Overdrive.

Tim, does that air resistance formula hold true for all shapes? Seems like there would be a curve based on the shape of the object, somehow.

I think Scott may find some improvement in range, like he is after, and will have the added capability of more gear selections off road. Not sure how of an improvement, but some. And, I personally, don't want to do more than 60ish in an old JEEP, regardless of it's condition. I'm curious to hear Scott's results.

 

I agree with Tim on Air Resistance which squares at velocity..............more interesting is the additional HP required using the same drag coefficient to make the vehicle travel another MPH once at the vehicles barrier.
Driving a Jeep down the freeway is about as aero correct as putting wheels on a barn door and trying to drive it.............Most small motors even at a lower operating RPM will not increase fuel mileage for the reasons first mentioned once increased drag is present................Two place's where the OD does shine..........if you have a low final drive ratio like 4.88's or lower and occasionally drive it on the freeway taking 25% off that final drive RPM is a Win / Win , but the real reason I'm a big fan of the Overdrive is being able to split the gears on the trail..............that in my eyes is where it really shine's...............recovery cost??

 

When I still drove my Jeep to trail runs (I was much younger then, now I usually flat tow) I took a trip to the Rubicon. 1987 Buick V6 w/carb, 4:88 gears and Warn OD. I averaged 65mph and was able to get between 22 and 25 mpg. Out of the 4 rigs that were on that trip I squeezed out the best mileage.

 

Wheelie. It goes up squared regardless of vehicle shape. The shape does affect the drag coefficient.

 

It's called the "drag equation" and it's generally true. There is a drag coefficient Cd that depends on the Reynolds number which depends on the shape. http://en.wikipedia.org/wiki/Drag_(physics)#Examples_of_drag

The Reynolds number Re is a dimensionless quantity that depends on the kinematic viscosity of the fluid (viscosity/density), the shape and the velocity v. At low enough Re, the Cd goes like 1/v and the drag is linearly dependent on v. At high Re, Cd becomes more or less constant and the v^2 dependence holds.

So yes the velocity dependence holds true for all shapes, but the character of the drag coefficient will change. So a smooth shape will tend to have laminar flow (low Re) at higher velocities than a rough shape. This makes sense intuitively - turbulence just sucks up power, so the smoother the shape, the higher the velocity for disturbed or turbulent flow, and the lower the Re for a given velocity. Clear?

Actually, the the drag equation sort of understates the problem of drag wrt horsepower, since it specifies the force required to overcome drag. The power needed to overcome that force is proportional to the cube of the velocity, ie P = F (dot) v.

 

I've crunched the numbers for most all jeep compatable stick transmissions in my attempts to fully understand the variables
That includes T90 A /T90 C /T86AA /T14 /T15 /T98 /T18 NR /T18 WR /T19 WR /SM420 and SM465
I've also injected all common engine output velocities.
And all FDR's into my calculations plus tire diameters
I have not delved into adding the aerodynamics equasions.

Primarily there is no substitute for correct engine selection.
Secondly the FDR must be calculated for desired optimum useage.
Thirdly the transfer case must be considered.
D18 is near optimum when used with an OD.
The D20 jeeps will always require a higher FDR than D18 jeeps.
There is an optimum transfer case reduction for most offroad jeep uses.

In general I will say that if your running a D18 go with 4.89 FDR and 25% O.D.
If you running a D20 transfer go with 3.73 FDR.
With D20 a 4 speed transmission is nearly a must along with Tera flex 3.15.

T19 WR has the best gears for use with 25% O.D.followed by T18.
The absolute worst transmission with O.D. is T18 NR.

Sorry I forgot exactly where I rated the other transmissions for OD compatability.
It's all jotted down in my notes somewhere.

 

Did ya' ever watch Beavis and Butthead?.........When one of them has an idea and it goes away as quickly as it came...........And they show a light bulb above his head..........and then you hear the zzzzzzzzztt.......and the bulb flickers out? That's I feel right now. I'll have to read that a couple dozen times to begin to understand. I do appreciate the very complete, thorough description though it is light years ahead of my formal education level. But, I am always ready to learn something new, and often do, here. In short, the formula basically holds true for all shapes. Now I have a bunch of words to look up. Good stuff.

......I never dreamed that this site would introduce me to the study of quantum physics.

 

Tim, I love it when you talk dirty...

"depends on the kinematic viscosity of the fluid (viscosity/density)" True - my 4-cyl YJ is significantly slower when the air is cold.

 

The best I have gotten with 4.88's, OD, and the 225 was also 22. With the sbc's, it doesn't seem to make too much difference on displacement, if you keep the rpms in the optimal range for the cam and don't go crazy with the rest of it, 4.88's or 4.10's (and I suspect anything in between) can get 18 mpg with an OD, at least up to 33" tires. I had the same results with 327, 350, and 381.

 

A Suzuki Samurai has almost the same drag coefficient as a CJ5, and a Samuria easily got over 30 MPG when new. The two vehicles weight are not even that dramaticly different.

I say this just to show that there is a lot more at work than engine HP (which the Samurai also had almost the same) and drag coefficients.

My Samurai when new, easily got 28 MPG all the time (I generally drove it around 70 MPH to work) and it could drive 95 MPH without any issue. I am pretty sure my CJ5 even when it was new never got over 15 mpg and I am certain it could not even drive 65 MPH let alone 95 MPH.

But, this is the sort of thing I love experimenting with - and by that I do not mean hypothetical situations on paper, that generally mean nothing in real life - I mean putting some different setups in my Jeep and see what happens. I have a strong suspicion I can get the CJ5 in the 20's for MPG but I am fairly sure there is no chance it will ever see 28+ MPG.

Speed is not something I am interested in testing on it - I will get it so it can go 70 MPH, it easily does 60 MPH now with the stock drive train and stock F134 so that is no major jump. I have considered getting an OD because I do not like the RPM's at 60+ MPH, no matter the engine. I will think on it, I have plenty of time.

 

If you actually measure the samurai surface area compared to a CJ surface area they are different with the CJ being larger. The Samurai is also significantly lighter. Btdt.
The amount of "dirty air" around the outside and under the vehicle's is different as well.
While similar types of vehicles they are different in many ways. Again, btdt.
The comparison you are making is analogous to a CJ and an IH Scout or Ford Bronco. Similar types of vehicles but completely different characteristics.


Sent from my iPhone

 

Huh. You're serious? Could be possible. The density of air changes about 20% from -25C (-13F) to 35C (95F). It would be hard to isolate that one variable. Engine performance should change with air density, and the humidity changes the density - less humidity is more dense. Likely additional factors to consider.

I'm just a dilettante wrt fluid mechanics, having taken only one course in grad school about fluid mechanics of biological systems. And that was, what, two decades ago. It was a great course, and I enjoyed it and learned a lot.

The Reynolds number is a really interesting thing. It's the ratio between the inertial forces (like billiard ball collisions and such) and the viscous forces that hold the fluid together. Low Re means that the viscous forces dominate, and you get the beautiful streamlines that you see in wind tunnel pictures. At high Re, the inertial forces dominate, and the parts of the fluid are thrown about by their trajectories, making the flow turbulent.

We are living in an ocean of air.

 

Alright I have resisted long enough to enter this technical discussion. Below is a chart I found in booklet originally published in 1931 Building The 100 M.P.H. Model "A" Ford, by George Riley it shows the power required to overcome air resistance.



Enjoy.

 

"Huh. You're serious? Could be possible. The density of air changes about 20% from -25C (-13F) to 35C (95F)."

Totally serious. And I agree there are other factors, such as rolling resistance of the tires, drag of cold oil in the powertrain, etc. But I also note that when riding a motorcycle the wind certainly seems far more resistant when the temperature is low. 10% or 20% could be significant.

 

Fluid dynamics is also what we use to explain sediment transport in streams, anticipated flooding depths, hydraulics, and countless other systems including such mundane things as a sediment analysis (sand, silt, clay %).

Interesting chart and the dashed example line is actually probably applicable to a CJ frontal area. But it falls apart at the higher speeds. I did have my CJ 5 up to about 90 mph on a couple of occasions and while I'm sure a 225 with a 4bbl and headers may get into the 200+ hp range, it was nowhere close to 300 hp as that chart would indicate.
As for air density changes and engine performance, there certainly was an effect on carbed engines, but most EFI systems virtually eliminate that factor unless you are pushing the system to its limits on air flow.

 

I would have thought it would be just the opposite. Less humid = less dense. It sure feels thick around here when the humidity is up. I'm thinking of drag racers, also, trying to cram all the air they can into their engines. Better for them at lower or higher humidity? Maybe I'm off on another tangent here and getting away from the topic.

 

He may have meant less oxygen dense, as air is displace by vapor, thus lowering engine performance.

 

That's how I envision the Jeep.
The Jeep has an aerodynamic limit that could be compare to a ships hull speed.
If you try going faster than the hull speed then the energy required becomes a huge concern.
I fully suspect that the Jeeps hull speed is down around 40 MPG.
Beyond that speed it really requires a lot more energy to overcome the wind drag.

 

My understanding is that water vapor is less dense than dry air, so when you reduce the fraction of water vapor, the air becomes more dense for the same pressure.

More water vapor would also reduce the fraction of O2 per volume (reduced partial pressure), so I'd think dry air would be better for performance.

 

Lower Humidity............when humidity / moisture is up the moisture takes up space that would normally be occupied by another molecule of air. Sea Level air or adjusted to below sea level is what you want in a perfect world on a race track..........more air / Oxygen allows you to burn more fuel at a ratio of about 15 parts Oxygen to 1 part of fuel................the more fuel you burn the higher the BTU's , more heat equals more Horsepower..........