Average or Not 2/1/2006
The two cam big twin Milwaukee based motorcycles in stock configuration will average 52 to 60 horsepower (HP) and 65 to 70 foot pound of torque measured at the rear wheel. This is a high torque low revolution per minute (RPM) type engine. The typical effective power band is 3100 to 4100 RPM. The average volumetric efficiency (VE) is 73% in stock form. The use of high flow air cleaner, rejetted carburetor or flash electronic fuel injection and high flow exhaust will bring VE up to 77% yield. This is referred to as a stage 1 upgrade.
The next level is the big bore or 95 cubic inch displacement (CID). This will yield 70 HP with VE of 71%. If you add an air cleaner, fuel and exhaust upgrades, 72 to 78 HP can be expected with VE of 79%.
Stroking an engine is when fly wheels are changed that move the piston farther up the cylinder and deeper to the bottom of the cylinder causing a larger swept volume, which increases CID. The 103 CID engine with cams, air cleaner and exhaust changes can make 85 HP with VE only at 76%. As you can see, as CID increases VE can suffer because the area being filled with air is getting larger and increasingly more difficult to fill.
The Sportster 1200 in stock, form 58 HP at 66 ft. lb. of torque with a VE of 87% has a small stroke and bore, yielding excellent efficiency. Looking to these averages you can see the laws of physics demonstrated. The law of diminished returns is at work. The larger the engine becomes the more difficult VE percentages are to keep up. This means as an engine grows in size it will become increasingly more expensive to bring VE back to the level you started with. Please, do not confuse HP and torque with VE. Large displacement engines may lose VE but torque and HP will increase, just in smaller and smaller amounts. Volumetric efficiency can be brought up by moving more air into the engine. The use of ported heads, aggressive cams, larger air inlets, exhaust that scavenges well and programmable engine management electronics all will increase VE. To recap, bigger is not always better, better is always better so any time you increase VE the by-product is horsepower and torque.
After covering the industry’s standards and bench marking their VE, let us explore some of our other options. Volumetric efficiency can only be increased by packing more air/fuel mix in the cylinders. This quest for efficiency can be divided into desired areas of operation, like low, mid or upper RPM. You must determine which of these RPM levels you use the most. Choosing the RPM range, we now have a target for component choices. Efficiency enhancements are just that. After market parts world is all about freedom of choice. Choices also mean compromise, so choose wisely when building a package of components that must compliment one another. For example; to have stronger low and mid-range efficiency the high RPM range will be shallow and weak. The components to accomplish significant increase for your choice will have to be specific in design application compatibility. There is only one law that rules efficiency, physics. The farther you deviate from the laws of physics, the lower yield of efficiency obtained. The internal combustion engine is the science of physics placed in motion to do work for a specific task. Choosing air cleaners or exhaust systems by merit of looks or popularity can hurt your desired results of efficiency. Should you decide to step out of the average and strive for a higher bench mark of efficiency, focus on the task and then choose the components. Good components with specific abilities for exceptional repeatable positive results will not be cheaper. Good products cost good money.
With the complexity of today’s technology I find myself becoming more specialized, as well as, the equipment, I must use for tuning. Other challenges of today are ambitious political entities with aggressive agendas, which may have wide spread repercussions. Trying to coexist between large corporations and federal mandates is an ongoing pursuit. Being careful to not sacrifice, freedom of choice, options of efficiency enhancements to the public is ‘the new adventure.’
Adventure, isn’t that part of the allure of motorcycling? Should you find yourself wanting something a little different or on the quest for maximizing efficiency it is nice to have options. The after market has lead efficiency enhancements as well as cosmetic adornments for well over 50 years. The new frontier is technical enhancements that offer greater yields while empowering freedom of choice.
Good Riding and God Bless
Steve Caldwell - steve@precisionvtwins.com
877.429.0399