Higher Altitude = higher speed?

[Carl Johansson]

Assuming you can get adequate air into your engine -  it seems to me that I have read here that the higher altitude makes for less dense air --  which equals less resistance -  which equals higher speeds!

Am I remembering this correctly -  anyone have a good source that talks about this -  like the tech series here -  some good discussion of altitude and speed!

Thanks guys
Carl "lightheaded" Johansson

[hotrod]

If you look at the Aero drag formula it is very clear that you are correct.

drag forumula for english units

Drag (lbs) = Cd x FA x ( 1/2 x ((air density lbs/cubic ft)/32.2) x velocity in ft/sec)^2 )


As you can see air density is one of the terms, in the equation, so air drag will vary directly with air density.

The issue of power is similar but the combination of the two sometimes leads to non-intuitive results.

In cars that are traction limited, sometimes the reduced power at altitude actually helps them, as they become more drivable, and in the case of the drag strip they can launch better because they do not over power the tires. Likewise at Bonneville the reduced air drag at altitude means it takes less power to go a given speed "all else being equal". Unfortunately rolling resistance on the salt is not equal to pavement so you end up balancing conflicting effects.

On some cars the combination will be to their advantage at altitude in other cases it will hurt them.

The altitude will help a car with aerodynamics of a brick more than it will a really slick body. Percentage wise the difference will be the same but a really small change in an already low drag number will not make much difference in top speed.

Larry

[rockstar]

am i correct in thinking that as you say high altitude = less dense air,but in a N/A engine does that mean less fuel/air mixture for the engine to burn resulting in a power loss,so any advantage in air resistance is negated by a drop in the power or is my basic education all to cock?

[JackD]

Assuming you can get adequate air into your engine - 

Am I remembering this correctly

Hasn't this been cussed and discussed here before, before, before, before, in the recent past ?

[hotrod]

am i correct in thinking that as you say high altitude = less dense air,but in a N/A engine does that mean less fuel/air mixture for the engine to burn resulting in a power loss,so any advantage in air resistance is negated by a drop in the power or is my basic education all to cock?

The answer is yes and no !

Yes NA cars will lose power at Bonneville, but that is not necessarily a problem.

For example:
Bonneville air 100 deg F, 40% humidity, 0.0601855 lb/cu ft
Standard sea level air density is  = 0.0765716 lb/ft^3 (at 59 deg F, 29.92 in Hg barometer, 0% humidity)

So based on air density alone, NA power would drop to about 78% of sea level power.

Now suppose the car is making so much power that with its current weight distribution, and salt conditions the tires break loose at 80% of its sea level power. Then the power loss due to the altitude would act like an unintentional traction control. At wide open throttle the car could just reach a bit of tire slip but not be uncontrollable. If the power required to break a record at the air density of Bonneville, is less than 78% of sea level power, then the power loss is not critical.

We see this sort of thing here in Colorado all the time, cars come in from the Mid west or east coast and run an all time best time at high altitude. It is most likely a case of the traction control effect I mentioned that at home they are overpowering the tires and not able to hook up on launch, the lower drag in the traps and the intangible of they are so concerned about the altitude that they really pay attention to their tuning and get the car properly dialed in.

Larry

[Sumner]

...............................So based on air density alone, NA power would drop to about 78% of sea level power...................Larry

Hey Larry since we aren't drag racing I'd like that 22% power back and I'll just add some weight if traction is a problem  .

Like Jack mentioned there was a really huge discussion on this a couple months back and a little searching should find it.

c ya,

Sum

[Mile High Talon]

Irrespective whether N/A, turbo, or blown, for all types of vehicles higher altitude equals lower speed as reducing air pressure reduces HP. N/A lose the most, ~20-25% HP, and turbos and blown rides lose ~12-15% HP up in Denver. Carb'ed cars and bikes also require larger jets up here than at low elev., and the coolant temp. increases.

Bandimere, our local NHRA 1/4 mi is at 5800'. A 13 sec. car at sea level will run 14's there. Faster cars lose less, slower cars lose more, so a 8-9 sec. ride will lose ~.5 sec and a 18 sec. ride will lose ~1.5 sec(IMO-its a waste of time to drag a mini-van, but some do anyways). Even the 5,000HP Top Fuel cars lose a few tenths up here.

The higher up you go from Denver, the more HP you lose. One example is driving over the Continental Divide coming up the hill (local term for a mountain) from Silverthorne to the Johnson Tunnel at 11,000'+. Anyone who's done it knows their ride makes significantly less power than it does going up Genessee or Floyd Hill near Denver at 6,500-7,000'. Or race up to the top of Mt. Evans or Pikes Peak, both over 14,000'. The HP loss is so great you'd swear you're only running on half the cylinders, and you'll likely boil your coolant if running at WOT. I've made more than 2,000 trips over the Divide on I-70 in Mile High Talon, quite/most often at WOT(thanks to my Escort Passport), and its been up Evans and Pikes a half-dozen times each at WOT, and the datalogger confirms it makes less HP and torque the higher up it goes.

In short, any slight decrease in aero drag is more than off-set by the large decrease in HP at higher elevations.
James 

[Dynoroom]

I'm not going to get sucked into this....     

[JackD]

I'm not going to get sucked into this....     

Except that..............?
Oh never mind.

[Harold Bettes]

ALL aircraft speed records are set at conditions where the air is thin!

The premise of power decreasing with higher density altitude conditions is valid, however that is not the only issue to consider.

IF one has an abundance of power available in excess of tractive effort (drawbar pull) capability, then the consideration of power alone is not the limiting factor. It is that old saw about "wheel spin" or something like that.

AND if turbocharged, methinks that one can perchance compensate for density changes with the wastegate or intercooler or both.

OH Geee! Mike, I fell for the bait!

Regards to All,
HB2

[ack]

I'm not going to get sucked into this....     

Me neither been there done that a few weeks back people believe what they want, facts be dammed.

[JackD]

"Ya gotta shake yer bait."

[Mile High Talon]

...Of course none of us are racing airplanes on the Salt, so that's not a direct comparison. While the premise may be theoretically possible, it just doesn't match up with the real-world track data.

Tony Schumacher has the TF 1/4mi. record of 4.428 sec. at Pomona, Joe Amato holds the Bandimere track record at 4.584 sec. In fact, every single NHRA 1/4 mi. class record has been set at a "low" elevation track, and none have been set at "high" elevation: http://www.nhra.com/stats/natrecord.html 

Believe whatever you'd like, and for anyone who'd like to try drag racing at mile-plus altitude, bring your car up here, I'll put you up at my place for free, Bandimere is <5 miles away, they run test & tune nights every Wed. from April-Oct. for $30, and you can check it out for yourself.

Have a great day!
James

[Dynoroom]

High, I'm glad you've collected data for your information but....

How does drag racing compare to LSR?

Dang, sucked me in anyway...   

[hotrod]

How does drag racing compare to LSR?

Good question! The strip records as posted above by Mile Hi Talon, would lead you to believe the change in air drag does not matter, or the lower air pressure is an insurmountable barrier to performance, it is really a very biased data set.
For every national class AA/F run up at Bandimere there have probably been 1000 runs by similar cars made at sea level conditions. I also race at Bandimere, and agree that the lower air pressure hurts total power output, but there is ample evidence that for some cars, the aerodynamic effects trump the power effects. That is why you have sea level racers come up here and occasionally run all time best runs for "their cars".

To beat the altitude effect on power, you need to understand how and why the engine is making less power and then take steps to compensate. For a purpose built car that is intended only to run at Bonneville that is more do-able than is it for a car that will spend most of its life at sea level with an occasional run at altitude. Some of the racers build high altitude motors for drag racing or run different gearing etc. to compensate, and some of them run very similar times at altitude as they do at sea level.

To a limit, on a blown/turbo charged engine, you can simple dial up the boost, but it is not as simple as that phrase makes it sound. It is not a freebe, the increased boost has its own consequences to engine performance and tuning and other allowances must be made.

The effects of lower drag due to changes in air density is why some very fast cars run in the heat of the day at Bonneville rather than in the cool of the morning, as at 200+ speeds the change in air drag due to lower density is useful if they have a surplus of power to tap on demand.

Probably the better example of the effect of low density air on air drag is the distance fly balls are hit at Coors Field compared to sea level ball parks (or the frustration of pitchers that depend on aerodynamic effects on their curve balls up here.)


Larry