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Backplate pressure question

Old 05-05-2012, 09:40 AM
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TampaRC
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Default Backplate pressure question

I thought I once read something about not to use the backplate pressure configuration with a throttled engine. Is there any truth to this?
Old 05-05-2012, 12:55 PM
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Toad
 
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Default RE: Backplate pressure question

I use it that way all the time. In my heli's and planes. 1/2a-.90's
Old 05-05-2012, 07:05 PM
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Big Al-RCU
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Default RE: Backplate pressure question

With or without a check valve in the pressure line?
Old 05-06-2012, 10:51 AM
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Default RE: Backplate pressure question

Most RC carbs will require a fuel pressure regulator (actually an on demand regulator) such as the Cline for proper operation with case pressure to the tank. A check valve is commonly used in conjunction with the regulator. For a straight venturi and needle valve, no check valve is needed.

The problem with case pressure with no regulator is that most carbs can't cope with the excessive pressure at low speeds and needle settings get to be very tricky if not impossible to maintain. Constant fuel pressure solves the problem.

The above applies to all size engines.
Old 05-07-2012, 02:25 PM
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Default RE: Backplate pressure question

Oh jeez, now I have to try one without a regulator to see if it is possible.  It may save me the trouble of tapping into a tuned pipe some day.
Old 05-07-2012, 07:45 PM
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Default RE: Backplate pressure question

I don't use a regulator on most of my stuff unless the engine came with one. No problems. when you think about how long the negative pressure pulse is compared to the positive, you would think the carb would load up at low rpms. maybe they do a little. seems to me the lowering of rpm and pressure associated seems to make it a non issue for me. try it. see what happens. cant hurt. Works fine on my OS .55 heli and my .91 OS powered Fury. (heli) in fact it worked so well on the .91 when I took off the pump it surprised me. It runs way better than it did with the pump which seemed to go lean a lot. The positive pressure pulse is longer than the neg by a bunch. So pressure on the fuel in the tank is consistent until rpm drops like when throttling down. Which in most cases isn't an issue being slightly rich on the throttle closing. I haven't had an issue with it at all.
Old 05-08-2012, 08:55 PM
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Big Al-RCU
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Default RE: Backplate pressure question

Toad,
I believe this answers my earlier question if you are using a check valve or not, (obviously not).
I was hoping (fishing?) for clarification of the generic term ‘backplate pressure’.
1) No check valve.
I agree with your analysis of the non-check valve setup. Very informative and I believe accurate.
2) With check valve.
In this case the check valve allows the tank pressure to pump up and hold to the max positive pressure pulse, (as opposed to the average pressure). This is somewhat as an electrical analogy a power supply cap being charged up from an A/C source thru a rectifier, i.e., peak vs. RMS.
The pressure with the no check valve setup should be proportional to RPM and in fact you have shown this from experience to be the case.
However with a check valve the pressure will bleed off slowly. In the electrical analogy the rate would be determined by the size of the cap. In our case it is the volume of air above the fuel which stores the energy. It would thus bleed off faster with a full tank, slower with a low tank.
It is NOT going to work for a throttled engine without a pressure regulator.
I only bring this up because I’ve not seen it discussed before and have seen confusion in the ‘backplate pressure’ terminology.
Hope this answers TampaRC's question.
Al
Old 05-09-2012, 05:25 AM
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Default RE: Backplate pressure question

I just make a .015 hole in the pressure tap so it dampens the pressure and fuel doesn't accidentally get in the crankcase.  A check valve would do that too.  I will admit I haven't tried it on a throttled engine yet, but it worked well with a centrifugal fuel switch and a pipe with a monster size hole in the carb. and control line goodyear racers.  I will have to try a throttled one, but will be busy for a while.  The new field isn't ready yet.
Old 05-10-2012, 04:25 PM
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Default RE: Backplate pressure question

Hmmmm, I'll have to discuss this further with my Heli's.. they must like me cause they sure throttle nice. I could put in an inline one way valve like YS does. see if the engine likes it better...if I can find one around here.....oh, thats been done before to just get rid of the perry pumps of the day...
Old 05-11-2012, 06:33 PM
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Default RE: Backplate pressure question

I just tried an old .15 Rossi with no muffler and kind of high timing (180 degrees exh.) It wouldn't idle under 10,000 rpm on backplate pressure but went down to about 5 or 6,000 with no pressure on a perry carb. So I would think of it as a cutoff more than a throttle. No check valve or regulator. It started easier on the backplate pressure though.

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