How-To: Bolt-On Horsepower—Getting the Most Bang for Your Buck

Taking a look at the most beneficial bolt-on mods for additional ponies

Horsepower and the dough spent to find it are always subject to the law of diminishing returns. Early on the gains are easy to come by and relatively cheap, but as the search continues you’ll find yourself spending a lot more to gain a lot less. Since most of us are bound by what the late Memphis Slim once described as the “rent situation,” it pays to spend wisely. The question is, where exactly do you get the most bang for your buck? Let’s take a look at some of the most common bolt-on modifications, what they cost, and what kind of horsepower gains you can expect to find, and see if we can’t come up with an answer.

Baby Steps—Jet Kits
Due to emission laws many bikes are jetted on the lean side. Depending on the circumstances you may find yourself with anything from a bike that runs perfectly, to one that spits and sputters like Popeye on a bender every time the throttle is opened. Assuming your bike isn't one of the perfect ones, and further assuming that you'd like it to be, a simple jet kit or fuel-injection tuner should be all you need to put things right.

As the name implies, a jet kit normally includes a selection of jets, both main and pilot, a new needle, any needed shims or springs and, most importantly, a full set of installation instructions and jetting guidelines, which should cover a wide range of scenarios from stock to heavily modified. There are also kits specifically designed to work with a given manufacturer’s pipe and airbox modifications. These are usually sold or supplied gratis in conjunction with those products.

Horsepower Gain: Marginal if the kit is being installed on a stock bike; however, it will smooth over any rough spots in the fuel delivery and get the bike running properly. On a modified bike, jet kits short-cut the tuning process and eliminate a lot of guesswork.

Cost: $75 to $150 depending on application.

Skill Level: You should have at least a passing familiarity with your carburetors, but generally these kits are easy to install. You'll need to drill out the anti-tamper plug over the pilot screw, which may entail removing the carburetor(s) from the bike, and disassemble the carburetor so you can replace the jets and needle, and in some cases modify the slide. But other than that these kits involve little more than swapping parts.

Jet kits, dyno, bolt-on horsepower how-to
The first step, installing a jet kit, basically got our stock 1200cc Sportster running the way God and its designer intended it to. With 55.64 horsepower and 66.0 ft.-lbs. of torque, this is a pleasant bike but a little doggy.Cruiser

Fuel-Injection Tuners
Although fuel-injected bikes have far fewer issues than their carbureted brethren, EFI systems are by no means perfect, nor are they designed to cope with modifications that radically affect their mapping, hence the popularity of EFI tuners. Tuners fall into two broad categories: those that are plug-and-play devices designed to be used with stock or lightly modified engines and those that can be programmed via a PC to meet a highly tuned engine's requirements. Tuners in the first group are an excellent choice if you contemplate riding the bike in stock trim, or perhaps adding only a pipe or airbox kit. Those in the second group are of the most value if you plan on taking your quest for horsepower beyond the "bolt-on" scene.

Horsepower Gain: Again, unless you're using one in conjunction with some other change, the horsepower gain won't be much, but in most cases they will improve overall ridability.

Cost: $189 to $329, although add-on peripherals will up the price.

Skill Level: Basically, if you can read the supplied instructions you can install it. Most of these devices plug directly into the stock wiring harness, although I can think of at least one that requires some splicing. Externally adjustable trim screws are provided, which makes fine-tuning these units to a particular pipe a snap, although custom mapping one will take it to the "should be an experienced tuner level."

dyno, bolt-on horsepower how-to
Not too shabby—installing the Screaming Eagle SE11 mufflers and jetting accordingly yielded an immediate 14-percent increase in horsepower and a 9 percent gain in torque.Cruiser

Pipe Job
Exhaust swaps are the single most popular bolt-on modification. The question is, do open exhausts make more power, or just more noise? And secondly, how do you go about selecting a pipe? Both good, if slightly loaded, questions. The short answer to the first is a qualified yes. Assuming that the pipe in question is properly designed, and further assuming that the pipe is suited for the application, chances are good that you'll see some sort of power increase. Unfortunately, far too many of us pick pipes based on esthetics instead of engineering. In those cases you're likely to get a lot more sizzle than steak.

Which brings us to the second part of the equation: How do you pick an aftermarket pipe? In broad strokes, a two-into-one pipe produces the most useable power, particularly in the lower half of the powerband, which is where 99 percent of riders spend 99 percent of their time. Generally they are also the quietest type of pipe, if that’s an issue for you (and it should be).

If you don’t like the way a two-into-one looks, I’d recommend individual pipes that incorporate some sort of muffler and have reasonably sized inner diameters. Last on my list would be the straight, baffled type of pipe.

I’d recommend staying away from unbaffled drag pipes and the very short, top-fuel-style header pipes, as well as those huge bazooka-type pipes. In the real world these pipes make too much noise and not nearly enough power, and when they do make power it’s biased toward the upper end of the rpm scale, often with an accompanying dip in the torque curve in the midrange.

Horsepower Gain: With a jet kit and airbox modifications, a 10- to 15-percent (sometimes more) increase is very possible.

Cost: Prices here are all over the map, in part because there are so many guys making pipes, and so many catalogs discounting them. Expect to see slip-on mufflers starting around $200 and full systems ranging from $350 to $1000.

Skill Level: Installing a pipe is entry-level stuff. The harder part will be installing the jet kit.

dyno, bolt-on horsepower how-to
Opening up the airbox and installing a less-restrictive filter netted another nice gain, 3.36 more horsepower and an additional 7.54 ft.-lbs. of torque.Cruiser

Open Wide
Because intake roar contributes to a motorcycle's overall noise level, manufacturers want to keep it to a minimum. One way they do this is by reducing the size of the airbox opening. The unfortunate side effect is that very often the intake plumbing becomes somewhat strangled, reducing the flow of incoming air. This becomes more of a problem when the exhaust is uncorked because the scavenging effect of the modified pipe is of the most benefit when it's not fighting an intake restriction. Be advised that some of the kits remove the entire airbox, leaving the filters exposed to the elements—not exactly a great idea if you do a lot of wet-weather riding.

Riders of some bikes can take this a step further. For example, Barons has a ported manifold available for the ’99–’05 Yamaha Road Stars that adds maybe 7 horsepower. It retails for around $125 and bolts straight on. Likewise, Patrick Racing has a carburetor kit for the same bike that includes your choice of a 42 or 45mm flat-slide Mikuni that’ll get you an additional 10 to 12 ponies, when combined with a ported manifold and the right pipe. At $646.99 (45mm carb, stainless steel cables) it ain’t exactly cheap, but considering the horsepower it’ll provide it’s not bad.

Horsepower Gain: In most cases modifying the airbox provides a modest increase, maybe 1–3 horsepower, but with the appropriate pipe gains of up to 5 hp may be realized.

Cost: Filters start at about $40; complete air-box-removal kits can up the ante to $300 or better.

Skill Level: Minimal to moderate. Installing one of these kits isn't rocket science but you will need a selection of hand tools, your shop manual and, with some bikes, a few hours of your time.

dyno, bolt-on horsepower how-to
The first ignition graph shows the Screaming Eagle module, with its modified advance curve, installed along with the pipe and airbox mods. It’s not too impressive, is it? Barely a 1-horsepower gain at the very top, and actually a slight drop in peak torque, but it did eliminate the little dip in the torque curve when the throttle was snapped open. Take a look at the second chart; you’ll see why absolute numbers can be misleading.Cruiser

A Jolting Development
Ignition is one area that's often overlooked, but if your sparks can't keep up with the rest of the engine, then all the hot-rod parts in the world won't be worth a tinker's damn. How much horsepower can be set free depends on how good the ignition system is to begin with, and how your engine is set up. If you're at the pipes and airbox stage, and the OEM ignition is up to snuff, then chances are the only improvement you'll notice is slightly stronger throttle response, and maybe a little smoother running. However, if you're riding an older bike with a marginal ignition system or your OEM system isn't what it should be, then you'll certainly pick up a few extra ponies. By the same token, if you've taken your mill beyond the bolt-on stage with things like high-compression pistons and radical cams, an ignition modifier, for example something like the Dyna 3000, which provides alternate ignition curves and selectable rev limits, may be indispensable. Be advised that aftermarket coils may not be compatible with factory-installed ignition systems or wires. Read the manufacturer's recommendations carefully so you know what you'll need.

Horsepower Gain: essentially it depends on how weak the OEM system is, and how highly tuned your motor has become.

Cost: Coils start around $100. The Dyna 3000 series ignition system starts at $289, depending on application.

Skill Level: Like a fuel-injection modifier, these devices are mostly plug and play.

dyno, bolt-on horsepower how-to
When speed is plotted against time, you can see that the high-performance module (shown in red) gives the bike a clear advantage. Because the modified ignition curve lets the engine come on harder, earlier, the bike is faster right up to the point where the rev limiter kicks in. In fact, at the 10-second mark it’s 6 mph faster. A huge advantage under any circumstances.Cruiser

Cam It
So far we've been concerned with straight bolt-on modifications. Here's where we separate the casual tinkerer from the serious gearhead. Swapping cams isn't a particularly difficult job, and ironically, they're sometimes cheaper than a good set of pipes. Furthermore, provided you pay careful attention to the job at hand, they're certainly not difficult to install. But there are several pitfalls. First, finding a performance cam for some bikes may be difficult. Second, while some aftermarket cams are "drop in," meaning they'll work with your existing pistons, valves and rocker arms, many more require high-compression pistons, heavy-duty valve springs or modified rocker arms before they can be installed. Some cams will only show a real increase if the head has been ported to match, which certainly takes them out of the bolt-on category.

Horsepower Gain: up to 20 percent in some cases, but pick the wrong one and it might be 2 percent.

Cost: $200 and up.

Skill Level: Cams should be installed only by an experienced mechanic, or someone who wants to become one.

dyno, bolt-on horsepower how-to
This one surprised me. The cams only yielded an additional 2 horsepower at the very top of the powerband and caused torque to fall off slightly. It’s a very small return for considerable expense and effort. Our suspicion is that the stock head simply wasn’t flowing enough air to make these cams worthwhile. I suspect that a little judicious porting here would work wonders.Cruiser

So What Have We Learned?
Candidly, I think most of us knew or at least suspected that the biggest bang for the buck was going to come from uncorking the intake and exhaust systems. But I have to admit that these experiments surprised me. While I certainly expected to see some gains from the intake and exhaust modifications, I didn't expect them to be so pronounced. If nothing else it makes a great case for using parts specifically designed for a particular model, and certainly for parts designed to complement each other. Likewise, I was puzzled that the cam didn't provide a bigger boost. Obviously the ports are the limiting factor, and I suspect an increase in compression might also help—food for future thought at least.

The bottom line is that the dyno doesn’t lie. Assuming that we’d stopped with the pipe, jet kit and air-filter modifications, which are certainly jobs that anyone with a few tools and a shop manual can perform at home, we’d have picked up an impressive 24 percent increase in horsepower and a healthy 12 percent increase in torque while spending less than a grand. Obviously your results may vary, but all things being equal, that’s an awful lot of bang for the buck, and certainly the first place I’d start looking for horsepower.

Big-Bore Kits
Like they say, nothing beats cubic inches (unless it's square dollars). Installing a big-bore kit is one way to beef up your mill. However, since the majority of the kits involve machine work and open-engine surgery, I don't really consider them a "bolt-on" option.

The Power Primer
Since horsepower is directly related to torque and torque is a direct result of how hard the expanding gases push down on the piston, it follows that the only place you can make horsepower is in the cylinder head. Now don't get your panties in a bunch; reducing internal friction or converting a greater percentage of combustion-produced heat into work, rather than letting it blow out the exhaust pipe, will increase the overall power. But that's not creating additional horsepower so much as liberating what's already there to do other things. Simply put, if we can get the engine to breathe, we'll make more power—everything else is just a detail.

Debugging Your Shield
Like spring, summer has sprung. That means the bugs are out in force, intent on splattering themselves all over your windshield. Looking through that layer of insect carcasses makes me slightly crazy, especially at sunset, but not nearly as crazy as seeing a motorcycle rider reach for the squeegee at a gas station to clean his windshield.

Unlike that hard shield on your helmet, your windshield is more prone to scratching, and should be cleaned using less aggressive means than that dirty squeegee at the self-serve. Better to ease the bugs from your windshield than to chisel them off. Carry a soft rag to wipe them away after you have hydrated them a bit. You can do that by soaking a rag in water and laying it on the shield for a minute or two. The last thing that went through the bugs’ minds and the rest of their remains will soak up some of that water and soften up, making them come off with less resistance.

I use Pledge (because it's cheap, softens up bugs well, and leaves a slight wax layer to make the next bug adhere a bit less tenaciously) and spray it on, giving the bug splats special attention. I give it a minute or so to work (the time it takes to fill the tank is a good interval), and then wipe if off with a soft, clean cloth. I avoid dragging the bug's hard little exoskeleton across the shield by turning the towels once the bug bits are captured. Those minute scratches can turn a clear windshield into a hazard in short order.
— Art Friedman