Cascade Crew

[Kage87Z]

OK... a series of 83 mph runs appears to be the result of...

... the gas cap.

Non-vented, replaced with the right cap, even my idle went up 150 rpm.

Back out on the 19th. Then we'll see.

[blue89]

Serious? Why would it do that?

[Twilightoptics]

[Al Hasse]

[91RSVert]

Hum, my cap makes a suction sound when I take it off. Maybe I need to look into this myself.


Wait, isnt this what the charcole(sp?) canister is for. Or is your car a carb?

[aaron_sK]

[Twilightoptics]

I think we're talking about his 1st gen aren't we? If so, no canister.

[blue89]

I fail to see how this would effect anything on a fuel system with a return. Returnless carburetor systems I guess.

[Twilightoptics]

[blue89]

First off, how much pressure are we talking? And low or high pressure lines? 1psi on a mechanical pump and carb would make a huge difference. But 1psi on an electric pump at 50psi, just don't see it making a difference. Only thing it would do is pressurize the tank.

[Twilightoptics]

And there is a point when the pressure in the tank exceeds the pressure the pump can make.

Kind of like how you need a boost referenced fuel pressure regulator when you run boost on a carb.

Under your pretence, the tank gets pressureized, and as the gas continues to leave the tank gets even more pressurized. It' would be a vicious circle until something blew.

First gen carbed car, mechanical pump. If you figure the tank gets "pressureized", it would start forcing fuel to the mechanical pump potentially exceeding the pressure maximum at the needle/seat.

However, with an electric pump, the tank would then become under vacuum. If the tank has a steadily increasing vacuum and reaches say 50psi vacuum... how's the pump going to pump 50psi postive pressure the other way? You've then got -----> <------

[chevymad]

A pump, any pump, works alot better pushing fluid then pulling it. That is one of the main reasons they're actually in the tank. Also fluid has a certain pressure that will cause it to boil. Like filling a syringe with water and pulling hte plunger back til the water boils. The water then turns to vapor at room temp in the syringe. Gasoline does this at an even lower pressure. On a mechanical pump system, the pump is at the motor pulling the fuel from the tank, then pushing it up to the carb. The fuel line has a certain resistance to fluid going through it. You have any height difference between the tank and pump, which may work for or against you. On take off you also have the mass of the fuel trying to stay in place as the car moves forward. All of this makes the fuel pump work harder and drops the pressure in the suction line. Now if you don't vent the tank and you suck a vacuum inside it, at some point the pump will either not be able to pull the fuel to it. Or the fuel will boil in the suction line and cause a vapor lock.

Now an EFI setup with intank pump avoids most of those pitfalls. However you still need to have some kind of vent, or you'll just suck the tank flat like a pop can. Any fuel volume used up by the motor needs to be replaced by something in the tank. Modern setups use a carbon canister. They're also set to hold something like 3psi of vapor pressure, then release into the canister. Once the motor is running vacuum is applied to the canister to purge the carbon of fuel vapors. If the tank has less then a certain pressure in it, then air is let into the tank, but vapor is never let out except to the canister.

The 3psi of positive pressure is why we hear the hiss when we open our 3d gen tanks. A first gen that isnt from california will be vented and that wont happen.

Not sure how clear that is. but a few thoughts.

[Kage87Z]

It's a carter pump located just outside the tank, running at about 5.5 pounds at idle with an add on regulator.

Dual 4 barrel, edelbrocks.

No return lines or charcoal canister.