Reply With QuoteBrian, that was a great post. I'd like to offer some of my experience, here if I may. While mechanically, you're dead on, there are other issues regarding rod length, and rod/stroke ratio "n" that play in, here, that I feel should be adressed. Not to say which is better or worse, as that debate has, and will go on forever, just offering more info. Mind you, some of this will be cut and paste to save me hours of typing.
The length of the rod is less an issue than is the ratio. There is a small range of ratios for most conventional piston engines: the rod is going to be between roughly 1.4 and 2.2 times the stroke length. The rod can't be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and, although used for racing,
aren't real practical. A greater rod angle will occur by installing a shorter rod or by increasing the stroke (smaller value of n). A reduced angle will occur with a longer rod or a shorter stroke. (larger value of n) When the n ratio value becomes smaller, it has several effects. This is a brief summary of the mechanical effects.
(here's the cut and paste)
Motors with low values of n (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high n motors):
» physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance)
» lower block weight (short deck vs. tall deck)
» higher level of vibration
» shorter pistons, measured from the pin center to the bottom of the skirt
» greater wear on piston skirts and cylinder walls
» slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes:
» intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
» on the negative side, a small or badly designed port will run out of breath sooner
» piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected:
» earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation
» the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)
Here's some pros and cons regarding long vs/ short rod. (more cut and paste)
Long rods
Pro:
» Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
» The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
» For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.
Con:
» They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshafts rotation.
» Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
Short rod
Pro:
» Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
» The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
» Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.
Con:
» Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
» Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.
Now you know why you see more Fords (typically with "n" numbers in the 1.7 range like the 289, 302, 351W, 429/460) with smaller ports and carbs as compared to GM products, which have few engines with anything higher than 1.6. (The 327 has an "n" of 1.76, 396/427 has 1.61, and the 454 has a 1.53)
I'll continue this more later.
Ah hell Scott ... oh nevermind!
This is an open thread asking for information and ideas, most people (I believe) will try to give you what they consider "good" information. Now it's up to you to research this information and ideas to decide if YOU think its feasible. 