How to calculate Boost

[Lextreme II]

I have been searching the net and didnt really find anything solid information about how to calculate boost. I know there are many factors affecting it but here is one example:

Engine Size: 5.1L
Compression: 10.4:1
Exhaust: Stock
Heads: Stock
Supercharger: Cobra M112
Crank pulley size: 5.25"
Supercharger Pulley: 2.90"
Redline: 6,000
Intercooling: None
Fuel: Stock with Extra Injector

Can anyone work this out? I have found this an example so if we just plug in the numbers would it work?

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

2.90" Pulley Size:
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 9.11”
Ratio = 1.811 / 1

@ 6000 rpm = 10,867 S/C rpm

M112 supercharger output capacity = 1.8 L/ rpm = 10,867 L (LPM*.03531 = CFM) = 383.71 CFM of air

5.1 engine can inhale 542 CFM of air (cubic inches (313)*RPM/3464)

Excess air = 383.71 – 542 = -158.4 “boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
-158 / 542 * 14.7 = -4.29 pounds boost

2.50" Pulley Size
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 7.85”
Ratio = 2.10 / 1

@ 6000 rpm = 12,600 S/C rpm

M112 supercharger output capacity = 1.8 L/ rpm = 12,600 L (LPM*.03531 = CFM) = 445 CFM of air

5.1 engine can inhale 542 CFM of air (cubic inches (313)*RPM/3464)

Excess air = 445 – 542 = -99 “boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
-99 / 542 * 14.7 = -2.68 pounds boost

According to the above formula even with the smallest supercharger pulley I am still in the NEGATIVE zone. Am I doing something wrong with the formula or the crank pulley is just too small or its just too large of an engine for the M112?

 

[rms]

David,
Engine consumption CFM is every 2 revs.
So your rpm in engine cfm calcs should be half actual rpm.
You will then see some boost.
Actual running boost will be less but is ballpark.

We are running a 1.6ltr/rev Whipple on a 2.2ltr VW, 2 to 1 ratio, 24psi boost at 8000 engine rpm.

 

[cribbj]

Also, in Corky Bell's book "Supercharged" there are all the formulas, including corrections for loss of efficiency due to heat of compression, "slippage" in the Roots blowers at higher ratios, etc.

 

[XR8tt]

Boost is a restriction of flow in this case.. It depends on V/E of engine....
Lb per min is best way of finding power levels as its more related to what the turbo / engine can breath....

 

[rms]

David,
2.9" pulley around 6psi
2.5" pulley around 9psi

%VE of engine and blower are roughly similar so the formula gives a general idea.

Re your previous post on injection pre blower.
The VW engine mentioned, running methanol has 4 - 1.6ltr/min port injectors and 1 - 1.6ltr/min pre blower injector. The blower injector only starts to inject above 3000rpm and 5psi boost and is used to control intake temps to around 50deg C. We are about to add a second injector to try and lower temps further.

The blower has run for 5 years with no problems. More than I can say for the rest of the engine!

 

[cribbj]

Erol, not to get OT here, but are those just normal injectors, or were they built for methanol? Also, how have they performed? Have you done any special maintenance like cleaning or purging the system after races?

 

[rms]

They are Bosch 'Indy' injectors for methanol. 2 - Bosch Motorsport 200ltr/hr at 8 bar fuel pumps. 2 - Bosch 3 bar regulators.

Everything seems to cope with 99.9% methanol OK.

The trick seems to be not allowing the system to run dry and let air or moisture get to things. No flushing etc, just fill tank with pumps running then plug tank breather.
Tank is not anodised, lines and fittings are Earl's, filter is Kinsler, never had any problems.

 

[Lextreme II]

RMS,

Sorry to get you back would you please check the "Corrected" calculation"

2.90" Pulley Size:
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 9.11”
Ratio = 1.811 / 1

@ 6000 rpm = 10,867 S/C rpm

M112 supercharger output capacity = 1.8 L/ rpm = 10,867 L (LPM*.03531 = CFM) = 383.71 CFM of air

5.1 engine can inhale 271 CFM of air (cubic inches (313)*RPM/3464) / 2

Excess air = 383.71 – 271= 112 “boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
112 / 271* 14.7 = 6 pounds boost

2.50" Pulley Size:
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 7.85”
Ratio = 2.10 / 1

@ 6000 rpm = 12,600 S/C rpm

M112 supercharger output capacity = 1.8 L/ rpm = 12,600 L (LPM*.03531 = CFM) = 445 CFM of air

5.1 engine can inhale 271 CFM of air (cubic inches (313)*RPM/3464) / 2

Excess air = 445 – 271= 174 “boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
174 / 271* 14.7 = 9.44 pounds boost

However from the above poster with 2.8 engine can inhale 285.6 CFM of air (cubic inches*RPM/3464). Am I doing something wrong here? 2.8L inhale 285.6 cfm and 5.1L only inhaling 271 cfm?

 

[pwd]

Dave, I am sure that I read somewhere that the 112 has a capacity of 2.1 L/rev

 

[Lextreme II]

Thanks pwd,

I think I made a mistake somewhere in the charger. Here is the corrected worksheet:

2.90" Pulley Size:
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 9.11”
Ratio = 1.811 / 1

@ 6000 rpm = 10,867 S/C rpm

M112 supercharger output capacity = 1.84 L/ rpm = 10,867 L (19995*.03531 = CFM) = 706 CFM of air

5.1 engine can inhale 542 CFM of air (cubic inches (313)*RPM/3464)

Excess air = 706 – 542= 164 “boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
164 / 542* 14.7 = 4.45 pounds boost

2.50" Pulley Size:
Crank Pulley Circumferance = 16.50”
S/C Pulley Circumferance = 7.85”
Ratio = 2.10 / 1

@ 6000 rpm = 12,600 S/C rpm

M112 supercharger output capacity = 1.84 L/ rpm = 12,600 L (23184*.03531 = CFM) = 818.62 CFM of air

5.1 engine can inhale 542 CFM of air (cubic inches (313)*RPM/3464)

Excess air = 818.62– 542= 276.62“boosted” CFM

Boost = excess CFM / normal CFM * atmospheric pressure
276.62/ 542* 14.7 = 7.50 pounds boost

Does the above calculation sound ok? How about this link http://www.eatonperformance.com/superchargers/Simulator/Landing.aspx

 

[Cobber]

160 hp from a 5.1 supercharged engine? I think something is a bit out there.

http://www.sprintex.com.au/files/Blower_Calculator.xls

This calculator I think is a bit generous but you can reverse engineer it somewhat. Also don't forget loss due to cam overlap.

 

[Lextreme II]

I ran through Corky's book today and I couldnt find the formula for it.

 

[pwd]

Have you checked with Andrew, Dave. He has a simple sounding formula for for working out boost, I just cant remember it at the moment.

 

[v8commy]

this may be of help if you guys haven't seen http://planetsoarer.com/boost/boost.htm