Service
SERVICE
Oval versus round
I have some questions concerning Honda’s oval piston technology: 1) Does the oval-piston NR750 produce more torque than the round-piston RVF750? 2) Does Honda have plans (besides endurance racing) for oval-piston technology? 3) Do more valves per cylinders generally increase valve-adjustment intervals? 4) Couldn’t any ring-sealing problem be solved by curving the straight area of the piston? 5) Does a 90-degree V-Four with a 360-degree crankshaft produce more low-end torque than the same unit with a 180-degree crankshaft?
Glenn Pady Porter, Texas
l) We don 7 know exact torque figures for the two bikes, but we expect torque output is similar and that the NR 750 makes its peak torque at a higher rpm than the R VF750. This is almost certainly due to engine tuning, and we don 7 expect either engine type to have an inherent torque advantage.
2) Unfortunately, Honda doesn't share its future plans with us; we wouldn 7 be surprised, however, to see a Honda street bike (for the Japanese home market) use an oval-piston engine in the next year or two.
3) More valves by no means guarantee longer valve-adjustment inter-> vtf/.sv in general, however, there are enough mechanical benefits to small valves so that valve-adjustment intervals should 6c longer for multi-valve engines, everything else being equal.
4) Judging by the way current ovalpiston Honda engines run, their piston rings must seal fairly well. We don 7 know if their sealing properties need further improvement, or if changing the shape of the piston would be beneficial.
5) Honda has uniformly gone to 360-degree crankshafts in its racing VFours, explaining that this configuration allows for exhaust tuning that provides a wider powerband. Honda's racing department has done that while the company's marketing department has pushed 180-degree crankshafts that provide a better exhaust note; so, from a performance standpoint,
Honda must believe that the-360 degree crankshaft has a performance advantage.
Not so mad Max
I raced the 1986 season on a 1986 Yamaha V-Max, and was not pleased with its quarter-mile elapsed times. When my manager and I purchased the bike we noted a 10.89second ET in your magazine by Jay Gleason. Eve run consistent 11.70second times, and have yet to see a 10-second pass. Let me note that I have raced the entire season in Ohio.
The bike is factory all the way down to the tires, and I come off the line at 4000 rpm in first gear, keeping the front wheel down. I weigh 145 pounds, and I take only the mirrors off before a race. The bike has 1473 miles on it today.
I don’t understand why the bike won’t reach the Tens. In fact, along with the 11.70-second ETs, it runs only 117 mph. What does Gleason know that I don’t?
Darrell Ray Campbell Amherst, Ohio
That 10.89-second quarter-mile time you refer to was turned by our 165-pound technical editor, not Gleason. That same day, on the same V-Max, Dale Walker, a professional drag racer, ran a 10.64-second quarter with a 128.57-mph terminal speed.
From the vast discrepancies in terminal speeds, it's clear that our test bike and your production bike run very differently; yours certainly isn V making full-rated power unless you're running on an uphill track on a scorching hot day. If that's not the case, we'd suggest a very thorough look at your bike's engine.
Are cam timing and ignition timing set to specification? Check the ignition timing with a strobe to see if it's advancing properly over the entire rev range. How do the cylinders rate on a compression or leak-down test? In our experience, bike-to-bike power variations often result from differences in piston-ring and valve sealing; if the leak-down or compression check yields substandard results, think about cylinder honing and re-ringing, or a good valve job. Finally, check the V-Max's V-Boost mechanism; if the system isn't working properly, the power will absolutely flatten out above about 6500 rpm.
We're sure you 'll find something to fix; and once you remedy the problem, your V-Max should start running 10second quarters.
Ultimate Vision
I am desperate for anything short of a trade-in that will turn my 1983 Vision 550 into a currently competitive middleweight sportbike. I want to run with Ninjas and Hurricanes, at least at the dragstrip.
Michael R. P. Verona
Summerville, South Carolina
Eddie Wilbanks, of Wilbanks Racing Products (1140 Wilbec, Memphis, FN 3811 7; [901] 6827409) is currently working to homologate a 750cc version of the Vision engine for AMA Class C competition. He also offers hop-up parts for the Vision ranging from the mild to a 90-horsepower-atthe-rearwheel, Weber-carbureted, bored-and-stroked, 750cc Vision engine. He'd certainly be eager to give you a hand in your quest to outdo the 600cc Fours. 0
