Big Inch Twin Cam Part 2 - Garage

Part Two
Big Inch Twin Cam Part 2
In our March issue, we followed along as Short Block Charlie modified Bill Carter's CVO 103ci Twin Cam engine by increasing displacement to 117ci. Since that time, Bill has been busy putting 1,300 easy break-in miles on the motor, so it is now time to dyno tune the engine to optimize the air/fuel (A/F) ratio and ignition timing for maximum power. In this installment, we'll accompany Ron Piner of Trask Performance in Phoenix, Arizona as he dyno tunes the 117ci engine.

If you've read Part 1, you should recall that the 117ci engine build-up was a collaboration between its owner and Short Block Charlie. Bill, being an old-time Harley racer, wanted to freshen up and increase displacement of his 103-inch stroker motor while maximizing mid- and low-range torque for stout pulling power while riding double with his wife aboard. Charlie recommended installing big-bore cylinders to increase displacement to 117ci because big cylinders would provide the best bang for the buck. Charlie also recommended installing cams with moderate duration and an early-closing intake for excellent torque. To that end, Axtell 4.125-inch bore cylinders and Dave Mackie 598G cams were installed.

The Mackie cams have 0.598-inch valve lift, 254-intake and 256-exhaust duration, respectively, and close the intake valves at a relatively early 50 degrees ABDC. For better top-end power on a 117ci street motor, cams with roughly 260 duration and intakes closing around 58 degrees ABDC would be in order, but the 598G cams were chosen to maintain excellent torque down low. While working on the bottom end, Charlie installed a Timken bearing in the left-side crankcase, freshened up the SE 103 heads and set the mechanical compression ratio at 10.5:1. The engine also has a 2-into-1 Vance & Hines Pro Pipe with free-flowing baffle and stock Delphi 46mm throttle body, along with a Screamin' Eagle Race Tuner for calibrating the ECM's air/fuel ratio and ignition timing.

(2.)Shown is Trask's Dyno room. The Dynojet 250 dyno is recessed into the floor for easy positioning of a bike. The dyno's large roller for spinning a bike's rear tire is in the foreground. The large square air vent on the front wall is for incoming fresh air for engine cooling and to help remove exhaust gasses. The two black corrugated pipes at the right foreground are connected to an engine's exhaust system to remove exhaust gasses. The white square-steel tubing in the foreground on the right and left sides are vents with blowers for removing air from the dyno room. Exhaust-tainted induction air reduces power readings.

Dyno Tuning
Dyno tuning an EFI engine can mean different things to different people. To some, dyno tuning means adjusting a few pods on an EFI add-on module and then quickly doing a couple dyno runs. To others, dynoing means downloading a generic fuel map (for a similar engine combination) into the bike's ECM.

Ron Piner is a long-time Trask Performance employee and a highly experienced dyno tuner, doing several dyno tunes daily, often on 150-plus horsepower Trask turbo bikes. During this dyno tune, Ron will go way beyond the first two dyno scenarios by building complete custom air/fuel ratio and ignition timing maps for Bill Carter's specific 117ci engine combination. Since Bill's bike already has a SE Race Tuner installed, Ron will use the Race Tuner to build the custom maps. Building custom maps takes several hours of dyno time, but that's the best way to optimize throttle response and maximize power on a modified EFI engine.

Types Of Dynos
There are several types of dynos, the most common being the engine and the rear wheel dyno. Engine dynos are excellent for the development and testing of new parts because they accurately measure crankshaft torque and then calculate horsepower. However, they are not practical for tuning street bikes because it takes a lot of work to connect a bike's engine to an engine dyno. On the other hand, rear wheel dynos are perfect for testing street bikes because the entire bike can be easily rolled onto the dyno and strapped down.

A rear wheel dyno uses a large roller that rotates with the bike's rear wheel and calculates torque and horsepower under full-throttle engine acceleration based on the time and acceleration rate of the roller. Keep in mind, however, that rear wheel power readings will be lower than crankshaft readings (generally 10-18 percent) due to power being lost in the bike's drivetrain (primary drive, transmission and secondary drive).