Jims Twin Cam Race Kit

JIMS Twin Cam Race Kit

CW EVALUATION

120-inch Harley-Davidson tune-up

IT’S AN AGE-OLD CLICHÉ BUT JUST AS true today as ever: You can’t beat cubic inches. Cams and valves, carbs and compression, pipes and ignition all add punch to a motor’s output, usually more in one rpm range than another; but when you add inches, the power program steps up everywhere.

If you subscribe to that bigger-is-better philosophy, JIMS Twin Cam Race Kit speaks your language. This “kit” is a complete Twin Cam powerplant, right down to the carburetor and ignition, and it packs a 120-cubic-inch wallop instead of the stock motor’s 88. It’s called a kit because it comes partially assembled in three boxes; final assembly is up to the owner or a capable mechanic.

This engine is the product of collaboration between JIMS Machining-manu-

facturer of aftermarket components and special tools for Milwaukee iron-and The Motor Company itself. For quite some time, JIMS has been supplying Harley with select components, including lifters for stock engines as well as stroker crankshafts sold through H-D’s Screamin’ Eagle accessory division and used in some of the company’s 103-inch Custom Vehicle Operations models. Harley also granted JIMS access to its engine technology to help develop larger-displacement version the Twin Cam 88 motor. The end result is the $8000 Cam Race Kit, which fits Dyna and FL models and is claimed to produce 125 rear-wheel horsepower and 121 foot-pounds of torque.

Although the kit is manufactured by JIMS at its headquarters in Camarillo, California, it is available only through Harley-Davidson dealers. Both H-D and JIMS clearly and emphatically state that this is not a street-legal engine; it’s for racing only. How much “wink-wink, nudge-nudge” you interpret into that claim is your call. But despite supposedly being a competition motor, it comes with a warranty that extends 90 days from the date of purchase.

Visually, the JIMS motor looks much like a stock Twin Cam engine, but it’s far from it; virtually every piece has been manufactured just for this kit. The engine cases are a JIMS product, beefier and with a much wider cylinder-stud spacing that can accommodate a bore of up to 4.8 inches; stock cases can only handle up to 4.125 inches, and even then, just barely. The kit motor, coincidentally, has a 4.125-inch bore and mates it with a 4.5-inch stroke-Vs-inch longer than that of the “standard” Screamin’ Eagle stroker crank.

So, too, are the cylinders, forged pistons and cams JIMS items, but designed in conjunction with H-D. The heads are Screamin’ Eagle castings with larger, 2.080-inch intake and 1.625-inch exhaust valves (the stockers are 1.940 and 1.560 inches, respectively) from JIMS. The compression ratio is a rather high 10.0:1, and the cams have considerable duration (264 degrees on the intakes, 262 on the exhausts) and a healthy, .635-inch lift. The carb is a cavernous 51mm Screamin' Eagle Mikuni, with hot sparks supplied by a Pro Adjustable Map system, yet another SE item.

With one exception, all necessary hardware is included as we found when installing a kit in a 2003 Dyna Low Rider, and it goes together just as easily as a stock engine. The lower end including the complete cam case and timing cover, arrives fully assembled and the valves and springs are already in the heads. If your skills with H-D engine assembly have gotten a little rusty, you’ll find the accompanying instructions extremely thorough and easy to follow.

That one exception is the exhaust system. The assembly and tuning instructions do not make any pipe recommendations, so we arbitrarily chose to test with both a Thunder Header and a SuperTrapp 2-into-1. Just be aware that commercially available H-D exhaust systems are not configured to provide ideal tuning with a 120-inch engine; this is why people who race big-inch Harleys generally use custom-made systems.

After the 120-incher was slipped into the Dyna, it underwent 500 careful breakin miles before we strapped it to our dyno to see if it lived up to its billing.

It came close enough to suit us. With the Thunder Header, the engine “only” managed 113 horsepower, but it ground

out 123 ft.-lb. of torque, exceeding JIMS’ claim by 2 ft.-lb. There was a sizable, l0-ft.-lb. dip in the torque curve between 2100 and 2800 rpm, though, that could be felt when riding in the taller gears. Not that there is anything wrong with the Thunder Header; its dimensions simply aren’t intended for engines larger than 95 or 100 cubes.

Neither are the SuperTrapp’s, but its ability to be tuned-achieved by varying the number of discs in its silencer-helps it at least partially overcome that handicap. With it bolted in place and 18 discs in the muffler, it allowed the 120 to belt out 120 horsepower and match JIMS’ claim of 121 ft.-lb. The torque curve was smoother, too, only dipping 3 ft.-lb. between 2200 and 2700 rpm.

That dip is too small to be perceived from the saddle; instead, all you feel, any time the gaping Mikuni is dialed wide-open, is a rush of big V-Twin acceleration that’s downright breathtak-

ing. The motor zips through first gear so quickly that it bounces off the 6500rpm rev-limiter before you even have a chance to shift. The rear tire spins practically all the way through first and partway through second, and third gear launches you and the bike down the road-er, track-like something let fly from God’s Own Slingshot.

So, is the JIMS Twin Cam Race Kit worth its 8 Large price tag? We certainly think so. It’s an easy-assemble, dropin package that performs beautifully and just about doubles the 62-64 horsepower of a Stocker. Plus, there’s considerably more performance there for the taking. The ports in the cylinder heads are ascast, so they’ve not been matched or smoothed whatsoever, and an exhaust system sized for 120 inches could easily extract another 6-7 horses, if not more.

Just remember, it’s for racing only-although nothing stipulates where you race...

DETAILS

Harley-Davidson Motor Co., Inc. 3700 W. Juneau Ave. Milwaukee, Wl 53208 414/343-4680 www.harley-davidson.com Price...$8000

Ups A coordinated “kit,” not a collection of disparate parts Tested and approved by the bike’s manufacturer Maybe the best instructions ever

Downs No recommended exhaust system Not exactly cheap Aw, gee, it’s for “racing” only