Let’s talk about fans and shrouds on gy6 engines

[rollingbender]

What are your thoughts on removal of the fan and shroud, specifically on a naked bike where air circulation is not hindered by body panels encasing the engine? There certainly have been a lot of air cooled motorcycles built without such contraptions that seem to get along just fine.

[aeroxbud]

I think the scooter design is designed to fit into a smaller package. If you compare fins on an air cooled scooter to a GY6, they have a lot more surface area.
The shroud acts as a conduit to get the hot air away. A sort of supercharged air force.
Lambretta


GY6

[GrumpyUnk]

I bought a Taotao that had been fitted with a BBK. The owner reported his friend decided it did not need the shroud and airflow from motion would keep the engine cool.
It no longer ran as it had detensioned the rings and had poor compression.
I replaced the cylinder, piston and rings, and fitted a new set of shrouds to make it work.
If you want to experiment running without shrouding, go ahead, but be ready to replace the parts when it stops working.
There is not enough airflow naturally occurring or the makers would not spend the extra $4 to install plastic panels to direct the cooling airflow.
The fins on a motorcycle with no forced cooling are several times as large as the fins on a 139QMB. They can radiate a lot of heat due to the surface area AND the airflow from moving down the road will be more than the air pulled across a radiator in a car/truck.
The shrouds are needed. On the edge of the cylinder head there is a formed gasket that keeps the airflow from leaking past near the rocker cover, and makes sure it goes past the cylinder fins instead. They REALLY want the air to go over the cylinder fins or they would not bother.
tom

[rollingbender]

The geometry of the fans on these things leaves a lot to be desired.

[GrumpyUnk]

If you want a fan that is supposed to produce more flow of cooling air, check gy6store dot com. I poked around there a week or so ago and they had several fans listed that were supposed to fit the 139QMB. There's not a lot of extra room inside the shroud, so changes are somewhat limited. I think it was designed for cost and fit more than the best airflow. The blades need something to hold the outer tip in place at 8,000 rpms... so there's a flat washer of plastic molded into the fan. The blades seem to contort around the mounting bolt holes for some reason that I cannot figure out. It may be to make the fan blades unequally spaced. If the blades were evenly spaced, they might set up some harmonic noise. The different spacing(just from looking at them) may preclude the development of harmonic vibrations and noises. They do loo a bit misshapen, but they seem to keep up with the needed airflow of standard parts. I do not know if they are up to 100cc BBKs.
Some have removed the tabs and bars across the inlet built into the shroud and fitted wire mesh in an effort to allow better incoming airflow. The plastic of the factory shielding does add some area that impeded airflow, so it seems a reasonable way to allow better incoming airflow.
If you get a 'performance' blower wheel, it would be interesting to note the difference. The air that has cooled the head and cylinder is exhausted downward under the engine. You should be able to feel the hot air flowing with the engine running at idle and feel how much heat is being exhausted. You can do the same thing under the cylinder, above the chassis/deck on lawnmowers to know how much heat they are producing. When running normally, the air is not too hot that you could not keep your hand in the airflow. Decades ago, a Corvair flat-six exhausted cooling air that was not exceedingly hot in a similar manner. So... If your exhausted air is very hot, you are running the engine hotter than normal. I think.
tom

[rollingbender]

I’ll be losing the squirrel cage part of the system on my next build. I have a plan developing for a different fan pointing directly at the jug/head. Just wondering how much if any shrouds need to be fabricated around the engine.

[FrankenMech]

What are you trying to accomplish by messing with the fan and shrouds?

[jackrides]

Look at the fins on a HD Sprint.