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Power Surge 06-24-2008 09:52 AM
Quote:
Originally Posted by MrMagoo (Post 854589)
The float bowl pressure should be equalized, that's what those two pipes sticking up out of the air horn are for, they vent to the inside of the air filter, thus keeping the pressure inside the air filter and inside the float bowls equal.
Really? How's that work? :p

MrMagoo 06-24-2008 10:02 AM
Sorry Sal, I may have stepped beyond my pay grade on this one. I read it in a Holley tuning book, and I will get the quote directly for you and maybe you can straighten me out on this. I try not to play pretend expert but sometimes the desire to be helpful gets you into dangerous territory :)

Power Surge 06-24-2008 10:08 AM
Quote:
Originally Posted by MrMagoo (Post 854892)
Sorry Sal, I may have stepped beyond my pay grade on this one. I read it in a Holley tuning book, and I will get the quote directly for you and maybe you can straighten me out on this. I try not to play pretend expert but sometimes the desire to be helpful gets you into dangerous territory :)
I thought maybe you had a brain fart :D.

The tubes are strictly vents. You can't have pressure equalization between them because the air cleaner is obviously open to atmosphere (otherwise the car wouldn't run). You'd need some kind of sealed enviornment for pressure equalization to occur.

Also, the higher end Holley based carbs with the upgraded needle/seats can handle 8-9 psi.

MrMagoo 06-24-2008 11:41 AM
Thanks Sal. I didn't know about the new needles and seats. Is there an optimal pressure? is 8 better than 6.5 on a carb that can handle it?

I'm going to dig out that book, I don't want to misquote it, but I believe the author was stating that there is positive atmospheric pressure in the world inside the air filter created by the air rushing down into the manifold. Something about the Venturi Effect.

Power Surge 06-24-2008 12:02 PM
Quote:
Originally Posted by MrMagoo (Post 854919)
Thanks Sal. I didn't know about the new needles and seats. Is there an optimal pressure? is 8 better than 6.5 on a carb that can handle it?

I'm going to dig out that book, I don't want to misquote it, but I believe the author was stating that there is positive atmospheric pressure in the world inside the air filter created by the air rushing down into the manifold. Something about the Venturi Effect.
It's dependant on your needs. Most street driven carbed cars don't need 8 psi, but it's also not going to hurt anything as long as it doesn't blow the floats open. I run 4-5 psi on my car, but I'm also not making 600hp either.

One thing to keep in mind, is that bowl level height and fuel pressure changes will affect your tuning.

As for the pressure thing, positive pressure does occur, but that's down in the venturi by the boosters, and doesn't have anything to do with the vent tubes up top.

undy 06-24-2008 12:06 PM
Quote:
Originally Posted by Power Surge (Post 854894)
I thought maybe you had a brain fart :D.

The tubes are strictly vents. You can't have pressure equalization between them because the air cleaner is obviously open to atmosphere (otherwise the car wouldn't run). You'd need some kind of sealed enviornment for pressure equalization to occur.

Also, the higher end Holley based carbs with the upgraded needle/seats can handle 8-9 psi.
Not true..

Actually you do have a pressure equalization between the two. The vents are common to the inside of the air cleaner which is under atmospheric pressure or a little less depending on rpm, throttle position and the negative pressure within the air cleaner. That all being said, they will equallize due to both bowls venting to a common area. If there wasn't a vent then under the right conditions the bowls wouldn't fill due to being "air locked".

Remember... Atmosphere (@ sea level) is around 14 PSIA, which is still a pressure

Dave

Power Surge 06-24-2008 12:19 PM
Quote:
Originally Posted by undy (Post 854925)
Not true..

Actually you do have a pressure equalization between the two. The vents are common to the inside of the air cleaner which is under atmospheric pressure or a little less depending on rpm, throttle position and the negative pressure within the air cleaner. That all being said, they will equallize due to both bowls venting to a common area. If there wasn't a vent then under the right conditions the bowls wouldn't fill due to being "air locked".

Remember... Atmosphere (@ sea level) is around 14 PSIA, which is still a pressure

Dave
Well of course you have atmospheric pressure, but that's not going to keep the bowls equalized in the case of different FUEL pressures between the bowls which is where I thought the conversation was stemming from. If you've got 8 psi in one bowl and 5 in the other from some odd fuel log, flow path, or heat related issue, atmospheric pressure is not going to "equalize" the fuel pressure in the bowls.

wtm442 06-24-2008 02:42 PM
Quote:
Originally Posted by Power Surge (Post 854931)
If you've got 8 psi in one bowl and 5 in the other from some odd fuel log, flow path, or heat related issue, atmospheric pressure is not going to "equalize" the fuel pressure in the bowls.
The pressure inside the primary and secondary bowls of a Holley carb are always at the same pressure .... atmospheric pressure (the pressure inside the air cleaner) ... which is approx 14.696 PSIA at sea level or Zero PSIG

Of course, the fuel pressure inside the fuel line just before the needle and seat of the two bowls "could" be different only if there was a tremendous restriction or blockage inside the fuel line(s) leading to the needle & seats.

Quote:
Originally Posted by Mr Magoo
I wrapped my line from the pump to the carb, added a Holley heat shield with spacer, laid a piece of reflective insulating pad underneath the fuel log and just generally tried to eliminate obvious points of heat transfer. I like the results. The car doesn't get doggy when it's hot like it used to.

I'm seriously considering getting rid of the braided steel lines for fuel entirely and going with rubber and insulation, but no urgency at this point.
No need to get rid of the braided stainless. With the heat insulation, the small amount of heat transfer that would be eliminated would be minimal. If you need further temperature reduction to avoid percolation you need to increase the amount of outside air that flows thru the engine compartment. But that is very difficult to do when the car is stuck in stop and go traffic. Therefore, the next step, if needed would be to add a return line to the gas tank.

MrMagoo 06-25-2008 07:22 AM
Thanks WTM, that saves me a lot of work, I was just thinking how little I feel like changing all of the hoses.

Sal, others who are interested:

Here were my sources for fuel bowl venting: "Holley Carburetor Handbook" by Mike Urich, page 7:

All bowls are vented to the carburetor air horn to maintain a constant "reference" pressure. This keeps airflow variations - like a dirty air filter - from upsetting the mixture.

Also this, from "Holley Carburetors, Manifolds and Fuel Injection" by Mike Urich and Bill Fisher, page 25:

Because the fuel bowl is maintained at near atmospheric pressure by the vent system, fuel flows through the main jet and into the low-pressure or vacuum area in the venturi.

Power Surge 06-25-2008 08:08 AM
Quote:
Originally Posted by MrMagoo (Post 855160)

Also this, from "Holley Carburetors, Manifolds and Fuel Injection" by Mike Urich and Bill Fisher, page 25:

Because the fuel bowl is maintained at near atmospheric pressure by the vent system, fuel flows through the main jet and into the low-pressure or vacuum area in the venturi.
Yeah, that's the main principal behind a venturi style carb. The venturi creates a low pressure point in the bore, and the booster accellerates the signal. Fuel is drawn into the bore from vacuum on one side, and "pushed" from the other side by atmospheric pressure.

I guess I misunderstood what you were responding to, I thought you meant that the vents could compensate for a mismatch in fuel pressure between the bowls.

danc30 08-07-2008 01:47 PM
Quote:
Originally Posted by decooney (Post 854879)
After reading through this thread, and great replies, I'm thinking about trying to cover and wrap my braided line setup from my fuel pump to my carb before scrapping it. Or, maybe just reroute the lines all together from the rear instead, which also seems like an excellent alternative.

A friend has the same setup that I have in his 390GT Mustang and was experiencing horrible fuel boiling. So, he actually took this stuff below and covered his lines with it. I've used it for my clutch hydraulic clutch lines too. Since it looked kind of ugly, he then wrapped it with that special black felt type electrical tape to make it look less noticeable. He says he does not have the boiling fuel problems any more. His engine bay gets as hot or worse than our Cobras and experiences the same hot start issues when it's just sitting in the sun on black pavement.

http://www.jegs.com/images/photos/800/893/893-18000.jpg
What is that insulation wrap and where did you get it?

decooney 08-07-2008 01:52 PM
Jegs or Summit
 
The photo is from Jegs.com

danc30 08-07-2008 02:29 PM
That was quick. Thanks Duane.

Michael C Henry 08-07-2008 06:36 PM
I bought a heatsheild sleaveing product made by DEI from Summit. I covered my BG fuel log and tubes to carbs with it.

Naumoff 08-08-2008 04:25 AM
Decooney,

I didn't go back and reread the whole thread but did you check your timing.
I remember having this issue years ago and it was that the timing to far advanced.


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