Does air-fuel mixture lower intake manifold temp?

[BradH]

And to be more specific, with a "spider" or air-gap style intake where the runners are isolated from the oil splash of the lifter valley. If so, anyone know of data that shows an example of by how much? Thanks - Brad

[pamotorman]

I would say the difference between the water and oil temp. this is what I ran because without it you could see the fuel distribution difference because 4 intake runner were exposed to the hot oil and 4 runners were not.

[MadBill]

Dynoing in cold humid weather, it's not uncommon to see frost forming on the manifold.

David Vizard has provided data on this in several of his books; I'll try to dig out the one with extensive insulating/isolating work reported. Fuel vaporization definitely lowers the mixture temperature but the fuel vapor displaces some air, so the net air mass flow may be less than if the fuel is mostly in droplet form.

[Carnut1]

Fuel that changes states from liquid to a vapor will absorb latent heat. Think of how a refrigerator or air-conditioner evaporator works. The evaporation rate will change depending on how high the vacuum is in the manifold. This is one reason carbs can make more hp than fuel injection. This is one reason I like t.b.i. fuel injection systems, they inject into a plenum and the evaporation of the fuel cools the charge a bit. The more the manifold heat soaks from oil and conduction needs to be looked at as well. Thanks, Charlie

[Circlotron]

So when you are running alcohol, what percentage of the engine's cooling requirement would be provided by the evaporation of the fuel? It must be quite a bit seeing many engines run the 1/4 mile with no cooling water, just a grout filled or billet block.

Some time ago I did some calculations for propane. It worked out that it provided almost 50% of the needed cooling courtesy of the hot water passing through the pressure regulator.

And nitrous. With extra power comes extra heat but also extra cooling. I wonder which one wins out?

[BradH]

I'm asking because the intake I'll be using was Jet Hot coated inside & out before I acquired it. However, the plenum & runner work it needs will leave the TBC only on the outside. Since the external coating will act both as a heat deflector on the outside (good) and a heat retainer from the inside (bad), I was trying to confirm what I thought I'd read in the past about a cooling effect by the air-fuel mixture that could offset -- to some extent -- the latter characteristics of an external-only TBC.

[Schurkey]

Anyone have a link to the photos of Smoky Yunick's 208 cid SBC-powered Indy car, with the turbos WAY BACK, and long intake piping?

The alcohol mixture would frost the long intake pipes. 1300 horsepower...on the dyno. When the mixture was right, he had all the power he needed.

Too bad the carb couldn't take the G-forces. By '75, he'd injected the engine.

[dwilliams]

I watched a friend's car idling one day. ~110F Florida day. Hot engine, hot air blowing back from the radiator. 2-inch carb spacer white with frost.

"Evaporative cooling" was much less theoretical after seeing that.

[pamotorman]

I watched a friend's car idling one day. ~110F Florida day. Hot engine, hot air blowing back from the radiator. 2-inch carb spacer white with frost.

"Evaporative cooling" was much less theoretical after seeing that.

that is why they have heaters on airplane induction systems.

[Ron Miller]

I can add this,,, back in the mid 80"s I met a Guy that worked at Holley Carb in the Race Division. He modified a 4781 850 c.f.m for a 427 Drag Race build. I gave him all the details on the engine & car. I will tell you this, I have never seen a intake or a spacer frost up before. This carb was so crisp and very responsive. It could idle for 20 to 30 minutes with no issues. You could wack the throttle full open with no hesitation or see a puff of black smoke. So I would say Yes when the A/F mixture is correct, you will have lower intake temps.

[David Redszus]

If an inlet manifold shows frost then some of the benefit of evaporative cooling is going into the manifold instead of the inlet air stream. Aluminum does not make a very good inlet manifold material due to its thermal properties.
Much better to use a polymer that will not conduct (or absorb) heat.

[David Redszus]

Thermal conductivity is predicted by the following formula:

Q = k A dT / D

where
A = area
dT = temperature difference
D = thickness of coating
k = thermal coefficient

Since the area and temperatures are fixed, only k and thickness determine thermal conductivity.

Some examples of k for various materials.
Aluminum...237
Silicon carbide ..150-190
Boron nitride..60-90
Iron....41
Alumina....30
Stainless...16
Zirconia...3

The lower the number, the better the insulating property of the material.

Coating an iron manifold with Alumina provides very little benefit unless the coating is very, very thick.
It would be much better to make the manifold out of stainless, and if need be, coat the inside with zirconia.

The ceramic insulating tiles used by NASA on the space shuttle are over 3/4" thick.
Think of how much thermal benefit results from a coating that is 0.003" thick.

The next problem to solve is the difference in thermal expansion of the ceramic coating compared to the base metal. Keeping the ceramic coating attached is difficult unless only a thin layer is applied.

NASA had to replace hundreds of ceramic tiles after every space flight.

[Olefud]

Thermal conductivity is predicted by the following formula:

Q = k A dT / D

where
A = area
dT = temperature difference
D = thickness of coating
k = thermal coefficient

Since the area and temperatures are fixed, only k and thickness determine thermal conductivity.

Some examples of k for various materials.
Aluminum...237
Silicon carbide ..150-190
Boron nitride..60-90
Iron....41
Alumina....30
Stainless...16
Zirconia...3

The lower the number, the better the insulating property of the material.

Coating an iron manifold with Alumina provides very little benefit unless the coating is very, very thick.
It would be much better to make the manifold out of stainless, and if need be, coat the inside with zirconia.

Right as to conductivity. But under the hood radiant heating can also be a significant factor. Shiny helps to minimize this.

[David Redszus]

Thermal conductivity is predicted by the following formula:

Q = k A dT / D

where
A = area
dT = temperature difference
D = thickness of coating
k = thermal coefficient

Since the area and temperatures are fixed, only k and thickness determine thermal conductivity.

Some examples of k for various materials.
Aluminum...237
Silicon carbide ..150-190
Boron nitride..60-90
Iron....41
Alumina....30
Stainless...16
Zirconia...3

The lower the number, the better the insulating property of the material.

Coating an iron manifold with Alumina provides very little benefit unless the coating is very, very thick.
It would be much better to make the manifold out of stainless, and if need be, coat the inside with zirconia.

Right as to conductivity. But under the hood radiant heating can also be a significant factor. Shiny helps to minimize this.

Heat will transfer as a function of temperature differential; the greater the temperature difference the more the heat transfer. Consider the difference in the exhaust system where gas temps can reach 1800F, and underhood temps that are usually slightly above ambient while the vehicle is racing. Pit stops allow substantial heat soaking to fuel lines, brake lines, carbs, etc.

Temperature management is often a critical aspect to racing survival (surpassed only by a lack of funds).

[jmarkaudio]

What about ceramic coating the inside and out of a manifold?