VCCA Home Forums GENERAL DISCUSSION General Discussions gas caps/ pressurized or not ?

Hi All, what is the difference & when do you need one? Thanks

The only time you need a pressurized cap that I know of would be with FI. These old carbureted cars need a vented tank.

I know for sure that during the 1970's vented gas caps were NOT used at the same time that carburetors WERE in fact still used.

The tank was vented through a canister under the hood near the engine where the gasoline vapor was supposed to be captured and held until it was drawn into the engine and burned.

I don't know if, or how well the system worked, but it was there.

Even though I don't think of cars from the 70's as being that old or antiques, I guess they are 50 some years old. 🙂

I stand corrected. Not having dealt with the mechanics Ole mentions I learned something today. At least I was half right.

MacBubba asked what's the difference between a pressure and unpressured cap. Simply put, the pressure cap has a spring-loaded plate on the underside that will rise up into the cap when pressure in the cooling system rises enough to force the plate up. To keep the system from exploding the pressure is vented into the overflow tube. I believe they came along when highway traffic speeds were increasing to the point that rads of the day couldn't cool the engine properly without boiling over. By pressurizing the cooling system the boiling point of the coolant was raised sufficiently that this problem was overcome.

Now I’m confused. Are we talking about cooling systems or fuel systems?

Shoot Rusty, I'm confused every day...

Looks like you and Peggy had a great week on tour. I can only imagine how that much fun can be disorienting!

NO gas caps were pressurized. Most were vented up into the '60's and the cars that had a non-vented gas caps had a different sorce of venting the tank
In either case the vent allowed air to come into the tank to take place of the gas (lIquid) leaing tthe tank.
If no air was allowed to enter a vacuum would build up and suck the sidess of the tank togther or if you were luckey the engine would stop runnig before that occured.

The gas tank on the early baby grand 1915 used a pressure fuel system, there was a pump on the dash for getting the pressure built up for starting and a pump that ran off a lifter for after it started. We were on a 4 cylinder tour with one and they had to use the dash pump some times for the big hills.

Sorry, Rusty for causing some confusion - Guess I forgot to read the title of the thread>

Thread is about fuel tank caps, pressurized or not to pressurize.