hsukaria

So, the intake manifold pressure increases with WOT (goes from extra vacuum at idle to almost atmospheric at WOT). But the exhaust pressure also increases with throttle opening and load. That maintains a positive delta pressure (pressure difference) between the exhaust and the intake. So, pressurizing the fuel tank with the exhaust ensures a positive pressure to push the fuel from the tank to the carb, regardless of atmospheric pressure variation.

Now, atmospheric pressure variations will affect the intake pressure and the exhaust pressure in a similar way. What atmospheric pressure variation will do is increase or decrease the absolute pressure in the cylinder, causing variation in engine output power. That's why somebody living in a high altitude will have less power from their engine and typically has to install a larger engine for the same airplane.

Rudolph Hart

Thanks for the drawings earl,but i still struggle to understand the theory/practice and since i want to run a shotgun pipe on my 220 i need too.I read here somewhere that exhaust pressure to the tank was less than one psi whereas atmospheric pressure is nearly 15psi.Hope somebody can talk me thru that one.

hsukaria

I don't know what the muffler pressure is, it will change with throttle setting. Somebody here might have good numbers for that. But when you connect the muffler pressure line to the tank, you are not swapping one pressure for the other. The muffler pressure is ADDED to atmospheric pressure at the tank.

earlwb

The muffler pressure varies with the type of muffler or exhaust system used.
You add the muffler pressure to the ambient air pressure. So you would have between 1 to 4 psi extra pressure. 1 extra PSI is quite adequate for most purposes.

hsukaria

Thanks earlwb, putting actual numbers on the muffler pressure says it all.

Rudolph Hart

Many t5hanks for the replies but i can;t think of another word right now except bollocks.The exhaust/pressure vent orofice thingy is usually positioned near the end of the exhaust pipe and i'm half betting that 15psi just outside the end of the pipe will have more influence than a maybe feeble 1psi the engine may blow past that fuel pressure nipple while you engine is supposedly peaked to the max and revving it's nuts off,as it should do

I'm interested in trying to find out,all please comment with your thoughts funny ones included please

NM2K

My Feedback: (14)

Not related to the previous post.

I was 19 when I got my first Enya engine. It was a .29 R/C. I can't remember if it was ball bearing or not, but it came in a plastic box and included a high compression head. It cost a whopping $15 at the Base Hobby Shop, which good money back then (1966).

Anyway, coming from drag racing before the USAF, I immediately switched to the high compression head for more power. Not knowing that the high compression head was for no-nitro fuel, I immediately began flying the engine burning Fox Missile Mist. As you can imagine, those were some very rich, but HOT break-in flights. Somehow it survived and became one of the best engines that I've ever owned. Luck of the Irish, I suppose.


Ed, NM2K

earlwb

I think many people are confused here.
We have about 15 psi atmospheric pressure all around us, everywhere.
Inside the fuel tank with the engine not running, you still have that 15psi pressure too.
When the engine is running it adds more pressure on the tune of 1 to 5 PSi or more even to the fuel tank, So now you have 16psi to 19psi pressure inside the fuel tank, but outside of it all it is still the regular normal 15 psi air pressure. So if we drop the 15psi normal pressure and just let it be a zero reference, then when the engine is running we are pressuring the fuel tank with something like 1 to 4psi or more pressure.

You can also thiink of it as all around us is a zero pressure reference, and anything that causes more pressure would be pressurizing it more. For example, usually the air pressure gauges like tire gauges and such show zero with normal atmospheric pressure. But you add 30 psi to the tire's pressure. technically there is 45psi in the tires but the guage doesn't read the normal 15psi atmispheric pressure as that is a zero reference.

hsukaria

Not to confuse the issue more, but would the variability of the muffler pressure cause tuning difficulty since it is varying from 1 to 5 psi? Is that why fancy engines use pumps? I have seen somebody selling fuel pressure regulators also, but they seemed expensive and complicated. Maybe that is what O.F. was leading up to.

earlwb

No, we have numerous different kinds of mufflers. A low resistance muffler would have less pressure and a high resistance muffler more pressure. The average pressure increase is typically around 1 to 2 psi. But some mufflers have more and others might even have less. Some tuned mufflers or tuned pipes can even boost the pressure a lot more.
The low resistance muffler is more noisy and the higher resistance muffler more quiet. But we have a almost infinite variety of mufflers in any case,
Basically one muffler might have 1psi of back pressure and another might have 4 psi of back pressure. But yes as the engine speed increases or decreases there is some variability in pressure. But the fuel fitting tends to act as a resistor or slow bleed valve and smooth out the variabiiity. You can negate, to some extent, the bleed valve effect by using a larger fuel fitting on the muffler with a larger hole in it. This can be advantageous in some cases where the engine tends to load up at idle when you chop the throttle quickly and the fuel tank is still pressurized. This can also work on some engines through the mid range too, as you give it throttle, it has a delayed action effect on pressurizing the fuel tank, thus the engine runs a bit lean in the mid range momentarily.

Years ago, before mufflers, the engine's carbs had smaller intake venturi's on them. But as the engines evolved over time, the carbs became larger and the intake venturi became larger as well. That resulted in more poor fuel draw and now engines have to have muffler pressure to aid in drawing fuel.

charlie1960

14.7 psi
If i remember correctly.

hsukaria

The smaller carb venturis might provide better fuel draw, but also cause power loss since the lower intake pressure causes a lower cylinder pressure. That is also why at high altitudes, the atmospheric pressure is lower than 14.7 psi, thus causing a reduction in power.

Rudolph Hart

Thanks everyone for your comments it's given me something to figure on for a while.Given that i definitely want to run the shotgun pipe i might fit a perry vp20 pump or a cline regulater if i can get one over here.Just wondering if a fuel pressure nipple on a short shotgun pipe would be sufficient,i'm not looking forward to a lean run with the 220

clytle374

I've got a 100 with a pressure fitting on a rcspecialties 90 deg adapter with only the stock manifold screwed into it. The engine runs fine from spins to hovers. I can't be getting much pressure that way. BTW a cline regulator will only lower the pressure.

I am debating using a vp20 pump so I can move the tank closer to the cg.

Cory

earlwb

My poor old Enya R 1.20 engine uses a simple kludged straight pipe and at the end of the pipe there is some holes and it is pinched on the tip as well. That seems to provide adequate muffler pressure for my engine to run in the plane it is in. I did not make this kludged exhaust system, someone else did before I bought the engine and plane. I actually have a proper exhaust pipe and muffler to go on the engine, but since it does work and it isn't all that bad looking assembled and covered up on the bipe, I left it as is.

Clancy Arnold

I have a technical question on a Saito 150.
A friend bought a Saito 150 with a known bad cam.  He ordered the new cam and asked me to reassemble the engine.  It was disassembled when he bought it.  There are two steel spacers on the Cam Gear Shaft. One is thin and one is thick.  Which one goes in front and which goes to the rear?

Clancy

w8ye

My Feedback: (16)

It is not too important. Some have two thin ones and I've seen them with no shims and also two thick ones.

I was going to say for you to try to center the cam lobes underneath the cam followers? The crankshaft gear has enough space for the cam gear to be anywhere within reason as long as it is in the front of the box.

hsukaria

Today has been a good learning day by reading this forum. Super helpful, thanks guys.

Hobbsy

My Feedback: (102)

The Cline system will make a considerable amount of pressure even with a straight pipe as the check valve traps the "peaks" of the exhaust pressure waves. Even the little Saito .30 will put a bulge in one of those stiff walled Sullivan tanks.

Clancy Arnold

Thanks
Iput the thick one in back and the thin one in front to center the gear in the space in the casting.

How do you like my Avatar? I am 5 Ft 8 In, the Taube is 7 Ft 4 In WS, I fly it in Control Line Sport Scale.
Clancy

rcdude7

My Feedback: (264)

I just bought a new/old saito fa-150, it has the choke plate, old style NV assy w/blunt needle, fire nozzle type muffler and short crankshaft.

This engine has a "B" stamped on the engine mounting lug. Would this be the 14:1 compression ratio engine? How can I tell without tearing it down? Should I replace the NV assy with the new style assy?

I know a little about saitos, but I'm no expert.

w8ye

My Feedback: (16)

I would just use it.

The high compression ones do fine as long as you don't let the mixture get lean.

flyingagin

Clancy Arnold
How hard does that bird pull? About 20 years ago I built a os .61 rear exhaust piped cl. It about pulled my arm off when it came onto the pipe.
Like the avatar

Clancy Arnold

Flyingagin
It is easy to fly because it is equipped with one of my U/Tronics Control 4 channel systems. Channel 1 is Throttle, channel 2 is Wing Warp, channel 3 is Rudders and channel 4 is unused. The Elevator, Wing Warp and Rudder are all controlled by scale cables. Iadjust the Rudders for line tension and the Wing Warp to level the wings.There are92 Scale Cables on the model. At 1/3 Throttle is is doing 7.5 second laps on 65 Ft lines for 37 MPH. The Jeannin Stahltaube is powered by an OS FS91 and weighs 10 Lb 2 Oz.

I have attached the access to my build log. Reply # 25includes a 30 second clip of it's first flight. The Taube did not crash, the photographer fell down. LOL
Clancy


http://www.rcgroups.com/forums/showthread.php?t=1319950

flyingagin

Thanks Clancy Arnold
Since pull is a function of model wight, line length and the square of the speed, 37 mph is going to make an incredible difference. I don't remember my speed but I bet it was like 110 - 130 so it pulled real hard. I flew it two handed. No throttle.