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Old 05-05-2005, 09:02 PM   #1
Join Date: Sep 2001
Location: At the end of the longest line
Age: 38
Posts: 7,454
Tech File: The joys o' N/A engines

Not a lot of sport compact fans out there ever consider going the naturally aspirated way to their performance goals. This is a justified line of thinking since it can inherently difficult to extract large amounts of power from a smaller motor without the aid of forced induction. However, for those who are looking to build up motors for N/A operation or to fortify your motor for boost/nitrous, here's a few tips for you to get all you can from the motor.

Interchangeable parts

It's a simple rule of economics that most automotive companies won't make every engine unique. It's too expensive to re-tool equipment when it's easier to just make certain parts that can be swapped between two engines. Take Ford's "modular" engine family. The name "modular" is derived from the fact that the engines are specifically designed to make mixing and matching parts easier. It not only helps you by making parts selection easier, but the company saves money in production costs.

For instance, the B16, B17, and B18 motors all share the same 3.189 bore and even share the same piston compression height of 1.181 (save the B18C5 which has a compression height of 1.195). What does this mean to you? B18 pistons can be used in a B16 block. The real difference in displacement comes from the crankshaft. It should be noted that rod lengths in the engines vary so you can't expect the components to operate exactly the same way if swapping them between engines. Also, it's noteworthy that just because one part will work in a given engine doesn't mean that the flipside is true. For instance, just because B18 pistons will go into a B16 block, it DOES NOT necessarily mean that B16 pistons will go into a B18 block. Just do your homework before attempting such mechanical cannibalism.

Rod selection

This is yet another automotive instance where bigger is not necessarily better. A lot of people out there are willing to fork over money for bulky H-beam rods in typical daily driver performance applications. This just isn't logical. Look...the truth is that if your name isn't John Force and you're not putting down 1,000hp+ you don't need an H-beam rod or the weight associated with it. Ever see a building under construction? Those aren't H-beam girders they're using to build the frame. They're I-beam girders. I-beam rods for the most part are lighter and stronger than a comparable H-beam rods. Of course, as a general rule in engine construction, lighter is better since it facilitates faster engine response.

Rod-to-stroke ratio

Oh boy...rod/stroke ratio. This is still a matter of debate amongst engine builders and bench racers. I think it's fairly safe to say that a better rod/stroke ratio is, at the very least, condusive to reducing friction. Aside from that, no one can really come to a concensus as to it's power production potential (try saying that 3 times fast). A rod to stroke ratio is made by simply dividing the rod's length to the throw of the crankshaft (or the "stroke"). A shorter rod will produce a higher rod angle, while a longer rod will produce a lower rod angle. What does this mean to you? A higher rod angle will side-load the piston more increasing friction on cylinder walls and lead to premature wear. Some argue that since a longer rod forces the piston speed (which we'll get to shortly) to increase, a longer rod gives you more torque potential (especially at higher rpm) by forcing air more air out and drawing more in since your motor is essentially a large vacuum pump. People will argue that point so it's not something I'm willing to claim as an outright fact, but it seems to have some merit. Care should also be taken if and when you pick a longer rod since the extra mass of the longer rod obviously adds weight to the reciprocating assembly. For math freaks, a retired aeospace engineer named L.E. Mayfield has come up with this equation:

M(x) = F * r*

sin(x)*(1-((r/l)sin(x))^2)^1/2 + cos(x)*sin(x)*(r/l)


M(x) = torque (ft-lbs)
F = Piston axial force (lbf)
x = crank rotational angle (radians), evaluated from TDC to
BDC (0-180 degrees)
r = crank throw (inches), stroke/2
l = rod length (inches) pin centerline to bearing journal center line

If you can figure that out (or happen to have a math CAD program), knock yourself out. It's all greek to me.

Piston speed

Piston speed is also another important thing to consider since it can effect the wear your engine will encounter. Just as the name implies, piston speed is thte speed at which the piston moves up and down in the cylinder. For instance, when the piston reaches TDC or BDC it momentarily remains stationary, so it's speed is zero. Maximum piston speed is reached at about the midway point in it's stroke. Ignoring rod angularity, the maximum speed is simply the speed of the centerline of the crank pin. So, on an engine with a 4 inch stroke, the crankpin is traveling in a circle 4 inches in diameter, so it will have to travel 4 x pi inches (sorry...I haven't a clue how to type the symbol for pi). This results in about 12.6 inches or 1.05 feet. If your engine is turning at 7,000rpm then the piston will travel about 7,000 x 1.05 = 7,350 feet per minute. I suppose it's worth noting that 7,350 fpm is about 83 mph. Normally, however, piston speed is refered to as mean piston speed. How far the piston travels in one minute is simply twice the stroke, and how often it makes the return stroke is a matter of engine rpm at a given moment. So the equation for mean piston stroke is as follows:

Vp = S x rpm / 6


Vp = mean piston speed
S = Stroke (in inches)

So if you use the example given, you get:

Vp = 4 x 7000 / 6 = 4,667fpm (or roughly 53mph)

What does it all mean? Well...there's no doubt that pistons account for most of the friction inside an engine and increasing that speed will only increase friction. Many older textbooks imply that there's more to it...like that there is a limit to piston speed (usually around 4,000fpm) and after that you're just asking for trouble. I'm not sure I can lend any support to that, but hey...they're experts (or they were if they're not still alive). Increasing piston speed also increases loads on associated parts since the piston has to start and stop and the bottom and top of each stroke. Like the fall from a tall building, it's not the speed that hurts, it's what happens when you stop.
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