Basically all you need to know is that engine power will peak at approximately Mach .5, then flat lines up to .550, then drops after Mach.550. The speed of sound can vary by temperature, altitude, and atmospheric composition. Mach 1 (speed of sound) at sea level and at 70 degrees Fahrenheit equals approx. 770 MPH or 1130 feet per second.
There are several good web sites that offer Mach on line calculators. Using these calculators, you will need to input the bore, stroke, valve diameter, max rpm, and maximum valve lift.
The way to adjust the Mach Index number is by increasing or decreasing the valve size or valve lift.
If you experiment with the calculator you will find that you will get better results by increasing the valve diameter compared to increasing valve lift. Basically I would shoot for a mach index to be at .500 at my predetermined maximum RPM.
Example: A stock 88ci motor using 3.75” bore, 4.00” stroke, .473” lift cam with a factory red line of 5,200 RPM. It has a Mach Index of .537 which is fine for that stock set up.
A 95ci engine with stock valves and max RPM of 6000 and a Mach Index of .500 would need a cam lift of .625”. Now using the same 95ci engine with 2.0” intake valves, you would only need a .570” lift cam to accomplish a .500 Mach Index.
Meeting your Mach Index number does not mean that your engine will make power to Mach .500. It only means that the engine will be capable of making power to Mach .500. You have to make sure that all of your engine components match your engine build.
ALWAYS REMEMBER THAT YOU WILL MOST LIKELY HAVE TO GIVE UP SOMETHING TO GET GAINS IN OTHERS
Is BIGGER BETTER when it come to engine parts? Think about this, if you are in your car and you turn on the AC. Do you feel the drag on the engine and have to give more throttle to keep the speed up?
Just think of ways to "turn off the AC" on your motorcycle so it could have more usable power. I’m sure that you don’t have AC on your V-Twin, but think of ways that the engine is robbed of power. What components in the engine can you modify that robs power with out going too far? How much power is robed by the oil pump being driven off of the crankshaft?
Have you have ever turned the engine over by hand with the spark plus out? It still takes a lot of energy to move the crankshaft, then it gets to a hard spot, then has a tendency to move more freely. What causes the spots that are hard to turn? Remember that the spark plugs are removed. The hard spots are caused by the crank turning the camshafts. The cam shafts are hard to turn due to the valve spring pressure along with the rocker arm ratio. Remember the rocker arms are just another version of a pendulum. If you are using a ten foot pendulum with the pivot point at 3 feet picking up a 100 lb weight, which end would you want the weight and which end would you want to have to apply the pressure to move the 100 lbs?
There is not a lot that we can do to the rocker arm as far as reducing pressure. Roller rockers will help some by reducing friction. That leaves the valve spring pressure. If you increase the valve lift, then the valve spring has to be compressed more, creating more pressure. Of course you have to change the valve springs if the lift is too high and causes coil bind. Coil bind is where the valve spring coils touch each other (bottoms out) if compressed too much. What would happen if you used a lighter pressure spring? Most engine builders use very high lift camshafts, and high pressure springs to help keep the valves from floating at a higher RPM.
One theory of making more power is by increasing an engine's ability to keep making power at a higher RPM. This would give the advantage to that vehicle in a race.
Example: Let’s say that you have two identical vehicles with the exact same horsepower with the same maximum RPM racing in straight line. In theory both vehicles should cross the finish line at the same time. This means that the wheels are turning at the exact same speed at maximum RPM on both vehicles. What would happen if one vehicle could make power 1,000 RPM higher than the other vehicle? Since the RPM is higher, then the wheel speed will be higher (faster) and that vehicle would win the race.
Let's create another scenario that will cause you to think in a different way. By increasing the valve lift and/or increasing spring pressure to keep valves from floating at higher a RPM, the extra pressure not only robs power from the crankshaft but also increases pressure and wear on the cam bearings, push rods, rocker arm bushings/rollers and on the valve itself. Have you ever heard the term used “The engine dropped a valve?" This is when the valve stem breaks, or the keepers let loose of the valve,'s or a spring has broken usually from too much pressure or stress.
Let’s change our way of thinking a little. Let’s build a 95ci engine using a 2.0” intake valve. The increase of the valve diameter will increase the weight a little, but shouldn’t matter much at a lower Maximum RPM. You could use a titanium valve which will compensate for the added weight of a larger valve. Why is it better to lower the weight of moving parts in an engine? It’s called Rotational Mass. The momentum of a heavy object in motion is harder to stop than if that object was lighter. An engine running at 6,500 RPM's pushes the valve open 54 times a second. When that valve is thrown open it has to stop, (sometimes floating) then returns to the valve seat sometime bouncing off the seat. A lighter valve would help eliminate this. Also lighter rockers and push rods would help eliminate valve float. This is probably why the newer engines like the 2007's have reduced rotational mass. A lighter flywheel and rods will also let the engine turn up quicker.
Now since we are using a 2.0” intake valve, we will not have to use an extremely high lift cam. According to the Mach Index Calculator, we can use a .550” lift cam and it will give us a Mach Index of .506 at 5,800 RPM's or Mach .480 at 5,500 RPM's.
By limiting the engine to 5,500 - 5800 RPM's, we can use less valve spring pressure and lighter push rods. (Remember we had to have more spring pressure to help keep the valves from floating on high revving engines.) In return there is less effort to turn the crankshaft resulting in more power and a quicker revving engine.
Now that the RPM range is determined, you can determine where to aim for maximum torque. The way to take advantage of a lower revving engine is to design your transmission gear shift points to fall back to the Maximum torque RPM band. This idea should work on short tracks, roll on’s, and stop light to stop light. , If your looking to break a world speed record, you may not want to use this configuration theory or you can get a lot more gears.
Design your shift points at close to maximum RPM and where the next shift begins as close as possible to peak torque RPM. This way you are always in the power band range through shifts. Have you noticed that crotch rockets have to really wind up to get into the power range? Just think if you strapped that fast crotch rocket engine on a heavy frame like a FLHT and then added a heavy rider, it wouldn’t be quite as quick then, would it?
Another theory to reduce load on the engine is using a thinner viscosity motor oil. There are two ways that the theory works.
First is a thinner fluid will travel through an orifice easier (less pressure) than a thicker fluid. Think of it this way: Would it be easier to suck a triple thick milkshake through a straw vs a cup of milk? So it takes less effort (power) to drink milk through a straw than a triple thick milkshake. Is part of the theory robs less power from the crankshaft when it is turning the oil pump.
The second par of the theory is: It the fluid is thinner, the crankshaft weights will travel through the fluid easier than traveling through thicker fluid.
Example: Which would take less effort, swimming across a pool filled with water or swimming through a pool filled with molasis?
Another thought is: The crankshaft weights (flywheels) will be covered in oil. What happens when this crank is covered in thick oil?
The thicker the is that is attached to the crank, the more weight that is added to the crank. A heavier crank takes more power to get it spinning. Remember the discussion about "Rotational Mass"?
Most automobiles are using very thin oils now such as 5W-20. Some are using 0W-30...I see where NASCAR is using 0 weight and 0W-5 oil
for the same reason listed above.
These are just a few ideas to get you thinking out of the "old school" way of thinking.