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Fuel Pressure and Volume

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Fuel Pressure and Volume

Postby Fatman » Sun Aug 10, 2008 6:44 pm

I've seen it come up a few times in the past few years where a racer says they have adequate pressure down the track and others say that they may have the pressure but not the volume. I don't get it.

If the fuel pressure measured at point A and there is no restriction between point A and the carb, then wouldn't the pressure and volume be adequate regardless of how you are getting the fuel from the tank to where it is measured?
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Postby Warpspeed » Sun Aug 10, 2008 6:58 pm

Yes, you are right, assuming the needle and seat also had sufficient flow capacity..
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fuel pressure

Postby bigjoe1 » Sun Aug 10, 2008 8:11 pm

Many years ago I was at Edelbrock. We used to go and test at the strip dirring the week when noone was there. Once I tested the affect of fuel pressure. After each run I reduced the fuel pressure by one pound. This was a SB Chevy with a two carb tunnelram setup.I started with 6 to 7 lbs, andd after 5 runs, the gauge read ZERO, but the car ran EXACTLY the same every run, five times in a roe. I can not explain it, but I never forgot it either. Fuel pressure is not very important. . On customers cars I have worked with, I tell them to set it at 6 to 7 lbs and forget it. I get phone calls where people go on and on about the problems they have with fuel pressure, I feel; soorry for them, they should be looking at something else.

JOE SHERMAN RACING ENGINES
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Postby Ron C. » Sun Aug 10, 2008 10:05 pm

Well I guess I'll come at it from a different perspective. If your fuel system is delivering enough volume to the engine to achieve the engine's power potential then it's meeting it's needs....bottom line.

But to really define the question about volume and pressure and how do they effect the end results let's take a closer look. If you have a 1/2" garden hose and you have 40 psi pressure and you time yourself filling a bucket. Then you hook up to a 2" fire hose and it also has 40 psi to fill the bucket which one will deliver more volume or fill the bucket first? There both 40 psi right. But the fire hose will deliver tons more water.

You run into this volume and pressure deal when you start running supercharged engines..especially a blow through carb application. You'll run into the need to not only have enough fuel pressure but it has to flow (volume) enough fuel at that pressure. Most electric fuel pumps as the pressure need goes up, the volume delivered goes down. This is what gets you into trouble. Yes the electric pump will pump up to 38 psi, but the volume is drastically reduced. So most go to a belt driven or cam driven fuel pump to get the volume back up to match the pressure....if that's what the application needs.

In carburated naturally aspirated you can get anyway with more in this area, a blown engine you'll start spitting pieces out the exhaust pipe if the volume does not match the pressure needs.

Hope this makes since to someone that's interested.....??

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Postby RayJE Carburetors » Sun Aug 10, 2008 11:22 pm

the most common problem guys have is not haveing enough float drop........ casued buy either incorrect fuel pressure or float level too low.............. a needle and seat is just like a valve and a seat in a head ... the further off the seat the larger the curtain area.. the greater the flow
eg a .110 thou needle and seat will flow 175pounds of fuel per hour at 7 psi and .250 thou float drop....

most of the time i see this with guys runing electric pumps on alcohol..........they try to hold 8psi on a .150 thou needle and seat and they have the flaot screwed to the floor to get get the needle and seat to seal then at WOT they have no float drop and dont have sufficient volume .....

Joe i disagree with you about fuel pressure not being important........ fuel level in the bowl is effected buy fuel pressure ......each time you raise or lower the fuel pressure the fuel level will raise and lower as well all things being equal......this has a major efffect on fuel curve....the higher the fuel is up the well the easier it is for the booster to pull
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Re: fuel pressure

Postby paul 308 » Mon Aug 11, 2008 8:51 am

bigjoe1 wrote:Many years ago I was at Edelbrock. We used to go and test at the strip dirring the week when noone was there. Once I tested the affect of fuel pressure. After each run I reduced the fuel pressure by one pound. This was a SB Chevy with a two carb tunnelram setup.I started with 6 to 7 lbs, andd after 5 runs, the gauge read ZERO, but the car ran EXACTLY the same every run, five times in a roe. I can not explain it, but I never forgot it either. Fuel pressure is not very important. . On customers cars I have worked with, I tell them to set it at 6 to 7 lbs and forget it. I get phone calls where people go on and on about the problems they have with fuel pressure, I feel; soorry for them, they should be looking at something else.

JOE SHERMAN RACING ENGINES

my guess would be the tunnel ram system used has two carbs, which means four fuel bowls, so there is a much greater fuel reserve which puts less demand on the fuel system over a short distance. i would say you would get a much different result with a single carb.
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Postby cfm » Mon Aug 11, 2008 9:18 am

I've seen a few different scenarios on good psi but not enough volume.

1) Using a dyno with 1 fuel flow meter. Motor would not rpm and ran like poop over certain rpm. Fuel psi gauge on dyno console and fuel psi gauge on fuel line just before carb fittings both showed 7psi.
After much investigation, phone calls, different dyno fuel system parts we made full dyno runs while I stood next to the motor with an eye right in frt of the fuel bowl glass window - thanks BG for clear sight glasses ! The fuel level would drop, come back up, and then turn into a frothy mess !

Removing one part at a time on the dyno fuel system, when we bypassed the fuel flow meter the frothy mess disappeared and the engine came right back to life without missing a beat.

So, the fuel psi gauges where showing correct psi, unfortunately air psi was part of this.

2) Cracks or poor sealing with fuel tank pick up tube and/or pump inlet line. PSI shows normal, but again there is air in the system.

3) Needle and seats not flowing enough fuel. PSI shows normal, but float bowls are emptying.

It is my belief if there is a restriction or issue before the pump, the fuel psi gauge may not be influenced. However, if there is a restriction after the pump, then the gauge typically will be influenced. Although, that dyno experience had the fuel flow meter after the pump so I still won't 100% believe if a fuel psi gauge shows correct psi that it is actually all fuel.

Got to love those large and clear fuel bowl sight glasses ! Really saved our azz being able to see what's happening in real time in the carb - where it counts.
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Postby cfm » Mon Aug 11, 2008 9:33 am

Kind of on topic and in case some don't know - I know many do - those liquid filled fuel psi gauges can play with you. If they get hot enough the gauge will start showing less psi even though your actual fuel psi hasn't changed.

This of course can result in a bad catch 22 to the unspespecting. They adjust their regulator so that the gauge shows the correct fuel psi. Problem now of course is that you are really running too much psi even though the gauge now shows proper psi.

That's why these gauges have some sort of bleed valve on the outside of the body, many times under a rubber cap of sorts.
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fuel pressure

Postby bigjoe1 » Mon Aug 11, 2008 9:58 am

I will say something about my example. I had a small fuel cell in front of the engine, so it probably had some gravity feed affect in my tunnelram test. I doo understand that single four barrels are much more critical about this.

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Postby stealth » Tue Aug 12, 2008 1:48 pm

Just to answer your question on pressure vs. volume. Consider the weight of the fuel in that AN10 or 8 line running all the way to the front of the car, now consider the considerable G-forces pushing that weight back at the pump while accelerating. You might have 12lbs of pressure at the pump and have the fuel standing still in the line. NO Volume with pressure at the pump right.

Joes front mouted tank would not have this problem.

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