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Big spark more power?

Would an engine with a powerful spark produce more power than an engine with an adequate spark, that is a spark that is enough to ignite the air/fuel mixture? My father says no because he thinks once the gas ignites it doesn’t care how big the spark was. I think it would make a difference because the gas would burn more completely. Who is right? I have a 1975 YZ125 with two spark plug holes, one of which is being used. Would my bike produce more power if both were used?

David Gravereaux New Canaan. Conn.

Your father is right.

There’s very little power difference as long as the spark is strong enough to start the mixture burning. The second tapped hole wasn’t put in your YZl25 for a second spark plug; it was provided so a compression release could be fitted.

If you actually put a spark plug in the compression release hole and added the electrical bits to fire it, the main thing you would change is the ignition timing required for best power. Two plugs would start the burning in two places, and less time would be required for the mixture to burn. The idea is to have the mixture completely burned just as the piston reaches the top of its stroke, so the two plug version of your YZ would have to fire the plugs later than the single plug version.

The quicker burn time could be an advantage if the engine was having detonation (pinging) problems. Detonation is the early > explosion of the mixture in the cylinder that the flame front hasn’t reached. Quicker burn times reduce the risk of detonation, and a two plug YZ could either run with lower octane gasoline or more compression ratio compared to a one plug YZ.

On some engines two plugs can be used with more compression to make more power, but the plugs by themselves aren’t a horsepower secret.

Contact area

In Steve Ferruggia’s letter on tire contact patch size (July, 1983) he mentions the size of the size of the contact patch is fixed regardless of tire width. According to his high school physics different tire widths would change the shape of the patch but not its area for any given tire pressure and vehicle weight.

Could someone explain how this is possible? It seems to me that variations are possible given differences in tread pattern, carcass profile, and sidewall stiffness.

Richard Ross

Cucamonga, Calif.

Steve Ferruggia was right that contact area is basically related to tire pressure and weight on the tire, and you're right in pointing out that variations in tire design can lead to variations from that ideal. If a tire were perfectly flexible, contact patch area would be constant for a given air pressure and load; the contact area times the air pressure would equal the load. A real tire isn’t perfectly flexible and has some stiff ness of its own. The tire sidewalls carry some of the weight, and the contact area is generally less than the ideal would predict. The tread pattern also has an effect, and a tire with a relatively open tread pattern has less rubber in contact with a road than a slick version of the same tire.

The perfectly flexible tire idealization is useful in understanding why larger tires have been used for racing and high performance applications. It’s not because they put more rubber in contact with the road at anv given time, but instead they allow wear and heat build up to be spread over the larger tire surface. This allows a softer rubber compound to be used for a given tire life.

Bigger carbs?

My 1978 KZ650 has a Bassani Superbike header with competition baffle,

K&N air filters, and stock carbs with revised jetting. Would bolting on a set of Maxi-bore or Smoothbore carbs make a significant difference in performance?

Jake Logan

Amarillo, Texas It all depends on what you mean by significant and what you mean by bolting on.

From our experience, larger carburetors might improve power 5 to 10 percent at the top of the rev range, and the engine might continue to

make power past the redline (not that over revving the engine would be any more desirable with the new carbs). The catches are that the improvements will only come about with correct jetting, and low speed engine power may be reduced. Until you have the motor carbureting well, it may be slower than it is now. And, as we’ve discovered, it’s possible to have a motorcycle that runs well on the main jets and will run quick quarter-miles while the idle circuits are so fouled up that the bike is miserable to ride around town.