regarding the article on rod length...if they really wanted to see if it makes a difference, all they had to do was compare a 351C and 351M - both have the same cleveland heads, you could find similar manifolds, etc. 351C = 5.7 rod, 351M = 6.5 rod (room for 7!). That is more that just a little different and I bet would have provided for a more meaningful test. Think they would re-run it...in the name of science?

 

doubt it....but to provide completely accurate results everything would have to be the same...but if things like the cam lift/duration, lobe seperation, valve timing, ignition timing and port size were all the same, the tests would be bias to whichever engine they were designed for. Naturally longer rods are going to be able to rev higher, so with the ability to rev higher you can tune the engine to make the maximum power up there. If you have an engine that can spin to 8000rpm safely, you can tune it to operate in the 4000-8000rpm range where you are definately going to make more power than the same sized engine with a 2000-6000rpm power band. Horsepower is, IMO, how much effort the combustion has, and torque is the mechanical engery created by the rotating assembly using the horsepower as its energy source.
Equal amounts of air/fuel mix take the same amount of time to combust. Higher rpm requires more mix, more mix takes longer to burn all the while releasing more energy. The longer burn creates a longer more powerful downforce on the piston during the power stroke, making more power naturally. So now factor in it takes less time from the piston to go from TDC to BDC because of the faster engine speed, you are now distributing that power more equally throughout the power stroke. Therefore it takes less time to make the power stroke, and youre doing it with more potential energy, which is why N/A vs N/A a longer rod engine will make more power.
....and all that is why I now say comparing a long rod engine to a short rod engine is like comparing apples to oranges.

 

I don't think there is one correct rod ratio for every combonation. The rod ratio is just one factor among many. If your building a high rpm motor, then a longer rod is prefered, but is there a point of diminishing returns? A longer rod weighs more. Excess reciprocating and rotating weight is a bad thing in a high rpm motor. A shorter rod is stronger in shear, provided the reciprocating geometry is okay. A lesser rod ratio has better volumetric effciency at lower rpm. Beyond a certian point, is the potential increase in horsepower per cubic inch at high rpm with a longer rod, worth the extra weight, and the other trade offs?

The Chevy 302, code Z28, had a 1.90 rod ratio, but it didn't outperform the Boss 302 with a 1.73 rod ratio. Actually the Boss outperformed the Z28 in race tune. Cylinder head design...ect.. are probably of greater importance.

Why did Ford go with a 1.63 rod ratio on the 351C, down from the Boss 302's 1.73? It's also less than the 351w's rod ratio. They may have considered that with the extra 50 cubic inches, the 351c could make the needed HP at lower, more streetable, engines speeds. However, the Boss 302/351C heads had poor low rpm port velocities, and a weak signal to the carb at low rpm. A lower rod ratio would help the huge Boss heads work better at lower rpm.

The 289 does make more HP per cubic inch than the 302 does, all else being the same. This shows that the higher rod ratio is more efficient at producing HP. However, even the 302 has more lower rpm tourqe than the 289. This illustates the trade offs on virtually identical engines with different rod ratios, but only 13 cubic inches different.

Usually a longer rod results in a more durable engine.

 

There are so many variables involved, rod length is difficult to discuss. But in general, some principles that reamain the same are: Rod angles. A longer rod has less extreme angles, both to the pin and to the crank. Because of this there is less frictional loss between the piston and wall. The combustion pressure is exerted more directly onto the crank. Also, because of the difference in angles, with the long rod, at mid stroke the piston travels faster. The higher piston speed forces the exhaust gas out through the port with greater velocity and the resulting scavenging effect actually helps to draw in the intake charge during the tdc dwell time. After tdc on the intake the higher piston speed creates a greater vacuum in the cylinder so the intake charge is accelerated into the chamber.

Certainly you can alter the breathing by improving flow through the ports and with different cam grinds, but I cannot see any way a shorter rod is going to result in greater torque or horsepower. The longer rod will always have the advantage of more direct application of pressure to the crank and an advantage in breathing AND less fricional loss.

 

I agree with almost all your points Bdox, and they are good points too.

To take better advatage of exaust pulse scavanging during the TDC dwell period, a long rod motor may be better served with long tube headers and a high overlap cam.

Shorter rod motors do get good cylinder filling at lower engine speeds, because the piston travels farther down the bore from TDC to 90* crankshaft rotation.

 

I just would like to throw in some reality. All the comments so far are very valid, but basically are not very feasable economically in anything but an all out race motor. On a street motor, I would spend my money elsewhere.

 

Good point WW. But it is also true that you can often get a good combination just swapping around some stock parts for a street motor.

 

true..........

 

I wouldnt get crazy on a street motor....but a properly built shortblock is definately the foundation for any type of performance oriented engine.