Fuel pressure testing question. (warning, long)

[JustJason]

Not sure if this needs to be in nonboat tech section but since the original post was in here i'll post this in here.
2 weeks ago I edited Don's "how to test your boats fuel system" and added some pictures of my own. (it's in the FAQ section here if you need to reference it. http://forums.iboats.com/showthread.php?t=283363 )

A fellow Iboater, who i've chosen to allow to remain anonymous unless he chooses to jump on this thread, advised me through PM that I need to make some changes to the way I do things. I do not agree, which is why i'm posting here. I'd like to hear what you all have to say, and by all means if i'm wrong i'd like to edit the fuel system thread so everybody, including myself, has the most accurate, most up to date information.

1. He stated

better change the gauge line to the same size of fuel line or ur readings are going to be off!!

I replied with

Huh? Are you talking about the post I wrote on boat fuel system checking in the FAQ section?
Honestly I've never heard what you just said before. My low pressure gauges, as seen in that post, uses something like 3/16-1/4 inch line. As well as my higher pressure fuel injection gauge, my oil pressure gauge that goes to 100psi, and my transmission pressure gauge, that goes to 400psi, all use realitively small line. Are you saying that all my gauges are incorrect? They are all brand name tools. A mix of Matco and Snap-on. Why do they come plumbed that way? There is no way to change the line size to the gauge.
Some I'm curious as to why you said that. A fuel pressure system is nothing more than a simple hydraulic system, in which the pressure is equal in all the lines regardless of size.

He follows up with

well the clear line needs just go to the gauge not the boat lines

anytime you reduce size in a fitting you will get a bad reading

i just replaced the nipple in my gauge

in all our factory traing this year, they pushed it hard, cuz guys were scewing up

im just letting you know the new standard now

and you must run the motor for at least 10 mins wot to check things

And I reply

I'm not looking to argue or anything, just learn, because I never heard what you are saying before.

Why would I need clear line to the gauge? The only reason I use clear line at all is to look for air bubbles and possisbly crap coming up from the tank. If I put clear line in between the gauge and the T-block I wouldn't see fuel flow there, and I wouldn't see bubbles.

On to the change in fitting size. Let me ask you a question, wether it's high pressure fuel injection, or a low pressure mechanical pump. Isn't it all just a basic hydraulic system?
Lets say for sake of arguement, that I had a mechanical fuel pump and a carb. Lets say I'm using 3/8ths line between the carb and the pump. With the seat in the carb fully closed, lets say the mechanical fuel pump is capable of 7psi. If i splice in a tblock and line that is 3/8th, but the line that goes to the gauge is 1/4 inch, my gauge is still going to read 7psi. Why are you saying it wouldn't be? I've never heard it before. In a hydraulic system isn't psi equal in all lines?

As far as running the motor for 10 minutes at WOT. I do agree that to fully accurate the test needs to be done in the water under a load. But In the post I said it can be tried on the muffs because alot of times things will show up. so no questions there.

On all my gauges, especially my high pressure gauges which use quick connects, the inlet to the gauge cannot be changed as they are built into the gauge housing itself. Even my snap on gauges are that way.

Are you doing your factory training in house or are they shipping you off someplace like MMI?
Just curious.
Jason

He replies

Why would I need clear line to the gauge? The only reason I use clear line at all is to look for air bubbles and possisbly crap coming up from the tank.

that is right, clear for air bubbles and crap, u will see it in any clear line. but you only need it at the gauge line, everything will flow that way if you keep the gauge lower than the place you are testing. u could use all clear, but the problem with clear is that it goes bad, so replacing all the clear is OK, I'm just saving cost shop wise

If I put clear line in between the gauge and the T-block I wouldn't see fuel flow there, and I wouldn't see bubbles.

u r wrong on that part, air rises, liquid sinks

On to the change in fitting size. Let me ask you a question, wether it's high pressure fuel injection, or a low pressure mechanical pump. Isn't it all just a basic hydraulic system?

hydraulic? i guess u can call it that way. so your theory is = 1" pipe at 100psi, if that went down to 1/2" pipe it would be 100 psi, wrong. just look at the trim and tilt system

As far as running the motor for 10 minutes at WOT. I do agree that to be fully accurate the test needs to be done in the water under a load. But In the post I said it can be tried on the muffs because a lot of times things will show up. so no questions there.

only air bubbles, cuz at wot u might not get over 3 psi at pump, but at idle you will

On all my gauges, especially my high pressure gauges which use quick connects, the inlet to the gauge cannot be changed as they are built into the gauge housing itself. Even my snap on gauges are that way.

Are you doing your factory training in house or are they shipping you off someplace like MMI?

MMI was training, did that learn a few things, the factory stuff is taught by factory guru's who see it everyday. That is what i take 4 times a year.I make the shop pay for it

my reply to that was basically I don't agree, and i'll take it to the boards. What do you guys think? Am I wrong? Are all my gauges plumbed incorrectly?
If I am I humbly bow, but i've been doing my tests the same way for years, and I haven't had a problem. In a nutshell... is pressure equal in all of the lines, regardless of size? I believe it is.
Either way my post in the FAQ section was about restriction testing, not pump output, so I don't know how the user got started on pump output. But if line size to the gauge also affects restriction testing (and again, i don't believe it does) Chime on in and say so!!
Jason

 

[Don S]

Re: Fuel pressure testing question. (warning, long)

In any type of hydraulic system, (including vacuum) the pressure (or vacuum) is equal througout the system. A 1" ID line will have the same P or V as a 1/16" line.

 

[JustJason]

Re: Fuel pressure testing question. (warning, long)

That's what I thought. Thanks Don!

 

[chiefalen]

Re: Fuel pressure testing question. (warning, long)

I think he was saying that a garden water hose say with 50 psi, with a twist nozzle on the end, with the nozzle half closed it squirts the water into a smaller diameter.

Thus increasing the water pressure, say to 60-70 psi.

Am i right is that what he is saying?

 

[tboss]

Re: Fuel pressure testing question. (warning, long)

Smaller lines or larger it doesent matter when it comes to pressure but it has an effect on volume or the flow at the head or the tool you are using. This is best explained to a novice like me with an air tool such as a grinder or drill motor. The spects on an air drill are 90psi with 40 to 50 cfm. You can go from one size line 3/8 to 1/4 inch and the drill will still have 90psi at the head but the voulme will decrease to the extent to where it will not have any power under a load. This also relates to hyd. pressure the size of the line doesnt matter if the smaller line can withstand 2300lbs thats what it will have same as the 1 1/2 main line from the pump. The only way to reduce the hyd. pressure is ot install a hyd. reg between the 1 1/2 inch line and the smaller line , you cant reduce ultimate pressure by installing a smaller line . This is what I done for a living for 15 years before I retired. Now if Im wrong there are a hell of lot of dump trucks and garbage trucks out there running around with a hyd. system that is installed wrong. It all boils down to the old bottle neck theory. The only way to chang pressure whether it be air , hyd. or water is with a regulator and to decrease volume or flow is a smaller hose. Chief if what you said is wht he is saying then he is wrong, only thing that will increase the pressure after the regulator is another driving force such as heat from the sun or another source of pressure such as another pump but with another pump then you have to consider the volume from the other regulator will it supply the volume to the second pump to keep it from calvating and a calvating pump is a dieing pump. This same thoery pretains to your fuel system IMHO Tboss

 

[chiefalen]

Re: Fuel pressure testing question. (warning, long)

I post the analogy, not that i agreed with him.

The pressure remains constant say at 50 psi.

Doesn't matter the size of the pipe, volume is alother sotry.

Airplanes have hyd. also. 3000 psi.

That 3000 psi would be there is the pipe was 1/4 inch or 2 inch's.

If you have a holding tank big enough that is.

Same as the town water supply it's constant in your pipe at home maybe reduced but constant.

You open faucet and the pressure is there. Reduced volume from the 2 inch pipe coming in your house main to a 1/2 or 3/4 pipe to your faucet.

Thus we all agree Jason is right as always.

 

[JustJason]

Re: Fuel pressure testing question. (warning, long)

I think he was saying that a garden water hose say with 50 psi, with a twist nozzle on the end, with the nozzle half closed it squirts the water into a smaller diameter.

Thus increasing the water pressure, say to 60-70 psi.

Am i right is that what he is saying?

That's not a correct way of thinking about it, and I can explain why.

Lets say your town water supply is 50psi.

First you have to define pressure. Pressure is nothing more than a pumps ability to push against a closed ended pipe, or a pumps ability to push through a restriction in the pipe. (pressure) and a pumps ability to maintain flow (typically in gallons per hour, or GPH). When you open up that closed ended pipe, or turn on that water valve, you release the pressure in the pipe. If the pump has enough GPH, the pump will maintain pressure. If the pump does not, flow will decrease and pressure will drop to zero. Just like volts and amps, pressure and flow (gph) go hand and hand.

Now you can't compress liquids, but you can pressurize liquids and you can accelerate liquids.

Now that we know what pressure is, lets look at the garden hose again. When the spigot is off there is no pressure in the garden hose, because there is no water in the hose. Lets say again the town water supply is 50psi. (and lets assume, to keep things simple, that the town's pumps can maintain enough flow to maintain the pressure)
When you turn on the hose, and water comes out the end, it does not come out at 50psi. It comes out at close to 0psi. We already said that pressure is nothing more than a pumps ability to push against a restriction. If there is no restriction in the hose, then there is no pressure.
Now if you had the strength to hold your hand over the end of the hose, so that no water comes out, then the pressure inside of the hose would equal the pressure inside of the pipe on the other side of the spigot, which equals the towns water supply pressure. Your hand makes the restriction necessary in the definition of pressure.
Now if you have a spray nozzle on the end of the hose, and you turn it on, what is the water pressure coming out of the nozzle?
It's always always always going to be less than the supply psi, it has to be. When the nozzle is wide open, a lot of flow comes out, but at very little pressure. When you tighten the nozzle up, flow decreases, but the pressure of that flow goes up. It will never, an can never, be higher than what you start with.
A lot of people get confused because the water appears to shoot out of the nozzle at high pressure. Really its just shooting out at a pressure close to, but not higher than, the supply pressure. All the water is really doing is being accelerated by the restriction in the nozzle. Your increasing the speed of which it exits the hose, but your not increasing it's pressure, you can't.

It definitely gets more complicated than this, especially with series and paralell hydraulic circiuts, because just like voltage, water pressure in a working circuit is subject to drops. But in a nutshell, that's the basic explanation of it all.

 

[Robj]

Re: Fuel pressure testing question. (warning, long)

Capt you are right. You must have spend alot of time thinking about this to make such a long post. For practical purposes, whether you use a 1/8 or 1/4 inch line, really makes no difference. Hopefully this person will chime in as well.

Have a great day,

Rob.

 

[Robj]

Re: Fuel pressure testing question. (warning, long)

Pressure is the resistance to flow.

With the nozzle, you maintain a higher pressure in the hose, because there is some resistance to flow. With no nozzle and the hose turned on, there is very little resistance to flow therefore there is low pressure in the hose. At no time can the hose pressure exceed the supply pressure.

Have a great day,

Rob.

 

[JustJason]

Re: Fuel pressure testing question. (warning, long)

--------------------------------------------------------------------------------

Pressure is the resistance to flow.

With the nozzle, you maintain a higher pressure in the hose, because there is some resistance to flow. With no nozzle and the hose turned on, there is very little resistance to flow therefore there is low pressure in the hose. At no time can the hose pressure exceed the supply pressure.

Have a great day,

Rob.

Uhmmm yeah rob.... that's what i was trying to say hehe