Changing the Subject - Sort of

Has anyone ever measured the suction of our engines at the needle at normal operating speed? I sure could use that value.

If not.... I'm thinking of doing the following:

1. Pull the glow plug.

2. Connect the crankshaft to a variable speed electric drill using small timing belt pulleys and timing belt.

3. Lubricate the engine with synthetic oil. (might dilute it with alcohol to improve pentration.

3. Connect the needle valve fuel nipple to an incline manometer.

4. Spin the engine at 10,000 rpm.

Or use the number if somebody has done it already!

There was a long thread on SSW a while back with a heading on airspeed in the venturi. MAN used to publish fuel suction figures which were how much you could lower the fuel tank and the engine would still suck fuel. Most were in the 9 to 12 inch range. I recall Frank Williams has done an experiment similar to what you propose and the results were quoted here.

>Has anyone ever measured the suction of our engines at the
>needle at normal operating speed? I sure could use that
>value.

I've seen it several times over the years. I don't recall where, or have the information at hand.

Of course, the pressure difference you measure is going to depend on how much you open the needle..

Wherefore, Dr. Taib?

You need to consider the impedance of the needle/spray bar seat. Fuel flowing though this (and the spraybar hole) will experience a pressure drop (just like electric current flowing through a resistor). If you lower the tank while the engine is running, the engine leans out because it takes energy to raise the fuel level up to the needle. However opening the needle (thereby lowering the flow imedance) will mitigate the effect. As a matter of fact one can keep lowering the tank and opening the needle until the engine can't be richened anymore. At this point the needle no longer is opposing the fuel flow, although the small diameter (and length) of the spraybar seat and the diameter of the spraybar hole still represent a potential pressure drop..

I suppose if I was an expert on fluids, I would try to figure out what diameter of the seat and hole I might choose, so that the effective pressure differential across them is insignificant. Then by seeing how low the tank can be moved, you are measuring the pressure within the venturi (at least at the place the spraybar hole is located.

LAST EDITED ON Dec-03-03 AT 02:09 PM (CST)
 
If you read my message #10, I guess it wasn't clear. It's a static measurement. I would measure pressure at the spraybar hole by hooking up a vacuum gauge to the spraybar instead of a fuel line. I'd supply fuel to the engine by another means for the test.

"Then by seeing how low the tank can be moved, you are measuring the pressure within the venturi (at least at the place the spraybar hole is located."

Well, you might come close, but that really wouldn't measure the venturi pressure. When fuel head = venturi pressure, there would be no flow. Again, you'd want a static measurement. But there's a way to do it: You could put fuel the engine by another means, described above, while your movable tank is hooked up to the spraybar and located way below the engine. You could even put water in the tank hooked up to the spraybar. Start the engine and slowly raise the tank hooked up to the spraybar until fluid starts to go into the venturi. Knowing the height difference between the movable tank and the spraybar and the fluid density, you can calculate the venturi pressure. I presume this is what you were telling me to do in your message. I have a vacuum gauge, so I'd use it instead.

I'm about to do that, but on a peculiar venturi. I'm trying to make a venturi that has the lowest possible pressure (highest vacuum) with reasonable pressure recovery. Suggestions are welcome.

My plan to test it with the engine running. I drill a little hole in the venturi and glue a piece of hypodermic needle tubing into it. This tubing works a treat for applications like this. It bends easily and you can solder a spraybar barb on the other end so fuel tubing will fit. I'll use one of these tubes as a spigot to put fuel into the venturi and another as a pressure measurement tap. To test a conventional venturi-spraybar setup, you could admit the fuel through such a spigot, using a bladder and remote needle valve. Then measure the pressure by hooking the gauge up to the normal spraybar.

The first round of pressure testing I did was to measure how well Tony Huber's regulator regulates (it regulates well) and to see how crankcase pressure varies with RPM. Curiously, crankcase pressure decreases with RPM on the engine in question.

Howard, you can use an engine with remote NVA (some Os Max LA for example), just disconnect tubing between NVA and spraybar and put a "T" where you can measue suction directly, without any modification on engine

I guess so, but you are taking the fun out of doing it the hard way.

I understand you. I would disappoint you even more if I tell you that you can just connect there a long transparent fuel tubing shaped to “U” filled by fuel and it will directly show you what is maximal distance for suction. So I will better not

You could also get an idea that you can measure difference between optimal mixture for 2 cycling and point where it goes 4 cycling, you will put one number here and you will kill our speaking about our “leaning” theories, and what we will do afterwards? So I never wrote this message OK?

>I understand you. I would disappoint you even more if I tell
>you that you can just connect there a long transparent fuel
>tubing shaped to “U” filled by fuel and it will directly
>show you what is maximal distance for suction. So I will
>better not
>
>You could also get an idea that you can measure difference
>between optimal mixture for 2 cycling and point where it
>goes 4 cycling, you will put one number here and you will
>kill our speaking about our “leaning” theories, and what we
>will do afterwards? So I never wrote this message OK?

Is it just me, or is this not one of the funniest postings we've had in weeks? Good job, I love it!

And Igor, helping Howard to do *anything* in the easiest way possible is an errand for fools.

LAST EDITED ON Dec-03-03 AT 07:30 AM (CST)
 
>I'm about to do that, but on a peculiar venturi. I'm trying
>to make a venturi that has the lowest possible pressure
>(highest vacuum) with reasonable pressure recovery.
>Suggestions are welcome.
>
>My plan to test it with the engine running. I drill a
>little hole in the venturi and glue a piece of hypodermic
>needle tubing into it. This tubing works a treat for
>applications like this. It bends easily and you can solder
>a spraybar barb on the other end so fuel tubing will fit.
>I'll use one of these tubes as a spigot to put fuel into the
>venturi and another as a pressure measurement tap. To test
>a conventional venturi-spraybar setup, you could admit the
>fuel through such a spigot, using a bladder and remote
>needle valve. Then measure the pressure by hooking the
>gauge up to the normal spraybar.

This type of set-up was my first thought. My only concern was that it changes the physical configuration of the venturi and might add enough restriction to skew the numbers. Would love to have your results.

>
>The first round of pressure testing I did was to measure how
>well Tony Huber's regulator regulates (it regulates well)
>and to see how crankcase pressure varies with RPM.
>Curiously, crankcase pressure decreases with RPM on the
>engine in question.

I'm not surprised about the decrease in crankcase pressure. The resistance of the venturi becomes more apparent as the volume of air flowing through the venturi increases. You are measuring between the "restriction" and the "pump" inlet. The flow rate is not linear with respect to the pressure differential because of that resistance.

And this was with a wide-open venturi, the fuel being pressurized by a bladder. I should have tried it with a restricted venturi, too.

>The first round of pressure testing I did was to measure how
>well Tony Huber's regulator regulates (it regulates well)
>and to see how crankcase pressure varies with RPM.

How do I get one?

Brad,

Did you get my emails? It seems to be wishy washy here lately.

>Brad,
>
>Did you get my emails? It seems to be wishy washy here
>lately.
>
>

Yes. I was on vacation yesterday.

No Worky!

You can always use the SSW private message system. It works real good.

>I'm about to do that, but on a peculiar venturi. I'm trying
>to make a venturi that has the lowest possible pressure
>(highest vacuum) with reasonable pressure recovery.
>Suggestions are welcome.
>
>My plan to test it with the engine running. I drill a
>little hole in the venturi and glue a piece of hypodermic
>needle tubing into it. This tubing works a treat for
>applications like this. It bends easily and you can solder
>a spraybar barb on the other end so fuel tubing will fit.
>I'll use one of these tubes as a spigot to put fuel into the
>venturi and another as a pressure measurement tap. To test
>a conventional venturi-spraybar setup, you could admit the
>fuel through such a spigot, using a bladder and remote
>needle valve. Then measure the pressure by hooking the
>gauge up to the normal spraybar.
>
>The first round of pressure testing I did was to measure how
>well Tony Huber's regulator regulates (it regulates well)
>and to see how crankcase pressure varies with RPM.
>Curiously, crankcase pressure decreases with RPM on the
>engine in question.

Ron B.
F4Fguy

Howard,glad to see someone actually testing instead of opining.

A couple of points:

(1)Re pressure tapping the venturi;be careful of the law of unintended results! As I'm sure you know ,a venturi is very shape sensitive,in all planes.Also ,the pressure change doesn't track the contour,but lags it to a degree determined by the amount of restriction and the shape.The point of lowest pressure is at the "vena contracta"(roughly translated narrowest flow)which will always occur some distance downstream of the narrowest point of the physical venturi,and as far as I know,can only be experimentally determined.You don't say what guage needle you plan to use,I'd recommend the smallest possible say #30(you don't need any flow,only pressure).

(2) CC pressure;If you track CCP with output,you'll find max pressure goes hand in hand with torque,as max pressure = max air delivery per cycle.Pressure will fall steadily until the HP peak,at which the pressure is no longer sufficient to overcome the flow losses,and delivery per unit time decreases.The foregoing is why too much crankcase packing is not a good idea.You can raise the top end slightly (sometimes)but,you'll restrict flow in the bottom part of the curve.There's a very good paper by Nomura(I think)at Yamaha on Delivery Ratio vs Output,in the SAE ca 1972/3.

Ron.B.

>The first round of pressure testing I did was to measure how
>well Tony Huber's regulator regulates (it regulates well)
>and to see how crankcase pressure varies with RPM.

How do I get one?

The City Smasher

Godzilla,

You've already got one. Or at least a copy of the very same unit Tony copied when making his regulators.

Also, unless I have missed something over the past couple years, Tony's would require modification, where the unit sent I sent to you has already been tweaked.

Does everybody now better understand how it takes Howard *years* to squeeze out each new Stunt plane? Good Lord...

Thank you. I'll look for tiny tubing. Should it be flush with the wall or poking out through the boundary layer (not that I'd know how thick the boundary layer is)?

What is the best shape for a venturi? I want both good pressure recovery and a heap of vacuum. It can be long, and there is no problem machining it.

>Thank you. I'll look for tiny tubing. Should it be flush
>with the wall or poking out through the boundary layer (not
>that I'd know how thick the boundary layer is)?
>
>What is the best shape for a venturi? I want both good
>pressure recovery and a heap of vacuum. It can be long, and
>there is no problem machining it.

Ron B.
F4Fguy

Howard,

I no longer have access to a good technical library,so most of this is from my increasingly C.R.S. memory,but here goes.

(1) flush or not?:You said it,at these sizes who knows.I seem to remember that 150FPS would give turbulent boundary flow.Below that the inference would be laminar,you may find that you need to "trip"the entry flow to get the best flow conditions.In any case I'd be careful of getting things too smooth.At these sizes,boundary can be a large portion of the cross section.
(2)The entry can have a significant effect on the total flow.it needs to be well rounded and regular.A parabola or ellipse appears to be the way to go.A fairly large entry "bell"accellerating the flow gradually is what we're after.
You can get very good recovery,even with smallish bores if the entry and exit are well configured(recovery coefficients of 0.83-0.87).
(3)Exit should be a smooth transition from the entry shape to an angle of 7 1/2deg or less(included).Obviously this is going to be a space consideration.Lowest pressure is going to be very close to the tangent of the exit and the entry curve,i.e.slightly below the narrowest point of the entry.
(4)Not mentioned yet,but our friend Joe Wagner notwithstanding,too long an intake tract is NOT good.Unless you want to get into another bag of worms and tune the intake,the shorter the better,in other words,raise the intake frequency to a point well above the operating range.
Another point;all the above is probably moot if you bung the NV through the venturi,as you then have a configuration almost impossible to predict,what with two wall penetrations,a round bar in the middle and a variable bleed in same.

Well,that's about all I can come up with at the moment.Hope it helps.

When Frank Williams was power plant editor in SN, about 1995, he did a good article on this.

If anyone has that article and could FAX or scan and e-mail to me, It would be appreciated.
Thanks

Not too many comments directed at the original post. Maybe additional info will help.

The reason for using the method that I proposed was to eliminate some variables.

The advantages as I see them are:

1. I could get readings directly in the spraybar.

2. I could experiment with various configurations and because of the simplicity of design should better be able to duplicate results. More accurately see cause and effect from changes.

3. Only outside variables as I see it are ambient air temp and barometric pressure. Record these and adjust to standard values.

4. Not limited to any one needle/venturi/spraybar configuration for testing.

5. Fuel characteristics aren't a factor.

My concerns are:

1. Is rotating the motor at speed without actually running it going to give me vastly different results when compared to normal operations?

2. Am I protecting the engine adequately to prevent damage?

3. Will Michigan win it's bowl game?

Given the reasoning behind the experiment... what do you think?