Engine wear and failure issues:
* Rod angularity affects piston side loading. Reducing the maximum rod angle (a) will reduce side loading.
* Reducing piston acceleration from and toward TCD (point of maximum acceleration) reduces tensile loading of the rod.

Engine ignition and breathing issues:
* A piston that dwells at TDC longer allows the air/fuel charge a longer time to burn. This allows less ignition timing for peak power. Less ignition timing is useful because it reduces detonation allowing (slightly) higher compression ratios.
* A piston that dwells at TDC shorter increases the speed of the exhaust gasses during the overlap period. This increases the scavenging effect at low rpm and the engine makes more torque at low RPM.
* Reducing and delaying peak piston velocity allows the intake valves more time to open more to fill the cylinder. This allows a smaller intake running volume (and plenum) and better high RPM breathing.

Effects of a longer Rod
* Less rod angularity reduces wear.
* Lower piston velocity and acceleration reduces tensile loading of the rods.
* Less ignition timing is required which resist detonation.
* Compression can be increased slightly before detonation is a problem.
* Less intake runner volume is required and high rpm breathing is improved.
* Reduces scavenging at low rpm (weaker low RPM power).
* Longer TDC dwell time. (high RPM efficency).


Effects of a shorter Rod
* Increased rod angularity increases wear.
* Increased piston velocity and acceleration increases tensile loading of the rods.
* Increases scavenging at low rpm (increased low RPM power).
* Reduced TDC dwell time. (Reduced high RPM efficiency).


Stroking an engine is when you increase the stroke of the engine at the expense of rod length. When stroking an engine the rod length is decreased because the deck height is constant.

Effects of stroking an engine
* Increased displacement.
* Increased rod angularity increases wear.
* Increased piston velocity and acceleration increases tensile loading of the rods.
* Increased scavenging at low rpm (increased low RPM power).

Stroking an engine can be a very effective way to make more power but close attention should be paid to piston velocities and accelerations. Stroking an engine is a "double whammy" on increasing the piston velocities, stroke is increased and rod length is reduced.

Yup, which is why increasing bore is the way to go for increasing power and also important, durability! I use a 4.15" stroke and also use the longest BBC rod I could find/fit... 7.1"!

this is gonna get good

Don't most stroked motors use a longer rod.

I'm in the camp whereby compression height rules over rod length , get ring placement right then get a rod to fit.

I have a 4" stroke with 6.123 rods in a smallblock. The pistons are shorter, not the rod.

Good info on rod ratios - Thanks for posting. Obviously wear is an issue, but the real issue with stroker motors is piston speed. That's what seperates the wear from the - "breakage". The formula: mean piston speed ( ft per min )= stroke X 2 X rpm divided by 12. and the rule of thimb is: Factory cast stuff = 3,750 ft/min, aftermarket cast =4,500, factory forged = 4,600, budget aftermarket forged = 4,800, Race aftermarket stuff = 5,500, high $ custom endurance race = 6,000, pro stock = 7,500. As an example, I am running a cast crank in my 408 with Eagle I-beam rods and KB Hypo pistons. Pretty much bottom feeder performance parts. I shift at 6200rpm. That gives me a piston speed of 4,133 ft/min. Theoretically, I should be safe. A lot of room for interpretation here ( I can already read the posts of those reving twice that high with stock stuff and having no problems ) and the numbers could be a little conservative. I think it's good ball park info though - and maybe it can keep you away from the " look what I broke" posts.

piston speed will be the same at any given rpm, irrespective of rod ratio, it's the rate of acceleration that will change with shorter rods.

rod ratio is relevant to stroke. a large stroke BB will struggle to obtain even stock rod ratio given deck height and piston compression height limitations.

The difference in piston position B/ATDC with even large changes in rod ratio are very, very small. I have yet to read an analysis that explains the changes in engine function.

Example 1: 4" stroke, 8" rod = 2:1.
In 10° rotation, the piston moves down .0379".

Example 2: 4" stroke, 6" rod = 1.5:1 (25% lower ratio).
In 10° rotation, the piston moves down .0404".

The difference is 2.5 thousandths, or 1/16th of 1% of the stroke. BFD.

It seems that everything is some form of trade off, Big cubes with a lot of stroke vs big bore and more rpm, the key is finding components that will live thru it, which ever way you choose to go

The rod's job is to connect the piston to the crank. Build the stroke you want for the cubes you need, find a piston CD that is liveable and order the rod accordingly. Nuff said

I thought that short rod ratios/ longer strokes provided more push on the crank pin @ a better point in the combustion cycle

Agree, rod just connects piston.
Nascar had to institute min. deck hgt rule to keep from getting out of hand.
the liter the assyembly the better.
the shorter the rod, less wght, more rpms.
as long as block can take it, no problem.
Till they put counterweight cheeks into ringland. Heard something like that.
Would like to know more about it.

Right, those annoying auto manufacturers have no idea what they're doing!
Why don't they follow your advice, and all use whatever rod they have lying around...

Hint: when designing a new engine, the deck height is frequently the last choice. Does that tell you something?

I tried that once in a desk-top program.
I need a 6" stroke in an LA motor, with 1.10" minimum piston CD, so I need a 5.50" rod, right?

The piston will separate the first time you start the engine.

This is from my HEMI book:


The infamous “Rod Ratio” and why use a longer rod

What the heck is a “rod ratio” and why do I care about it? Good question. The rod ratio is the center-to-center rod length divided by the stroke.

There’s a lot of talk about this rod ratio thing, but if you think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance - think again. The most important thing about a connecting rod is whether or not the rod bolts are torqued! You don’t design an engine around a rod ratio. For a high performance hot street engine you want cubic inches, which is the easiest to get from stroke. Maximize the stroke and bore, then make the compression height large enough to keep the wrist pin out of the oil groove and that determines your rod length. End of calculation!

Nevertheless, since curious minds want to know, so lets examine this phenomenon:

The rod ratio determines the length of time, or dwell, at TDC. As the crank rotates through TDC, a short rod will be at a greater angle than a long rod so as the crankshaft rotates, the piston will be pulled away from TDC faster. So the longer the rod, the longer the "dwell", and the longer the dwell, the longer the piston sees pressure. In other words - long rods are good. Right? Obviously the length of the rod is limited by the deck height, stroke, and the compression height of the piston.

A longer rod reduces the rod angle and less rod angle will reduce piston side loading. A longer rod also gives better average leverage on the crank for a longer period of time resulting in a flatter torque curve. A shorter rod will yield a higher peak torque but the torque won’t last as long.

Short rod ratio engines tend to be “peaky” which can be desirable in an oval track car. Long rod ratio engines have a flatter torque curve, and generally desirable for drag racing and hot street applications. So if long rod ratios are good why are so many pro-stock engines, and production engines, built with short deck blocks, which would limit the rod ratio? Simply because the shorter the deck the stiffer the block, and shorter pushrods mean less stress on the valve train. In other words, the general stability of the engine outweighs the importance of the rod ratio.