Blown
Elite Explorer
- Joined
- December 6, 2007
- Messages
- 1,074
- Reaction score
- 829
- City, State
- Montrose, CO
- Year, Model & Trim Level
- 2000 X-Edge
I ran a large pulley to get it running safe and thought it time to maybe run my smaller pulley to get more boost. The calculations with the info. here from Kenne Bell gave me the confidence to run the smaller pulley for winter.
((14.7air pressure psi+ Boost in psi)/14.7 air pressure) X (engine liters/2)/2.2 supercharger in liters=Theoretical Pulley Ratio at sea level
14.7 is the air pressure at sea level. I live in Colorado. I found a chart and the air pressure here is 12.0psi at 5,500' altitude. That makes a huge difference for you flat landers out there. The supercharger was a big innovation in WWII to boost aircraft power at altitude. The opposite is true, think about dropping to sea level, one could blow-up a supercharged rig tuned for high altitude! The calculation for my rig reveals 10.3lbs of boost at sea level with the 2 3/8" pulley, a bit too much without more octane or intercooler! The boost gauge concurs with the calcs at 5,500'.
I run an F150/Bronco crank pulley at 6.5". I have a 4" pulley and a 3 3/8" pulley, ratio's are 1.63:1 and 193:1. I revisited the pulley/boost calculation over the weekend, Went through Kenne Bell's tuning stuff, put on the smaller pulley, tuned and gained 4lbs of boost! I am able to do so because of the altitude and cooler temps. According to Kenne Bell, 1lb of boost increase is 17-20 horsepower or 68 to 80 horses with an additional 4 pound of boost!!! It is noticeable!
Calculations:
Sea level ((14.7+10.3)/14.7))X(5/2)/2.2= 1.93
5,500' ((12psi +8.4lbs boost)/12)X5/2)/2.2= 1.93
Kenne Bell stuff from here: Jim Bell's Supercharged / Turbocharged Performance Guide | Kenne Bell
KB stuff:
1 psi boost requires 1.5 octane (minimum) 1 psi boost = .5 point CR (effective CR)
1 AF ratio = 2 octane (example 12.5:1 to 11.5:1)
1° advance = 1/2 – 3/4 octane point
20° ambient = 1 octane
10° engine coolant (160° -180° range) = 1 octane
1000′ altitude = .5 psi (2″ Hg)
I used the above from KB to check my tune by octane. There are other variables to consider if tuning for a set time and conditions in the Kenne Bell info. One assumption is that you are already using premium unleaded.
At 5,500' 8.4lbs of boost X 1.5 Octane = 12.6 points increase in octane required.
5,500' elevation X .5psi = 5.5X.5= 2.75 octane points
1.3 increase air fuel ratio from stock X 2 = 2.6 octane points
Winter Temps 20 degrees = 1 octane point
12 degrees spark retard X .625 octane = 5 (I split the difference between .5 to .75)
12.6 - 2.75 - 2.6 - 1 - 7.5 = -1.25 octane points = A good tune for here, but if I drive it down in altitude.............or the temps go up.........yikes!
I think I can make this pulley work. I am going to put a cooler thermostat in it and may build an intercooler later. The following lines from KB shows how this would help.
10° engine coolant (160° -180° range) = 1 octane
20° charge temp reduction through intercooling = .5 psi additional boost with same octane
((14.7air pressure psi+ Boost in psi)/14.7 air pressure) X (engine liters/2)/2.2 supercharger in liters=Theoretical Pulley Ratio at sea level
14.7 is the air pressure at sea level. I live in Colorado. I found a chart and the air pressure here is 12.0psi at 5,500' altitude. That makes a huge difference for you flat landers out there. The supercharger was a big innovation in WWII to boost aircraft power at altitude. The opposite is true, think about dropping to sea level, one could blow-up a supercharged rig tuned for high altitude! The calculation for my rig reveals 10.3lbs of boost at sea level with the 2 3/8" pulley, a bit too much without more octane or intercooler! The boost gauge concurs with the calcs at 5,500'.
I run an F150/Bronco crank pulley at 6.5". I have a 4" pulley and a 3 3/8" pulley, ratio's are 1.63:1 and 193:1. I revisited the pulley/boost calculation over the weekend, Went through Kenne Bell's tuning stuff, put on the smaller pulley, tuned and gained 4lbs of boost! I am able to do so because of the altitude and cooler temps. According to Kenne Bell, 1lb of boost increase is 17-20 horsepower or 68 to 80 horses with an additional 4 pound of boost!!! It is noticeable!
Calculations:
Sea level ((14.7+10.3)/14.7))X(5/2)/2.2= 1.93
5,500' ((12psi +8.4lbs boost)/12)X5/2)/2.2= 1.93
Kenne Bell stuff from here: Jim Bell's Supercharged / Turbocharged Performance Guide | Kenne Bell
KB stuff:
1 psi boost requires 1.5 octane (minimum) 1 psi boost = .5 point CR (effective CR)
1 AF ratio = 2 octane (example 12.5:1 to 11.5:1)
1° advance = 1/2 – 3/4 octane point
20° ambient = 1 octane
10° engine coolant (160° -180° range) = 1 octane
1000′ altitude = .5 psi (2″ Hg)
I used the above from KB to check my tune by octane. There are other variables to consider if tuning for a set time and conditions in the Kenne Bell info. One assumption is that you are already using premium unleaded.
At 5,500' 8.4lbs of boost X 1.5 Octane = 12.6 points increase in octane required.
5,500' elevation X .5psi = 5.5X.5= 2.75 octane points
1.3 increase air fuel ratio from stock X 2 = 2.6 octane points
Winter Temps 20 degrees = 1 octane point
12 degrees spark retard X .625 octane = 5 (I split the difference between .5 to .75)
12.6 - 2.75 - 2.6 - 1 - 7.5 = -1.25 octane points = A good tune for here, but if I drive it down in altitude.............or the temps go up.........yikes!
I think I can make this pulley work. I am going to put a cooler thermostat in it and may build an intercooler later. The following lines from KB shows how this would help.
10° engine coolant (160° -180° range) = 1 octane
20° charge temp reduction through intercooling = .5 psi additional boost with same octane