Painting the engine potentially affecting cooling?

Red Beard

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I have seen a lot of painted cases out there and was thinking about doing it myself, but I remembered I read somewhere back in another forum that painting cooling fins on an air cooled motor will adversely affect the ability of the engine to shed heat. I'm no rocket scientist :shrug:, and therefore turn to the collective wisdom of this forum for input.

Thoughts anyone?
 
My guess is that the theory would be that a layer of paint will add insulation thus raising the temperature?
Bear in mind that when painting you shouldn't be applying the material too thickly as this will be more prone to chip and crack. Ideal is just sufficient to cover the substrate. This should only be several microns thick, i.e. the thickness of a sheet of paper.

To try and understand what effect painting will have you would need to know the emissivity coefficient (The emissivity coefficient - ε - indicates the radiation of heat from a 'grey body' according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal 'black body' with the emissivity coefficient ε = 1) of bare aluminium versus that of painted.
According to this chart, the painted surface will transfer slightly more heat than an oxidized surface and significantly more than a polished one.
What the difference is in the real wold I have no idea but suspect nothing significant.

Other factors such as ambient temperature and relative humidity may well cause greater temperature fluctuations.

You could always invest in an infra red thermometer then you could compare the before and after readings (adjusted for the relevant ambient conditions) thus contributing the knowledge base here.

Folk have been painting motors for many, many years so (IMHO) you have no need to worry.
 
My guess is that the theory would be that a layer of paint will add insulation thus raising the temperature?

Yes, that's right, with the exception of a certain FAA-approved engine paint and a certain specialized Kal-gard paint.

Coupla relevant threads:

http://www.xs650.com/forum/showthread.php?t=31774
http://www.xs650.com/forum/showthread.php?t=34230

Shedding engine heat is a fascinating science. Yes, flat black supposedly has the highest emissivity, but the heat has to get through it to the surface first. According to the emissivity charts, wrapping the engine in asbestos would improve its radiated emissions, but we know that would actually produce a lump of molten aluminum.
 
I would also agree but how much heat does it keep in?? And how much is too much paint. I have painted motors and cylinders all my building life and never had one over heat or crack. MANY MANY MOTORS...
I think the test would be if you had a heat gun test a stock unpainted motor and then test a complete painted or power coated motor and see the TEMP difference. I don't think its going to be a lot. I have run oil temp gauges on bikes and really didn't see a big difference of a motor that was painted or not and the reason I did it was I powder coated a set of cases/jugs/head on a CB750 and a buddy brought up the same question . ISN'T the motor going to run hotter. It did but so minimal that it was hard to tell from the oil gauge. They sell this thing that's called a oil cooler and most older bikes that install one drop there TEMP 40 to 70 degrees or JUST RIDE FASTER. Me I will still keep painting motors until I see one melt.
 
To try and understand what effect painting will have you would need to know the emissivity coefficient (The emissivity coefficient - ε - indicates the radiation of heat from a 'grey body' according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal 'black body' with the emissivity coefficient ε = 1) of bare aluminium versus that of painted.
According to this chart, the painted surface will transfer slightly more heat than an oxidized surface and significantly more than a polished one.
What the difference is in the real wold I have no idea but suspect nothing significant.

Holy crap, thanks for dropping that science bomb Bill Nye! Good to know the physics behind it, so thank you!


TwoManyXS1Bs; said:
a certain specialized Kal-gard paint.
Shedding engine heat is a fascinating science. Yes, flat black supposedly has the highest emissivity, but the heat has to get through it to the surface first.

Very good read on that kal-guard, never had even heard of it before, interesting.....

if you had a heat gun test a stock unpainted motor and then test a complete painted or power coated motor and see the TEMP difference.

Sounds like I have an experiment to run sometime in the future...

Thanks for the input y'all, I think I might actually invest in a heat gun and check out the results, I have all our local density altitudes, Pressure altitudes, humidity, and temp relations daily here, so if i can match the conditions on the day I test unpainted and painted it would make for a fun experiment!
 
Cylinders and heads are rough cast because it provides more surface area for cooling. I wouldn't lay the paint on thick so the surfaces came out smooth. You want them rough for more efficient cooling. That's why it's not a good idea to highly buff and polish these areas to a mirror finish. Looks good maybe but doesn't cool as well.
 
Cylinders and heads are rough cast because it provides more surface area for cooling.

Walking around the NASCAR garage this weekend I noticed this. Radiators, Intakes, Heads,,,,etc are not painted.
If you look at hi-end engine builder's motors take notice what they paint and don't paint. It isn't for appearance.

IMO, For a daily XS650 rider that commuted back and forth to work on pump gas....paint the cases like a boss. I'd sand blast the cylinder fins leaving a rough finish almost like as 5twins says "a rough cast look".

Has anyone determined the benefit, if any, the finned oil filter cover has in heat reduction?......Perhaps painting and the use of that would offset?
 
Was thinkin' about making a thread on this, but this thread is perfectly relevant.

Just a wintertime thaw thinking game to keep the brain juices flowing.
I love these kinds of physics/logic puzzles.

Apologies in advance to our Metric friends.

*** 1 ***

You have a new-fangled pistol-grip hand-held laser-beam-aimed InfraRed (IR) temperature sensor.

Your refrigerator went kaplooey. Down the street is the budget used/refurb refrigerator store. They have 3 equal-sized and equal-priced refrigerators on display, plugged-in and running for customers to evaluate, ready to go. You want the best refrigerator for your money and decide to be scientific about this. So, you shoot your IR gun at the middle of each refrigerator's door. You also note that the store's room temperature is 75°F.

Refrigerator #1's door shows 70°F
Refrigerator #2's door shows 65°F
Refrigerator #3's door shows 60°F

Which refrigerator do you buy?

*** 2 ***

It's a calm/still severely cold evening, -30°F. The weather guys are predicting that it'll get even colder overnight. You're concerned about your 3 elderly neighbors' well-being, but don't want to intrude unless necessary. Using your IR gun, you stroll outside and take readings of their front doors.

Neighbor #1's front door shows -20°F
Neighbor #2's front door shows 0°F
Neighbor #3's front door shows +20°F

Which neighbor are you concerned about?

*** 3 ***

You want to purchase a passive room heater, like one of those no-moving-parts oil-filled heaters. The heater store has 3 models on display, plugged-in and heating. You whip out your trusty IR gun and take readings of each heater. You also note that the store's room temperature is 75°F.

Heater #1 shows 130°F
Heater #2 shows 125°F
Heater #3 shows 120°F

Which heater do you buy?

*** 4 ***

You're out ridin' your XS650, with 2 buddies who are also ridin' XS650s. It's a super-hot 106°F summer day, and ya'll are concerned about overheating damage. After a highway-speed ride, everybody comes to a stop, you whip out your IR gun and take quick readings of the cylinders of all 3 bikes.

Your engine shows 290°F
Tom's engine shows 270°F
Harry's engine shows 250°F

Whose bike is in danger of overheat damage?
 
Twomany you got TOOOOOOO much time on your hands
When referring to race motors CARS BIKES doesn't matter Paint is weight and that's why a lot of guys don't paint anything. I have built many race bikes and motors and if I add paint I got to add in the weight. My frames/motor/body parts don't have paint for weight reasons when I got to add in bike weight-rider weight. Many of the cars are wrapped and so are bikes and there is a difference on the plus side of wrap than paint.
Just think about it you got to primer the parts spray 3 or more coats sand then clear coat. Do your own test get a flat piece of metal and weight it. Then put two coats of primer-three coats of color- three coats of clear test the weight may not be a big amount but its there. Now take the same metal and put a sticker on it that's about the size of wrap and weigh it. It has come down to ounces but increase speed. Weird right.
I built a Hayabusa last year took my test rider who is 220 lbs and he went 9:70 in quarter mile. Same everything and I put 140lb rider and he went 9:40 Same horsepower. Weight is the key and paint in racing a NO-NO.
As far as the oil cooler I would think you would get some benefit from the one in the cover but one in front of the motor will work way better. CLEAN AIR
 
Twomany you got TOOOOOOO much time on your hands...

Hahaha, yeah, I cooked that up back when it was 19°F outside, and I couldn't think about anything else other than trying to keep warm.


OK, so here's an added twist to *** 3 & 4 ***

*** 3 ***

You want to purchase a passive room heater, like one of those no-moving-parts oil-filled heaters. The heater store has 3 models on display, plugged-in and heating. You whip out your trusty IR gun and take readings of each heater.

Heater #1 shows 130°F
Heater #2 shows 125°F
Heater #3 shows 120°F

*** Update ***
*** Heaters #1 & #2 are the size of golf balls, heater #3 is the size of a washing machine. ***

Now, which heater do you buy?

*** 4 ***

You're out ridin' your XS650, with 2 buddies who are also ridin' XS650s. It's a super-hot 106°F summer day, and ya'll are concerned about overheating damage. After a highway-speed ride, everybody comes to a stop, you whip out your IR gun and take quick readings of the cylinders of all 3 bikes.

Your engine shows 290°F
Tom's engine shows 270°F
Harry's engine shows 250°F

*** Update ***
*** Harry's engine is painted black ***

Now, whose bike is in danger of overheat damage?
 
Now, the answers.

*** 1 ***

You have a new-fangled pistol-grip hand-held laser-beam-aimed InfraRed (IR) temperature sensor.

Your refrigerator went kaplooey. Down the street is the budget used/refurb refrigerator store. They have 3 equal-sized and equal-priced refrigerators on display, plugged-in and running for customers to evaluate, ready to go. You want the best refrigerator for your money and decide to be scientific about this. So, you shoot your IR gun at the middle of each refrigerator's door. You also note that the store's room temperature is 75°F.

Refrigerator #1's door shows 70°F
Refrigerator #2's door shows 65°F
Refrigerator #3's door shows 60°F

Which refrigerator do you buy?

The correct answer is: "I dunno".

Since all 3 are colder than ambient, you can assume that all 3 are cooling. But, without knowing the actual temperature inside the refrigerators, you only have a piece of the puzzle, since you don't know the cold leakage (insulation value) of each refrigerator's door.

#3 could be the coldest, or
#1 could be the coldest, with the best door insulation.
#3 could be the warmest, with the worst door insulation.
Or any combination of these.

*** 2 ***

It's a calm/still severely cold evening, -30°F. The weather guys are predicting that it'll get even colder overnight. You're concerned about your 3 elderly neighbors' well-being, but don't want to intrude unless necessary. Using your IR gun, you stroll outside and take readings of their front doors.

Neighbor #1's front door shows -20°F
Neighbor #2's front door shows 0°F
Neighbor #3's front door shows +20°F

Which neighbor are you concerned about?

The correct answer is: "I dunno".

Since all 3 are warmer than ambient, you can assume that all 3 homes have some sort of heating. But, without knowing the actual temperature inside their homes, you only have a piece of the puzzle, since you don't know the heat leakage (insulation value) of each neighbor's door.

#1 could be the coldest, or
#1 could be the warmest, with the best door insulation.
#3 could be the coldest, with the worst door insulation.
Or any combination of these.

*** 3 ***

You want to purchase a passive room heater, like one of those no-moving-parts oil-filled heaters. The heater store has 3 models on display, plugged-in and heating. You whip out your trusty IR gun and take readings of each heater. You also note that the store's room temperature is 75°F.

Heater #1 shows 130°F
Heater #2 shows 125°F
Heater #3 shows 120°F

*** Update ***
*** Heaters #1 & #2 are the size of golf balls, heater #3 is the size of a washing machine. ***

Now, which heater do you buy?

The correct answer is: "I dunno".

Since all 3 are warmer than ambient, you can assume that all 3 heaters have some sort of heating. But, heater #3 has a much larger radiating surface, and will probably put out more total heat.
But it won't fit under your computer desk to warm your feet.

*** 4 ***

You're out ridin' your XS650, with 2 buddies who are also ridin' XS650s. It's a super-hot 106°F summer day, and ya'll are concerned about overheating damage. After a highway-speed ride, everybody comes to a stop, you whip out your IR gun and take quick readings of the cylinders of all 3 bikes.

Your engine shows 290°F
Tom's engine shows 270°F
Harry's engine shows 250°F

Whose bike is in danger of overheat damage?

*** Update ***
*** Harry's engine is painted black ***

Now, whose bike is in danger of overheat damage?

The correct answer is: "I dunno".

Since all 3 are warmer than ambient, you can assume that all 3 bikes have been running. But, without knowing the actual temperatures inside the engines, you only have a piece of the puzzle, since you don't know the total heat dissipation (convective and radiant) of each bike's engine. You also don't know the engine tuning/conditions and demands for highway speeds, which gives insight into the engine's power utilization.

It's logical to assume that all 3 engines are similar, with exposed aluminum cylinder fins, and in that scenario, you'd think that yours is in imminent danger.
However, the engine needs to shed its internal heat. A good heat conduction, poor insulation value, will let that heat escape. As such, that would drive the external temperatures higher. The air-exposed engine with the highest external temperature is radiating the greatest heat, and could be running cooler inside than the others. Then again, it could have a problem and really be hotter than the others.

If you want to reduce the externally measured temperature of your engine, wrap it in a blanket of asbestos. That'll do it, keep all that heat in, the outside will be much cooler. Works great on exhaust pipes, keeps you from roasting your legs. You'll also have a ready supply of molten aluminum. With the exception of properly applied Kal-Gard and certain FAA approved engine paints, expect most engine painting to act as a heat blanket. What does that say about Harry's engine?

A good adjunt to monitoring engine temperatures is an oil dipstick thermometer.

********

This kind of test demonstrates the fascinating 'inductive logic' nature of human thought. When trying to solve a puzzle, in the absence of a complete picture, the human mind will, often unknowingly, 'fill-in' the missing pieces, and lead it down an interesting path.

So, for all of you who read this and thought "I don't know", congratulations. You aced it.
 
The Horse done an article about a year ago comparing bare, vs paint vs powder coat. Bottom line, bare is best. Bare and textured for more surface area, as alluded to above.
A word on "temp guns" while handy reference devices, comparisons can only be made with the exact same target area and distance to the receiver. Surface texture, color, reflectivity, and curvature all will affect them. They are relatively weak infrared beam devices, and as they are measuring a light spectrum wave length, anything that will distort the beam is a variable that will affect readings. We use them occasionally for observations on aircraft, but a direct contact indicator (tempi-laq stick, or thermo-reactive temperature tapes) is a far more reliable device.
 
About the fridges, my wife would pick the one with the colour she liked and I'd pick the one that held the most beer.
About painting the fins, our old Britbikes had cast iron heads and barrels that came painted with cylinder black.
We used to re-paint them aluminum so they'd look like the aluminum race engines that we couldn't afford.
I thought it quaint that my Heritage Special had it's aluminum head and barrels painted black so it looked like an old Britbike although the fins' polished edges gave the game away.
I suppose that as black is good and polish is bad for cooling the Heritage's fin paint job broke even?
BTW it took 18 years for the black to wash off back to the natural aluminum finish they seemed to be ashamed of.
 
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