Right On Time
The heart of an engine is its timing - port timing, pipe timing, and ignition timing. This means that an engine should be designed for a specific purpose, with all the components targeted for a certain power band. The flow rate of the ports, the induction size, the exhaust scavenging, etc., all contribute to this power band. The combined weight of the machine and the rider (the weight to horsepower ratio) has to be considered. The size of the impeller and the nozzle diameter, as well as the ride plate and intake grate, play definite roles in performance.
It's always true that if you do a job right the first time, you'll save yourself a lot of grief. Remember that speed costs money, so the first thing you need to decide is how fast you really want (and can afford) to go. The wider the power band, the higher the cost. There are a few rules of thumb which I think apply to all engines. These are:
1) Select the fuel you'll be using, and stick with it.
2) Choose your lubricant carefully, and stick with it. Don't mix brands.
3) Choose a tunable exhaust system (if possible), designed for your particular application (limited, sport mod, or all-out racing).
4) Blueprint the engine cases.
5) Port the cylinder to complement the exhaust system, using the proper Port Time Area for a usable power band.
6) Design and machine the head's combustion chamber to complement the engine's hoped-for speed and Squish Velocity.
7) Match precisely the exhaust manifold to the cylinder port. Do the same with the carburetor's intake manifold.
8) The ignition timing should be set precisely to correspond with the engine load.
Let's look at upping the performance of a Yamaha 701cc engine. By installing a good exhaust (I'm picking a Factory Pipe unit), we can realize as much as a 10-12 hp increase at 6500 rpm. I've also found that in raising the fuel octane and doing a little head modification you can increase the torque, but you have to be very careful not to over-compress this large-bore cylinder, as it's very difficult to seal the combustion chamber on larger bores. For me, when a head modification is necessary, I use an 0-ring system, because I feel more secure with that.
Okay, let's get to work.
STEP ONE
Any engine deserves the best fuel and lubrication you can give it. Start with your fuel. If you're using pump gas, stick with a reliable company's 92-octane petrol. If you're making the move to high- octane racing fuel, there are any number of excellent choices out there. Find the one that works for you and don't change brands unless you have to. (I personally use VP's C-12 Racing Fuel, 108 octane.)
STEP TWO
Lubrication is every bit as important as fuel. As with the fuel, decide which brand you're going to use, and stick to it. Some people swear by synthetics; some don't think they're necessary. The final choice is between you and your engine builder. I would stay away from off-brands you've never heard of. The name-brands may be more expensive, but you'll always know what you're getting.
STEP THREE
Selecting the proper pipe can make a tremendous difference in your machine's performance. Which pipe is right for you? The only way to really find out would be to buy a bunch and test them yourself. This would cost a lot of money and take a long time. You can shorten the process by reading the tests published in PWCI and the other magazines, and by talking with your mechanic about exactly what you're after. There are cast pipes, stamped pipes, good pipes, and less-good pipes. The thing to remember is that a good pipe can suddenly become a less-good pipe if it's not a good fit for the rest of your components. Everything has to match. That's why talking to your mechanic is so important.
All in all, I lean toward tunable pipes because they give me the ability to further customize my exhaust to my riding. There are people who disagree, but that's been my experience. (Lately, I've been using Factory Pipes with excellent success.)
STEP FOUR
Call me crazy, but I don't think the installation and expense of large reeds (which can cost as much as $200) is worth the extra few miles per hour they deliver in a limited or sport mod application. If you have to have them, I've had good luck with the larger factory reeds used on the WaveRaider.
STEP FIVE
Do yourself a favor and choose the best head you can find (and afford). Spend a little extra if you have to, it'll wind up saving you money later. Your mechanic will help you make the right decision.
Also, decide which fuel delivery system is right for you. You can go with the traditional carbu- retor, or you may want to opt for a fuel injection system. The newer fuel injection systems are very reliable, and can be reset without too much difficulty.
STEP SIX
I believe the ignition on a modified engine is very important. A big-bore cylinder requires a strong ignition. Combine that with a high-compression, high- octane fuel, and you can see why that hefty spark is important.
On a stock engine, just about any system will work, but as the compression is increased, you need to increase your ignition's output as well. Otherwise, the higher compression may extinguish the spark, which, in turn, will fail to ignite the fuel/air mixture to full potential. This results in the loss of the unburned charge through the exhaust pipe, and the loss of the power you're trying to build.
For most people, a charging ignition is the right choice, I think, because of the ease of use and reliability. However, if you really start getting into high compression and horsepower, you'll probably be needing to make the move to a total-loss setup. The total-loss systems are more difficult to use, requiring external chargers and so forth, but they may be the answer to your ignition question. (I use an MSD ProGrammable ignition, a total-loss system which allows me to program the point and rate of retard, allowing the utilization of practically any power band.)
WRAP UP
Of course, these aren't all the steps involved in building a motor, but they should help you understand what your mechanic's talking about when you start the process. What I can't stress enough is that the most important thing you can do when it comes to building an engine is to ask questions and learn everything you can, before you see your mechanic, and throughout the engine-building process. Knowledge is power. Use it.
George Grabowski HPT Sport USA