# Aerodynamics/anyone recognize this as an old post?

Discussion in 'The Garage' started by dyeager535, Aug 16, 2004.

1. ### dyeager5351 ton statusPremium Member

Joined:
Dec 13, 2000
Posts:
27,833
433
Location:
Roy WA
Was searching my computer for another file, and ran across this one. No idea where I got it, but at one time I knew a guy that worked/had worked at GM Powertrain a long time in developement, and this may of come from him.

Just thought it was interesting, didn't know this info was out there. Very clear from the numbers that slowing your vehicles MPH down will save you gas, even though the weight is so high. As many posts as there are on the subject, looks to me like slowing down is the most effective way to increase economy. /forums/images/graemlins/smile.gif

"Here's the Aerodynamic Resistance Demand Horsepower figures for a 1973-87 GM
truck at different speeds. Data from GMC Truck Selction Manual.

MPH Hp required

10 .2
20 1.5
30 4.9
40 11.7
45 16.6
50 24.8
54 28.7
58 35.6
60 39.35
65 50.1
70 62.6

So, good economy will grow difficult at highish speeds.

That said, an axle that keeps the engine operating at ~400 or so below the
torque peak (probably 2800 rpm is the peak, so shoot for 2400ish) at the
fastest speed you usually drive will help.

I'll assume you still have the 9.50 16.5 tires which turn 677 rpm, or
similar size equivalents.

Lets say you want best economy at 68 mph, which is pretty much the point of
no return anyhow...

Heres the math;

60/68 = .882 (this is speed correction)

2400/677 = 3.54 (this gives the ideal axle for 60mph)

3.54 x .882 = 3.12 (this corrects the ideal to 68 mph)

this suggests the best ratio at 68 mph is ~3.12:1 for this example. A 3.08
axle might work well for this example. Beats me if ones availible however.
Perhaps a 3.23 is, which isnt too bad a second choice."

2. ### TSGB1 ton status

Joined:
Apr 22, 2002
Posts:
12,527
0
Location:
Centralia, Washington
Wow. I've never seen actual statistics on information like that. Great info! /forums/images/graemlins/deal.gif

3. ### 83ZZ502_Jimmy1/2 ton status

Joined:
Feb 7, 2001
Posts:
3,103
0
Location:
Richmond, VA
Good Info again Dorian.. I remeber discussing this a loonng time ago, but I dont recall this chart. /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif

John

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
It looks like it's time for some education.

The aerodymanic drag is a squared function of velocity. That means that everytime you double the vehicle speed, the power required to maintain that velocity is 4x as great. At lower speeds, things like accessory drag and "pumping loss" are dominant. You also have to take into account tire rolling resistance. Every vehicle will have an optimum speed for fuel economy. Generally, the worse the coefficient of drag, the lower that speed will be.

When choosing an axle ratio, you are comprosing between acceleration/towing performance and fuel economy. The trade-off is rather severe. For example, when you are already geared low, going lower gives you a little more power, but uses way more gas. This chart shows an example based on three different engine sizes. The little markers are the standard available gear ratios.

Assuming everything is set up reasonably and running OK, fuel economy is determined primarily by vehicle mass. As this chart shows, 87% determined by mass. 13% is your engine efficiency, aerodynamics, etc. This is based on EPA cycles, so drag would make more of an impact at high speeds.

To get depressed, look at how less than 13% of your \$2/gallon liquid gold goes to propel the vehicle:

5. ### doctor4x41/2 ton status

Joined:
Jan 22, 2002
Posts:
1,450
0
Location:
Los Angeles
Man am i gonna mess with heads with this info
Thanks guys /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif
R

6. ### dyeager5351 ton statusPremium Member

Joined:
Dec 13, 2000
Posts:
27,833
433
Location:
Roy WA
I don't know that I agree with much of that information. Or perhaps just the way it is presented. I'm sure it's true, but only sofar as it is packaged/intended.

Sure, the formula's may all be well and good, but there is a LOT of assumption going on. There are way too many engine variables for the urban energy breakdown to be correct. A carbed 502 will have significantly different fuel consumption than a fuel injected 262 I6 in the exact same vehicle, and engine economy alone is a large part of that.

Saying that 13% of a vehicles energy goes into propelling the vehicle assumes that all vehicles have the same coefficient of drag, which is not nearly correct. As well, tire contact surface plays a part as you mention, as you can see with economy car tires being extremely narrow. Yet I don't see where in those equations rolling resistance is calculated, except perhaps as an average. If you don't average all the data out, and use exacts, the end result I suspect will be far different based on two vehicles.

Even if you increased the weight of say a Camaro to match that of one of our K5's, and used the exact same drivetrain, the amount of energy spent fighting drag would be significantly different. Thus, all calculations based on drag have to be adjusted for every vehicle, since you cannot "average" drag between vehicles that might have a COF of .28 vs. .50.

With the way the EPA calculates things, more than likely they work off of some sort of average, since CAFE IS an average across the makers vehicle line. I notice in "economy as a function of mass" that for given weights, there are some dots (assuming those are individual vehicles sampled) that are WAY out of the range, around 20% different in some cases. Averaging those vehicles into the mix may make the numbers more easy to read, but it doesn't mean anything if your vehicle is the one dot way outside of the norm.

Most of the data presented basically gives the automakers a crutch as to why the new trucks need to be so fuel INefficient. With all the advances made in drivetrain design, car economy has increased at least 25% in many vehicles, (compare say, MPG of a chevette to a corolla, both of which are light front wheel drive vehicles) yet truck economy is scarcely better than when our trucks were built, and the only real improvements seem to be in the city, which is where mass plays a HUGE role in economy.

I'm no math whiz, but I do know that averaging data out really doesn't make an accurate picture.

7. ### ntsqd1/2 ton status

Joined:
Nov 28, 2002
Posts:
3,381
0
Location:
So. CA
The best Brake Specific Fuel Consumption (BSFC) for any given spark ign engine happens at or very near the peak torque RPM. BSFC is a measure of how much HP per volume of fuel can the engine produce. The lower the number the more efficient the engine is.
Same may also be true for a diesel, but I don't know that.

So gearing to be at/near peak torque RPM at your hwy cruise speed is a logical thing to do.

Most of those Energy Flow Charts that I've seen are based on passenger cars. They give you an idea of where all the losses are and the respective proportions of each loss avenue, but you can't use the numbers for anything more than a rough idea.

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
[ QUOTE ]

Saying that 13% of a vehicles energy goes into propelling the vehicle assumes that all vehicles have the same coefficient of drag, which is not nearly correct. As well, tire contact surface plays a part as you mention, as you can see with economy car tires being extremely narrow. Yet I don't see where in those equations rolling resistance is calculated, except perhaps as an average.

[/ QUOTE ]

Wind drag and rolling resistance all come out of the 13%. The point is that an internal combustion engine does not get all the energy out of gasoline and that you actually produce more heat than rotational power.

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
[ QUOTE ]
I notice in "economy as a function of mass" that for given weights, there are some dots (assuming those are individual vehicles sampled) that are WAY out of the range, around 20% different in some cases. Averaging those vehicles into the mix may make the numbers more easy to read, but it doesn't mean anything if your vehicle is the one dot way outside of the norm.

[/ QUOTE ]

The statistics are accounting for the "outlyers". That is where the term "maximum error = 17%" comes from. It's not saying that a vehicle can not be built that has worse economy than this (i.e. your big block in a small car). It just means that none of the vehicles tested were outside this range 17% range.

Also remember that changing to a different engine usually changes the vehicles weight. Not just from the engine, but then it has to have upgraded drivetrain, more sheetmetal for crash absorbtion and then bigger brakes to stop this mass, which adds additional mass. This is known as mass compounding. These studies are based on production vehicles, not modified or home-built ones.

10. ### dyeager5351 ton statusPremium Member

Joined:
Dec 13, 2000
Posts:
27,833
433
Location:
Roy WA
[ QUOTE ]
Wind drag and rolling resistance all come out of the 13%.

[/ QUOTE ]

They may come out to 13%, but is that with 20 ft of frontal area, or 40? I know I'm being nitpicky, but again, the averages presented could very well be WAY off the mark based on vehicle construction.

If we had the same data for another GM car with equivalent power then an accurate comparison of power spent combating drag couyld be made. IIRC the Monte SS made 185HP some years, weren't the trucks around that number as well? Now to find those HP vs. speed numbers...

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
No, you're looking at it wrong. If you have huge drag, then the total gas consumed goes up, but 13% of the total power in still accounts for the drag and rolling resistance. All of the power numbers will go up, but the percentages will be about the same. When you go from a corvette to a suburban, not only do you send more HP to the wheels to maintain the same speed, you are also creating more heat, engine losses, etc.

Of course a larger vehicle requires more fuel to overcome air drag and the diagram does not violate this.

Joined:
Nov 28, 2000
Posts:
3,122
2
Location:
Western Massachusetts
[ QUOTE ]
Was searching my computer for another file, and ran across this one. No idea where I got it, but at one time I knew a guy that worked/had worked at GM Powertrain a long time in developement, and this may of come from him.

Just thought it was interesting, didn't know this info was out there. Very clear from the numbers that slowing your vehicles MPH down will save you gas, even though the weight is so high. As many posts as there are on the subject, looks to me like slowing down is the most effective way to increase economy. /forums/images/graemlins/smile.gif

"Here's the Aerodynamic Resistance Demand Horsepower figures for a 1973-87 GM
truck at different speeds. Data from GMC Truck Selction Manual.

MPH Hp required

10 .2
20 1.5
30 4.9
40 11.7
45 16.6
50 24.8
54 28.7
58 35.6
60 39.35
65 50.1
70 62.6

So, good economy will grow difficult at highish speeds.

That said, an axle that keeps the engine operating at ~400 or so below the
torque peak (probably 2800 rpm is the peak, so shoot for 2400ish) at the
fastest speed you usually drive will help.

I'll assume you still have the 9.50 16.5 tires which turn 677 rpm, or
similar size equivalents.

Lets say you want best economy at 68 mph, which is pretty much the point of
no return anyhow...

Heres the math;

60/68 = .882 (this is speed correction)

2400/677 = 3.54 (this gives the ideal axle for 60mph)

3.54 x .882 = 3.12 (this corrects the ideal to 68 mph)

this suggests the best ratio at 68 mph is ~3.12:1 for this example. A 3.08
axle might work well for this example. Beats me if ones availible however.
Perhaps a 3.23 is, which isnt too bad a second choice."

[/ QUOTE ]

Cool info! I still can't figure out what they were thinking with my 3.42's though /forums/images/graemlins/grin.gif

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
Here's some similar information:

When looking at fuel economy, generally the higher the gearing the better, because you reduce accessory and pumping losses in the engine. At too low of rpm, the engine can get less efficient, though. This is for a passenger car and really shows how lower speeds improve mileage.

14. ### dyeager5351 ton statusPremium Member

Joined:
Dec 13, 2000
Posts:
27,833
433
Location:
Roy WA
[ QUOTE ]
Of course a larger vehicle requires more fuel to overcome air drag and the diagram does not violate this.

[/ QUOTE ]

OK, so the 13% we are talking about is basically "to the ground", no? Which doesn't/can't (generally) take into account variables like 4wd, auto vs. manual, transmission parasitic loss, axle parasitic loss, etc, correct? The 62.4% "engine loss" figure would never change with the same engine, but the driveline figures could vary considerably, which would change the 13% figure.

Just making sure I understand this.

Joined:
Jul 26, 2000
Posts:
9,895
1,129
Location:
Grand Rapids area
Build(s):
1
Yes, they are showing 5.6% in "driveline" which is probably just transmission (probably automatic), driveshaft and rear axle. If you throw in a transfer case, then that number increases and the rear wheel number goes lower than 13%.

Of course in 4WD the driveline losses are even worse.

16. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,176
2,386
Location:
Austin, TX
Build(s):
3
Subscribed....to create a BTT bump also.

17. ### Thunder3/4 ton status

Joined:
Jul 20, 2000
Posts:
8,946
2
Location:
So I guess the ground effects package, big ass wing, and 12" exhaust tip, I put on my k5 arn't doing me much good?

Joined:
Nov 20, 2007
Posts:
10,254
634
Location:
Roseland VA/PtPleasant NJ
Build(s):
5
55 is the way to go...

19. ### tRustyK5Big meanieStaff MemberSuper ModeratorGMOTM WinnerAuthor

Joined:
Jul 23, 2000
Posts:
37,293
2,909
Location:
E-town baby!
Build(s):
1
I used to have a freeway drive to and from work a number of years ago. Used to always run 70-75 all the way to work and would get maybe 10 mpg.

figured I'd try one tank where I stuck to the speed limit...which is 100km/h (62 mph) and see if it made a difference. I picked up 3.5 mpg on that tank. It was only taking me an extra 4 or 5 minutes to get to work too, so I kept doing it.

Pushing air is hard work, and this era of truck is as aerodynamic as a sheet of plywood.

Rene

20. ### h0wl0ngcanitbe1/2 ton status

Joined:
Jun 2, 2008
Posts:
316
0
Location:
Washington
all those calculations are takin into account a stock setup right? or does this apply to just whatever gear setup you have? example 35" tires on a 4:56 axle? how would you apply those calculations to those specs?