Fuel tank question

[mark]

My fuel tank is just about finished but im unsure if it needs a vent in it? I thought i heard someone say before that if you didnt have a vent to let air in that the tank would be under a massive vacuum.

So my question is does it need a vent with a one way valve in it or will the filler cap (aero style fitted to body) do the same job?

Im running bike carbs with bike fuel pump if that makes any difference.

Any comments or advice greatly appreciated as usual

[Talonmotorsport]

Short answer yes you will need a vent otherwise your fuel pump will create a vacum, deform or collapse the tank then stall the engine through fuel starvation.

[les g]

Talon is spot on
also a good size vent is best as this will help when filling the tank
ie. fuel in air out
stops the pump clicking off all the time or a splash down your trousers or bodywork
cheers les g

[Talonmotorsport]

Some thing else to consider is the angle and the curve of the tube that joins the back of the aero filler cap. Lad I know made one from 2" stainless tube from 3 mitered bits of tubing, filled the car up at the pump and jammed the nozzle in the fuel cap. Que at tesco was not happy,the AA man found it funny, I gave up after an hour and a half and walked home!

[spud69]

The only thing to watch out for Mark is that the one way valves can stick, what i did for the SVA is the rubber tube that you use for the vent if you attach it to one of the uprights to the top of the chassis then do a u-turn and bring the tube down to a level a few inches below the tank, so that if the car does tip over fuel does not drain from the tank. My Mr SVA did look for this and was pleased with the way it was done.

Andy......Soon be on the road

[mark]

Thanks for the replies

What diameter vent do people recommend?

[les g]

most road car tanks seem to be approx 12/13 mm up to 19mm
but i suspect that there is no right or wrong here
the bigger the vent the less likely of blowback when filling
but you could get away with one the same size as the fuel outlet in reality
as air wont need to get in any faster than fuel gets out
its the filling side that can be a pain as the gun at the pump keeps clicking off with any backpressure
cheers les g

[flyerncle]

Most modern car's have the vent pipe routed from the tank to the filler neck and then to a sealed system incuding a carbon filter so as not to vent petrol fumes to the atmosphere, look down the fill neck and you will see a small pipe poking through the wall of the filler, another thing to consider is the venturi effect if the pipe is placed in the airflow to the rear as I learned the hard way after walking into a stream of Avgas leaking from a Cessna tank vent when the wind was blowing from behind the tail.

Spud's idea sounds excellent and if it keeps Mr IVA happy c'est la vie.

[chriseyj]

Sorry to jup on this post but is it a possibilty to use a polyethylene boat tank like those found on ebay, or would that be really naff. Production cars use plastic tanks. Have seen a few but dimension aren't appropriate on most of them.
Cheers Chris.

[flyerncle]

Good idea but I think you will have a problem with IVA you will not be able to fill it from outside without fitting some sort of filler pipe.

Modified beer keg !!!!