The length of the rod is depicted as L1 in the figure. Similarly the length of the crank arm is denoted by L2. Observe that L2 is not equal to the stroke. The stroke is in fact twice L2.

The ratio of the rod length to stroke is called "rod ratio" and is a useful term to quantify the kinematics (relative motion) of the piston as the crank completes a cycle. Rod ratio can also be used to quantify the dynamics of piston motion (the relative forces) but that is for another article.

The equation for the rod ratio is as follows: Rod Ratio = rod length / stroke 

The total distance that the piston moves down the bore is solely determined by the stroke of the crank. But both the speed, and the acceleration of the piston are dictated by the rod ratio. The piston speed and acceleration can have numerous effects on the performance of an engine. The velocity of the piston (it's speed) can be important in determining how the intake charge is pulled through the ports and past the valves. A fast moving piston will pull harder on the ports, creating a larger Delta-P to "suck" air into the cylinder on the intake stroke. Here one might think of correlating the point of maximum piston speed to the point of maximum valve lift for example.

The acceleration of the piston on the other hand, leads to forces on the rod and main bearings, as well as on the wrist pin. These forces put a limit on the rpm's that the bottom end of the engine can reliably withstand. The rod ratio also determines the "dwell-time" of the piston at top-dead-center (TDC) during combustion. This means that the position of the piston relative to the point at which maximum combustion pressure occurs can be altered through changes in rod ratio. This could be used to try to correlate the point of maximum combustion pressure to the point at which the piston has the greatest mechanical advantage on the crank for instance. In fact, the very nature of the combustion process can be affected by the position of the piston, and how long it dwells at TDC. These are all interesting topics, but in this article we restrict ourselves to an investigation of piston displacement and velocity, and we concentrate mostly on piston acceleration.
 

The equation that governs piston position along the bore is readily determined. A "back of the envelope" derivation is shown in the following link: 
Click here for a derivation of the piston kinematics equation... 

Once the piston displacement from TDC is known, it is a relatively simple matter to determine the piston velocity and the acceleration. This was done via an Excel spreadsheet which generates curves of piston position, velocity and acceleration as a function of the crank angular rotation. A case with all three curves shown together is presented in the following link. Note that the piston acceleration was divided by 1000 to keep it on the same scale as the other two curves. The situation modeled here is an Evo III spec S14 engine with 87 mm stroke and a 144.25 mm long rod: 
Click here for piston displacement, velocity and acceleration curves... 

Observe that the velocity and acceleration curves are not perfect sinusoids. They approach being perfect sinusoids as the rod is lengthened (increasing the rod ratio). Also, the maximum piston velocity occurs well before 90° ATDC. Furthermore, the maximum piston acceleration occurs at TDC, being roughly twice as large here as compared to BDC. But the peak piston acceleration at TDC occurs very briefly, while near BDC the piston is accelerated upwards at a relatively constant rate for almost 70 degrees of crank rotation. This curve was constructed for 8000 rpm. Clearly at a lower rpm the velocity and accelerations would be smaller, while the displacement would remain the same. 

So, on to the question of interest: Can we use a longer rod to decrease piston acceleration, and thereby build a bottom end capable of reliably sustaining higher rpms? 

Click here for piston acceleration curves as a function of rod length... 

This series of curves shows that a longer rod reduces the maximum piston acceleration. An infinitely long rod (approximated here as one that is 10 meters long) will reduce the peak acceleration by 23% (relative to a factory Evo III configuration). But that's as low as the acceleration can be made to go with an 87 mm stroke at 8000 rpm. As the rod gets shorter, on the other hand, the max. piston acceleration is increased, but only at TDC. At BDC, the piston acceleration is actually reduced by a shorter rod (at least intially). The piston acceleration curve also begins to form a characteristic "double-hump" shape. If one were to keep making the rod shorter until it was only as long as the crank arm radius (a shorter rod than this would prevent the crank from completing a rotation), then the piston essentially would come to a "sudden" stop at 90° ATDC and it would "suddenly" start moving upwards again at 90° BTDC. These sudden stops and starts lead to infinite acceleration at 90° after and before TDC, and this is what the double-hump is starting to show. Of course this is all pure theory, as in practice the piston and rod consume space which makes the previous example impossible to achieve. But looking at the theoretical limits of an engineering problem is always instructive. 

Now a seasoned engine builder might consider trying to package a longer rod into the existing cylinder block. The reasons for wanting to try this can vary, and one of them might be to try and reduce the max. piston acceleration in an attempt to allow the bottom end to safely maintain higher rpms. So let's say we want to try this on an S14 engine with an Evo III crank. If we work real hard at squeezing the ring pack together, possibly pushing the wrist pin up past the oil scraper ring, and we reduce the OD of the wrist pin to a minimum, then we might just be able to wedge in a 1 cm longer rod. Having accomplished this we could be quite proud of ourselves in having built an S14 capable of higher rpms due to the reduced max. piston acceleration. But how much has the max. piston acceleration really been brought down? This is easy to determine with our spreadsheet, as shown in the following link: 
Click here for max. piston acceleration with a 1 cm longer rod... 

The curve is blown up to concentrate on the region near TDC (0° crank rotation) in order to better see the change in max. piston acceleration. And the answer is somewhat discouraging. The acceleration is only reduced by roughly 1.5% after all our efforts to lengthen the rod. Now this result should not be considered inconsequential. For example, if the previous redline limit for bottom end integrity had been, say 8200 rpm, then it is now raised to 8323 rpm. That's something you can hang your hat on. But depending on your application, it may or may not be worth the effort required. Remember too that, as previously mentioned, there are additional reasons why one might want to try a different rod ratio. 

The piston kinematics spreadsheet is fun to play around with. Should the reader desire to perform further experimentation, the spreadsheet can be downloaded via the following link: 

Click the following for more...

4500 feet per minute for a well built (internally balanced, 4340NT crank, 4340 rods with cap bolts, steel pins, forged pistons, and a suitable valvetrain) street/strip engine is around the limit for reliability. Production engine's are generally good up to around 3800-4000 feet per minute. Racing engines such Nextel Cup, F1, Indy, and even sportbikes have piston speeds exceeding 4800 feet per minute and maybe exceeding 5000.

Piston Speed in Feet per Minute = (stroke x 2 x rpm)/12

Rod Lengths/Ratios: Much ado about almost nothing. From Isky Cams

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.

6.125" Verses 6.250" Rod Length Comparison Chart

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