Rod Slots

[bishopg]

I would like to know from one of the Mercury guru’s on here what is the purpose of cutting rod slots. I have seen it mentioned on a couple of post and would like it explained to me if at all possible.

[us1]

The purpose is to straighten out the flow in to the boost ports. Cut just what is needed. Three way rod slots do nothing but reduce crankcase compression. What’s important is the best possible feeding of the boost ports alone. Cutting towards the transfers gains very little flow and seriously reduces the crank case compression and low end. This is just what I have found on my dyno.

[Superdave]

So John, explain one-way and 2-way rod slots with the advantages and disadvantages of both.:rolleyes:

[W. Tripp]

Think of the rod slot as a port. There is a larger volume in front of it (the crankcase) and a large volume at the end of it (the area under the piston).

Like any port, the entry shape is the most important. This is where you can get the most disturbance and reduction of efficiency. The shape and volume of the port (compared to the volume behind it) determine the velocity that will be developed in the port.

If you ever took a basic physics or chemestry class, you were told that energy is never lost, it only changes form. The same is true of the energy (velocity) in the port. The mixture velocity in the port will change to pressure in the volume under the piston. How much pressure is based on the efficiency of the port. The port walls (shape and angle), entry shape and outlet shape determine how much (or little) energy is changed into friction (heat energy).

The outlet shape can determine how well air/fuel mixture slows down and makes the turn to the entries of the transfer and boost ports. Air does not like to change direction more than 15 degrees at a time. This is why a radius turn is better than a single angle or even a series of angles. If forced to turn more than 15 degrees (in one movement), the air will become turbulent, friction is developed, and the fuel will seperate from the air.

The delicate balance between the amount of material removed and shaping the rod slot for improved efficiency is choosing the better of two evils. This is very important when you are working with crankcase volume, which is exactly what the rod slots are.

Another issue is the location of the entries to the boost ports (all three of them)compared to the rod slot location. If you have ever pulled a sleeve out of a well run engine, you may have noticed the coloring on the back of the sleeve from fuel and oil. This coloring can show you the areas of the transfer and boost ports that are active and the areas that are not. Studying these areas can help to better understand the flow in the ports and how to better use them. Work to the rod slots and port entries can make better use of the ports for additional power, and both are closely related.

The volume of the crankcase and in the various ports determine the velocity in the ports at a specific rpm. Up to a given speed (usually given a percentage of Mach number to relate it to the local speed of sound in the port), more port velocity at a given rpm (without losing flow) will ALWAYS give more torque at that rpm. Since power is a function of torque at rpm....this is the road to power, but a difficult one to balance well.

I hope this helps,
-Wayne

[Jay Smith]

I agree with John I have also found that the boost ports are the key to overall best flow of fuel vapor to the engine. I have seen motors accuatually loose bottom end grunt and snap out of the hole due to the rod slots being cut either too big or they were started too high in the slot to give an "approach" to the port . Too much material removed from this area helps nadda and creates tons of volume which make for a LAZY motor . Had a guy a couple of months ago try his hand at rod slotting with the old addage bigger is better and he called crying. It was on a bassboat and the thing would hardly untrack compared to stock. I band aided the problem by cutting the from case half .075" putting the reed closer to the rods thus disipating space and reducing the volume the bottom came back to a little better than stock so his whole efforts were in vein ! Just a thought !

[bishopg]

Thanks to all for taking the time to answer my post. One more question I have is do any of you have pictures for the "perfect" "balance". Once again thanks to all for your time.

[Jay Smith]

It has all in the world to do with a bunch of factors ( weight of boat, how many rpms is expected or required , all pertinant informantion about the rig and the motor ) If you asked 10 engine builders how they do rod slots you would get 5 different answers and ways "they" feel is best..... SORRY BUT thats an open ended question and all that will answer in thier mind and practices is correct and thier method works best for them through many trial and error processes.........

[props4u2]

That Wayne guy can really type, would take me all night to peck out that much. LOL.
Great info guys.

Lee

[jimmyj]

Plagirism :D

[merc326]

can anyone post any good pics of the rod slots, i have done a search and cant find any good pics, i thank you....