Should i have coated my intake manifold?

I recently coated my cast aluminium intake manifold black and i am wondering what affect this has had on performance. The intake manifold is much cooler to touch i would say about 30*C as before i could touch it for maybe 1 second now i can touch it infinitely.

Does this mean the heat conducted from the head is being transfered to my intake air raising my IAT's? I say this because the manifold is still bolted to the head and previoulsy the heat would be both transferred into the engine bay through convection from external surface of the manifold and through the inside to the intake air also via convection but now it seems most would have to go into the inside toward the intake air since effectively i partially insulated the outside of the manifold with black paint?

Ceramic thermal coatings on exhaust manifolds are designed to retain heat in the exhaust to lower underhood temps amongst other things but i don't think we would want to retain heat in the intake manifold so we can keep air densities high as possible.

What are peoples thoughts? Am i getting more heat transferred into the air that the engine breathes thereby losing a few % of power? I have been told 7*C=1% power drop.

Cheers

I don't think you're looking at it correctly. Before when it was unprotected and uncoated the heat generated from your exhaust manifold would cause the intake manifold to heat up which would then heat up the air and reduce performance. By coating it you basically put in a heat shield to protect the cool air coming in from the heat of the engine creating a little more power because the air would be cooler. It does the same thing as if you wrapped the exhaust with the heat wrap stuff only opposite. When you wrap the exhaust it keeps the heat inside the exhaust to reduce heat under the rest of the hood, and when you leave the exhaust unwrapped but wrap the intake then it allows heat into the rest of the hood but not into the intake.

I'm sure it didn't affect HP even the slightest bit, unless you coated the inside as well which would have altered the surface texture.

I don't think you're looking at it correctly. Before when it was unprotected and uncoated the heat generated from your exhaust manifold would cause the intake manifold to heat up which would then heat up the air and reduce performance. By coating it you basically put in a heat shield to protect the cool air coming in from the heat of the engine creating a little more power because the air would be cooler. It does the same thing as if you wrapped the exhaust with the heat wrap stuff only opposite. When you wrap the exhaust it keeps the heat inside the exhaust to reduce heat under the rest of the hood, and when you leave the exhaust unwrapped but wrap the intake then it allows heat into the rest of the hood but not into the intake.

My exhaust is ceramic coated and shielded so there is no direct radiation path to the manifold and underbonnet temps are reasonable. IMO more significantly the manifold is heated by conduction through the head and heat transfer is occuring through convection over the internal and external surfaces.

By insulating the outside sure it is now insulated fom the underhood temps (these may not be higher than the manifold temp anyway so it may be a secondary thing) but the manifold is massively heated by conduction throught the head which now has nowhere to go but into the air the engine breathess so i wonder if an internal thermal coating might help this but then where does the heat conducted to the manifold go. Ideally we want a fully insulated manifold inclusing special gaskets to prevent conduction.

I'm sure it didn't affect HP even the slightest bit, unless you coated the inside as well which would have altered the surface texture.

the outside of the manifold has changed temp by ~30*C so surely the inside has changed aswell (which way i don't know but IMO it may have increased due to above reasons) and a few % either way seems possible to me.

The external coating is holding heat in the manifold. That means that the actual casting may be holding more heat, but that doesn't mean its shedding that heat into the intake charge. It might, but the amount of time the air spends in the intake tract is so minimal that it doesn't really affect that much. Air Gap intakes were all the rage, but the truth is, they still get just as hot as regular intakes, it just takes a little longer.

The other thing to consider is this... underhood temps can pretty easily reach 200*F or more, so its possible that you're protecting the intake gasses from excess heat soak from the outside.

But my guess is that if you tested your manifold alongside an identical non-coated manifold, you wouldn't notice any difference.

I agree with curtis here. It is beneficial to coat your exhaust manifold/header, turbo exhaust housings, and SVs where you want to hold the heat in to help keep up exhaust velocity and reduce heat soak. However, I just don't see it making a difference in the intake. I would assume you would rather have the hot air escape the intake manifold.

That just seems more logical to me.

The external coating is holding heat in the manifold. That means that the actual casting may be holding more heat, but that doesn't mean its shedding that heat into the intake charge. It might, but the amount of time the air spends in the intake tract is so minimal that it doesn't really affect that much. Air Gap intakes were all the rage, but the truth is, they still get just as hot as regular intakes, it just takes a little longer.

The other thing to consider is this... underhood temps can pretty easily reach 200*F or more, so its possible that you're protecting the intake gasses from excess heat soak from the outside.

But my guess is that if you tested your manifold alongside an identical non-coated manifold, you wouldn't notice any difference.

If the casting is hotter more heat would transfer to the intake charge, i would not think a 20*C manifold increase would give a 20*C increase in intake charge temp (i might do some basic calcs to see).

As far as performance goes i lost some performance ~5% according to the dyno with the painted manifold that flows ~20% better (it is supposed to........lol) the dags, bumps etc were removed then a light extrude hone (casting texture is still very evident in most parts so runner diamater, length and plenum size are really unchanged except for variations between castings (the painted and unpainted manifold were different castings). More than likely there is something else at play.

I agree with curtis here. It is beneficial to coat your exhaust manifold/header, turbo exhaust housings, and SVs where you want to hold the heat in to help keep up exhaust velocity and reduce heat soak. However, I just don't see it making a difference in the intake. I would assume you would rather have the hot air escape the intake manifold.

That just seems more logical to me.

thats why i thought it might be best to coat the inside only to keep heat out and not do the outside so you aren't keeping the heat in. There seems to be a market for insulated gaskets (Hondata makes them) to reduce IAT.

If the manifold is insulated better the heat would go somewhere else (a different thermal path), IMO insulating just means it takes longer to heat up it also won't heat up as much

Is this a carburetor engine or EFI?

In a carb engine, or a throttle body EFI engine, the intake manifold downstream of the carb needs to be WARM, to allow the fuel vapour to remain in suspension.

In an multiport EFI engine the intake tract must be as cool as possible for max power.

Therefore, a cooler intake will reduce a carb or throttle body engine power, but boost a muti port engine power, (in theory)

Is this a carburetor engine or EFI?

In a carb engine, or a throttle body EFI engine, the intake manifold downstream of the carb needs to be WARM, to allow the fuel vapour to remain in suspension.

In an multiport EFI engine the intake tract must be as cool as possible for max power.

Therefore, a cooler intake will reduce a carb or throttle body engine power, but boost a muti port engine power, (in theory)

EFI port injected...........cold air definately = power

thats why i thought it might be best to coat the inside only to keep heat out and not do the outside so you aren't keeping the heat in. There seems to be a market for insulated gaskets (Hondata makes them) to reduce IAT.

If the manifold is insulated better the heat would go somewhere else (a different thermal path), IMO insulating just means it takes longer to heat up it also won't heat up as much

IMO the paint is reducing heat transfer.
However, black paint conducts heat better than other paints, especially on aluminum. This is why many aluminum air cooled engines (motorcycles, lawn mowers etc) are painted black to better radiate heat.

You may get better results through better thermal insulation if you paint the manifold white.

I recently coated my cast aluminium intake manifold black and i am wondering what affect this has had on performance. The intake manifold is much cooler to touch i would say about 30*C as before i could touch it for maybe 1 second now i can touch it infinitely.

I probably don’t need to tell you, but fingers are pretty shitty thermal couples. They are poor for measuring heat transfer and even worse for temperature. So while it certainly may be cooler, you should consider a more objective method of measuring the difference. It may be as simple as a change in thermal effusivity (http://electronics-cooling.com/html/2007_nov_techdata.html).

On the other hand if it is really cooler, it may be that the paint has increased the thermal resistance between the heat source and outside of the intake. Would a few microns of paint really make that big of difference, I wouldn’t think so. But if it is cooler on the outside because the paint is acting as an insulator then it is possible that more heat is being transfered into the intake air.

However, black paint conducts heat better than other paints, especially on aluminum. This is why many aluminum air cooled engines (motorcycles, lawn mowers etc) are painted black to better radiate heat.

I am assuming by conducts you mean radiate in the first sentence.

Black paint will defiantly radiate better than unfinished aluminum. However, it depends more on the paint and less on the color, as the emissivity of paints doesn’t vary greatly between color. Most paints have an emissivity of around .85 to .98. Where as aluminum tends to be around .05 to .40.

It should also be kept in mind that in many cases the aluminum has a much lower radiative absorptivity. For under the hood we could probably assume that emissivity equals the absorptivity for both the aluminum and paint, respectively. So while the aluminum does not radiate heat away well, it doesn't want to absorb radiated heat either.

I probably don’t need to tell you, but fingers are pretty shitty thermal couples. They are poor for measuring heat transfer and even worse for temperature. So while it certainly may be cooler, you should consider a more objective method of measuring the difference. It may be as simple as a change in thermal effusivity (http://electronics-cooling.com/html/2007_nov_techdata.html).

On the other hand if it is really cooler, it may be that the paint has increased the thermal resistance between the heat source and outside of the intake. Would a few microns of paint really make that big of difference, I wouldn’t think so. But if it is cooler on the outside because the paint is acting as an insulator then it is possible that more heat is being transfered into the intake air.



I am assuming by conducts you mean radiate in the first sentence.

Black paint will defiantly radiate better than unfinished aluminum. However, it depends more on the paint and less on the color, as the emissivity of paints doesn’t vary greatly between color. Most paints have an emissivity of around .85 to .98. Where as aluminum tends to be around .05 to .40.

It should also be kept in mind that in many cases the aluminum has a much lower radiative absorptivity. For under the hood we could probably assume that emissivity equals the absorptivity for both the aluminum and paint, respectively. So while the aluminum does not radiate heat away well, it doesn't want to absorb radiated heat either.

yeah i know you can't feel temperature (thats why a 10*C bit of steel feels colder than a 10*C block of wood) but i am able to hold the manifold continuously where as previously i was unable to do it for more than a second so its defintely hotter (remains to be seen how different it is when cruising with hood closed). As for radiation i don't think its an issue since radition is T^4 function so its more to do with the thin layer of paint being a increase in thermal resistance IMO. The only real way is to place a probe in the end of the runner and also the start of the manifold to see the temp increase due to manifold under different conditions (cruising , WOT) but i am not concerned enough to start putting holes in the manifold maybe if need to i remove it again it might be an interesting exercise.....

Keep in mind that temperature and heat are two different things. Just because the surface of the manifold has a lower temperature than before does not mean that the amount of heat is any different.

your example of the steel and wood are a perfect analogy. They are both the same temperature, but the steel has more heat in it and a higher heat capacity, making it feel colder. Another analogy is reaching your hand into a hot oven which doesn't burn, compared to touching the rack in the oven which does.

The coating on the intake is preventing heat from transferring out of the manifold and into your hand, but that should not necessarily be an indicator of how much heat (or temperature) is contained the aluminum "meat" of the intake.

Keep in mind that temperature and heat are two different things. Just because the surface of the manifold has a lower temperature than before does not mean that the amount of heat is any different.

your example of the steel and wood are a perfect analogy. They are both the same temperature, but the steel has more heat in it and a higher heat capacity, making it feel colder. Another analogy is reaching your hand into a hot oven which doesn't burn, compared to touching the rack in the oven which does.

The coating on the intake is preventing heat from transferring out of the manifold and into your hand, but that should not necessarily be an indicator of how much heat (or temperature) is contained the aluminum "meat" of the intake.

That is what I was trying to get at with the link to thermal effusivity, but there was a typo in the address that is now fixed.

Per the link:
It is important to realize that while the two expressions contain the same parameters, they are quite different. Diffusivity is related to the speed at which thermal equilibrium can be reached. Effusivity (sometimes called the heat penetration coefficient) is the rate at which a material can absorb heat. It is the property that determines the contact temperature of two bodies that touch each other.
For example, it explains the well-known but often misinterpreted effect that a metal feels cold to the touch and wool warm, even when both are at room temperature. Our finger is a very poor temperature sensor (that is, of the temperature of the body it touches, not of the contact temperature), but a sensitive heat flux sensor. The contact temperature is lowest for materials with a high thermal conductivity and a high thermal capacity, explaining why metal feels cold.

yeah i know you can't feel temperature (thats why a 10*C bit of steel feels colder than a 10*C block of wood) but i am able to hold the manifold continuously where as previously i was unable to do it for more than a second so its defintely hotter (remains to be seen how different it is when cruising with hood closed). As for radiation i don't think its an issue since radition is T^4 function so its more to do with the thin layer of paint being a increase in thermal resistance IMO. The only real way is to place a probe in the end of the runner and also the start of the manifold to see the temp increase due to manifold under different conditions (cruising , WOT) but i am not concerned enough to start putting holes in the manifold maybe if need to i remove it again it might be an interesting exercise.....

I agree that for this case radiation is probably not that significant, but not for the same reasons.

Assuming the outside of the intake is really cooler; then it is probably safe to say that more heat is being transferred into the intake air stream. The only question is does it really matter (that is, is it too hot now)? So unless you are going to measure the air temperature or try to calculate how much power is being transferred to the air, there really is no easy way to figure it out.

i know this is a little off topic but a friend of mine painted his intercooler black with just regular repco (like autozone or napa) spray cans. if paint has an effect on how a unit retains or expends heat, does this mean he's done something bad?

EDIT: this is a front mounted (OEM lancer evo 3) so i wouldnt think it's affected by engine bay temps.

why no image?