id say combustion. motors that run alcahol run alot cooler. now the alcahol helps cool the motor also but if friction was the heat builder then they would still run as hot i would think. the rings and piston skirts are the only real amount of friction in a motor so dont think that they are generateing more heat than the combustion process wich is in the 1000 degrees farenhight level. why do you ask?
Just one of those things I wonder about when I am driving. Light why does the exhaust volume increase when you accelerate, and not just the tone or pitch of the exhaust. But I assume that under a full load, the release of the exhaust gas is so much more pressurized causing more volume...
The exhaust port in head is the hottest spot on the engine. You might be wondering then, why does the coolant, when it's the coldest right out of the radiator, enter the bottom of the engine and move to the top where the metal is the hottest? A couple of reasons. 1) You want the cylinders to be the coolest part of the engine. This is becaue aluminum pistons will expand more (not faster) than the cast iron or steel cylinder. So, a 700*F cylinder needs a lot more cold clearance than a 180*F cylinder. The more cold clearance, the worse it runs, and the more risk you're going to rattle a piston and knock the skirt off. If you pass coolant over the exhaust port first, you're going to have hotter water flowing over the cylinders on the way down and out. 2) As coolant passes over the exhaust ports, it boils. It is very difficult to force this gas down into the block, so there is a risk of cavitating the coolant system. In order to keep the coolant from boiling too much, you need to flow a LOT of coolant. If you allow this boiled coolant to continue to flow upward, it will cool, condense and continue to happily flow through the system. Boiling is not a bad thing either. Phase change from a liquid to a gas will use a LOT of energy without increasing the temperature. There are some down sides to the upward or standard coolant flow system. The first being that gas pockets can form above the exhaust ports if the head is not designed properly. When this happens, metal temps go up very quickly and valves get burnt. That is why it is very important to put the head gasket on right so the restrictor is in the front. The water will move from an area of high pressure at the back of the head to an area of low pressure at the front of the head. This might not be enough though, and some people wil actually drill and tap the water jacket above the exhaust port to ensure that any boiled coolant will have a way to escape out of the head. The Ford 2.3T SOHC head is particularly bad about this. The downside to a reverse flow system, like in the Chevy LT1 is that the water pump uses twice the normal horsepower to flow the water needed to keep the cylinders cool. The high water flow, and increased cylinder temps in the reason chevy only did this on one engine ever. If your engine is over heating the first thing someone will want to do is put on a high flow water pump. Why does this very rarely ever work? The coolant system is just that, a system. The radiator has a maximum heat rejection capability, and the water was a maximum heat absorption capability. If you've hit the limit of your radiator, flowing more water won't help. What will help is flowing more air. Air flow over the radiator is much more important than water flow through the radiator. That is why it's very very very important to make sure you clean the nasty bugs and animal guts out of your radiator every once in a while. Having the proper fan shroud makes all the difference in the world too. Since the water's heat absorption capability is largely due to phase change, and not just flow, it rarely helps an engine to increase water flow.
I think you may be too close to that exhaust volume. Friction. Coming off a motorcycle at 150mph generates a lot of heat. Combustion. Blowing up a motorcyle creates even more heat. I choose combustion.
