Service
SERVICE
We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your "Service” letters to technical subjects only, and keep them as brief as possible. Send them to: "Service," CYCLE WORLD. 1499 Monrovia Ave., Newport Beach, Calif. 92663.
TO EACH HIS OCTANE
An item in your May issue suggested ways of mixing different available grades of gasoline to obtain approximately 98 RON (Research Octane Number) for motorcycles needing leaded premium gas.
How about engines with higher-than-stock compression ratios? Do they need more lead or octane? After some engine modifications my Norton 850 Commando
has a c.r. of 10:1. Is the suggested mix of
three parts unleaded premium to one part regular leaded sufficient for my engine? Or is an octane booster (if available)
necessary
Tim Gibbons Niagara Falls, Ont.
You'll have to experiment. The method mentioned does work, that is, mixing gasoline of different octane ratings results in a tank of fuel with an octane rating somewhere between that of what you start with and what you pour into it.
First, though, the lead content isn't the
vital part. Lead is simply an additive. Put lead into the base gas and you raise the octane rating of that base. There are other (more expensive) ways to raise the octane rating, so you can have an unleaded gas with a higher rating than a leaded gas has.
Next, an engine's c.r. is only one factor in its octane requirements. Combustion chamber design is another. So is camshaft timing, and so is the ignition's advance curve.
As a rough rule, wild camshafts mean you can run lower grade gas on the same c.r. because the engine isn 7 getting its full intake charge or full compression when the revs are low and the engine is under load. A vacuum advance lets you use a high c.r. because opening the throttle retards the spark and reduces octane demand. The water-cooled, high-rev, high-compression Honda CX500 uses 90 octane no-lead, for instance, while one of our crew has a bored XL250 that demands 94 octane premium, although both engines have 10:1 c.r.
In your case, the Norton engine was designed before the engineers worried about
octane ratings. A 10:1 c.r. is borderline with today's 98-octane RON premium, leaded ófïy
not. We'd suggest you try the mix, and if it doesn't work well, retard your initial ignition setting and use stiffen advance curve springs to slow down the rate of spark™ advance.
IN SEARCH OF POWER
I’m planning to install a longer duration. higher lift camshaft in my Twin. Stock compression ratio is 9.5:1. which yields 150 to 155 psi on my compression gauge.
My mechanic suggests I install Venolia
pistons of 11:1 compression ratio to mak^_, up for the loss of pressure incurred by the later closing of the intake valve. I want as
much as I can get without also getting a
detonation problem. What would yoiHsuggest?
Bill Bowling
Oakdale, Calif.
We'd suggest caution. Your mechanic knows his theory. As mentioned above, longer camshaft liming means less pressure when the engine is running below its designed power band. A nd of course as the revs go up, so does the ability of the engine to use less octane for the same compression.
But there's no reason to go too far. The power gain between 10:1 and 11:1 won't be worth the chance you'll be caught without a good brand offuel. Especially now that the best isn 't as good as it was and we sometimes must buy the grade they've got, or walk. We'd go with the Venolia piston, but we'd order them in the 10:1 c.r.
STARVED FOR OIL
I have a 1977 Kawasaki 1000. Lately when drag racing the oil warning light
comes on when I hit second gear hard.
How can 1 fix this? (A couple of my friends have the same problem.)
Marty Pavilonius Chicago. 111.
It’s a common racing problem. Your engine is so powerful the force of the bike leaping forward is stacking the oil into the back of the sump. Away from the oil pump pickup.
Do this often enough and your bike’s wrist pins will gaul and seize. Because the drag and road race guys out-run their oil easily, many of the racing performance shops, including R.C. Engineering, and Yoshimura, will install flapper valves to hold oil near the pickup. Check in your area for a shop dealing with racing equipment.
