Over 40 years of building engines, I have developed opinions about what the effect on cylinder head temperatures would be if they were painted, powder coated, or left bare. I had thought that leaving the cylinder and heads raw might run lower temps than engine paint. Most times, if painted, the cylinders and heads would be black in color. The color black absorbs heat and doesn’t aid in a cooling effect. Even though I own a powder coating business I would not powder cylinders and heads because I thought it would hold in the heat of a running engine. But, were my opinions correct? I decided to run tests to find out. I felt like a high school kid doing a science project and the results were eye opening.
The first coatings I wanted to test were the painted cylinder/head combo vs. the bare, raw aluminum cylinder/head combo. I had a Yamaha RD-350 cylinder and head that were freshly painted and because it was a 2 stroke with an open intake, it was easy to introduce the heat into the cylinder combustion area. As you can see in the picture, I used a heat gun that had an output temperature of 750F. The heat gun was held in place with a square tube welded onto the work bench. This way, the conditions would be similar when I changed to the bare cylinders. I partially restricted the exhaust port with a steel rectangular tube to aid in heat retention into the cylinder. The cylinder was placed on 2 aluminum plates to partially close off the bottom of the cylinder.
First, I turned on the heat gun to start building up some cylinder heat. I wanted no air movement in this area so I blocked it off with some cardboard. The head was placed on top and a spark plug tightened into the head with the thermal couple under the spark plug gasket. The thermal couple gives very accurate temperature readings. Before heating, both cylinders had a room temperature of 68 degrees. Both cylinders were heated for exactly 30 minutes with the heat gun setup. At the end of 30 minutes, the black cylinder had a head temperature of 165F and the bare cylinder had a head temperature of 164F.
To do the next part of the test, I turned on a floor fan on low. The fan was 7’ away and was the same direction the air would hit across the cylinder head if it were on a motorcycle running down the road.
The cylinder head results with the fan on:
Time Black Paint Cylinder Head Bare Cylinder/Head
- 0 Minutes 165F 164F
- 1 Minute 160F 158F
- 3 Minutes 146F 145F
- 4 Minutes 141F 140F
- 5 Minutes 136F 136F
- 7 Minutes 132F 131F
- 10 Minutes 126F 125F
As you can see, there was really no difference between the black cylinder head temps and the bare aluminum cylinder head temps. This was nice to know, since my next bike will have the cylinders and heads painted black.
The next test was performed to determine if there is a difference in heat dissipation between a painted aluminum piece and a powder-coated one.
For this test, I had an aluminum strip that was half painted black and the other half powder coated blue. The reason blue was chosen was to distinguish it better for you, the reader. On the strip, I also milled out four 5/16” areas to allow temperature readings on the aluminum itself. These areas were the same distance from the center mounting hole.
After placing a 3/8” diameter bolt in the center hole, this assembly was secured in place with a drill press vise. An acetylene/oxygen torch was held in place with another drill press vise and the flame directed at the center mounting point. I will call the painted end “section 1,” and the powder-coated end, “section 2.” The milled areas closest to the center will be called “A” and the outer milled sections will be called “B.” See the photo for clarification. After lighting the torch, I took measurements every minute and, after 5 minutes, placed a fan 8” away and directed towards the center of the strip.
Here are the results.
Time Paint 1A Power Coat 2A Paint 1B Powder Coat 2B
- 1 Minute 87F 96F 86F 95F
- 2 Minutes 89F 105F 89F 98F
- 3 Minutes 92F 160F 91F 149F
- 4 Minutes 165F 270F 162F 259F
- 5 Minutes 169F 430F 167F 406F
- 1 Minute 147F 279F 127F 242F
- 2 Minutes 129F 254F 112F 208F
As you can clearly see, powder coating does act as a heat insulator, trapping heat in the aluminum.
Just think about all that heat trapped in your cylinder and head which could cause major engine damage! Based on my tests, I concluded that it doesn’t really matter if you paint an aluminum cylinder head or leave it bare, but I would definitely not powder coat one.
*This article was originally published in The Horse Magazine Issue #128 March/April 2013