Supercharger Boost Calculation

[SHO-Z]

Here is a simple boost calulation for Superchargers, This does not allow for other losses.

 

((Super Charger Disp/(1/2 Engine Displacement)) * Ratio * 14.7) - 14.7 = Boost Pressure

 

3.5" Pulley on Eaton M62 Supercharger 1 Liter Capacity 2.8L engine

3.5" Pulley on Eaton M62 Supercharger 1 Liter Capacity

Drive-------PS--------- Ratio

Pulley----- Boost------ 3.5"

5----------- 0.3--------- 1.428571429

5.25--------1.05------- 1.5

5.5----------1.8-------- 1.571428571

5.75---------2.55------ 1.642857143

6------------3.3--------1.714285714

6.25------- 4.05------- 1.785714286

6.5----------4.8-------- 1.857142857

6.75---------5.55------ 1.928571429

7-------------6.3------- 2

7.25---------7.05------ 2.071428571

7.5-----------7.8------- 2.142857143

7.75----------8.55----- 2.214285714

8--------------9.3------ 2.285714286

8.25------- 10.05------ 2.357142857

8.5--------- 10.8------- 2.428571429

8.75--------11.55----- 2.5

9------------12.3------- 2.571428571

[TimZ]

That's a good general equation.

 

Note that it assumes that the engine is operating at 100% volumetric efficiency. It's interesting to note that if your engine has less than 100% VE (i.e., it doesn't breathe very well), your resulting boost pressure will be higher than what the equation predicts. So, you should be able to use your measured boost pressure to give you a rough idea of what your VE actually is.

 

Also, note that with this type of setup, for a given supercharger displacement and pulley ratio, the less boost you make, the more power you will have, since this indicates that the engine is breathing better, and the supercharger doesn't have to compress the air as much to achieve the flow rate dictated by the pulley ratio and blower displacement.

[280zedx]

Here's how i calculated my setup. It's pretty much the same result.

 

Crank Pulley Circumferance = 17 7/8”

S/C Pulley Circumferance = 8 1/16”

Ratio = 2.217 / 1

 

@ 5800 rpm = 12,858 S/C rpm

 

M62 supercharger output capacity = 1 L/ rpm = 12,858 L (LPM*.03531 = CFM) = 454 CFM of air

 

2.8 engine can inhale 285.6 CFM of air (cubic inches*RPM/3464)

 

Excess air = 454 – 285.6 = 168.4 “boosted” CFM

 

Boost = excess CFM / normal CFM * atmospheric pressure

168.4 / 285.6 * 14.7 = 8.7 pounds boost

 

In reality i'm getting just over 6 at redline. I'm losing some through the intercooling and setup as well as other losses.

 

Word of caution with too big of a crank pulley, a 2.5 ratio puts the supercharger rpm past it's "redline".

 

You're formula comes out roughly the same for an M90 as well....19lbs boost

[SHO-Z]

Both calculations are real close for a 2.21 ratio. I have a 3.5" Diameter pulley on the Mercedes SLK supercharger and with the electic cluth would be difficult to change. It is good to know that the pulley size that I am looking at will work.

[kj280z]

Hey guys...

 

This was really helpful... I just recently had a problem with my crank pulley. I'd like to hear what you guys have done to mount your crank pullies.

 

Currently I have a 6 inch serpentine pulley that fits onto the front of a 77/78 crank pulley. I'm planning to start with a 2.75 inch blower pulley (2.18 ratio)... based on some quick calcs on my 3 liter motor it looks this will yield 6.7 pounds of boost.

 

I'm using the MP62, and have had discussions with several folks who say this blower can be spun to 18,000 rpms... I'll probably have a smaller blower pulley made to test this... (my calcs say that the 2.61 ratio will yield 10.87 psi)

 

I'd love to hear what you have done! I might need a better crank pulley option so please share what you've done and if ya have an extra I'd consider buying one!

[premo-s12]

hey guys, just want to make sure i have this right. im trying to find out my boost with a 2.1 ratio. its an eaton m90 and im going off those specs down below.

 

SC: 5800rpm * 2.1 = 520'ish cfm?

VG: 5800rpm = 306cfm

Excess air: 520 - 306 = 214

boost: (214 / 306) * 14.7 = 10.28psi

 

that look about right?

 

this is with a vg30e by the way

 

eaton m90 specs:

http://www.automotive.eaton.com/product/engine_controls/superchargers/M90.asp

[Guest ZFury]

If you are really worried about CFM. Get one of these.

http://www.toolexperts.com/Airflow/Airflow_and_Wind_Meters/Page_1/CFM.html

 

If not, just guesstamate. It produces enough for your car. Just use BOV to meter the PSI.

[SHO-Z]

I get the same boost using the original calc.

 

Have fun on the project.

[premo-s12]

sweet. thanks guys!

[HoustonZ]

Any update on this project? I'm looking at a mercedes supercharger for my project as well.

[SHO-Z]

Just got thru the AZ summer and will start working in the garage now. Really do not have anything done in that we are also selling our house and do not want to have the Z half done then take it on a road trip. Will keep everyone informed on the progress.

[andy mason]

was wondering if you had more pics of the Green z I am in the proses of the same thing sore no pics