Rod Length Woes
Contributed by Mike Drew (md) CT Member #1292
The subject of rod angularity almost always comes up. I see it all the time and the opinions vary greatly. I think that in order for a person to determine what is an expectable rod length, he/she should be aware of what the rod actually does and the theory behind the logic of short vrs long rodding an engine. I’ll try to explain in this article the rational behind short and long rodding. I don’t claim to be an expert on this subject and as I research it, I find myself asking more questions. I would welcome any thoughts that you may have.
Rod Angle
The connecting rod transfers forces between the crank and the piston in both directions. From the piston to the crank during the power stroke and from the crank to the piston during the exhaust and compression strokes. These forces exert a tremendous amount of tensile stress to the rod. Primarily when the piston is at mid stroke and the crank is 90* from TDC.
The amount of tensile stress that the rod experiences is amplified when the angle of the rod is increased. Another undesirable effect of excessive rod angle is cylinder and piston wear. For the sake of simplicity and familiarity, I’ll reference the SBC for the following discussion.
A 350 with a stock 3.48" stroke and 5.7" rod length has an angle of 17.51 degrees when the crank is 90 degree away from TDC. I have personally owned vehicles with a 350 that have lived a healthy life of 150,000 miles without excess oil consumption or rotating assembly failure. I know that I’m not alone here and have heard of 350’s living to the ripe age of 300,000 miles and more. This tells me that GM got something right with the 350, so I’m confident that 17.51 degrees of rod angle could be used as a baseline for an acceptable rod angle. I have also read many times during my studies from various respected sources that 18 degrees or less is preferred. Anything over 18 degrees and excessive wear to the piston major thrust area and opposing cylinder wall will result. The 400 in stock trim is an example of this wear. The 400’s stock 3.75" stroke and 5.565" rod yields an angle of 19.33 degrees. The 400’s were prone to early ring failure and excessive cylinder wall wear.
When building an engine where the stroke or rod length is changed, the resulting rod angle should always be considered. If we use the 350 as an example of "acceptable" rod angle, it would be prudent to maintain an angle as close to that as reasonably possible. Rod angle is calculated by dividing the stroke by 2 then divide that result by the rod length, then compute that number by sin. You’ll need a scientific calculator to do this. The formula will look like this for a 350: (3.48 / 2) / 5.7 sin = 17.51
More commonly referred is the stroke to rod length ratio and it’s easier to calculate. Simply dived the rod length by the stroke. I’ll use the 350 again: 5.7 / 3.48 = 1.6379 or, 1.64 rounded up.
When increasing the stroke of an engine, clearances and space become issues very quickly. If it is desired to maintain a rod angle of 18 degrees or less, or even the stock ratio prior to a stroke increase, an increase of rod length will be required. When rod length is increased, the added length will force the piston pin to move closer to the top of the piston. If the piston’s compression distance (distance between the pin center and the top of the piston) remains the same, the piston will surely be pushed out of the cylinder bore. Therefor, pistons must be matched to the stroke and rod. When the pin is moved closer to the top of the piston, it begins to encroach the piston rings. Piston manufactures have tightened the ring land areas, placed the pin in the oil ring groove, moved all rings closer to the top of the piston and even eliminated the second ring to accommodate the hot rodder’s never ending pursuit of pushing the envelope of OEM engineering designs.
Torque
Torque is a measurement of twist. For an analogy, imagine yourself trying to break a nut loose with a 6" wrench. Now try it again with a 12" wrench. It’s much easier to break loose with the longer wrench because torque is increased. You will also notice that you must turn the longer wrench a greater distance to rotate the nut the same amount as with the short wrench. If that analogy is fuzzy, replace your steering wheel with a small 12" GT style. You will be able to maneuver the car quicker, but the resistance will increase. That’s why large trucks have large steering wheels. You need a little help turning them big ‘OL front tires. The same thought can be used with an engine. The longer the wrench, or increased stroke, more torque will be developed.
When rod length is increased, the piston must travel further to turn the crank the same degree of rotation that the shorter rod will around TDC/BDC, so in theory, a longer rod will apply more torque during this point of crank rotation. Conversely, the piston with a shorter rod must travel further to turn the crank the same degree of rotation that the longer rod will around half stroke. So again in theory, the short rod will apply more torque during this point of crank rotation.
When the fuel/air mixture is ignited, the expanding gasses will create increased pressure in the cylinder, forcing the piston down. The point in the cycle where the most effective torque can be applied to the crank is when the piston is halfway down the cylinder and the crank is 90* ATDC. By this time, the expanding gasses have used up a large amount of their useful energy. Therefore, it is important that the remaining useful energy from the combustion process be efficiently used at this point. This is where many engine builders argue that a shorter rod will increase torque over the long rod. They feel that the engine will rev faster and develop more torque from idle through the mid range rpm’s by using a shorter rod.
The other thought centered around torque is when the piston is around TDC. It’s argued that the longer rod will develop more torque when the piston is between TDC and around 45 degrees ATDC. Some believe that after the rotating assembly is spinning, the longer rod will utilize this theoretical increased torque at the engine’s upper rpm range. There may be some merit to this argument. From the very few Winston Cup engine builds that I’ve seen, they use fairly long rods with rod ratios of up to 2.0. Winston Cup engines spend the majority of their life spinning above 5000.
I have not seen or heard of any dyno sheets that support the theory that rod length has any effect on torque/hp. If there are, I would love to see them! Out of all the books, articles and web pages that I’ve read in regards to this train of thought, I just don’t feel all that warm and fuzzy about any of it. It's all based on theory, opinions and basically thinking too darn much about it. Until I see actual dyno sheets from identical engines where the only difference is rod length, I’ll leave the torque arguments out of my rod length decision making process.
Piston Speed and Dwell Time
Piston speed is expressed in feet per second, feet per minute or miles per hour. The formula for finding feet per minute piston speed is: stroke x 2 x rpm / 12 = fpm. This formula will give you the mean (average) piston speed but not the maximum speed that the piston will be exposed to. The piston’s maximum rated exposure speed is determined by its’ composition. A standard cast piston will not sustain the same speed that a hypereutectic cast or forged piston will. The most common number I’ve seen is a maximum of 4000 fpm for stock pistons. A 3.75" stroke at 6500 rpm will push a piston 4062 fpm, where as the 3.48" stroke at 6500 rpm is 3770 fpm. As you can see, piston speed increases with a stroke increase due to the piston traveling further within the same amount of time per revolution. Always consult the piston manufacture to ensure the piston used is rated for your intended application.
As the piston approaches TDC/BDC, its’ speed will decrease while it makes the transition from up/ down stroke to down/up stroke. The maximum speed seen will be around mid stroke. While rod length will have no effect on mean piston speed, it will affect the amount of time where the piston is in its’ travel per each revolution. When rod length is increased, the amount of time that the piston spends around TDC/BDC is longer and its’ time spent around mid stroke is shorter. When rod length is decreased, the opposite affect occurs. There are differing opinions regarding piston speed. I personally don’t believe a dyno will notice a rod length change unless it is 10% or greater, but I’ll cover the theory anyway and you can decide for yourself.
When the piston travels up the cylinder during the compression stroke and the spark plug fires, the fuel/air mixture is ignited and starts to expand. If the piston were to travel away from TDC too quickly, it would outrun the advancing flame front and cylinder pressure will be lost. A longer rod will allow the piston to "dwell" around TDC longer than a short rod, which in theory.
Another "theoretical" consideration of piston speed is the piston’s transition from stroking up and down the cylinder bore and vise versa. As the piston travels to the end of its’ stroke, it comes to a complete stop and then speeds away in the opposite direction. This is referred to as instantaneous velocity. If this transition can be slowed down, the mechanical stresses to the piston and rod can be reduced. I doubt that a change of rod length less than .500" would make a noticeable improvement, but I’m not an engineer and am just guessing.
Flow and Pumping Losses
Theoretically, a change in rod length will affect how the engine breaths. If piston speed can be increased around TDC by using a shorter rod, the piston will pull harder on the intake and exhaust ports during the beginning of the intake stroke. At low and mid range rpm’s, the velocity of the intake charge of the fuel/air mixture will effect the amount of torque produced. If velocity is increased, the intake charge will be greater which will increase cylinder pressure and torque. If rod length is increased, piston speed will be slower around TDC which will reduce the intake charge during this point of the intake stroke. The other school of thought is that the piston will pull harder around mid stroke with a longer rod than a short rod. The intake valve will be fully open around 110 degrees past TDC where the long rod engine will be pulling harder. Which rod will fill the cylinder better? I don’t have a clue. If you do, let me know.
It is also believed that rod length will affect exhaust gas evacuation. Towards the bottom of the power stroke, the exhaust valve starts to open. This is where the short rod is pulling and pushing harder than the long rod. Some builders think this will overwhelm the inherently small exhaust valve. These builders feel that the exhaust gasses will be more effectively pushed out of the cylinder if piston speed if increased around mid stroke where the exhaust valve is near its’ full open position. The opposite side of this discussion is, increased piston speed around mid stroke will overwhelm the exhaust port. Again, I don’t have a clue who is correct.
Summary
As you can see, there’s pros and cons for just about all aspects of the rod length debate. I think that for the most part, this subject is overworked and based on speculation and too much idle time thinking about it. I do feel there is some merit to improving rod angle, especially when stroking an engine, but I seriously doubt that a dyno will show any noticeable differences.
If your set on long rodding an engine, I would suggest that you don’t get too carried away. Make sure that you take a good look at the pistons you will have to use. A piston with a .750" compression height and a two ring package just might convince you to use a tall deck block, cut back on the rod length, or go with a shorter stroke.
If you think a short rod engine is better suited for your application, make sure you compute the rod angle. If it gets much greater than 18 degrees, I would suggest you get a good set of rods that will take the tensile stress it will be exposed to.
Michael Drew
