNDriven]

Turbo math always confused me. One statement will say VE can exceed 100% on a supercharged engine, but then all the formulas used to calculate HP and air flow, says to use NA VE numbers like 80% for our 231 Buick V6.

 

[bison]

Turbo math always confused me. One statement will say VE can exceed 100% on a supercharged engine, but then all the formulas used to calculate HP and air flow, says to use NA VE numbers like 80% for our 231 Buick V6.

The reason it's confusing is because everyone refers to mass efficiency as volumetric efficiency. When you refer to air you should only be referring to mass flow in regards to cylinder filling ability. Air takes up the entire space of what it's in. 1 gram of air in a 500ci cylinder takes up the entire 500ci. 3 grams in that same cylinder takes up the same space. It's too bad some f uck tard started using the term volumetric efficiency when referring to the engines ability to fill it's cylinders air. A turbocharger/supercharger will increase the mass efficiency of an engine tremendously. It however will have zero effect on airflow through the engine. Well it can effect it a little with the cam profile but it's not worth mentioning. You can have 80 lb/min of air at 750cfm or you can have 1 gram/hr and still flow 750cfm. Both have to be looked at. Mass flow is relative to ambient pressure and or a blower pressurizing the engine beyond ambient pressures ability. Air flow however is almost totally controlled by the engines displacement and speed. If you are flowing 80lb/min through an engine you are kicking ass.

 

[GNDriven]

Thanks, You just shed some understanding for me on something I have been thinking about for years. I sure some author somewhere stated that, but the way you explain it was understandable, you should right a Turbo Buick Book, with turbo math, performance upgrades i.e your pitch on 4 bolt exhausts, that pertains particularly to the Turbo Buick. I have countless books that reference the Buick 231 and stage motors, but only to make certain points. I would pay good money to have a book written strictly for Turbo Buicks and what turbos do what, what power levels break what, what mods do what. Seems like there is enough info out there to write a great book with a lot of excellent examples.