The question...run in the cool dense morning air or less dense afternoon air??

[Sumner]

I've always felt the morning was better but then people bring up the other side of the equation where it takes less HP to run through the less dense afternoon air.  I spent yesterday morning trying to figure this out and came up with the following and as I've mentioned before math isn't a strong point of mine.

First I used an often used formula that determines a cars drag force for a certain speed.  It uses Cd, frontal area, air density and velocity desired to determine the drag force.  I put in values that might be close to Hooley's Stude...... Cd of .40, frontal area of 24 sq. ft., 220 mph and 'standard' air densities for sea level, 4600 feet (morning) and 7200 feet (afternoon) and came up with the following:

..Altitude........Drag Force (lb/ft3)
.... 0 .............. 1152
.. 4600 ............ 974
.. 7200 ............ 882

From that we can see the drag force does change significantly with altitude and less dense air.

Next I took the Drag Force numbers and used them in a formula that determines HP needed to run a certain speed.  This formula uses the Drag Force number along with the cars weight and rolling resistance to take that into account also.  I used 6000 lbs for the weight and .20 for the rolling resistance and came up with the following HP numbers.

.. Altitude.........Drag Force...... HP Needed
.... 0 .................1152 .............. 732
..4600 ................ 974 .............. 628
..7200 ................ 882 .............. 574

From that we can see that the HP needed goes down as the air density goes down with altitude.

So the next question is how does the altitude effect the HP?  Can we overcome the effects of altitude and less dense air by making more HP in the denser morning air.

I found a formula that calculates the loss of HP due to effects of altitude and less dense air. I started with 732 HP at sea level and then calculated what it would be at 4600 feet and 7200 feet and guess what?

.. Altitude ... HP
.... 0 ......... 732  Original HP
.. 4600 ...... 631  HP is down to this number at 4600 ft.
.. 7200 ...... 574  HP is down to this number at 7200 ft.

The conclusion I found using the formulas I found and used  is that it is a wash.  We loose HP at increasing altitude at the same rate that it is needed to run our 220 mph.  We still run the 220 regardless of the altitude if we have the 732 HP needed to run it at sea level.

You need less HP at the higher afternoon density altitude but your HP will be down to the point that you could run in the morning with the same results as the HP gain there offsets the effects of the less dense air in the afternoon.

I guess this is a case where "air density" has the same weight or value in the equations to figure drag force and in HP loss.

Now there are other factors that could still come into play such as humidity.  One thing to also consider is traction.  It has been my feeling that traction is up in the morning and can go down in the afternoon as the heat brings water to the surface of the track.  Also driver comfort...I'd much rather be running in the morning vs. the afternoon,

Sumner

[Milwaukee Midget]

Sum -

I'll repost my observations from 2013.  This is for the low power, normally aspirated, aero challenged MG, and there were other changes that were made - tire pressure, jet size, etc. 

But for the baby brick, I found the thinner air gave me the best speeds.

. . . I'd prefer to run in cooler temps - it gets a tad warm in that little kipper can.  But as it sat this year, it appears as though the aero advantage of less dense air proves to provide better speed than the power of denser, colder air.



Logic dictates it shouldn't look like this, but this is the data I collected on my 9 competitive runs.

[SPARKY]

Sum

Years ago I read that what made B'ville special --- its is real close to the cross over point of what you are trying to evaluate: HP vs Drag  and that is was just about optimal---makes having a good intake system pretty important to be able to knock of about 1000' of DA

[Sumner]

Sum -

I'll repost my observations from 2013.  This is for the low power, normally aspirated, aero challenged MG, and there were other changes that were made - tire pressure, jet size, etc.  

But for the baby brick, I found the thinner air gave me the best speeds.

. . . I'd prefer to run in cooler temps - it gets a tad warm in that little kipper can.  But as it sat this year, it appears as though the aero advantage of less dense air proves to provide better speed than the power of denser, colder air.



Logic dictates it shouldn't look like this, but this is the data I collected on my 9 competitive runs.

So that brings up did you have a better tune for the afternoon air vs. the morning air?

The formulas assume you keep the proper tune as the altitude changes.  I think that what most of us are seeing is that our 'tune' happens to be better for one altitude vs. another other wise show me with the math where what we are observing defies science and math  .  

If one person is making optimal tune (HP) at the lower density level vs. the higher one they will run faster in the morning and if the other, maybe you, has the optimal tune (HP) at the higher density then you will run faster in the afternoon.

If I'm reading your graph correctly I'm seeing a 2 mph difference max.?  If that is right 2 mph might also be something else maybe?

Sum

[GH]

I think that you just go out there a run the snot out of it. I never looked at the tach or any other gauges, just looked where I was going and hoped for the best. I couldn't tell any difference between a 230 run and a 248 run.

[Sumner]

Sum

Years ago I read that what made B'ville special --- its is real close to the cross over point of what you are trying to evaluate: HP vs Drag  and that is was just about optimal---makes having a good intake system pretty important to be able to knock of about 1000' of DA

If you are gaining say 1 # of boost at 220 mph then you would still be back to the fact that that is less HP gain at the lower air density (high altitude) vs. more HP at a higher air density (lower altitude) and we are back to the fact of it is a 'wash' since as the HP goes up the drag increases to offset it.  The car will run faster though but both at the lower or higher density altitude with the corresponding proper tune. 

With a better intake we are making more HP and the car will run faster than say the 220 but the rules of air density going down (need less HP) and HP decreasing (due to increased altitude), with neither one having an advantage over the other still applies,

Sum

[Milwaukee Midget]

Absolutely - It's less than 2% difference - the only consistent feature being the higher the adjusted altitude, the faster the car went.

It's a small sample, certainly not absolute, and there are variables not accounted for, but it does indicate a trend in this particular instance.

Here's how I broke it down last August . . .

Okay, let's go back to the run data -
Track orientation sw to ne

Date   Time   Temp   Humidity   BP   D A   Wind    Wind Speed   Mile 2   2 1/4   Mile 3   Note   Air jet   Timing
12-Aug   8:37   70.6   15   25.73   5863   ssw   1   102.464         Rookie   180   37
12-Aug   14:28   88.9   8   25.7   7008   s   4   109.781   114.844   115.886      180   37
12-Aug   17:08   89.8   7   25.66   7106   ese   9   111.12   115.185   116.698      180   37
12-Aug   18:42   90   9   25.65   7145   sse   10   112.234   117.155   118.069      180   37
13-Aug   9:25   71.9   31   25.78   5943   nnw   1   111.944   115.336   115.878      180   37
13-Aug   11:41   77.8   20   25.78   6279   ne   5   110.386   114.275   115.028   push start   180   37
13-Aug   15.29   88.4   9   25.73   6946   sse   5   113.319   116.444   117.293      175   37
13-Aug   17.57   91.5   6   25.68   7177   sse   11   113.586   117.161   118.257      175   37
14-Aug   11:07   79.2   22   25.77   6391   n   0   113.241   117.077   117.816      170   37
14-Aug   13.56   88.5   11   25.73   6965   n   3   114.456   118.141   118.693      165   37

Let's toss the highlighted rookie run and the push start as outliers, then separate by jet size -

On the 180 “rich” jets, my best speed was with a bit of help from a 9-10 mph sse wind, and we'll toss those –

12-Aug   14:28   88.9   8   25.7   7008   s   4   109.781   114.844   115.886      180   37
12-Aug   17:08   89.8   7   25.66   7106   ese   9   111.12   115.185   116.698      180   37
12-Aug   18:42   90   9   25.65   7145   sse   10   112.234   117.155   118.069      180   37
13-Aug   9:25   71.9   31   25.78   5943   nnw   1   111.944   115.336   115.878      180   37

We leaned out the mixture - we'll dump the wind enhanced outlier -

13-Aug   15.29   88.4   9   25.73   6946   sse   5   113.319   116.444   117.293      175   37
13-Aug   17.57   91.5   6   25.68   7177   sse   11   113.586   117.161   118.257      175   37

Switch to 170 jets
14-Aug   11:07   79.2   22   25.77   6391   n   0   113.241   117.077   117.816      170   37

Switch to 165 jets
14-Aug   13.56   88.5   11   25.73   6965   n   3   114.456   118.141   118.693      165   37

Final dataset -


date       time      temp  rh  bp      adj        wind   1mile    2mile        3mile              jet     timing
12-Aug   14:28   88.9   8   25.7   7008   s   4   109.781   114.844   115.886           180   37
13-Aug   9:25   71.9   31   25.78   5943   nnw   1   111.944   115.336   115.878     180   37
13-Aug   15.29   88.4   9   25.73   6946   sse   5   113.319   116.444   117.293       175   37
14-Aug   11:07   79.2   22   25.77   6391   n   0   113.241   117.077   117.816        170   37
14-Aug   13.56   88.5   11   25.73   6965   n   3   114.456   118.141   118.693        165   37

I'm going to take some credit here - most of the actual "driving" of the car is in the first mile - the one mile numbers are consistent enough for a rookie.

Compare the two best 1-mile times - and note the adjusted altitude, and wind direction and speed -

13-Aug   15.29   88.4   9   25.73   6946   sse   5   113.319   116.444   117.293       175   37
14-Aug   13.56   88.5   11   25.73   6965   n   3   114.456   118.141   118.693        165   37

I think we were getting pretty close on optimal jetting.  I never did anything with the timing other than to take out the rev limiter after the rookie run.

I know this does not lead to a conclusion, but I think it leads to a direction.

[Sumner]

Absolutely - It's less than 2% difference - the only consistent feature being the higher the adjusted altitude, the faster the car went.

It's a small sample, certainly not absolute, and there are variables not accounted for, but it does indicate a trend in this particular instance.

Am I reading the first graphic wrong?

115.886 at 7008 and 115.878 at 5943 (about same speed but different DA's)

and

117.293 at 6946 and then a faster 117.816 at a lower DA of 6391

If those are right then maybe we are seeing something else come into play,

Sum

[SPARKY]

highlights the old adage

clean cars like the cooler air---dirty cars like the hotter air

[Milwaukee Midget]

highlights the old adage

clean cars like the cooler air---dirty cars like the hotter air

That's where I think it's heading in my case.

I also think the database is too small to make any absolute conclusions - especially if you throw in driver inconsistency - which I'll own.

[SPARKY]

I thought you were going to take issue with me accusing you of NOT washing your pride and joy often enough 

[Sumner]

highlights the old adage

clean cars like the cooler air---dirty cars like the hotter air

Is it an adage that is true or just old  .  The same physics are going to apply to the clean car in less dense air or more dense air and also the HP will be effected the same.  Also the same for the dirty car.

One thing is for sure....the clean car will run faster than the dirty one on less HP if the frontal area is equal.

Before exploring this I would of swore on running in the morning, now I can't prove their is an advantage as far as the density altitude is concerned nor according to what I've seen with the various formulas a benefit to running in the afternoon,

Sum

[USFRAMONTE]

Can't speak much about the air density question but I can tell you about the salt conditions between cool morning and warmer
afternoon. Having pulled the drag at all hours of the day and night, the track is much firmer in the cool morning hours or at night
once the sun goes down. The only time I have seen a race vehicle on course after dark is when we were towing it back to the
pits after it got stuck in the mud about the 11 mile. If I remember right that was about 10:30 or 11:00 pm.
If I have my choice I would make my runs as soon as possible in the mornings. It would be interesting to see how many of the
record return runs are faster on the morning return run?

[Sumner]

... Having pulled the drag at all hours of the day and night, the track is much firmer in the cool morning hours ... It would be interesting to see how many of the record return runs are faster on the morning return run?

Thanks and that is what we have noticed also.  A better track can mean less rolling resistance (less HP loss) and maybe better traction that can help a car that can't run the last mile at a terminal speed and can use all the course it can get.

Hooley's mid morning qualifying run...



... and the early morning backup run....



... about the same terminal speed as I think we were at the limits of that engine combination but a higher speed at the 2 1/4 which could be traction or who knows?  DA was about 600 feet lower for the second run and the humidity was up,

Sum

[SPARKY]

My favorite time to run is just after the record return runs--  the course has blown clean by the rr cars and the sun is high enough it doesn't blind one & the air is still pretty good