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Thread: Purpose of Cowl Bumps

  1. #11
    Senior Member efwd's Avatar
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    Default Re: Purpose of Cowl Bumps

    Joe, I will see if I can get a good pic of the inside clearance of the smooth engine cowl with the bump installed. It is the fuel rail for the fuel injection that gets very close to the top cowl. When I built mine, I seemed to have everything lined up but gravity pulls my cowl down about 1/4 inch at the prop spinner such that I can see the back side of the spinner a little bit. Back at the fitting of the cowl bump I seem to recall I had about 1/4 inch of space between the cowl and that rail fitting. I am not certain that the cowl would not rest on that rail if my bumps weren't installed given the fact that gravity pulls my cowl down a bit.
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  2. #12
    Senior Member JoeRuscito's Avatar
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    Default Re: Purpose of Cowl Bumps

    Quote Originally Posted by efwd View Post
    Joe, I will see if I can get a good pic of the inside clearance of the smooth engine cowl with the bump installed. It is the fuel rail for the fuel injection that gets very close to the top cowl. When I built mine, I seemed to have everything lined up but gravity pulls my cowl down about 1/4 inch at the prop spinner such that I can see the back side of the spinner a little bit. Back at the fitting of the cowl bump I seem to recall I had about 1/4 inch of space between the cowl and that rail fitting. I am not certain that the cowl would not rest on that rail if my bumps weren't installed given the fact that gravity pulls my cowl down a bit.
    Thanks for the explanation. Pictures would be awesome! Ill have mine on soon enough and I will keep in mind the sag and try to offset.

  3. #13

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    Default Re: Purpose of Cowl Bumps

    I installed them with a 912iS because the right hand fuel rail is about 3/8 inch from the upper cowl half. So the "bump" was positioned to provide more clearance. Of course the left one was positioned to match but serves no useful purpose.

  4. #14
    Senior Member JoeRuscito's Avatar
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    Default Re: Purpose of Cowl Bumps

    Quote Originally Posted by neville View Post
    I installed them with a 912iS because the right hand fuel rail is about 3/8 inch from the upper cowl half. So the "bump" was positioned to provide more clearance. Of course the left one was positioned to match but serves no useful purpose.
    My right side is also closer, in addition my cowl settled about 1/8 to the left... so it looks like I will be installing cowl bumps!

  5. #15
    Senior Member efwd's Avatar
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    Default Re: Purpose of Cowl Bumps

    Since you have decided these are probably not needed but here is the pic I promised. Looks to me that you would be fine without.
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    Eddie Forward
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  6. #16
    Senior Member JoeRuscito's Avatar
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    Default Re: Purpose of Cowl Bumps

    Quote Originally Posted by efwd View Post
    Since you have decided these are probably not needed but here is the pic I promised. Looks to me that you would be fine without.

    Eddie thanks! Your spacing (without bumps) looks similar to mine. I do wonder, while the bumps add head room for the rail at its center, the side edge of the bump is still just as close, it would seem that a big wider bump would have made more sense if adding spacing was their purpose. Im still contemplating if they are needed...

    Does anyone know what a good/recommended min gap from cowl to any part of the engine is?

    As I have just installed my radiator/oil cooler today it would seem 3/16" is gap enough. This is what the manual calls out for the gap below the radiator... I will say mine ended up much higher, about 3/8 in... still falls within the duct and I may add baffles to cover the lower gap completely if cooling is ever an issue.

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