In
my previous post, I implied diving into a project, starting with the engine, is usually a mistake that frequently leads to less than satisfactory results.
Often, the root cause of much difficulty originates before anyone picks up a wrench or screwdriver, orders or purchases parts, or talks to the machine shop. All too often, people dive into a project without a good idea as to what they're trying to accomplish.
To avoid this problem, and increase your chances of success, you need to sit down and
honestly think about what your needs are and what the vehicle needs to be able to do to satisfy those needs.
If you take this crucial first step and manage to get things right your first time out of the gate, congratulations, you've done better than me. (People who successfully complete projects without apparently taking this initial step simply know themselves well enough that they complete this step without conscious thought and effort.)
My current project is just the latest (and hopefully final) iteration in an effort that spans the last 45 years. Initially, my goals were vary vague because I really didn't know, and hadn't really thought about, what my needs and wants were. Experience over that time, via a wide variety of vehicles, allowed me to steadily refine my goals. However, the process of buying and selling various vehicles to figure out what I really wanted was unnecessarily expensive and time-consuming. (45 years! I'm sure nobody else, especially younger people, want to spend that much time trying to get things right.)
On my current project, one of the goals is to squeeze the maximum possible fuel economy out of the motorhome, without giving up a reasonable amount of power or reasonable speed. For the purposes of this project, reasonable amount of power is defined as being able to climb Turnagain or Thompson Pass without having to slow down significantly or having to downshift more than one gear. Likewise, reasonable speed is defined as being able to cruise for several hundred miles at speeds of 55 to 65 mph.
Well prior to buying the new heads, I picked the brains of many people whose opinion I respected to get a general idea as to how to proceed. These discussions included engine alternatives (318, 360, 383, and 440) as well other modifications that would help me achieve my fuel economy, power, and speed goals.
One person in particular, the manager of the local Schuck's/CSK/O'Reilly/whatever-their-name-is-this-week parts stores, provided a lot of guidance based the decades she spent in the southern California car culture. She recommended staying with the 318 I had with little or no internal modifications. (She even recommended I run tests to see if I really needed to rebuild the engine.) She also recommended switching to a small four-barrel carburetor, dual exhaust (although she said I should stay with the same size pipes), and consider Tri-Y headers. (She told me to stay with the stock exhaust manifolds is I couldn't find Tri-Y headers, pointing out that most readily available headers weren't really suited to my application and goals.)
Note: Once I decide to stay with the 318, different unleaded gas heads were a given ... I just happened to get lucky when I stumbled across a set of unused military-surplus heads on eBay. I only mentioned buying them (above) to provide a reference point in terms of the project timeline.
Once I established a basic framework to serve as a starting point, I began exploring various engine modification and component alternatives to narrow down the range of possibilities.
That's when I ran into significant problems. No matter how much I searched and read, I couldn't find anything that gave sound indications whether any of the possibilities would help or hurt my efforts to achieve my goals.
That's when my systems analysis and design experience kicked in. Difficulties such as I was experiencing is a good sign I'm approaching the task from the wrong perspective. Simply put, whenever you find yourself in a quagmire of seemingly endless possibilities, without any sign of clear path out, it's an indication you're approaching the problem from the wrong angle.
A technical article on designing and building race engines to ensure they ran at RPMs close to the peak torque in their power bands provide a glimmer as to where I was going wrong. Engine torque follows a curve, which is part of the power band, that varies from one engine configuration to the next. (As engine RPMs increase, its torque also increases, up to a certain RPM, after which the torque drops off as RPMs continue to increase.) The rounded peak, located somewhere along the engine's RPM range, indicates the RPM at which the engine operates the most efficiently.
Suddenly the task got much easier. All I had to do was make sure the combination of engine components and modifications put the peak torque close to the RPM at which the engine was expected to operate.
The article addressed setting up cars to help win bracket races, so the discussion on each of the examples started with a target speed and tire circumference. (For those of you who were asleep in various classes, tire circumference is the distance around the outside of the tire.) The discussions then went on to include rear axle ratios and transmission output ratios (typically 1:1) to round out a set of calculations to match up peak torque with engine RPMs.
I quickly recognized the set of calculations in this article are effectively identical to those I had previous used to determine proper speedometer gears for various combinations of tire circumference and rear axle ratios.
I think I'll stop here for now. In my next post, I'll wrap this up with by walking through the calculations, as well as decisions on engine configuration that I believe will give the best chance at achieving my goals.
Before closing, the more knowledgeable people out there will realize I drastically simplified a complex discussion, so save your breath and resist the urge to nitpick.
For others, I recommend doing some research to verify what I said above. After all, I've been married for over 38 years so I've obviously been proven to be wrong at times. :p
Finally, for those who are wondering why I'm going through all this trouble for what may turn out to be only a few mpg improvement in economy, consider this is just one aspect in a complex project. Also, consider engine manufacturers have to design their engines to suit the needs of a large, diverse group of customers. On the other hand, I have a rather limited need and can build an engine intended to suit that specific need.
