Carolina Motorcycle Events - Performance Through Research

Carolina Motorcycle Events - Tech Talk

Performance Through Research

By Dennis Thurlow and Staff

COMPRESSION

First we will look at the compression of the motor. Through test and tune research, we have found that a good useful street compression to be between 9.0:1 to 10.0:1. These work well with pump gasoline. There are several ways to raise compression in an EVO motor. The two most common are to mill the deck surface of the combustion chamber, thus reducing the size of the combustion chamber. The other is the use of aftermarket dome pistons. Most aftermarket dome pistons are expensive. They are made of forged aluminum metals. These pistons often have to have more running clearance in the cyclinder, thus making for more engine noise, due to loose fit. Loose fit decreases piston ring life by rings rocking in the cylinder. High dome pistons reduce turbulence in the engine, which is highly desirable for good air-fuel burn. Last of all, too much compression is more heat, too much heat is harmful to the engine. We have found that milling the deck surface of the combustion chamber to be the most effective way to raise compression, and least expensive. Through our research on compression ratios, we have found that milling between .020-.065 (depending on the components used), worked the best. This gives us compressions between 8.8:1 to 9.5:1 which we feel is good dependable ratios. A lot of times we are asked about combustion chamber shape. From dyno tests and tunes, we have found the stock chambers to work very well, especially with stock flat top pistons. Through research, we have run stock chambers on stock bore and stroke 1340cc EVO motors and have achieved over 100 horsepower. These motors also had stock valve sizes. Most of our performance packages run compression ratios of 9.0:1. These motors are capable of making 70 to 80 horsepower and 85 to 90+ foot pounds of torque at low rpm range where it is more useful.

VALVES PORTS SEATS

We could go on and on about this subject. We would like to talk about what we found works best for street use. The key here is valve seat area. We can't stress enough how important a good valve job is to a motor. Here we do a multi-angle valve job. The angles we use and where we locate them on the valve and the valve seat come from invested hours of dyno and flowbench research. When we perfect our valve and valve seat angles, this means valves and seats flow more air-fuel at less given cam lift. You get good idle and low rpm response from a lower lift bolt-in street, and higher rpm power of a larger cam. Also the valve themselves are subjected to multi-angle cuts. In logic, it would seem the bigger ports, the better, however, not so true. Cylinder bore diameter dictates the valve and port size. Large ports may flow well at high valve lifts on a flowbench, but at lower lifts, port velocity is down greatly. Thus poor cylinder fill at lower rpms, resulting in a sluggish bike at low to mid-range speed where most people do their riding. The best way to achieve good port velocity is to only modify the surfaces that are impending the flow, this keeps port velocity high and lets in the most air possible during intake valve opening. This is to give you better understanding of valve and port areas.

IGNITIONS

There are several ignition systems out there. Some good, some not so good. We have found that for the money and simplicity, we choose to use the Dyna 2000. It is capable of single or dual fire. Electronic advance with stock V.O.E.S., adjustable curve settings, adjustable rpm limiter, and also a warranty. It works well with any motor combination.

EXHAUST SYSTEMS

We could write a book about this subject. We will talk about what we have found to work, and more than the fundamentals of exhaust systems. One of the best, but often overlooked is the stock system. This system works very well with free flowing mufflers, such as Harley Screaming Eagle or aftermarket. In the past few months we have been working with the Dresser Dual Exhaust system, and found that slip on mufflers did well on these. Most two into one collector type exhaust work well. Thunder Header, Super Trap, Kerker, and Arlen Ness. Staggered pipes, such as Cycle Shack, Python II and III work well. Pipes that don't do too well are large straight type pipes, such as Porker, Vance & Hines, and Sampson. These pipes on the dyno at 6000 rpm may show more horsepower than others, but have very poor low to midrange torque and horsepower output, and that is where you do most of your riding.

CARBURETORS

The principal function of a carburetor is to regulate the flow of gasoline into the engine, and help atomize the fuel with fast moving air. Like ignitions, pipes, etc., there are many choices of carburetors on the market. One of the most overlooked is the stock CV H-D carb. Harley has used these carbs since 1988 on XLS, and 1990 to present on Big Twins. 1990 and later have 40 mm venturi diameters. We have had good success with these carbs. As people who know, we are constantly improving on these carbs. With some jet and needle changes, these carbs can perform. We have had 1340 motors using stock carb, with jet and needle changes show horsepower and torque readings into the low 90's. Once these carbs are jetted to the engine they are pretty much trouble free. The name CV came from the use of constant velocity, variable venturi. This carb uses a vacuum operated round piston that regulates the size of the venturi along with a butterfly and shaft that regulates air flow. At low speed, the round piston is down in the venturi, thus keeping the air velocity high. This results in quick throttle response at low speed. The round piston rises to expose greater venturi area to satisfy the engines' greater demands. Remember, high and low altitudes do not effect this type of carb.

S & S CARBS

S&S carbs have been around since 1967. We feel they are a very good choice of carburetor. They are butterfly style carbs. S&S E carbs have a venturi diameter of 39.6 mm, the G carb 44.5 mm. Both carbs have two fuel metering circuits, an intermediate and a high speed (or main) jet. It also has an adjustable idle mixture screw. Both have adjustable accelerator pumps. They have a cold start enrichment circuit. These carbs can be modified with thunder jets, air bleed jets. We are now researching Thunder jets on H-D CV carbs. We will have results soon. We have used both of these carbs on motors as small as 61 cu. in. all the way to 100 plus cu. in. You can buy these carbs in kit form at very reasonable prices.

We hope we have helped with making some decisions on your performance outlook. We have put much time and research in testing and tuning to come up with a performance to fit most any application from street to race. Feel free to talk to Cape Fear Harley-Davidson staff about your needs. See ya out there. Ride safe!

CF H-D Staff