Poked holes in my cowl ! (spoiler)

The original 2-stroke Genuine scooter and its 4-stroke manual and automatic offspring

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Mr.FixIt
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Poked holes in my cowl ! (spoiler)

Post by Mr.FixIt »

A couple weeks ago viney266 and I were sitting around the table with a bunch of locals enjoying some cold adult beverages. The subject of Stella engine cooling came up.

The engine cooling fan moves quite a bit of air, as we all probably know the engine runs cooler if you run with the engine cowl off. We can't all run around with the engine cowl off, that just wouldn't be right. Fitting vented Stella Auto cowls isn't a commodity everyone has access to, but I do have a sheet metal punch. The nice thing about having a scooter that has already been wrecked once, is imperfect, and resurrected from the dead... is that I don't mind making modifications.

A few carefully placed pilot holes and an afternoon of wrenching yielded a surprisingly discreet result. I chose to knock a few square holes right on the black squares of the checkerboard pattern on the cowl. You don't even see the holes unless you look for them. This allows air from above the cowl to enter and change the airflow pattern around the engine. The hope is that there is more cool air entering, and hot air leaving under the bike.

I haven't done a real scientific test yet. I want to make a run with the engine starting at the same CHT reading each time. I will first take the run with the holes taped off with blue painters tape. Then make a second run with the tape removed to see if there is any difference.
Holly:
Well, the thing about a black hole - its main distinguishing feature - is it's black. And the thing about space, the color of space, your basic space color, is black. So how are you supposed to see them?

Rimmer:
But five of them? . How can you manage to miss five black holes?

Holly:
It's always the way, innit? You hang around for three million years in deep space and there hasn't been one, then all of a sudden five turn up at once.
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Eric
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Christopher
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Post by Christopher »

It's been awhile, any updates? Did the holes help?
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Mr.FixIt
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Post by Mr.FixIt »

The extra ventilation did help. A few more holes may approach the cross-sectional area of a factory-vented cowl, but this little bit did help.

Without performing the test running up a hill over and over with holes covered vs holes open to get a good statistical sampling, the difference was barely 9 Fahrenheit degrees on the one run I did.

Part of it may not be a ram-jet source of air to the flywheel fan, but a place for fresh air to enter so that the hot air can get out without being re-circulated under the cowl.

CHT without holes = 315 °F
CHT with holes = 306 °F

As the days get cooler the concern of overheating becomes a concern of next summer. My son jumped on the back yesterday for a quick trip to the store. The engine certainly likes the cooler weather and hauled both of us comfortably on the straightaways.

It is fortunate that the original Vespa cooling fan design and cover is as well thought out as it is. It does seem to provide adequate air flow. After working on cooling improvements for my friend's 1960's British Sunbeam 175 scooter, the refined nature of the Italian design became apparent to me.
Eric
(aka Mr. Fix It)
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