JohnnyC

I did a calc in another post showing the effect of the heated throttle body on air intake temp, and several people suggested putting it in the Advanced Tech section. There has been quite a bit of discussion about how much the TB heats up the air entering the engine, so I figured why guess and did a quick calc. I basically modeled the TB as air convective heat transfer inside a cylinder with a constant surface temperature. There are a few numbers here that you can play with, but the overall result is still the same; as you would expect, the TB does not raise the air intake temperature significantly
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 72.24K, (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) (72.24°K)
(0.055 kg air / sec) (1007 J / Kg-K )


Tout = 295 °K = 70.5°F

To recap; that’s an increase of just 0.5 degrees at the worst-case, low-flow idle condition.

At a higher airflow of 500 CFM the temperature only rises 0.14 degrees to 70.14°F!

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

Revelation Z28

iTrader: (10)

jesus christ.. do you know how to bake a cake perfectly?


well when i did the TB bypass, i felt a little difference in mornings. i did it during the summer, but now that its cooler, i feel a little more.

GOaT Cheese

In your equation, you list the outside air temperature at 70 deg.. The throttle body is heated with engine coolant to improve driveability in very cold climates (30 deg. and colder) It is also part of the early fuel evaporative emissions system for the same cold climates. The heated throttle body was not intended to have any significant effect on intake air temperatures when ambient temperatures are higher than freezing. Since GM does not know where the car will ultimately live out its life, most, if not all throttle bodies ('98-'02 F-body) are heated.

Alvin

iTrader: (2)

Did whoever teach you heat transfer explain to you how unrelable convection coeffiecents are? outside of a text book and in the real world your lucky to guess a real H within 50%

I'm not trying to be mean or anything but convection is extremely hard to predict as it depends on way to many different things...

nemanracing

+1 I learned that in Thermal Dynamics. I'm a Mech Engr Major.

JohnnyC

O.K., fair enough. Plugging in a more extreme intake temperature of 0 degrees F (255.37K) produces a new Tave value of 111.13K. At this extreme outside air temperature the heated throttle body would heat the intake air to a value of 0.59 degrees F. A whopping 0.59 degree increase. The point here is that the heated throttle body was never intended to increase intake air temperature. The TB is heated to prevent water vapor from causing ice build-up on the throttle body itself. This can occur due to the venturi effect in the TB when outside air temperatures are at extreme values. The heated TB also helps to prevent carbon from building-up and causing the TB to stick.

Yes, I know that the convection coefficient is a crap shoot at best (I am also an ME), but this model is fairly simple and I didn't want to get into correlations in order to determine the exact value. Even if you double the value of h that I used, you still only get an increase in intake air temperature of 0.85 degrees F at idle conditions. The small heat transfer surface area of the TB, and the large mass flow rate of air dominate the equation.

I did this calc because there was some misconceptions about why the TB was heated to begin with. I just wanted to show that the heated TB has no significant effect on intake air temp.

Valkyn

iTrader: (8)

If having the TB heated has such a small effect on air temp, why is there such an increase in horsepower after bypassing the TB?

http://www.ws6.com/mod-8.htm

JohnnyC

This is the only dyno I have seen that is based strictly on this mod. My equation shows only the temperature of the air just after exiting the TB. My guess is that heating the TB does not effect the air temperature of the air that exits the TB itself (as proven above), but it may effect the temperature under the hood. This could theoretically change the temperature of the intake and other components by a small value. The dyno is not an exact science, either (many variables). I would need to see a lot more data and runs in order to come to a conclusion on a final HP gain for the mod, if any. I guess if you made me come up with a theory then I would agree that the mod is definitely not hurting anything, and any small gains are probably due to reducing under-hood temps.

treyZ28

simple solution:
someone get an LS1 intake, tap it with thermocouples.
run car for a while. record AIT. bypass TB. Record AIT

andereck

Interesting, but refigure for 19 cfm or 10 grams a second. This more accurately duplicates a warmed over engine at an idle. Its good to see someone taking a scientific approach with some math even if the entered data is a bit inaccurate.

GOaT Cheese

I agree completely. Reduction in underhood temp is probably the only reason for any gain.

Phoenix 5.7

iTrader: (7)

what about heat soak from an aluminum intake manifold(weiand)? that is proven and documented. the air theoretically only stays in there for a tiny bit of time yet can cause a few hp loss.

that being said, i think the real reason that ppl should do the TB bypass is to make takin off the TB easy. i noticed nothing with the bypass as far as power.

Alvin

iTrader: (2)

A better way to handle this problem would be to measure the temperature of the coolant going into the throttle body and coming out. You can then solve it out by specific.

From intuition I would guess that it would make a very small difference.

EdmontonSS

From a fellow who lives in a cold climate... If GM wanted to heat the intake air they would have put an intake air heater in the car, and/or stuck to a metalic intake manifold with coolant passages through it... The inlet air temperature will have no effect on drivability in any vehicle with electronic fuel injection and working temperature compensation. The throttle body is heated to keep ice from forming on the metals of the body and blade, as moisture in the air can and will condense and freeze on the metals, if they happen to be cold enough. Even though I don't drive my car in winter, I have not and won't bother bypassing my throttle body... I don't need my car launching into a snowbank because the throttle is stuck open, assuming by some miracle I had traction...

My car after a short drive around the block...

RedFirehawk

iTrader: (5)

What about the Air That comes in contact with the throttle blades and shaft? Dont see that in the equation.

1 FMF

iTrader: (1)

somebody give me a mesh and i'll run it through fluent

1 FMF

iTrader: (1)

my coworker says, "if you're gay enough to drive your f***'ing camaro in the winter, go to home depot and buy a 3/8" ball valve and hook that up to your throttle body hose, and you can shut the valve in the summer so it doesn't heat"

but, we are currently doing calculations.

1 FMF

iTrader: (1)

forgot to ask,
somebody post reliable numbers for the following,

throttle body size, 80mm ok? What is stock size on the LS1?
ballpark length of the throttle body,
mass air flow rate at 3000, 4000, 5000, 6000 rpms.

we'll figure worst case for flow parameters that'll impart the most heat from throttle body into the airflow.

who can I bill for 4 engineering hours? there's 4 of us now working on this, 2 really (1 busting ***** and 1 doesn't give a crap but there not doing work).

jimmyblue

iTrader: (11)

Can you please do this calculation, only for an aluminum intake
runner at 1, 10 and 100CFM? Questions in the External Engine
forum about the "heat soak" factor.

1 FMF

iTrader: (1)

it's 5pm and we are going drinking, sorry.






we'll pick up monday if possible,

but where do you get 1, 10 and 100 cfm from?
for an intake runner? what rpm?

I can do the calculations with relative accuracy but I need good numbers to plug into equations for it to be worth it.
To do just a throttle body, the subject being the throttle body coolant bypass theory increasing hp, is sort of a joke. Assuming that you all think you're loosing hp, at 3000-6000 rpm, because the TB is hot is not true. The air is flowing too fast through the TB to gain any significant heat to cause a power loss, what they guy said above like 0.5 to 1 deg F the air is heated through the throttle body is correct. I get a chance I'll try to duplicate the calculations. We are going into the throttle body business though, and will be selling 40mm throttle bodies, because the small diameter = less surface area which will impart less heat into the incoming air = more power. So our smaller TB will give more hp then your 90mm TB, solid marketting strategy ! We are partnering with pulstar plugs, mentioned in general maint. section, there million watt spark plug + our smaller TB, oh yeah.

if you want heat imparted into air for a whole sheet metal intake or aluminum intake, send me a cad model of your intake or at least, we need ballpark surface area numbers of the intake runners.