Calzone
Gear Whore #1
Back when I had a 1993 Jeep Cherokee, I consulted with a fluid engineer (who happened to be the guy who sold me the Jeep) about maximizing fuel mileage. The Cherokee had the awesome 4.0L AMC inline 6 cylinder engine, known for extreme longevity, but not known for great fuel economy. This was back right at the beginning of our second military field trip to Iraq and gas prices shot up over $4.00 in California (adjusted for inflation it was still more expensive then than it is now.) Anyway, the seller was also a designer of supercharger systems and had been contracted by Shelby, so he knew his stuff. He explained to me that thinking about the engine as a pump is what is key. So after schooling me on some airflow knowledge, he recommended that I hook up a vacuum gauge direct from the intake manifold to measure efficiency. I did that then he showed me where the "vacuum sweet spot" was for volumetric efficiency and I started using the gearbox a lot more to keep the vacuum signal to the gauge as steady as possible. What this taught me was that even in a truck with a mid-size engine, and a body that is built like a literal box, I could get decent fuel mileage. I drove back and forth from central California to New Mexico a few times and up to the Seattle area as well. I maxed out fuel economy in that Cherokee regularly at 30-32mpg, because I drove slower and I used my vacuum gauge + gearbox + RPM as a speedometer instead of blindly just driving. As cyclists, we feel wind resistance on our bodies build even after a few mph headwind; in a vehicle this really takes hold above 38mph, and builds logarithmically, if I recall correctly. This is how 55mph was established as a "fuel efficient" speed limit back in the 70s, but it was done by eggheads in a lab, not by American drivers. The DOT found by the early 90s that only a few percent had been saved by the speed limit, but that was only because people weren't maniacs who were using their heads and ears as an engine management system using a vacuum gauge. Most people just put the car in gear and drive.
I think I drove that Jeep across the continent maybe 4 or 5 times. I really liked that truck and wish I could find another. Chrysler steel and paint during the XJ era was not well suited for any kind of salt exposure and these days most of the XJ Cherokees have built-in air conditioning a-la rust holes.
One of these days I want to find a Willys wagon to bring back to operating condition and drive.
Phil Forrest
Phil,
Your understanding of physics holds true. Drag follows inverse square law. The difference between drag at 50 MPH verse’s 100 MPH is 4 times the drag. This is not linear at all.
In New Mexico I filled my Jeep Scrambler with 23 gallons of High-Test. Note I advanced the timing to compensate for high altitude on a carborated 258 Ci I-6. Also note that the 258 had a squarish bore and stroke and redlined at 3K RPM. Only 100 HP, but 220 foot pounds of torque.
I was on a shortcut from Santa Fe heading to an interstate to drive to New York about 2000 miles. The Jeep had a 2 1/2 inch lift and big tires; the speedometer only went to 85 MPH, but I figured out my speedometer had a 20% speedometer error so when I pegged the speedometer at 85 MPH I figure I was just doing over 100 MPH, and in a Jeep Scrambler with the lift that is mighty fast.
It was a deserted road that was lined with barbed wire on both sides, and like a boat I basically burned 23 gallons in an hour doing sustained 100 MPH. The expansion cracks pounded out a punk rock rythem, and one in a while the Scrambler would catch air in a dip in the road and we would skip a few beats. Kinda hairy, but also mucho fun. It required intense concentration
I am acquainted with a vacuum gauge, I installed one when I changed the engine to a ZZ3 Crate Motor (Corvette engine) that had 350HP 400 Foot Pounds of torque. If you learn how to jet a carb for maximum efficiently and smooth power a vacuum gauge is your friend. The idea is to maximize your timing right to the point of detonation, but also maximize vacuum for efficiency.
This process involves lots of trial and error as well as testing. Detonation is bad and can blow a hole in a piston. The air to fuel is too lean and ignition just kinda happens on its own.
I had a serious problem with the Scrambler. Without the doors and half cab it only weighed 3000 pounds with a full tank of high test, even though I had a 200 pound cast iron tranny and a heavier 350 Ci V-8. The problem was wheel hop under hard acceleration and this could lead to twisting the leaf spring so much that it would snap violently and cause a crash.
So tuning carbs and understanding how vacuum plays an important role in efficiently and power. An engine is nothing more than a pump.
My Scrambler beat a SVO Mustang in a drag race. The Mustang driver was surprise and annoyed. He wanted a rematch. I only needed two gears, while he needed three. Also to be fare I gave him the lead (head start). The Corvette engine had so much torque that I could start off in second gear at a stand still, and then redline the engine to 85 MPH.
I had an amazing tranny in that Jeep, a NV4500, which is basically a 3-speed tranny with a 6.32 under drive and an overdrive. With the gear multiplication of 2.6 from the transfere case, and the 3.50 gears in the cut down Ford 9 inch rear I figure I had 60:1 gear multiplication just in case I wanted to nose up into a telephone pole and push it down into someone’s living room who I didn’t like.
So if you were a car, and since you are gifted having a very high VO2 Max, you would be a car with a supercharger that can sustain high outputs for extraordinary amounts of time.
Snarky Joe would kinda be a truck with a diesel because he has lots of horsepower and torque.
If I were a car I might be a Porsche 550 Spyder a car that raced in 24 hours at Lemans in the late 50’s. It is the same car that James Dean got killed in. Weighing only 1250 pounds it had a 220 HP dual overhead cam “pancake” engine and pretty much because of its light weight performed like a go cart with great acceleration, handling, and braking.
So now the “Green Mean Machine” (C-10) is pretty much an old geezer that looks good still and held up well. Kinda funny how I personify cars and trucks.
Cal