In Search of Torque - We Bore-Up
Let's say you are stuck in traffic and have been putting along in fourth gear at 2,000 rpm or so. You finally get an opening and want to get up to 100 km/h quickly, so you can be on your way and dry some of the sweat you've been collecting under your arms. At 2,000 rpm your engine might be able to make, say, depending on you motorcycle 15 horsepower. If you downshift twice and raise its rpm, you'll have at least twice as much power and will get up to 100 km/h much more quickly. That is easy...This is what most motorcycle manufacturers tells us to do. It is what we know works. It is obvious, But few of us, including myself, do this simple thing. Instead we strain our poor engines at a too-low rpm and rail at the manufacturers gods: We want more torque!
Well, the first step to correct the lean air/fuel mixture in the 15 to 40-percent throttle range. Enriching the mixture by 10 percent greatly improves throttle response and acceleration, and the simple needle change along with some idle mixture adjusting can accomplish this. In the case of fuel-injection, only a slight recalibration is necessary.
When you bore the cylinder for larger displacement, be sure you get more compression with you selection of pistons for the bore job. You want to end up with a cranking pressure between 180 and 190 psi. Cranking pressure is the number you get when you do a standard compression ratio but is affected by head-gasket thickness and piston selection. In general, the higher this number the better – until detonation becomes a problem, which is likely to occurs in contemporary engines from 195 psi and higher.
What is the downside, you ask? Sure, there always is a downside. Because of its higher cranking pressure, this engine is more sensitive to fuel quality and often cannot be given full throttle below 2,000 rpm or so without detonating. However, this is seldom a problem and the cure is to shift down a gear. There's nothing like increased combustion size and compression for making more torque.
