JIMS 120-inch Twin Cam Hop-Up

(1.)To stop press-together cranks from shifting, Charlie TIG-welds both ends of the crankpin and then retrues the flywheel assembly.

(1.)To stop press-together cranks from shifting, Charlie TIG-welds both ends of the crankp

Short Block Charlie's in Tempe, Arizona, is quite accomplished in building JIMS 120-inch Twin Cam engines. Charlie has built a slew of them, ranging from 120 inches to 135 inches in displacement. JIMS TC engines have proven to be stout crate motors, and with a 51mm CV carb and free-flowing exhaust, they typically produce about 1hp and a tad more than 1 lb-ft of torque per cubic inch on pump gas. Customers upgrading from 95-inch or 103-inch TC engines often want a little bit extra in performance when installing a JIMS 120-inch engine. One such customer of Charlie's is Leo Shewchuk from Calgary, Canada.

Leo owns a 2003 Twin Cam Road Glide that he occasionally uses for exploring around Arizona's scenic Sonoran desert during the winter. Since Leo's bagger already had a fuel-injected 103-inch stroker engine, he thought a mild hop-up on a JIMS 120-inch engine (4.125-inch bore x 4.5-inch stroke) might be his ticket to nirvana. Charlie and Leo decided that the hop-up objective would be to increase airflow and efficiency using modified heads, a larger throttle body, different cams, higher compression and a free-flowing exhaust. Since Leo already had a set of Andrews 64G gear-driven cams and a Kryakyn 57mm throttle body, he had Charlie build the engine around those parts. Since the stock JIMS 120-inch engine is rated at 121hp and 125 lb-ft when using a 51mm CV carb and SuperTrapp exhaust with 26 disks, the major unknowns for the modified engine would be dialing in the fuel injection system and selecting the right exhaust system. A Screamin' Eagle race tuner was selected for remapping the TC's EFI system while dyno testing would be used to determine the best pipe.

(5.)Charlie torques the case bolts down to 19 lb-ft in two steps.

JIMS Twin Cam engines, both the 120-inch and newly released 131-inch, are available as a fully assembled engine or as a kit. Charlie ordered the 120-inch engine kit for Leo. The kit version includes a fully assembled bottom end and requires the builder to assemble the top end. Since stock Twin Cam cranks and even some aftermarket cranks have been known to shift out of true, Charlie beefs up all his TC cranks to eliminate any problems. Although Charlie has never had a problem with a JIMS 120 TC crank, he still modifies them--like all others--to eliminate any potential problems.

JIMS 120-inch crankcases are stout and designed to handle much greater power levels than stock factory cases. Beefier walls, larger radiuses and thicker mounting pads provide greater strength while allowing cylinder spigot sizes up to 4.8 inches. The JIMS stroker crank is a pressed-together assembly with two one-piece mainshaft and flywheel units held together by a pressed-in crankpin. Beefy I-beam connecting rods connect the crankpin to the pistons.

Since the JIMS stroker crank comes installed in the crankcases, Charlie started by disassembling the new cases and removing the crank. To eliminate any potential crankshaft shifting, Charlie TIG-welds the crankpin on each end and then retrues the pressed-together crank to within 0.0005 inch. Fortunately, the stout JIMS cases already include a left-side double-tapered Timken bearing, which is much preferred over the 2003-and-newer factory roller bearing for keeping a TC crank straight and true.

JIMS engines come equipped with chain-driven cams with either spring-loaded chain tensioners (early versions) or hydraulic tensioners (late models). Stock JIMS 120-inch cams are Screamin' Eagle 264 with .635-inch lift and 264/262 degrees duration. These cams will be replaced with Andrews gear-driven cams with .640-inch lift and 272/276 degrees duration. Gear-drive cams provide more accurate valve timing and eliminate any potential problems with cam chain tensioners.

Charlie performed his Vortec II head modifications to the stock JIMS head castings. The stock 2.080-inch intake and 1.625-inch exhaust valves were retained, but Charlie modified the ports for increased airflow, welded and machined the chambers into a modified kidney shape for improved squish and flow, set the chamber volume for a 10.5:1 static compression ratio (stock JIMS 120-inch compression ratio is 10:1) and installed a valve spring package compatible with the 64G cams.

The throat of the stock factory Delphi throttle body is roughly 46mm. Performance throttle bodies with throats ranging between 48mm and 60mm and even larger are available. Since fuel injection is only flowing air from the air cleaner to the intake port, throttle body sizing is not as critical as carburetor sizing. A 54mm to 57mm throttle body is in the ballpark for this size of street engine. Leo's Kryakyn 57mm throttle body is in the ballpark, but you don't need to go any larger for this application. Although high-flow injectors were dyno tested, the stock factory injectors gave the best power when tuned with the SE race tuner.

Finding a stylish exhaust system with excellent power production on a big-inch street engine is an arduous and costly task. It's hard to find both good power and good looks in one system, because the pipe must flow a large volume. And you can easily spend $2,000 to $3,000 buying only a few pipes to test. Nevertheless, the pipe must be compatible with the engine's displacement, cam timing, induction system and the application's rpm range. Several exhaust systems were dyno tested, and power varied significantly. Leo finally decided on using a Vance & Hines two-into-one Pro Pipe.

Despite what some would have you believe, building a custom big-inch EFI engine takes much dyno tuning to get acceptable power. Make no mistake, making power on a JIMS 120-inch engine is easier and less costly using a 51mm CV carb than EFI. Moreover, many of the magazine dyno charts for the JIMS 120-inch engine are for carbureted versions with unknown exhaust systems. Leo's EFI engine was dyno tuned on three different days: twice in the Phoenix area and once in Vancouver, British Columbia, Canada. The first session lasted all day. Several exhaust systems were tested, stock and high-flow injectors were tried and even a stock 46mm Delphi throttle body was installed at one point. Best power was found using the V&H pipe, stock injectors and the Kryakyn throttle body.

Truth be told, horsepower started out at only 100, gradually climbed up to 109, then up to 118. Finally, horsepower topped out at 131.64 at 6100 rpm and torque at 134.44 at 4500 rpm. The clutch was slightly slipping during the final dyno session. With a new clutch pack and a bit more dyno tuning, 135 to 140 streetable horsepower is realistic for this modified JIMS 120-inch engine.

Overall, making 131.44 real horsepower with a fuel-injected street engine running on 91-octane pump gas and a street exhaust system isn't too shabby. And this is especially notable considering all the big power claims being bantered around. Leo rode his JIMS-powered bagger back to Calgary, Canada, from Phoenix before dyno tuning the engine a final time in Vancouver and was impressed by the engine's power and smoothness. He also rode the bike to Sturgis and turned some heads with its outstanding performance. Clearly, with the right combination of parts, you can make 1.10 to 1.15hp/ci on pump gas with a large EFI Twin Cam engine. Just be prepared to do a bunch of dyno tuning to capture that hidden power.

This is the final dyno chart for Leo's modified JIMS 120-inch engine. Horsepower peaks at 131.64 at 6100 rpm and torque at 134.44 at 4500 rpm. The engine starts making 100 lb-ft of torque at about 2300 rpm and maintains at least 100 lb-ft to 6500 rpm. Torque doesn't drop below 125 lb-ft until about 5252 rpm. The same basic engine combination with 13:1 compression, stepped 21⁄4- to 2-inch headers and race gas can make an honest 150 rear wheel horsepower.