Say you attach a length of pipe (or hose) to a balloon.
You attach a funnel to the other end of the hose.
You hold the funnel out the window of a moving car.
What will happen to the balloon? Will it begin to inflate, at any speed?
Can you create pressure inside the balloon by ‘forcing’ air in quickly? Would anything happen at all?
If you go fast enough, sure. It’s really no different that blowing into a tube attached to a balloon. You’ll have to drive pretty fast to build up enough pressure to inflate a balloon. At slower speeds you won’t have enough pressure to overcome the resistance of the balloon material. It’s similar to blowing not-so-hard into a balloon, it’ll inflates slightly then stop expanding. You have to increase your blowing pressure to overcome the material’s resistance.
Is there a particular speed the average party balloon would begin to inflate at? ( I understand there would be lots of variables in this).
Could you do this with a car, at say 50mph? 80mph? 100mph?
Or are you talking about much higher speeds?
Okay, how fast would you need to go to fully inflate a standard balloon?
I think the solution lies in considering that the pressure in the balloon will equal the pressure at the funnel, acting as a kind of ram, which will increase as the speed increases. But as to how you calculate the pressure in a concave vessel with air being forced into it, I’m afraid I have no idea. I am neglecting the dynamics of the ballon actually expanding.
**Aro[b/], one suggestion. Get someone else to drive while you hold the balloon in the passenger seat. It might be tricky to drive and inflate the balloon at the same time. Could be exciting for other road users, though… 
I thought about trying this on the way home, but don’t have a funnel with me at work. Thought the guys on the SDMB could sort this out without the need to experiment…
That’s a question of pressure. Variables involve speed, surface area of the wider opening of the funnel, what material the balloon is made of, how large it is, that sort of thing.
So no, can’t answer your question.
I bet you could do it by standing in the kitchen and spinning around. Here, let me try…uh, nope. 
Well, you ought to be able to make a pretty good guess at the speed required. Ignore the funnel for a minute (which I suspect won’t increase the pressure all that much, anyway: see below). You can apply the Bernoulli equation, which in this case simplifies down to:
V[sup]2[/sup]/2 = P/[symbol]r[/symbol]
where V is the inlet velocity (or velocity of the car, if you prefer), P is the pressure in the balloon, and [symbol]r[/symbol] is air density. this page says that “a balloon inflating for the first time needed about 30 Torr for inflation.” 30 Torr = 0.579 psi= 83.4 pounds/sq. ft. [symbol]r[/symbol] for air is 0.00233 slugs/ft[sup]3[/sup]. So the velocity required to inflate a balloon ought to be:
V[sup]2[/sup] = 2*(83.4pounds/ft[sup]2[/sup])/0.00233 slugs/ft[sup]3[/sup] = 71600 ft[sup]2[/sup]/sec[sup]2[/sup]
V = 268 ft/sec = 182 mph.
Testable Hypothesis: Unless you have an Indy car, you ain’t gonna do it.
Having a funnel may help a little bit, but I suspect that any advantage will be small, because the entire funnel and ballon is effectively a dead-head, where there is no air velocity and constant pressure throughout. If the funnel helps at all I think it would mainly be in capturing and directing the air, regardless of angle of attack.
Oh, something I just thought of: As a practical matter, having a funnel would shield the inflating balloon from an increase in external pressure due to Bernoulli’s. So if you really did have an Indy car, you’d probably want some kind of funnel or shield anyway.