Driving at altitude

Hi Mike, 5,500 feet is not very high. Most of the continental USA is about that altitude and you can drive to around 12,000 feet near Denver in the family car with no special arangements or precautions. As you say you do lose some power but these days most vehicles have heaps of hp and unless you are towing you don't really notice it. Full throttle altitude is about 7,000 feet so after that it would be nice if you could get more oxygen or alter the mixture & timing on a petrol engine to maintain power. I don't know what was happening when you opened your tank filler cap, did it suck in air? That is what I would expect it to do and it may have sounded like air was escapeing when it was actually being sucked in.

obviously the tanks is sealed, and you would have done this at or near sea level, therefore you had sea level pressure in your tank, and as you drove up the air pressure is reduced, therefore there is more pressure in you tank than outside, pressure flows from high to low therefore out of your tank. its like taking a basketball on a plane. i guess the only way to prevent this would be every thousand feet say, get out and release the pressure, the only consideration is that if the pressure was to get too great it could crack a weld, but i doubt it at that level.
Also your engine power will drop, as the air loses density, your manifold pressure will fall, and without a turbo there is no way to retain rated power. As for full throttle height, i remeber from my flying days, full trottle height on an N/A plane was around 5000' but this is where you want the beuty of being able to close the waste gate and bump the power back up......hope this helps
Wayne-o

This can be correct Mike.

Petrol tank is meant to run always at a slightly high pressure , and equalize through charcoal cannister.

When engine turned off most tank/pump systems now have an outlet valve which shuts off maintaining this higher pressure - this makes starting easier next time
as don't have to re-pressuize system.

When going to higher alltitude a bigger pressure differential occurs.
So more whoosh.

I don't know your car type , so this is generic , but as also for power
engine management systems compenstate for lower air pressure and you
should notice the small power drop unless foot flat to floor. You
may notice foot in slightly different place on accelerator, this is the giveaway.

It can take a few minutes to compenstate though.

In early more primitve management systems, I've had the case of hard
and not starting unless air filter removed to give more air, until compensation.

Worth noting.

Robin Miller

Nothing much that you can do about it !

The fuel caps are made so when the tank contracts at night or in cold weather it can suck air in, but it CANNOT release air this is purposly designed like this.

The reason is so you dont have fuel vapors being released fire hazards

Can't realy over pressurise the tank they are reinforced with Baffels inside, swelll pot, ect Part of the construction they are quite strong

You would blow a breather hose or an o-ring on the tank if the pressure was too high anyway

Mike

Went up over Hotham Heights from Bairnsdale to Bright in 1969 151 miles ,done in 3 hours in my 1969 GT Cortina at 6030 ft it was gutless , had no life , I do believe that when the 1970 London-Mexico cars were in the Andes they made adjustments to the Carby's to compensate for the thinner atmosphere .

1 Carburettors Altering the mixture is a problem which may require consideration if long sections are run at high altitude or if numerous high passes are included in the route. Approximately 20% power loss will occur at 6,000 ft, with considerable richening of the mixture.

On the 1970 London - Mexico World Cup Rally (with many miles at 15,000 ft), some cars fitted with SU carburettors had a device fitted which enabled the driver to weaken the mixture from the dashboard. Details of this very simple device can be obtained from Don Law, ex-SU Racing Department, who was involved in the original design.

The alternative is to carry weaker jets or needles and change them for the higher sections of the route. On an SU carburettor, a needle change would take at least 5 minutes per carburettor, if you are not tired or suffering from altitude sickness!

www.hero.org.uk/info/appendixa.html

Doug

ever seen a bag of chippies blowing like a balloon going from Melb to the hills??

Yes to much time on your hands ..LOL.. I think

Given that it has to survive without bursting to pass the crash test in ADR's I'd very much doubt you will ever see a problem with the insignificant pressure developed by altitude.

A sled hitting the rear of your vehicle at 56kmh for ADR generates A LOT more tank pressure than that.

Dave

Never thought about the fuel tank, must admit I have been worried about the Twisties packets exploding though!