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April 2010 Vol 10 - No 3 - Tech Talk: Is a bigger engine for you? PDF Print E-mail
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Thursday, 08 April 2010 21:19
Article Index
April 2010 Vol 10 - No 3
Star of the Month: 116 bhp in Purple
Motorcycle Diaries
Tech Talk: Is a bigger engine for you?
Globalizer: When promotion is a good notion
Tech Talk: Safety Equipment
Pic of the Month: World’s liveliest Indian
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Tech Talk: Is a bigger engine for you?

Second in a series of articles

By Ben Harper,

Star Cruiser Co-Editor,


As you already know, choosing the right combination of performance modifications for your bike is a personal choice. Some riders prefer to add only a set of exhaust pipes for the sound, and leave it at that. Others like me become obsessed with power increases and are willing to spend the thousands of dollars, or the monetary equivalent, to get every bit of power they can. We have already discussed carburetors and exhaust pipes; now we move on to the next level; engine modifications.

I have modified virtually everything I could to my Road Star. While I don't expect most of you to follow in my rather big footsteps, some of the following information may be of use to you anyway.

In order to extract the most power from your engine, it is necessary to understand the principles behind a working engine. Your engine is basically an air pump; if you increase the amount of air moving through your engine, you increase the power coming from your engine. In order to do this, there are several things you can do to your engine.A good way to increase the power from your engine is to simply make your engine bigger, i.e., increase the displacement of your engine. There are many so-called "big bore" kits available today, ranging from a series of "swap out, swap in" 108 cu. Inch kits to large big-bore kits that involve engine boring and cutting of the cylinder skirt. Prices depend on the degree of work involved, but these modifications usually start at around $700.00 and go up rapidly.

Increasing the size of your intake valves is one step in the process of increasing the power in your engine. As with your choice of carburetors, a small increase in valve size will reap benefits suitable for street use. There are cylinder head modifications that go way beyond the increases I've suggested, but unless you plan to race your vehicle on a closed race course, this will make your bike almost unrideable in normal situations, and cost much more than you really need or want to spend. A head job with reasonable valve sizes will easily const $800.00 but with a good carburetor and exhaust, and will increase your power by better than 20%.

Another improvement is the addition of performance camshafts. These increase the valve lift when the valves open, giving you increase airflow and greater power. When combined with larger valves, you can pull 10- to 15% more power when compared to stock motors. Installation of cams usually requires milling the engine case to accommodate the larger cam lobes, which means this modification costs around $1000.00, but in this author's opinion, it is one of the most efficient performance increases you can make.

While you're at it, this is a good time to remove the exhaust gas recirculation tubes, if your bike came so equipped. This system is designed (poorly) to make the bike's exhaust cleaner, but it interferes with the performance modifications you will make. Yamaha dealers carry the plugs to remove this system, and it's a good investment for you under all conditions.

As you can see, power is extremely costly, and will only become more so in the near future. My engine was 130 cubic inches with 11:1 forged pistons, ported big valve heads, Orient Express cams, 45mm Mikuni flat-slide carburetor, high-flow oil pump, EGR removal kit, and more polished aluminum and chrome than I can explain.

But there are ways to economize, so please learn from my mistakes. Try to combine any projects that will save you money over the long term. In all I had 

my cylinder heads off four times, and that is a lot of labor for which I paid dearly. Also try to keep an eye out for sales from manufacturers, garage sales at dealerships, and other opportunities to save money on parts you intended to buy anyway.

I repeat, these modifications are costly, but the configuration I am discussing is for a moderate engine that is eminently usable on the street in all conditions. I have found that anything more radical tends to make the bike less enjoyable on the street, but if you're willing to sacrifice rideability for power, by all means go for it.

These modifications, in conjunction with the carburetor and exhaust system changes discussed last month, will substantially increase your engine performance without lessening your enjoyment, which is what this series of articles is all about. Next month we'll discuss suspension and brake improvements to help you control the increased performance you have built into your machine.

Till then, ride safe and keep your eyes open for other Stars. The owners just might want to talk to you about your improved Star.



Last Updated on Friday, 09 April 2010 05:27
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