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BBV Thrust engine cover
« on: Dec 30, 2019, 9:40 pm »
I am making a cover for the thrust engine on my BBV and I am upcycling an old caravan gas canister fairing for this purpose. Before I progress it any further I wonder if there are any comments  from people who have more experience than myself!

Re: BBV Thrust engine cover
« Reply #1 on: Dec 30, 2019, 9:44 pm »
I am making a cover for the thrust engine on my BBV and I am upcycling an old caravan gas canister fairing for this purpose. Before I progress it any further I wonder if there are any comments  from people who have more experience than myself!

Re: BBV Thrust engine cover
« Reply #2 on: Dec 31, 2019, 6:30 am »
Hi

Looks good in principle, it should help keep the elements off the engine and reduce engine noise too. I have just three things to say! Air flow, air flow, air flow!

Start by buying one of these: eBay 312703518274 Before changing anything. First measure the cylinder head temperatures in the vicinity of the exhaust port. This will be your target temperature.

Next, think about the three air flows you need:

  • Cooling air in
  • Cooling air out
  • Combustion air

You need an unimpeded flow of air into the cooling fan intake, and an equally free exit out of the rear. You also need a supply of cool air to the carb inlet.

When you think it's ok, run it up and monitor the temperature in the same place as before, ensuring that it remains reasonably the same as before, then you'll know you are good airflow.

Ian
Ian Brooks
Gloucester, UK

Re: BBV Thrust engine cover
« Reply #3 on: Dec 31, 2019, 7:47 am »
Hi Ian, I have looked at the engine cover on the video of the BBVs that was posted earlier, I could only see two small slots in the side of the cover, from your experience does that seem about right?
The cover I have created is totally open at the back (see enclosed photo) is this how the original cover would have been?

Re: BBV Thrust engine cover
« Reply #4 on: Dec 31, 2019, 7:48 am »
And the other side

Re: BBV Thrust engine cover
« Reply #5 on: Dec 31, 2019, 9:13 am »
It'll need air inlets at the front that are equal in area (at least) to the area of the cooling air inlet on the front of the engine. Without this, the cooling fan will attempt to recirculate air around the engine, drawing hot air from the exhaust. This will cause the engine to run hot - likely much too hot, resulting in a dropped exhaust valve. The hovercraft community went through a phase of this when people messed about with the cooling of Briggs engines in the early naughties. It must have plenty of cool air coming in from the front, and I would strongly recommend measuring the head temperatures before and after the mod, so you can tell if you have enough air coming in.

Also worth doing is measuring the thrust before and after the change, as it will reduce airflow into the fan. Probably see a 10-15% reduction at a guess, although I have seen oversize engine covers cause a 25% thrust reduction. As long as this is tolerable then nothing to worry about, but worth knowing so that if it refuses to go over hump you'll know why.

Ian
Ian Brooks
Gloucester, UK

Re: BBV Thrust engine cover
« Reply #6 on: Dec 31, 2019, 9:25 am »
Thanks Ian, after looking at the video with the original engine cover - it is probably about the same size. Thanks for the info - Im off to get a temp gauge and a spring balance. Have a good new year.
Cheers
Neal

Re: BBV Thrust engine cover
« Reply #7 on: Dec 31, 2019, 4:07 pm »
Keep in mind the oil cooler on the side of the Briggs's  just a thought .  Tom

Re: BBV Thrust engine cover
« Reply #8 on: Dec 31, 2019, 6:15 pm »
Thanks Tom, good thought - Ill have to have a think about that.
Cheers
Neal