Stock 883 Sportster cylinder heads work OK for a stock bike, but on a hopped-up 1250cc motor, they need some help. Help in the form of runner porting, unshrouding the valve-seats, machining the combustion chamber, decking the heads, a set of larger valves, and new performance valve springs. A company called Revolution Performance, based in Plymouth, Wisconsin, was dying to get the opportunity to show how they grind, port, and modify a Sportster head to completely transform the Motor Company's least performance-oriented motor. Normally when building a higher-power motor, Revolution Performance recommends a Buell Thunderstorm head, but that wasn't an option for this build because the 883 heads were already polished to match the rest of the motor.
Revolution Performance went all-out when modifying these old '93 heads. They started by matching the top of the piston to the combustion chamber of the head and milling some material off the bottom of the head to raise the motor's compression from stock to 10.5:1. Then the valves were unshrouded, meaning material was removed from around the valve seats to ease the transition of the fuel mixture into the cylinder and back out as exhaust. Each head was machined and upgraded with a full set of Kibblewhite 1200 valves. High quality Viton valve seals were slid over the valve-stems with steel locks, retainers, cups, and locators. Assembled with high-performance valve springs, the beehive shape reduces weight and makes for a better shape for the spring retainers, plus the high-tension springs are better because they keep the valve timing more accurate and prevent the valves from "floating" at high rpm. "Floating" is the term used for valves that do not shut completely because the valves springs are weak and lack the tension to create compression-in effect, they hover in limbo between open and closed. Both the intake and exhaust runners were ported to allow for higher velocity and reduce friction into and out of the cylinder. The original runner casting has a rough, sand-paper like finish, but Revolution Performance smoothed out the surfaces to speed up the velocity of the air and fuel mixture by reducing friction in the runner. The difference is like the top speed a bike can ride on a dirt road compared to the much higher speed a bike is able to reach on a paved road.
The gasket surface of the...
The gasket surface of the head's deck was machined in conjunction with custom machined CP piston raising the compression of the motor to 10.5:1.
Contouring the area around...
Contouring the area around each valve helps to ease air in and out of the cylinder-essentially making the head/valve transition more aerodynamic, thus allowing for a more dense air/fuel mixture into the cylinder and creating more horsepower and torque. Once finished with the major machining in the combustion chamber, the volume of each chamber was matched at an equal 65ccs of volume.
883cc heads=smaller valve...
883cc heads=smaller valves
1200cc heads=bigger valves....
1200cc heads=bigger valves. RevPerf machined the valve seats to handle the larger Kibblewhites with a 1.715-inch intake valve and a 1.480-inch exhaust valve.
883cc head's intake runner...
883cc head's intake runner in stock form with rough surfaces.
RevPerf smooths out the runner's...
RevPerf smooths out the runner's rough surface to allow for a higher velocity air/fuel charge than a stock sand-cast head. All the work is done by hand; in this case it was performed by Dusty Werner.
883cc head's stock exhaust...
883cc head's stock exhaust port.
The valve's guide into head's...
The valve's guide into head's exhaust runner was contoured, and the runner was ported and smoothed like the intake runner by Dusty Werner. Revolution tested the intake and exhaust flow of each head's runners on their Port Flow Analyzer v3.0. They were able to squeeze 233cfm into the intake runner and 159.7cfm out the exhaust at .600-inches of lift.
Stock valve springs on an...
Stock valve springs on an 883 head.
Revolution Performance makes...
Revolution Performance makes beehive- shaped valve springs; they're called beehive springs because the top of the spring tapers in towards the valve stem and keeper. Each spring is rated for 180-pounds of pressure at .600 lift and Revolution claims that the springs enable higher revs, longer life, cooler-running, and make less noise than other "performance" springs.