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Author Topic: Ideal pointing of McCoy 19 spray bar orifice  (Read 1100 times)

Offline Allen Eshleman

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Ideal pointing of McCoy 19 spray bar orifice
« on: June 17, 2013, 09:31:07 AM »
I am wondering what the ideal aim of a one orifice spray bar should be on a McCoy 19. I know the hole aims down into the venturi.  However,  should it be aimed parallel to the aim of the venturi, forward a bit, or backward a bit? 
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Offline Andrew Tinsley

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #1 on: June 17, 2013, 10:05:32 AM »
Rotate it until the hole just disappears. This should put the hole in the maximum fuel draw position i.e. the lowest area of pressure. I don't find any observable difference if it is rotated towards the front or rear of the engine.

Andrew.
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Offline Bootlegger

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #2 on: June 17, 2013, 12:14:10 PM »

  Something that I have found to work real well is set the spray bar hole at 5 o'clock from one side or 7 o'clock from the other side.
  When setting the hole correctly, be sure that it point's toward the prop.

  Marvin Denny told me about this several years ago and it works...

  P.S.  I mark the hole of the spray bar with a dremel cut off wheel when I have the spray bar out of the engine, by doing this I can tell where the hole is located when I install it..
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Offline Chris Wilson

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #3 on: June 17, 2013, 06:50:51 PM »
Rotating the hole backwards in a front induction 'tractor' engine makes the most sense to me as that is the way that the air is inducted (back towards the case) and the airstream will entering the venturi from a forward angle.

But this probably only makes a minute difference with short venturis and maybe part of the reason why a lot of purpose built stunt engines have long intake tubes that stabilize and straighten the air.
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MAAA AUS 73427

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Offline Jim Kraft

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #4 on: June 17, 2013, 07:21:32 PM »
Something I have wondered about. Is it better to have the most suction at the spray bar hole with the needle closed down to regulate, or have it straight down with less suction and open the needle to keep a larger orfice and keep it from clogging up so easy? I know with a filter it should not make a difference. Just a thought.
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Offline BillLee

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #5 on: June 17, 2013, 07:54:54 PM »
Hoping to dispel a lot of old wives' tales:  Please read http://www.nclra.org/TechTopics/NeedleValvePlacement/NeedleValveAlignment.pdf

Tests repeated by others, written up once (I believe) in Stunt News by Frank Williams.
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Offline Chris Wilson

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #6 on: June 17, 2013, 08:46:39 PM »
Hi Bill,
         I am not disputing that test for that particular environment but I have often wondered how a precisely placed hole really works when presented with a pulsed system where the intake is more than half the time closed and a new stunt type of environment (as opposed to the racing one aimed for on the test) that is constantly changing by virtue of maneuver G forces?

Surely the broad chaos of flow reversion and the constant stripping and reattachment of fuel droplets may not like any narrow approach that is precisely 'tuned' in the laboratory?

Just putting it out there ..........
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MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
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Offline Jim Thomerson

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #7 on: June 17, 2013, 09:39:19 PM »
i would expect an engine to run well so long as I could not see the hole in the spraybar.
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Offline Allen Eshleman

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #8 on: June 20, 2013, 06:26:51 AM »
Thanks guys,

       I will try turning it just to where I can't see it, perhaps a bit more - and toward the prop.   If that doesn't work well, I will experiment and do it toward the wing and may try other settings in between. 

I have always just turned them straight down into the venturi after marking them, opposite the orifice.  According to this info that may not be an optimum setting.

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Offline Ronald Eshleman

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #9 on: June 20, 2013, 12:35:20 PM »
How do you set one which has two orifices?
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Offline tom hampshire

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #10 on: June 20, 2013, 12:58:26 PM »
     Like a stunt Fox.  First look at the spray bar.  Note carefully that the cross hole in the center is NOT on the centerline of the entire piece, it's offset a little to one side.  You want the spray bar oriented in the venturi so that the offset side faces downward, toward the crankshaft.  I like to file a tiny notch in the end hex oriented in the center of the offset holes.  Or you can get it close by looking in to the venturi and watching the holes disappear/reappear as you turn the spraybar.  I'm not sure whether the offset was by design or accident, only that it exists in nearly all stunt Fox spray bars.  Not so for most other engines.  (The Brodak uses two holes drilled right on the centerline.)
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Offline Reptoid

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Re: Ideal pointing of McCoy 19 spray bar orifice
« Reply #11 on: June 20, 2013, 04:03:27 PM »
  Both Holley and Mikuni (carb manufacturers) did extensive testing on both a flow bench and on real time running two strokes and four strokes (so reversion was factored in). They published papers on venturi design and optimization which I read back when I was racing and tuning motorcycles (2 & 4 Stroke). The data concurs with Bill's test data listed above and for those who like diffinitive #s like me, the maximun point of vacum (or fuel draw) was found to be .030" downstream from the point of maximun restriction regardless of venturi shape. I have found in most cases facing forward to be slightly stronger draw, most likely due to flow in venturi being biased to the rear rather than a perfect 90 degrees to spraybar.
    Will it run OK in other positions? Yes, but it may have a negative effect on fuel draw so why not use the best setting possible and eliminate one more variable?
    They also covered Venturi shape in depth but I'll refer you to the info Randy Smith has posted previously on this forum since his explanation is almost verbatum what they found. Search and it will come up. H^^
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