Can someone explain the roll of Barometric pressure for drag racing? For Example, what difference does it make if the Barometer is 29Hg or 31Hg?? I wrote my Senior Thesis for college on what the role of weather has on drag racing and explained that the higher the barometer the better. But my physics professor tells me that I am wrong for thinking that. Am I wrong?

Any info would be helpful.

Thanks,

Aaron

 

You are correct and your professor is wrong. Pressure, simply stated, is the "weight" of the atmosphere. The more pressure, the more atmospheric weight. The more atmospherice weight, the more atmospherice density. The more dense, the more air molecules you have. The more air molecules you have, the more fuel you can burn. The more fuel you can burn, the more power you make. The more power you make, the faster you go.

Pretty simple really.

I'm a meteorologist. You are welcome to put your professor in touch with me.

 

Bob, do you mind if I quote you? And instead of simple terms (laymans) can you be more specific? This would be GREATLY appreciated!!!


Thanks,
Aaron

 

Raptor, thanks for backing up my argument!!



Aaron

 

Higher barometric pressure will always make more HP as perfectly explained by Bob.

With a higher BP you also get a higher air density. It's harder to push through the thicker air. I think your professor may be confusing that for BP. The increase in HP produced from higher barometric pressure is greater than the loss of power trying to push through higher air density.

Low sleek race cars won't notice the extra drag going through a higher air density but if you were racing a brick like a full size van it would probably be noticable.

I race at altitude. On a typical race day the density altitude is around 5000 feet. Because of the lower pressures, the engine doesn't make as much HP as it would at sea level. I estimate I'm only putting 420 hp to the ground but if I ever get on a dyno it will give a corrected HP of around 500 hp. Dropping down in altitude to a lower track is like increasing the barometric pressure. More BP builds more HP.

Barometric pressure around the country is all calibrated to sea level and is all based on station pressure and altitude. If it wasn't, then high altitude airports in a low pressure cell would have so low of a barometric pressure that it would go below most instruments.

Pilots use density altitude more than racers do. They need to calculate how much HP the engine produces to know how much runway they'll need to take off. Low density altitude producing less hp needs more runway.

Since barometric pressure, humidity, temperature and altitude all factor into creating density altitude, all of these are important. Barometric pressure is the biggest factor as mentioned above. Humidity is a weird one. It's how much water is in the air. The percentage isn't a good way to measure it since it varies with temperature. There's more water in the air at 50% humidity at 70* than there is at 50% humidity at 40*. Knowing the dew point is better.

So even though barometric pressure affects hp for racers and pilots, there are more factors involved. Tie them all together to come up with density altitude and it becomes a simple number to use. Just because the barometric pressure is high doesn't always mean you'll make more HP. Hot, humid temperatures can countereffect that high BP and lower the HP.

 

Remind your professor of "the perfect gas law".... the density of the air is directly proportional to absolute pressure, and inversely proportional to the absolute temperature (degrees Rankine in the "english" system).

Standard barometer is generally taken as 29.92" Hg. At a temperature of 32degF (492degR) the density of air is 0.0808 Lb/cu ft. Increase the pressure to 30.5"Hg, and the density of the air becomes 0.08237 Lb/cu ft. That means for a given volume of air (let's say one cylinder full), you have increased the mass of air in the cylinder by almost 2%. As noted in the explanation above, that 2% more air means you can add 2% more fuel mass, and make roughly 2% more energy... which the piston converts to HP.

If you lived in Denver, you could see 25"Hg barometric pressure. In this case, the pressure ratio is 25.0 / 29.92 = 0.8356, or you have lost about 16.5% of the mass of air in the cylinder.... or roughly 16.5% of the HP.... that is a big hit, as Stephen has already pointed out.

For temperature, its just the opposite. As absolute temperature increases, the density of air decreases. I mention 32degF (492degR) as a "standard" Keep the barometric pressure fixed at 29.92"Hg, and increase the air temperature from 32degF (492degR) to 100degF (560degR) and the air density drops by the factor:

492 / 560 = 0.8786

You have lost more than 12% of the air mass in the cylinder, and you have lost roughly that much HP, because you have to reduce the amount of fuel by 12% as well.

All of this assumes either you (for a carb'd engine) or the computer (for an EFI engine) is making the corrections on the fuel addition rate, or you will experience even greater losses as the A/F ratio gets thrown out of whack.

 

You're welcome to quote me...just insert the more technical stuff that others have posted into my more idiot-proof way of saying them and you should be good to go.

 

You guys rock!!!!

Thanks a bunch!!!

 

Now that's all assuming the engine is naturally asperated. Since the piston is trying to draw in atmospheric air, there's only so much it can pull in.

Once you use a forced induction system all those numbers go out the window since you're artificially increasing the air density inside the cylinders. 10-15 pounds of boost from a turbo or blower is a lot of air. More air in the cylinders means more fuel can be burnt producing more power.

 

I based my paper on a Naturally Aspirated fuel injected motor with an A4. I received a "B" on the paper.