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View Full Version : So let's talk about compressor maps

mikeb
11-13-2006, 01:08 PM
Say I have a turbo with the following compressor map:

http://home.earthlink.net/~mvburress/map.jpg

Looking at the A. Graham Bell book Forced Induction Performance Tuning on selecting a turbo by a compressor map we find the following formula to calculate CFM of airflow:

CFM = (L X rpm x VE x Pr) / 5660

Where:

L = Engine size in litres
rpm = max rpm
VE = volumetric efficiency
Pr = pressure ratio

So lets say we have a 7.15 litre engine, max rpm of 6700, a VE of 93 (?), and pressure ratio of 1.68 (10 lbs of boost).

(pressure ratio is calculated by adding 14.7 to your desired boost and then dividing the result by 14.7)

So 7.15 * 6700 * 93 * 1.68 = 7484677.2

Divide by 5660 and the answer is 1332 cfm.

To convert cfm to lb/min you multiply by .07

.07 * 1332 = 93 lb/min

Map 93 lb/min with a pressure ratio of 1.68 on the compressor map and you are off the right side of the map. Not good.

If you redo the calculations using a 4000 rpm max value you get 55 lbs/min, which does map into the 74% efficiency island at 1.68 pressure ratio. Better.

So what i'm surmising is that this turbo on the hypothetical engine would make good torque down low (or would it? impeller speed seems to be on the slow side and we are way to the right of the surge line) but would begin to run out of steam just over 5000 rpm. In other words it would be a good street turbo on a larger engine.

Am I interpeting this correctly?
JamesB
11-13-2006, 03:15 PM
Heres your answer:

Call Precision Turbo
Ask for Harry
Tell him every detail about your car and class regualtions(if any)
give him your credit card number
The right turbo will be at your doorstep in a couple of days


:D
momo stallion
11-13-2006, 03:23 PM
based on your calculations, that turbo is too small for your application :)


lol, i know that this doesnt help you much but what motor is this and what turbo?
usmcluke
11-13-2006, 03:35 PM
Sounds like that one isn't going to work, just give it to me so its not in your way. If you have any nitrous parts left let me know, I may be interested. :D
mikeb
11-13-2006, 03:38 PM
based on your calculations, that turbo is too small for your application :)


lol, i know that this doesnt help you much but what motor is this and what turbo?

It's a borg-warner S400 GT45 (75mm). I actually already have the turbo; it was part of a kit that I bought (kits for pontiacs are hard to come by). I had already guessed that it was too small before I bought it; I finally found a compressor map for the turbo and did the math and confirmed it. The engine is what my IA2 buildup is going to be.

I'd rather start with a small street setup to learn on before going to a "real" turbo anyway, so that's ok. I can sell the kit when i'm done with it. I think I may scout around for some low comp OEM heads and freshen my existing 455 and play with that for awhile. It won't set the world on fire by any means but I can learn from it.
Dawkins
11-13-2006, 07:00 PM
It's a borg-warner S400 GT45 (75mm). I actually already have the turbo; it was part of a kit that I bought (kits for pontiacs are hard to come by). I had already guessed that it was too small before I bought it; I finally found a compressor map for the turbo and did the math and confirmed it. The engine is what my IA2 buildup is going to be.

I'd rather start with a small street setup to learn on before going to a "real" turbo anyway, so that's ok. I can sell the kit when i'm done with it. I think I may scout around for some low comp OEM heads and freshen my existing 455 and play with that for awhile. It won't set the world on fire by any means but I can learn from it.

124cc low enough compression for you? ;)
mikeb
11-13-2006, 08:10 PM
124cc low enough compression for you? ;)

6H's?

Kind of thinking about 6x-8's or something in the 100cc range but 6H's might work with some milling.

Let me do some crunching on those.
truestreet5.8
11-13-2006, 08:12 PM
Heres your answer:

Call Precision Turbo
Ask for Harry
Tell him every detail about your car and class regualtions(if any)
give him your credit card number
The right turbo will be at your doorstep in a couple of days


:DSounds like a plan to me! lol
AL P
11-13-2006, 08:19 PM
Based on that map I'd say that engine is too big to use that turbo.

I'd bet that turbo would work fine for a 301 cube motor but I'm too lazy to calculate it.
Dawkins
11-13-2006, 09:14 PM
6H's?

Kind of thinking about 6x-8's or something in the 100cc range but 6H's might work with some milling.

Let me do some crunching on those.

I might be able to find a set of 6x-8's for you but the 6h's are yours if you want them.
mikeb
11-14-2006, 06:40 AM
I might be able to find a set of 6x-8's for you but the 6h's are yours if you want them.

I did some more math and by my calculations the best I could do is about 485 hp spinning the hell out of the turbo with turbo discharge temps > 130c :eek: Not worth it.

301

AL P for the win. I have 1/2 of a twin turbo setup :) The plot for a TT falls exactly right in the middle of the center efficiency island.