Fuel injection versus carb

Mike Kimball · Post by **Mike Kimball** » Fri Feb 17, 2012 11:02 pm

This discussion is just what I needed to convince myself that I am doing
exactly what I want. (There are always doubts.) Gravity feed to a
carburetor. No high pressure fuel pumps, no recirculation pumps, no high
pressure fuel lines (bit of a safety issue), no pressure regulator, no
header tank, no return lines, no computers. Simple. I am going to set up a
test with a 5 gallon fuel can and fuel lines routed the same way they are in
the airplane from the tank to the carb and measure the pressure and then
decide whether to use a regular low pressure fuel pump or none at all. I'm
not going to mind a bit that there may be a slight performance loss with the
less efficient carb.

Mike Kimball
SR #044

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]On Behalf Of
Rickhm at home
Sent: Saturday, May 10, 2003 8:27 AM
To: murphy-rebel@dcsol.com
Subject: RE: V8 PSRU discussion

Ken,

Great feedback. I see your point. I'm not sure exactly how I will address
this, but your suggestion is a very viable option. I may put a pump on
each side before the header tank so the gas is constantly recirculating
between the main tanks and the header. I saw this as a recommendation. Bu
doing this the fuel in the header is constantly cycling with that in the
main. Any bubbles will hopefully be caught and sent back to the main tank.
These recirculation pumps are not high pressure pumps like that of the
fuel injection side, but more traditional pumps.... I still need t think
about this a bit. The part I am still pondering is failure analysis and
what if's.....

Thanks again!

RIck
SR70

-----Original Message-----
From: klehman@albedo.net [SMTP:klehman@albedo.net]
Sent: Saturday, May 10, 2003 7:08 AM
To: murphy-rebel@dcsol.com
Subject: Re: V8 PSRU discussion

Yes there be bubbles !

I do know of one large (several gallon) unvented header tank
successfully flying in a high wing aircraft but even he has admitted
that he must be on the ground to reliably vent the tank manually and
restablish flow if he runs it dry.

Fuel is a mixture of compounds. When it is heated in the pump and fuel
rail all is fine as it is under increased pressure. However the more
volatile components will form vapour bubbles when the pressure drops
back to ambient across the fuel pressure regulator. With a large enough
and cool enough tank, most will recondense if they are returned to the
tank. High altitude, mogas, and warm fuel from lots of recirculation
won't help them recondense. With no fuel return, too many bubbles may
shorten the fuel pump life and has caused loss of prime as the pump
tries to suck vapour. So yes there can certainly be bubbles but most are
fuel vapour. In turbulence, there might be some entrapped air as well.

With a header tank vent, slugs of air from turbulence, side-slipping, or
low fuel also become a non issue as they can vent back to the cross tank
vent on my installation. I'd highly recommend that if a header tank is
used, it be vented. I think of a carbuerator float bowl as a header. It
separates vapour, provides a small fuel reservoir, and is vented one way
or another.

Ken

rickhm@mindspring.com wrote:

I read somewhere on the web that many use UN-vented Header tanks that the

return line flows back into. Additionally, they recomended a circulation
pump between the header tank and the main tanks.

I guess I have a fundemental question. Is the return fuel full of air

bubbles? Ken your comments suggest that this is the case. If true, I
guess I find this surprising since the fuel is at 50-60 PSI as it hits the
jets.

Rick Muller
SR70

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klehman · Post by **klehman** » Fri Feb 17, 2012 11:02 pm

Nothing wrong with a carb! We have dyno evidence that proves there is no
performance loss with a carbed ej22 if properly set up. I believe this
is true of all engines. It is usually much harder to set up a carb with
wet intake runners to achieve this compared to dry intakes and port
injection though. A stock efi manifold may not work well with a carb. It
is even harder to optimise mixture across all operating conditions with
a carb. But who really cares as long as you can achieve max power and
adjust your cruise mixture manually without large cylinder to cylinder
differences...
Ken

Mike Kimball wrote:

This discussion is just what I needed to convince myself that I am doing
exactly what I want. (There are always doubts.) Gravity feed to a
carburetor. No high pressure fuel pumps, no recirculation pumps, no high
pressure fuel lines (bit of a safety issue), no pressure regulator, no
header tank, no return lines, no computers. Simple. I am going to set up a
test with a 5 gallon fuel can and fuel lines routed the same way they are in
the airplane from the tank to the carb and measure the pressure and then
decide whether to use a regular low pressure fuel pump or none at all. I'm
not going to mind a bit that there may be a slight performance loss with the
less efficient carb.

Mike Kimball
SR #044

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Bob Patterson · Post by **Bob Patterson** » Fri Feb 17, 2012 11:02 pm

Hi Mike !

Do the flow test for your peace of mind, but there should
be NO problem getting <at least> 30 ++ gallons/hour flow from a S/R
wing tank to the engine room !

As for the prop, I agree with Ken - you'll likely get
better performance if you keep it down to about 2,700 rpm max.,
as efficiency drops quickly as speed increases, and the noise
increases sharply too ! :-)

And I can't resist a biased observation ... modern automotive
electronic ignition systems are light-years ahead of the stone-aged
stuff in use in GA. Those magnetos came off J. I. Case Tractors in 1930
or thereabouts !!! Use the modern, reliable, solid state stuff !! ;-)

I'm inclined to ditto that on the modern fuel injection
systems - yes, there's a bit of planning needed for return lines,
but you'll still get more reliability with less hassle if you
just LEAVE IT STOCK !! I've flown Rebels with Subarus that had
the stock Sube fuel pumps submerged in the wing tanks, just like
the cars, with much of the stock plumbing - worked GREAT !!!

I just can't understand why anyone would want to take a
modern engineering marvel that you can get in, turn the key, and
run anytime, in almost any weather - and try to turn it BACK into
something stolen off a farm tractor in 1930 !! Unless, of course,
you are at least 40% smarter than the automotive engineers ...
(meaning you hope to get 40% more power, OR 40% more reliability !)

;-^) ;-) :-)

------ end of tirade ---- down off soapbox !!

Sorry, went and got emotional there.... ;-)

As Bill Johnston said to me a few weeks ago ...

"We all make decisions based on emotion
- then justify them using logic ! "

Guess that's what makes homebuilding ! And everybody has
their own reasons for doing things..... and we all gain from those
who DO try something different. Sure hope you do get a simple,
reliable V-8 aircraft installation - the world needs it !

......bobp

------------------------------------orig.------------------------------
At 12:49 PM 5/10/03 -0800, you wrote:

This discussion is just what I needed to convince myself that I am doing
exactly what I want. (There are always doubts.) Gravity feed to a
carburetor. No high pressure fuel pumps, no recirculation pumps, no high
pressure fuel lines (bit of a safety issue), no pressure regulator, no
header tank, no return lines, no computers. Simple. I am going to set up a
test with a 5 gallon fuel can and fuel lines routed the same way they are in
the airplane from the tank to the carb and measure the pressure and then
decide whether to use a regular low pressure fuel pump or none at all. I'm
not going to mind a bit that there may be a slight performance loss with the
less efficient carb.

Mike Kimball
SR #044

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]On Behalf Of
Rickhm at home
Sent: Saturday, May 10, 2003 8:27 AM
To: murphy-rebel@dcsol.com
Subject: RE: V8 PSRU discussion

Ken,

Great feedback. I see your point. I'm not sure exactly how I will address
this, but your suggestion is a very viable option. I may put a pump on
each side before the header tank so the gas is constantly recirculating
between the main tanks and the header. I saw this as a recommendation. Bu
doing this the fuel in the header is constantly cycling with that in the
main. Any bubbles will hopefully be caught and sent back to the main tank.
These recirculation pumps are not high pressure pumps like that of the
fuel injection side, but more traditional pumps.... I still need t think
about this a bit. The part I am still pondering is failure analysis and
what if's.....

Thanks again!

RIck
SR70

-----Original Message-----
From: klehman@albedo.net [SMTP:klehman@albedo.net]
Sent: Saturday, May 10, 2003 7:08 AM
To: murphy-rebel@dcsol.com
Subject: Re: V8 PSRU discussion

Yes there be bubbles !

I do know of one large (several gallon) unvented header tank
successfully flying in a high wing aircraft but even he has admitted
that he must be on the ground to reliably vent the tank manually and
restablish flow if he runs it dry.

Fuel is a mixture of compounds. When it is heated in the pump and fuel
rail all is fine as it is under increased pressure. However the more
volatile components will form vapour bubbles when the pressure drops
back to ambient across the fuel pressure regulator. With a large enough
and cool enough tank, most will recondense if they are returned to the
tank. High altitude, mogas, and warm fuel from lots of recirculation
won't help them recondense. With no fuel return, too many bubbles may
shorten the fuel pump life and has caused loss of prime as the pump
tries to suck vapour. So yes there can certainly be bubbles but most are
fuel vapour. In turbulence, there might be some entrapped air as well.

With a header tank vent, slugs of air from turbulence, side-slipping, or
low fuel also become a non issue as they can vent back to the cross tank
vent on my installation. I'd highly recommend that if a header tank is
used, it be vented. I think of a carbuerator float bowl as a header. It
separates vapour, provides a small fuel reservoir, and is vented one way
or another.

Ken

rickhm@mindspring.com wrote:
I read somewhere on the web that many use UN-vented Header tanks that the
return line flows back into. Additionally, they recomended a circulation
pump between the header tank and the main tanks.
I guess I have a fundemental question. Is the return fuel full of air
bubbles? Ken your comments suggest that this is the case. If true, I
guess I find this surprising since the fuel is at 50-60 PSI as it hits the
jets.
Rick Muller
SR70

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