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Under cowl heat reduction printer friendly version
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jb92563
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Location: Lake Elsinore, CA, USA
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Posted Monday, February 27, 2012 @ 12:26 PM  

In preparing for a season of flying after my annual I was tinkering with the 4's engine tuning the mixture for for optimum settings and smooth acceleration.

After 30 minutes of adjustments I decided all was as perfect as can be and put the cowl on so I could go fly.

Not 2 minutes of running with the cowl on while taxing to the runway the engine started running rough and just cut out.

Went back to the hanger pulled off the cowl and looked for problems. I touched the fuel system fittings and they were so hot I could barely
keep my fingers on them. It was the dreaded fuel vaporization issue again.

Then I remembered that I had bought some exhaust tape from Aircraft Spruce last year.

Wraping the exhaust tubes in this tape ought to reduce the radiated heat heat from the hot exhaust gases from over heating my cowl
and fuel system.

I also put some loose fitting aluminum heat shields around the fuel system plumbing. (not in these pictures)

The result was great, as I ran the engine cowled up for 30 minutes on the ground as before and no more bubbles in the fuel.

Using higher octane fuel would also minimize fuel vaporization so I was wondering if I could run the engine on 91 Octane instead of the ussual 87 octane. The manual says to use 80 octane which I believe is about the same as the US 87 Auto fuel octane.

What are other folks using for fuel Octane rating?

The new home for my RF4D, better protected from the elements and birds of prey talons.

[Edit by jb92563 on Monday, February 27, 2012 @ 12:27 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Jorgen
Captain

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Location: Lund, Sweden
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Posted Monday, February 27, 2012 @ 12:39 PM  

Good thinking Ray,
might there be a risk that the heat is concentrated at the end of the tapewrapping, were the cracks in the short exhausts are usually formed? Keep us posted, the idea sure is interesting!

May the 4's be with you/ Jörgen

Bob Grimstead
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Posted Monday, February 27, 2012 @ 09:16 PM  

Hi Ray,

Great new hangar!

I'm not sure that vaporisation depends on octane rating. Vaporisation depends upon volatility, which is somewhat different, although motor fuel (Mogas) is generally a bit more volatile than Avgas

Octane rating is all about pre-ignition, and varies with compression ratio.
The original 1200cc Rectimos only had a 7 to 1 compression ratio, so 80 octane fuel was fine.
If you're now running 1400cc cylinders & pistons, your compression ratio will be around 8.2 to 1, so you should be using a higher octane fuel.

I normally use approximately 3 parts of 91 RON auto gas (the lowest octane available on the forecourt here) mixed with one part of Avgas. I am told that 100LL Avgas has around twenty times the amount of lead in it that leaded auto fuel had, so I guess that will be more than enough to delay pre-ignition (knocking, pinking or pinging).

You may remember that, when flying solo, I time my climb for the first couple of minutes after take-off to ensure my enigne is performing adequately. Over the years, I have noticed that if I am using more than my usual amount of Mogas (too little Avgas) the climb rate becomes less. I can't hear any pinging, pinking or knocking, but the performance definitely drops off, so recently I've tended to err on having too much Avgas rather than too little.

I know none of this addresses your fuel vaporisation problem, but it should be of general interest.
Of course your vapour lock problem will go away immediately if you use a standard carburettor.

To be honest, with no fuel pump and no flop-tube tank pick-up, I really don't see the point of having a slide injection/carburettor with no float bowl, particularly when there is such a strong reason for not having it.

Yours, Bob

Yours, Bob

--------------------

jb92563
Second Lieutenant

Gender: Male
Location: Lake Elsinore, CA, USA
Registered: Mar 2007
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Posts: 582

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Posted Monday, February 27, 2012 @ 11:48 PM  

Thats exactly the reason that I have kept the zenith carb, just in case I want to switch back.

For now the problem seems to be gone and once airborne there should be plenty of cooling airflow to
not have to worry about it.

It was just the ground situation that was annoying.

I'll use the higher Octane from now on to prevent the ping/knock issues.

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

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