# Turbo Cfm

maximum turbo cfm output = (3.14)(turbo compressor wheel size ÷ 2)2
3

spool time in seconds = turbo compressor wheel size2
(exhaust psi)(100)

Exhaust psi can be assumed to be 30 psi for the above calculation.

turbo weight = turbo compressor wheel size + 100

maximum turbo boost in psi = (14.7 psi)(maximum turbo cfm output)
engine cfm The red sections indicate exhaust passages. Exhaust flows from the outer, red portion of the turbo, through the outer edges of the turbine (the thing with the blades on it), to the red center. The blue indicates intake passages. Air flows from the center of the compressor wheel to the outer edges, speeding up as it goes into the intake system. So what's happening is, the exhaust is spinning the turbine on the exhaust side, which is attached to the compressor wheel on the intake side, so the compressor wheel spins. The compressor wheel on the intake side draws air in faster than it would normally come in and packs it into the cylinder.

## What you do:

Find a balance between turbo compressor wheel size, turbo cfm (cubic feet per minute or ft3/min) and waste gate psi (pounds per square inch or lbs/in2). Turbo compressor wheel size ranges from 56 - 91 millimeters (mm).

Factors to consider:

1. The larger the compressor wheel size, the longer it takes to spool up. Spooling up is when the turbo compressor wheel gains rpm (revolutions per minute). Turbos can spin up to 30,000 rpm, so the time it takes to spool up can be significant. This is what causes turbo lag. Turbo lag is a delay in the delivery of power. You hit the throttle, but the engine delays a second or two before producing significant power.

The HiPerMath equation is:

spool time in seconds = turbo compressor wheel size2
(exhaust psi)(100)

Exhaust psi can be assumed to be 30 psi for the above calculation.

2. The larger the compressor wheel the more cfm the turbo can put out.

The HiPerMath equation is:

maximum turbo cfm output = (3.14)(turbo compressor wheel size ÷ 2)2
3

3. Waste gate psi should be set where you want the turbo to stop adding psi. This is useful to control power. In the real world, it is also useful to prevent damage to the engine.

4. Turbo boost is measured in psi and determined by how many cfm the turbo is pushing in compared to how many cfm the engine would normally draw in. Turbo boost adds pressure to the air, which makes the air more dense. Denser air can have more fuel added to it, which will add more power.

The HiPerMath equations assume the turbo is spooled up. The equations are:

maximum turbo boost in psi = (14.7 psi)(maximum turbo cfm output)
engine cfm

You can assume the cfm will range from 500 - 1500 cfm, depending on your engine specs. Calculating actual engine cfm is massive, so estimating is a good idea.

5. Weight. The larger the turbo you have, the more weight is added to the total weight of your car.

The HiPerMath equation is:

turbo weight = turbo compressor wheel size + 100

This includes all the piping and an intercooler.

Definitions:

cfm = cubic feet per minute = ft3/min
psi = pounds per square inch = lbs/in2
rpm = revolutions per minute
mm = millimeter which is one thousandth of a meter.