Flat-Out Twin
Flat-Out Twin
QUIET RIOT SPECIAL SECTION
San Jose BMW knows the way to build a better Boxer
BRIAN CATTERSON
Has IT REALLY BEEN 27 YEARS SINCE REG PRIDMORE rode a BMW R90S to the inaugural AMA Superbike Championship? You'd never know from the way ol' Reg flogs a bike (a Honda nowadays) around the racetrack at his CLASS riding schools, nor from the progress the Bayerische Motoren Werkes has made on the performance front. The latest fuel-injected, cam-in-head, four-valve-per-cylinder flat-Twins are definitely a generation ahead of the carbureted, pushrod two-valvers that Pridmore raced, but they're still pretty tame.
No one knows that better than Chris Hodgson, proprietor of San Jose BMW (www.sjbmw.com). A dyed-in-the-wool Beemerphile who has been hopping-up Boxers for two decades now, Hodgson operates an oldtimey shop that doesn’t exactly fit the BMW corporate mold, complete with an oil-scented machine room that’s frowned upon by the suits back in Munich. But the metal shavings produced therein have resulted in a number of successful roadrace bikes, not least the two-valve “Wrecking Ball” that won the AHRMA 750cc Sportsman vintage national championship in 1990 before setting a 155-mph record at Bonneville a decade later. Not to mention the R1 lOORS-based “Silver Hammer” that Kurt Mund campaigned in Pro Twin roadraces in the ’90s.
Hodgson drew from that experience to build the RI 100S “street sleeper” you see here. We’ll gloss over the chain of events that delayed the bike’s arrival at the CW offices (forgetting all about the original stars-and-stripes paint scheme, and wishing Chris a speedy recovery from his road rash!), and get down to the details.
Starting with the numbers: When first run on the CW dyno for our October, 1998, road test, a stock RUOOS made 87 horsepower at the rear wheel; the San Jose bike makes 99. Torque increased as well, from 65 to 73 foot-pounds. That’s a 14 percent increase in peak power and a 12 percent increase in peak torque. To put it in terms more comprehendible by the masses, that’s the equivalent of powering up a Hayabusa to 182 horsepower!
How did Hodgson achieve this dramatic gain? To hear him tell it, it’s simple; in fact, his 19-year-old son Willy actually built this bike after Chris crashed the original during a test ride and his hands were too battered to build another.
First stop was the flow bench. By adding epoxy to the intake-port floors and removing metal from the ceilings, the fuel/air mixture was given a more direct shot into the combustion chambers. Subsequent reshaping of the ports and the addition of 2mm-larger valves amounted to a 21 percent increase in flow.
When the overly restrictive stock air-intake system was found to negate these gains, Hodgson fabricated a larger-diameter ram-air duct and a pair of alloy endplates that hold a higher-volume K&N filter. That brought flow back up to 18 percent over stock.
Internally, the engine was treated to 30-gram-lighter forged pistons that raised compression from the stock 11.3:1 to 12.0:1-still low enough to run on pump gas. The top-ends were balanced, and new cams keep duration close to stock, but advance the timing slightly while boosting lift by .060-inch. Crowning glory is a set of brightly polished Remus mufflers that poke out from under the seat. Total cost of the engine mods is approximately $3000.
The R1 lOOS’s chassis was left largely stock, save for the addition of fully adjustable Penske shocks front and rear and a MAP Engineering Paralever torque arm. Together, this increased ride height .75-inch in the rear, making for tiptoe stops even for 6-footers. The more-than-adequate stock Brembo brake calipers were upgraded with EBC pads, and the stock rotors were mounted on lightweight Marvic wheels shod with Metzeier Sportec M-l tires. Carbon-fiber fenders and cylinder-head trim pieces added a racy finishing touch.
My first experience with the San Jose BMW came on the street en route to a photo shoot. Unlike many modified motorcycles, the R1100S fires right up and runs fine off the bottom, without any blubbering or hiccuping. Power begins to build around 4000 rpm, then grows noticeably stronger as the tach needle swings past 6500 before tailing off approaching the 8000-rpm redline. Though the engine mods were aimed at increasing peak power, there’s no perceptible loss in midrange; to the contrary, the engine is noticeably freer-revving, and the six-speed gearbox seems to shift better, too.
The term “flat-Twin” describes this bike’s exhaust note as well as the cylinder layout, because it sounds like a cross between an inboard/outboard boat motor and a British sports car: BwwWaaap!
Handling was noticeably improved, the raised rear end effectively reducing rake and trail, lightening steering. Cornering clearance also was increased, a suspicion confirmed when I took the bike to a Club Desmo track day at Buttonwillow Raceway and didn’t drag a cylinder head all day. That may not sound like a big deal to sportbike riders whose knee pucks touch the ground more often than their kickstands, but as anyone who’s ever tried to ride a BMW fast knows, it’s quite an achievement. More than a few riders on front-line sportbikes were surprised to be passed by the big, black Beemer that day.
If the rumors are true, BMW’s engineers are developing a next-generation Boxer that promises to be the sportiest yet.
If so, I’d suggest they take a good, hard look at the San Jose BMW RI 100S, and build a bike just like it. Get it done by 2006 and they could call it the 30th Anniversary Superbike Championship Edition.
I bet even Reg would approve.
