Building the winning Engine Masters Challenge motor means successfully applying theoretical knowledge. This sounds reasonable, but there are stumbling blocks-two, in fact. First, the theory has to be right, and secondly, it has to be functionally applied. Assuming the smarts for the first part, the second does not seem too difficult until a specific component is found not to function quite as well as hoped. At this point, things can get complex real fast. It's here that the difference between a totally theoretical engineer and a practical engineer really shines. If you've followed the EMC through the years, it's clear that whoever wins has to be well versed in both theory and practice. What we're going to do is start with applied theory and see how far it will take us before we run into a problem that might derail a winning combination and force us to make a hardware compromise to get better results.
This Year's Rules
The way this year's rules are structured means the contest boils down to a power-percubic-inch race. In simpli-fied form, the contestant can build an engine from 300 inches up. If it's just output we are looking at, then big inches has the best chance of winning, but if it's output per cube, then a whole different scenario applies. Just so you know where I'm coming from, I can be pretty specific about the application of the theoretical side based on experience. Quite a while back, I did a project for Chrysler UK which involved developing a 90-cube four-cylinder engine to optimize power bandwidth, driveability, and output. The car was tested by two magazines and deemed outstanding. The powerband was steam-engine like, starting at 400 rpm, and going all the way to 8,000 rpm. That said, let's start with the theory behind why smaller cylinders make more power per cube and work our way through the rest of the engine's components to see what's most likely optimal, and why.
The starting point for maximizing power should always be at the cylinder heads. shown here
Small Vs. Big Cylinders
At the end of the day, a successful high-output engine is all about geometry. That geometry has to most effectively allow the engine to inhale the maximum amount of air per cube, mix fuel into it, compress it, and burn it. At this point, the pressure in the cylinder must be transmitted efficiently via the connecting rod to the crank.
The first point on our list is inhaling air and how geometry affects that. let's assume as a starting point we have a 4-inch bore. For most practical purposes, the combined size of the valves in a typical 4-inch bore V-8 is 91 percent of the bore size. This is distributed as 51 percent for the intake, and 40 percent for the exhaust. using a stock bore and stroke 5.0 Mustang engine (302 inches) as a starting point, we find that this works out to a 2.04-inch intake and a 1.6-inch exhaust. If we scale up the bore and stroke by 25 percent, we now have an engine with a 5-inch bore and a 3.75-inch stroke, for 589 inches. At the same proportions, the intake valve size will be 2.55 inches, and 2 inches for the exhaust. At this point it looks (at least at first sight) like all the proportions have stayed the same, so nothing has changed in terms of cylinder size and breathing capability. unfortunately, nothing could be further from reality. The problem is that the valve diameter may have gone up proportionally, but the volume it has to feed has gone up by the cube. The result is we have valves 25 percent bigger, but the cubic inches have gone from 302 to 589-that's almost double!
Bore here are the typical proportions of valve sizing in relation to the bore diameter. W
At this point, someone is going to argue that it should be the valve area we have to consider. Namely, for a valve made 25 percent bigger, the area goes up by 56 percent, not 25 percent. unfortunately, the valve area does not come into play here in quite the way we might expect. The breathing capacity of a valve is dictated by the curtain area, and the curtain area for a given valve lift goes up in the same proportion as the increase in valve diameter. The extra area of the valve is not realized until it has been lifted higher than 25 percent of the diameter of the smaller valve we are considering. The bottom line is that the bigger the cylinder, the less valve breathing capacity we can get per cube to be fed. so at this point, assuming we can optimize everything else, we want the biggest valves possible in the smallest engine allowed by the rules.
Heads For Torque
Still on the subject of size here, what we should now consider are the port and valve sizes needed to make the most output between 2,500 and 6,500 rpm. First, let's dispel the myth about sizing valves for a motor that must make maximum low-speed output as well as high speed. smaller valves don't make more low-speed power than big ones. If a back-to-back dyno test shows otherwise, it's because the big valve situation was not optimized. If maximum output is the goal, the valves need to be as large as possible within the confines of the bore. What is critical is the sizing of the ports. Get this a little too far off and an attempt to win an EMC could be dead in the water.
The curtain area as shown here is the most important factor in an engine's entire chain of
Starting at the intake port, we need to develop the port so that it has the minimal cross-sectional area to meet the 6,500-rpm needs. Achieving a minimal cross-sectional area, along with enough flow for the top-end output means having a port that flows big numbers because it is efficient-not big. That port velocity is not just important for low speed-it's vital. here's a good clue to what is needed. When the engine's peak power rpm has been dictated by a requirement (just like it often is by the restraints of a street car budget), then the port size needs to be optimal for the cylinder it has to feed, not the valve-and there is a difference. When developing the port, it should have the highest average velocity possible and the least amount of velocity difference between the slowest and fastest part. The port should direct flow past the valve into the cylinder in such a way as to generate good, but not excessive swirl.
As for the valve and seat form, this needs to deliver strong flow right off the seat to a point a little higher than the valve lift to be used. If the flow continues to increase past the max lift point, it's a sure-fire bet the port is too big! As for low-lift flow (this is where I get to stir things up), this needs to be as good as possible. here is not the time to debate the subject in full, but the stronger a valve flows at low lift, the more the cylinder thinks it has a higher acceleration valvetrain. ultimately, strong low-lift flow ends up being the next best bet to variable valve timing with no extra moving parts. To make it work means reevaluating the valve event timing. Typically, the lobe centerline angle (lCA) needs to be widened, and the duration shortened.
Swirl Generation Cylinder Wall Good swirl generation is an important factor toward low-s
At this point, we have to consider the combustibility of the mixture. This inevitably means checking out our porting work on a wet flow bench. A wet flow bench will always show problems. It's the nature of the beast, but the real trick is determining whether or not there are any big problems. Making the best of a fuel combustibility situation starts at the carb booster, and works its way to the combustion chamber. Minimizing the port size for the job in question is a good move toward a more ignitable and combustible mixture. As the charge enters the cylinder, it is important for it not to wash out the spark plug. In this context, the placement of the plug is critical not only to avoid too much wet fuel flow, but to minimize detonation. here we find that the combustion chamber's ability to resist detonation can be affected for better or worse. The charge is always hotter on the exhaust side of the chamber, so it burns faster. Also, detonation is also more likely to occur on the exhaust side because it is hotter. By biasing the plug toward the exhaust side, the charge is burned there first, and the flamefront progresses from there across to the cooler side by the intake. since this side started off cooler, it is less likely to detonate. All this may look easy to do, but in placing the plug, it has to be remembered that the charge and the flame-front are rotating, so everything is not quite as obvious as just looking at it and saying, "let's stick the plug here." This is where transparent cylinders and in-cylinder pressure measurement can really pay off. This used to be tech that only the big car companies could afford, but times have changed and top engine builders are now in a position to use such equipment. Then there is computer simulation of the burn characteristics, but that is a whole other story in itself.
Having burned the charge effectively, it now has to be expelled as effectively as possible. This brings us to the exhaust port, which in many ways needs to have many of the same attributes as the intake. An important one is that the port should not only have good flow, but also good anti-reversion properties. This is usually achieved by making sure the port is no bigger than it needs to be for an engine that will peak at 6,500 rpm. Making sure of this is a good move toward better low-speed output without any compromise in top end.
For an intake manifold, a single-plane is probably the way to go if the cam ultimately used is not too big. runner cross-sectional areas will be critical, as will the size of the carb and the booster selection. Making sure the carb is no bigger than needed will be the best bet toward good results at the 2,500-rpm checkpoint. A well set up vacuum secondary could also be the key to success here.