Originally Posted by JohnnyC

I was tired of hearing everyone talk about the "laws of physics", so I decided to actually post some of them.

Here is a simplified (believe it or not) calc showing the effect of a 200 degree throttle body on intake air temperature:

Assumptions:

1) Outside air temp (Tin) = 70°F (21°C, 294°K)

2) Throttle Body is maintained at a temperature of 200°F (93°C, 366.5°K), which maintains the average TB to air temperature difference (Tave) at 72°C (345.2°K, conservative).

3) The throttle body flows 100 CFM of air at idle.

4) Heat Transfer Coefficient (h) of 6.81 W/ M^2-K is used for air cooling steel.

The following equation describes internal flow heat transfer through a thin-walled tube with a constant surface temperature:

h = (mfr) (Cp) (Tout – Tin)
( 3.14 ) (Dia) (Length) (Tave)

Where:
h = Heat Transfer Coefficient (6.81 W/ M^2-K is used for air cooling steel).

mfr = mass flow rate of air, calculated using an air density of 1.16 kg / m^3

Cp = specific heat of air at 21°C and atmospheric pressure = 1007 J / Kg-K

Dia = throttle body diameter

Length = throttle body length

Tave = average temperature difference between the TB and the air

mfr = (air volumetric flow rate) x (air density)

mfr = 100 CFM = 100 ft^3 / min = 2.83m^3 / min

mfr = [2.83m^3 / min] x [1.16 kg / m^3] x [1 min / 60 seconds] = 0.055 kg air / sec

By rearranging the equation above we can solve for Tout and finally determine just how much the hot throttle body heats up the air going to the engine:

Tout = Tin +

(h) ( 3.14 ) (Dia) (Length) (Tave)
(mfr) (Cp)

Tout = 294.3°K +

(6.81 W/ m^2-K) ( 3.14) (0.076 m) (0.102 m) (345.2°K)
(0.055 kg air / sec) (1007 J / Kg-K )


Tout = 295 °K = 71.7°F

To recap; that’s an increase of just 1.7 degrees at the worst-case, low-flow idle condition.

At a higher airflow of 500 CFM the temperature only rises 0.4 degrees to 70.4°F!

Hopefully this will help people see how little of an effect that heating the throttle body has on the intake air temperature.