Gernot Schuh has kindly answered some of my technical questions regarding the modifications to the J.A.P. KTOR engine in what was formerly a normally aspirated, racing 1925 Zenith. The first incarnation was surely built under the supervision of Freddie Barnes himself, who personally oversaw all of the 'big gun' racers which exited his factory doors.
While we don't yet know the early history of Super Kim in England, it's useful to recall that this small company holds the distinction of more Brooklands 'Gold Stars' (for a lap over 100mph during a race) to its name than any other. This Zenith was originally built for a purpose - going fast, preferably faster than any other machine available at the time. Thus, it is no coincidence that the Sigrands chose a Zenith (over say, a Brough Superior, MacEvoy, etc) to bring home to South America; the Zenith had the reputation as the Fastest Machine You Could Buy for track racing in the 1920's. George Brough may have claimed differently, but the records at Brooklands tell the tale. Zenith was the Bomb.
Some technical notes (my words combined with Gernot's here):
The bore and stroke of this monster are 94.9x120mm, giving 1700cc, with a compression ratio of 6.6:1. Combine this with the blower's nominal delivery volume, this would give a corrected compression at maximum revs of 15.8:1 ! 
How can this work, especially with the carburettor setup, which we found to be for petrol rather than alcohol? The 'racing' cam timing holds the clue; the supercharger's efficiency, combined with extensive valve-overlap from the engine's unblown racing days. An 8.8mm valve lift on both exhaust and inlet valves at maximum 'bump' leaves about 4mm of valve opening at TDC! The incoming charge, blown past the partially open valves, would theoretically keep the exhaust valve in particular just a bit cooler, helping it survive in the very harsh combustion environment with such a high C.R.
The gearing pencils out for 260km/hr @ 5000rpm (156mph) in third gear. Wheelspin above 120mph probably would keep the maximum a bit below the theoretical top speed. Super Kim uses a three-speed Sturmey-Archer 'Super Heavyweight' gearbox, one of six ever built for record-breaking, and I believe, the only on not in use on a Brough Superior. Was it originally specified thus from Freddie Barnes? We don't know yet, as we don't know whether the machine was purchased new from Zenith, or in 'used' condition after a few seasons at Brooklands.
At maximum revs of 5000rpm, the piston speed is 20m/sec, a figure typically recommended as safe for racing in the engineering literature. The extensive piston drilling is a relic of 'Teens and Twenty's racing practice, when lightness was a premium (especially during the days of cast-iron pistons!), and engine revs were fairly low. Such drilling was also typical on connecting rods, and just about any other moving part in the engine! By the 1930's, as metallurgy improved, such 'cheesework' became obsolete and unneccessary. The pistons in Super Kim are clearly made in-house, as they're of a design I haven't seen, although it's possible, with such a large bore, that they came from a car.
Overall weight dry is 215kg (473lbs), which is really light for a Land Speed Record machine of this capacity; weight per se isn't a factor in top speed, only acceleration, and in fact, greater weight is an advantage in keeping the motorcycle stable, with both wheels on the ground, and helps minimize wheelspin. Having ridden rigid-frame racers at well over 100mph, I can attest that they aviate! A little avoirdupois certainly couldn't hurt, unless there is limited space possible for a straight-line run - a point completely relevant to a speed record in Argentina, which has few roads (or beaches) capable of a full-throttle, multi-mile record attempt. Quick acceleration might have been essential if only 3 or 4 miles of smooth straightaway were available.
During his rebuild, Gernot replaced the half-time pinion in the cambox, and the right-hand main bearing, and all the ball races in the drive side, along with the chains, tires, valves and springs, ignition cable, and about a dozen smaller parts. The camshaft and roller followers were refaced. All of this work took 320 hours, which includes 120 hours of deep cleaning and rust removal!
Asked if there were any surprises, Gernot says, "Suprises? The capacity, sure. Hmmm....and the twisted history, that is absolutely fascinating to me. That there is contact with the family. And so much appreciation for my work."
And how does it SOUND? "Like a tractor on drugs; on revs, it lets out a ROAR..." He adds, "The most important thing is: it runs, despite so many experts, who claimed it would not, and looks more like a racingman's hot dream. The inspection showed that it was used, at least for testing. There were traces of use on the cylinder walls, oil carbon here and there, and in the gearbox there was heavy wear between the mainshaft and the tubular shaft where the sprocket sits. This odd plate, that clamps the gearbox at the pin, were the kick start was once located, was not part of the original design."
To this I would add my own story with Super Kim. I was first introduced to photographs of the bike at Jay Leno's garage, 18 years ago. The seller, Hector Mendizabal (r.i.p.), claimed it was a Brough Superior, and it clearly wasn't, but we didn't at the time know what exactly it was. A bit of research on my own revealed it to be a highly modified Zenith, but the asking price was over the top at the time. By coincidence, I purchased a BS SS100 engine from Mendizabal a year later, and the Zenith came up again, still over-priced. I purchased the machine 11 years ago from the man who actually posessed Super Kim, for half the original price, after a tip-off from a Brough-owning friend in Germany.
I was warned at the time by 'experts' that Super Kim was a fake, a lash-up, a South American piece of trash, not worth the money, and that I would surely be defrauded at best, possibly shot at worst! I've spoken on this site before about the 'aura of fraudulence' around machines from South America, which hovers in the mind of North American and European bike collectors. That mindset persists to this day, and is evidenced by Gernot's own comments above regarding 'doubting experts'.
When I attempted to sell Super Kim 9 years ago, I was met with the same resistance, doubts, and scare talk. One Austrian fellow backed out of a signed purchase agreement, after being told by such an expert that the machine was basically worthless! I am fairly certain he regrets his decision nowadays... given that the last big-twin Zenith with racing history (unsupercharged) sold for $325,000.
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