# Rod Stroke Ratio

Three variables affect piston movement: Crank Angle, Stroke, and Rod Length.

Four variables affect piston velocity (and acceleration): angular velocity, Stroke, and Rod Length.

The mean speed of the piston is determined by RPM and stroke.

Mean Piston Speed = 2* stroke * RPM/12 (ft/minute)

This number is simply the calculated average speed of the piston. If the piston(s) were moving in 1 direction (or in a perfect circle) at one speed this would be that speed (circular velocity). This simple calculation can be helpful in quickly comparing average piston speeds and the affects of stroke on piston velocity. Unfortunately this calculation is often misused by the uniformed and used as proof that Rod Length (Rod Stroke Ratio) does not affect piston speed. We know this is not the case.

To understand the motion of a piston it is helpful to visualize the actually problem. Below we have a diagram of a piston, rod, and crank.

P is the y axis position of the piston (blue line)
L is the rod length (black line)
R is ½ the engine stroke (yellow)
q is the crank angle
a is rod angularity Figure 1: Piston, Rod, & Crank Diagram.

We know the stroke and rod length so for any given crank angle we can determine the piston position using the law of cosines.

P2+R2-2 RP cos(q) = L2 (Law of Cosines)

So P = R cos(q) + (L2 - R2 sin (q)2) 1/2

To find velocity and acceleration of the piston we need to convert the crank angle (q) to angular velocity (w) and time (t).

q = wt

for simplicity sake RPM = U

This gives us an equation for position as:

P(t) = R cos(2pUt) + (L2 - R2 sin (2pUt )2) 1/2

To determine the position at q degrees (p) insert a time of 60s/((U)(360/q))

To determine velocity and acceleration we need to derive the position formula.

V= d/(dt)(Position)
A= d/(dt)(Velocity)

```
V(t) = -R sin(2pu)pu  - 2 R2sin(2pU)cos(2pU)pU
(L2 - R2 sin (2pUt )2) 1/2

A(t) = -4 sin(2pu)p2u2 - 4 R4sin(2pU)2cos(2pU)2p2U2  - 4 R2cos(2pU)p2U2      + 4 R2sin(2pU)p2U2
(L2-R2sin(2pUt)2) 3/2          (L2-R2sin(2pUt)2)1/2   (L2-R2sin(2pUt)2) 1/2
```

OK, so what does this calculus mean?

It means BOTH stroke and rod length affect velocity and acceleration of the piston.

If you increase stroke, you increase piston velocity and acceleration.
If you reduce rod length, you increase piston velocity and acceleration.

Rod stroke ratio (RS) is often used to describe an engine's piston motion. RS is just the Rod Length/Stroke. Rod stroke ratio is important because it determines wear, velocity, and acceleration of the pistons. From figure 1 we can see that RS ratio directly affects rod angularity (a) along with Rod length and Stroke (definition of RS). Figure 2:Acceleration Rod Stroke Ratio comparison. Stroke constant at 3.5", Rod length (5" & 7")

Note: All velocity and accelerations graphs in this article were created using 8000 RPM.

Above we can see how rod length affects piston acceleration. The graph details the piston acceleration for two engines on with a 5" rod and the other with 7" rod; both with strokes of 3.5". Note the graph is plotted in g forces. The G's on the pistons is fantastic. From the graph you can see that the negative acceleration after TDC is the greatest. This explains why Rod failure often occurs at the point. Figure 3: Piston Velocity Rod Stroke Ratio comparison. Stroke constant at 3.5", Rod length (5" & 7")

Above we can see how rod length affects piston velocity. The graph details the piston acceleration for two engines on with a 5" rod and the other with 7" rod; both with strokes of 3.5". The short rod engine (blue) achieves a higher peak velocity and achieved the velocity sooner (which affects engine breathing).

Engine wear and failure issues:
* 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.

Figure 4 & Figure 5 below demonstrate this double whammy effect on piston acceleration and velocity. The comparison used below is between a Honda B20 block and a B16A block. Note: that the block height of the B20 is actually 7mm taller than the B16a. A B16a block with the B20 crank would have even slightly higher piston velocities. Figure 4: Rod Stroke Ratio Acceleration, B16a (blue) vs B20 (red)

This graph clearly illustrates why engine wear is increased and rod failure is possible if rev limits are pushed to far with B20 blocks. The g forces the rod experiences are significantly more (~20% more) at the same RPM. A lot of misinformation exists on the web about the effects of RS ratio and Rod Length's effect on piston velocity. We at FTLRacing hope you've found this article useful. Further discussion of Honda hybrid (stroker) engines can be found in the Honda Engine section.