"Online - Technical Support Guide"

PULLEYS, BOOST AND FUEL

 

 

The difference between blower pulley profiles

How much boost should I run?

 

The difference between blower pulley profiles:

There are 4 different pulley profiles used on blower applications. They are as follows:

1/2" Pitch (click on image for picture)

This pulley has a square tooth profile and is primarily used in street only applications. This profile works well for street applications under 7,000rpm and cost less than other profiles.

8mm (click on image for picture)

This pulley has a half round tooth profile and is the most widely used pulley and belt system. This profile is capable of handling higher rpm, in excess of 7,000 rpm, and more power than the 1/2" pitch pulley. 8mm also has the widest variety of pulley combinations to choose from. Applications ranging from street & strip, drag racing, marine, and off road applications should use 8mm pulleys and belts.

13.9mm and 14mm (click on image for picture)

These two pulley also have a half round tooth profile, except the teeth are about twice the size of 8mm. Another words, you can easily fit a pencil into the valley of a 14mm tooth pulley, where it would not come close to fitting inside the 8mm tooth pulley. 13.9mm and 14mm are very close to being the same size. !3.9mm is still widely used on BBC motors, where as 14mm fills the rest of the gap due to the increased belt selection. Primarily, these type of pulleys are used in very high horsepower applications utilizing Kevlar blower belts. Some offshore marine customers utilize the 14mm with intercooled motors. The Kevlar belt also prevents rubber dust from building up inside enclosed engine areas. This is the most expensive but most stout pulley and belt system to run, but also can be overkill in a lot of applications where it's just not needed.

 

How much boost should I run?

How much boost your run depends on a few things. 

The application of the blown motor.

The static compression ratio of the motor

What type of fuel will be used in the motor

Will the engine block or cylinder heads be o-ringed in conjunction with copper head gaskets.

If you do not know how to choose or what static compression ratio should be used in your motor, then go to the Compression Ratio section, and then come back to this section using the link at the bottom of that page.

The chart below shows how much boost you can run with the fuel you anticipate using. The purpose of different octane levels is to help prevent detonation with higher boost levels.

 This chart is for normal blown engines using standard roots type blowers. 

There is an example below the chart to help understand how to use it.

The far left column number represents static compression ratio. The top row represents boost. The number in the boxes is your final compression ratio (at sea level). The different colors represent minimum octane levels recommended. The color key chart is below the chart.

 

    BLOWER BOOST
    4 5 6 7 8 9 10 11 12 13 14 15 16 18 20 22 24
  7 8.9 9.4 9.9 10.3 10.8 11.3 11.8 12.2 12.7 13.2 13.7 14.1 14.6 15.6 16.5 17.5 18.4
C 7.5 9.5 10.1 10.6 11.1 11.6 12.1 12.6 13.1 13.6 14.1 14.6 15.2 15.7 16.7 17.7 18.7 19.7
O 8 10.2 10.7 11.3 11.8 12.4 12.9 13.4 14.0 14.5 15.1 15.6 16.2 16.7 17.8 18.9 20.0 21.1
M 8.5 10.8 11.4 12.0 12.5 13.1 13.7 14.3 14.9 15.4 16.0 16.6 17.2 17.8 18.9 20.1 21.2 22.4
P 9 11.4 12.1 12.7 13.3 13.9 14.5 15.1 15.7 16.3 17.0 17.6 18.2 18.8 20.0 21.2 22.5 23.7
R 9.5 12.1 12.7 13.4 14.0 14.7 15.3 16.0 16.6 17.3 17.9 18.5 19.2 19.8 21.1 22.4 23.7 25.0
E 10 12.7 13.4 14.1 14.8 15.4 16.1 16.8 17.5 18.2 18.8 19.5 20.2 20.9 22.2 23.6 25.0 26.3
S 10.5  13.4 14.1 14.8 15.5 16.2 16.9 17.6 18.4 19.1 19.8 20.5 21.2 21.9 23.4 24.8 26.2 27.6
S 11 14.0 14.7 15.5 16.2 17.0 17.7 18.5 19.2 20.0 20.7 21.5 22.2 23.0 24.5 26.0 27.5 29.0
I 11.5 14.6 15.4 16.2 17.0 17.8 18.5 19.3 20.1 20.9 21.7 22.5 23.2 24.0 25.6 27.1 28.7 30.3
O 12 15.3 16.1 16.9 17.7 18.5 19.3 20.2 21.0 21.8 22.6 23.4 24.2 25.1 26.7 28.3 30.0 31.6
N 12.5 15.9 16.8 17.6 18.5 19.3 20.2 21.0 21.9 22.7 23.6 24.4 25.3 26.1 27.8 29.5 31.2 32.9
  13 16.5 17.4 18.3 19.2 20.1 21.0 21.8 22.7 23.6 24.5 25.4 26.3 27.1 28.9 30.7 32.5 34.2

Pump Gas 93 Octane

Race Gas 106-110 Octane

Final compression ratio's exceeding 15:1, should have block or heads o-ringed  

Race Gas 110-114 Octane

Race Gas 114-118 Octane

Alcohol (Methanol)

EXAMPLE:

So lets say we have 454 Big Block Chevy. We want to use the vehicle in a street and strip application. In this application we will use a static compression ratio of 8:1. Lets also say the block is not o-ringed. We want to use pump gas for cruising around the street, but also want to step up the performance at the track. So now, we look at the chart to see what boost levels and what type of gas we need to run in this application.

Looking at the chart, we can run 8 lbs of boost on the street using pump gas. At the track, we can run up to 12 lbs of boost (no more than that because our heads are not o-ringed) using 110 octane.

The actual equation for Final Compression Ratio is:

F.C.R. = (Boost "divided" by 14.7 + 1) x Static compression ratio

 

ALTITUDE ADJUSTMENT

There is an important factor that might come into play, and that is the elevation your going to run your motor at. The final compression ratios listed above are at sea level. To adjust for elevation, it's quite simple. For every 500 feet above sea level, subtract .1 from your final compression ratio. So, if your running at 6000 feet above sea level in the example above, 8 lbs of boost with 8:1 compression would equate to a final compression ratio of 11.2 instead of 12.4. So, by using the chart and knowing you can subtract -1.2 from the final compression ratio, you can safely use 10 lbs of boost, on pump gas, to give you a true 12.2 final compression ratio. 

The actual equation for F.C.R. altitude adjustment is:

F.C.R. Altitude Correction = F.C.R. - (Elevation "divided" by 1000 x .2)

THE NEXT STEP

So now you know how much boost you can run, we need to now how do we get the blower to produce what we want. This done with the blower pulleys and drive ratio's. From here, we need to go and find out how fast we need to turn the blower on our motor to achieve what we want to do. From here, go to the Blower Boost Charts.