4 mil crank

[screaminshee]

Hey guys. I was looking into setting up a 4 mil motor but had a question. Whats the main difference between using 110 rods and 115 rods? Is it the fact that with longer rods the movement of the piston is less, as in from side to side and the chance of the piston turning on its side in the cylinder is reduced? I have no idea if this is even close to true, thats why im asking. But also what is yours guys preference and why? Please dont just say one or the other unless you have reasoning. Thanks!!

[gotta_goatsfast]

Hey guys. I was looking into setting up a 4 mil motor but had a question. Whats the main difference between using 110 rods and 115 rods? Is it the fact that with longer rods the movement of the piston is less, as in from side to side and the chance of the piston turning on its side in the cylinder is reduced? I have no idea if this is even close to true, thats why im asking. But also what is yours guys preference and why? Please dont just say one or the other unless you have reasoning. Thanks!!

 

 

 

You got it. It is a reliability issue, with a slight (mathimatically correct but doubt you'd feel it power increase). And here's why. Are you any good with Trigonometry? If so, lets take three points of interest and form a right traingle with them.

 

First point is crankshaft axis (center)

Second point is wrist pin axis (center)

Third point is Crankshaft journal axis (center)

 

Draw the triangle so that the right angle sits on the crankshaft's center. The adjacent side runs from the center of the crank to the center of the wrist pin. And the opposite side will be the crank's center to the center of the crankshaft's journal. And the hypotenuse... you guessed it, the wrist pin center to the journal center. (this is all with the crankshaft 90 degrees after or before top dead center).

 

Now we can form a math forumula. What we are interested in, is the angle that is formed from where the hypotenuse meets with the adjacent side. Well, we know that a 4mm stroker produces a 58mm stroke. From the center of the crank to the center of the journal will be half of this, so 29mm is the length of the OPPOSITE side. We also know that a 115mm rod is, well 115 mm from center to center, meaning the HYPOTENUSE side is 115 mm. (remember, crankshaft is 90 degrees out of TDC) Using trigonomic functions we can find the angle of the connecting rod. Whipping out my trusty calculator AND... the angle is 14.61 degrees. If we substitute out the connecting rod for that of a 110mm, we come up with a slightly larger angle of 15.29 degrees. Is this a lot? Well, yea it kind of is. There is less force applied to the side of the piston (which you can consider stress to the skirts) and more applied to the crown.

 

Why do I say it creates more power? Well, the engine is converting and/or transfering energy in every step. In order to transfer 100% of the energy onto a lever, you need to always be 90 degrees from the lever. This simply is not possible with a piston engine. The closest we can get is 90 degrees ATDC. Clear as mud, yet? I will explain the reasoning as best I can. You have a 2" nut that you need to break loose. A large wrench is provided. Are you going to press down at a 90 degree angle from the wrench (making an "L" shape where your arm is vertical and the wrench is horizontal). Now, place your hand so you are pressing on it at, say 75 degrees (or 15 degrees off of vertical). Now you are transfering most of the energy to rotating the nut, but you are also transfering some energy to PULLING the wrench away. Make sense? Now say you are 30 degrees. Most of your energy is trying to push the wrench away, while a small portion is actually applying force to the nut to turn.

 

How does that relate to the crank? Well we can use trig formulas to figure out that the angle of the adjoining hypotenuse (con rod) and opposite side (horizontal line between center of crank and center of journal). On a 58mm stroke with a 110mm rod this comes out to 74.71 degrees which is 15.29 degrees out of perpendicular (90 degrees). What about a 115mm rod on the same stroke? 75.39 degrees or 14.61 from perpendicular. As you can see, there is a slightly less energy being used to "pull" the crankshaft out of its center and slighty more transferred to rotating it.

[4MiLLDraGSheE]

Holy crap thats a wright up. Hes had that on his computer for months waiting for someone to ask this question. Either way u run different pistons depending on rods. I run a long rod but ive actually heard that the engine runs faster with the short rod. I dunno id have to see and equation for it.

[Zwiser]

great write up!!!

[gotta_goatsfast]

Holy crap thats a wright up. Hes had that on his computer for months waiting for someone to ask this question. Either way u run different pistons depending on rods. I run a long rod but ive actually heard that the engine runs faster with the short rod. I dunno id have to see and equation for it.

 

 

Haha, no. Just had to kill some time before a 5:30p work meeting. Short rod might free-rev a tad faster because the lesser mass of a rod that is 5mm shorter? Either way, the main reason for a longer rod is reliability. The ladies love a long rod too! Heres the proof for that...