350cc For the Honda
350cc FOR THE HONDA
HONDA CONVERSION
FOR SOME TIME, rumors have been floating around concerning a “350” kit for the 305cc Honda. Now, at last, the rumor is a reality, and a very attractive reality it is, too. The Forgedtrue company, makers of pistons for nearly everyone who goes fast and wants reliability, is now manufacturing a complete kit for Honda 305 (and 250) owners that will stretch their motorcycle’s engines out to the customary class limit of 350cc. With the Forgedtrue kit, the bore is brought out from the stock 2.30-in. to 2.52-in., increasing the displacement to 347.4cc; a gain of just over 12-percent.
The kit is expensive, about $135, but it contains first-rate hardware. The purchaser gets a pair of forged aluminum pistons, with rings, centrifugally cast-iron sleeves, special valve springs with damper-type retainer cups, an eight-page set of instructions, and a couple of decals that tell other Honda owners (and not a few others)
that this is a machine not to be trifled with. The instructions alone are very nearly worth the price: they are complete to the smallest detail, profusely illustrated, and anyone who is to make a mistake in installing the kit will simply have to ignore the. procedure outlined therein.
In actuality, there isn’t all that much to do. Just heat your Honda’s engine block, upside down, until the old sleeves drop out, then bore the block to the specified dimensions, hone to a fine finish for good heat transfer, reheat the block to 250-350 degrees and drop in the new sleeves. The rest of the job is straightforward assembly; no further machine work is required.
The pistons supplied with the kit are, as we have said, of the forged type, which gives the best strength for the least weight. They are grooved for three rings: the first is a dykes-pattern ring, expensive to manufacture but definitely superior in performance. The second ring is a combination compression and oil-scraper ring; and the third is a conventional oil-control ring. Standard Honda wrist-pins are used, and they are off-set in the pistons to reduce slap. The compression ratio is 9.0:1.
It may seem strange that special valve springs are included in the kit, when there is no special camshaft. There is a reason: the stock Honda springs have an unfortunate tendency to settle, and lose tension within altogether too-few miles. The replacement springs supplied in the kit are made of superior steels, and have a much longer effective life.
The sleeves included in the kit are made by the same firm that produces cylinder sleeves for Honda, and are made to the same specifications. The sleeves are of a high-density, centrifugally-cast alloy iron, and have excellent strength and wear-resisting properties.
Forgedtrue 350 kits are to be available from Webco, who have an exclusive distributorship for this product. To make a performance evaluation of the 350 kit, we prevailed upon Webco to loan us a Honda 305 Super Hawk and we ran some tests fjrst on the as-stock bike, and then repeated the tests after installation of the kit. Webco also distributes Harman and Collins camshafts, and because we thought that anyone willing to spend the money for the 350 kit would probably want to go “whole-hog,” we asked that the engine also be equipped with their #7164 camshaft and racing valve-springs. These springs, incidentally, are sold with light-alloy retainers, and they push the point of valve-
float up to a crank-speed that no rider in his right mind would ever try to reach.
To make certain that both stock and modified engines were given every chance to show their stuff, they were dyno-tuned at Modern Cycle Works, in Los Angeles. Power and torque readings were taken, and they indicated that the 350 kit, with the Harman and Collins camshaft, was indeed quite an improvement over “stock.” Valve-float, occurring at 8500 rpm, prevented the stock engine from developing its nominal rated horsepower, and we would guess that this was about par for a standard, well broken-in Honda Super Hawk. With the stock engine, 23.5 bhp (at 8500 rpm) was available at the rear wheel, and at the drag strip we ran consistently at 80 mph, with an E.T. of 17 seconds; not quite as fast as the very sharp 305 we tested last year, but good performance nonetheless. Following this, Richard Stanworth of Harman and Collins popped in the 350 kit and hot cams, giving 31 bhp at 9000 rpm, and we returned to the drag strip. After a bit of fiddling with jets and spark plugs, we ran through the standing quarter at a sizzling 88 mph, and the E.T. was brought clear down to 14.9 seconds.
This confirmed an opinion that we had already formed; that the modified engine was still relatively mild, and that it had exceptionally good torque — which accounts for the phenomenally good E.T. The Forgedtrue/Harman and Collins/Webco Honda engine is a tremendous performer, and it does the job without a trace of
temperament. We ran all tests with the mufflers in place, and the bike was easy to start and ran, if anything, more smoothly than when in stock trim. With, megaphones and some alterations in gearing, this modified Honda would almost certainly turn in 1/4-mile runs at over 90 mph. Some indication of the performance potential of this combination is seen in the competition record of one Tony Murphy, who rides a Honda outfitted with the prototype of this kit in AFM road races. Tony has been beating everything in his class, and many of the bigger bikes. Watching Tony haul down a long straight, running a dead-heat with a 500cc Manx Norton, tells the onlooker that there is power to be had out of the inexpensive Hondas.
And what of reliability? Once again Tony Murphy’s record can supply the answer; the modified Honda runs, and runs, and runs. It is, if anything, more reliable than a stocker. Actually, this is not too surprising; on a very high-speed engine, the heaviest strain comes from inertia forces, and in this context we should mention that the Forgedtrue pistons are lighter than their stock counterpart. Thus, at any given rate of revolution, the modified engine is running at a lower level of stress. In all, this is a kit that merits our unqualified approval. It is not cheap — but then it is not cheaply made, and Honda owners who seriously want that extra performance they are forever writing our technical editor about would be well advised to get off an inquiry to Webco. •
