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since I know very little about heat exchange and necessity of finned exhaust nuts I wanted to bring it to this forum. Is it necessary to have finned exhaust nuts on an air cooled motor? VW motors don't have them. Do the finned nuts really do that much cooling?

Look at it a different way; the exhaust port of the head is probably the hottest point on the engine. It is then perhaps the first place one would consider putting fins. On a "clean air" VW engine, not only is the cooling air being directed only over the cylinders, but the exhaust heat is being used for temperature control.

twist wrote:since I know very little about heat exchange and necessity of finned exhaust nuts I wanted to bring it to this forum. Is it necessary to have finned exhaust nuts on an air cooled motor? VW motors don't have them. Do the finned nuts really do that much cooling?

Apples and oranges. You could as easily say "Do VW's need a cooling fan? BMW motorcycle engines don't have one, so does it really do that much cooling?"

THEORETICALLY - the airhead motor was designed to have one, so it's probably best to keep it. Things like the thickness of the header connection to the head could, again theoretically, be designed differently depending on whether or not the designer expects that finned nut to be there.

OTOH, it is unlikely that it would make much difference, but it is entirely possible that, under some high heat situations, it might actually make an important difference.

while it's true the VW engine has a fan, it's not all that big and the engine sits in an enclosed compartment and has no cooling fins. The air head engine is in the direct flow of air and has finned cylinders and heads, and some have oil coolers. So this is why I brought this up, because I don't seem to think like normal people. While the VW has directed air flow, can it really be that effective? I ask after looking at the motor in a 50's era Porsche. It looked to me that that motor would get mighty hot in there and there are no fins at all. By contrast, the air head engine has cooling fins on the heads right up to the exhaust. I'm trying for a nice clean look as I redo this bike. I have a set of very nice billet aluminum exhaust clamps from Germany, no fins. They are used there bu I have no idea if they are the cause of engine failure due to excessive heat.

I guess it is just me, but I am mind reading impaired.

It is impossible for anyone to give you a resonable answer since we have no clue as to the year and model. Some year/models had marginal cooling and I would prefer to have those exhaust fins. Others never seemed to have a cooling problem.

Has anyone ever suggested that our bike have too much cooling?

twist wrote:while it's true the VW engine has a fan, it's not all that big and the engine sits in an enclosed compartment and has no cooling fins. The air head engine is in the direct flow of air and has finned cylinders and heads, and some have oil coolers. So this is why I brought this up, because I don't seem to think like normal people. While the VW has directed air flow, can it really be that effective? I ask after looking at the motor in a 50's era Porsche. It looked to me that that motor would get mighty hot in there and there are no fins at all. By contrast, the air head engine has cooling fins on the heads right up to the exhaust. I'm trying for a nice clean look as I redo this bike. I have a set of very nice billet aluminum exhaust clamps from Germany, no fins. They are used there bu I have no idea if they are the cause of engine failure due to excessive heat.

VWs have fins! (borrowed pic)



Most Porsches do as well. And the fans can draw enough air to starve the carburetors!

The only finless bits I've heard about are cylinders used in VW drag race applications without any cooling fan or shroud, and I understand some Porsche motors of the 956/962 era had no fins on the windward side of the cylinders.

Perhaps Duane could enlighten us as to the plain /2 bikes not having the finned nuts and the R69(S) having finned nuts. For cause or show?

melville wrote:
Perhaps Duane could enlighten us as to the plain /2 bikes not having the finned nuts and the R69(S) having finned nuts. For cause or show?

I was thinking about the /2 nuts too, but those (non-S) engines are putting out a fraction of the power of the later motors, thus less heat as well. Even an R75/5 is putting out nearly 20% more peak HP than an R69S (which had the finned nuts), and almost double the HP of my old R50/2 (which did not). That's just for an R75. The bigger motors make even more power - and heat.

The "hotest" bike with no finned nuts was the R60/2 and it was famous for having heat problems. As far as I know the lack of finned nuts wasn't a major factor. In any case, B&S offered a finned nut that clamped over the stock non-finned nut. I wasn't able to see any difference in the failures.

The R69S was far more reliable than the R60/2, but it wasn't the finned nuts that made the difference, it was the superior design.

How well a finned nut transfers heat to atmosphere would depend on how well the heat is transferred to the nut from the header?

ME 109 wrote:How well a finned nut transfers heat to atmosphere would depend on how well the heat is transferred to the nut from the header?

No, not just from the header, but from the head. If there were any heat issues which actually impacted reliability, I would guess it would be about heat in the head around the exhaust port and valve. The whole point of the "Oil head" is to pass oil through that area to aid in cooling that hottest of spots. That kind of implies that they had reached a point in the Airhead where it was a limiting factor (although the 4-valve head is an additional factor).