FREQUENTLY ASKED QUESTIONS

How much power will I make?

This is probably the most common question we get asked. We have a lot of experience with various combinations and have a pretty good feel for the range of results that come from each and can give you some guidance. But at the same time, it's important to understand that like a chain is only as strong as it's weakest link, the power of an engine is only as strong as the part that's holding it back.

For example, you can have a set of heads on your bike that's capable of supporting 120 horsepower, but if you have an 80hp pipe, the bike will make 80hp. A good result depends on all of the parts working together to support the final number. Sometimes the hardest part of getting a good result is figuring out exactly what's holding the motor back. But it's essential, because working on anything else is unlikely to help much if any.

Don't neglect the tune-up, either. It makes no sense at all to spend thousands on motor work and then leave 10hp or more on the table because you didn't spend a couple hundred more for a proper dyno tune. A proper dyno tune also protects your investment by making sure your combination of fueling and timing aren't setting you up for motor-destroying detonation. This is not a place to cut corners!

How will this change the reliability and longevity of my motor?

Modifying an engine is lot like drinking red wine or eating steak. Done in moderation, it's causes no problems and can even be healthy. Done in excess, it can shorten your life. That said, the XL engine has enough design margin that you can typically get a pretty substantial increase in horsepower and torque without doing anything in excess.

Here's a real world example to illustrate this. Let's say you're going to install a new set of cams. A set of Screamin Eagle .536 lift cams requires only moderately strong springs for proper valvetrain control up to say 7200rpm. Parts in the valvetrain that have their wear accelerated by higher spring pressures (valve tips, rocker bushings, lifters & lifter rollers, cams and their bushings, cam gear teeth, etc) are hardly going to notice, you'll get minimal extra wear from using a 150lb seat pressure spring instead of the factory 120lb springs. The lift is low enough that you won't cause any measurable increase in valve guide wear from side loading, either. The bottom line is that a motor with these cams should last about as long as a stock motor. And yet, we've had many many customers break 100 horsepower using SE .536 cams, and some have broken 110hp.

On the other hand, if you want to build a 120+ horsepower bike, the SE 536's aren't going to cut it. Maybe you'll choose something like a Wood W68S, which has .678 lift, and you need to spin the motor 7500+rpm. You're going to need a fairly healthy set of springs to support that, probably something on the order of at least 220lbs on the seat. The wear of all those aforementioned valvetrain components is going to go up dramatically, and you'll be lucky to get even half the life from them. You'll also increase the risk of breaking something in the valvetrain.

The same comparisons go for other areas of the engine, including the top end and bottom end. Done in moderation, mods won't measurably affect engine life or increase the risk of failure. We've got combinations that have repeatedly made 100-105hp on 1250cc XL engines with no change in the longevity or reliability.

What will happen to my gas mileage?

Making an engine bigger and/or more powerful does not necessarily mean lower fuel mileage and in fact many times the opposite happens, because the efficiency of the engine is increased.

To make a given bike go down the road at any given speed requires a fixed amount of horsepower. Let's say, for example, your 883 at 60mph requires 15 horsepower. As a function of the overall gearing, your engine is turning a specific rpm at 60mph, and with your right hand you're regulating the torque of the engine such that the torque times that rpm, divided by 5252, comes out to 15 horsepower. Think of torque as how much fuel is being burned each cycle and rpm is how often it's being burned (the frequency). The combination of the two is the horsepower the engine is producing.

Well, when you make the engine bigger and/or more powerful, the bike still needs only 15 horsepower to go 60mph. If you haven't changed the gearing, it will still be turning the same rpm at 60 mph and therefore be burning fuel at the same frequency. So to make 15 horsepower, each gulp of fuel will be the same size. Now if the motor is bigger or the breathing is better, it may want to take a bigger gulp of fuel each cycle, but you're in control of that with your right hand. To keep the bike at 60mph will require less throttle input by you. In the end, you end up burning the same size gulp of fuel each cycle and at the same frequency. Nothing has changed except you're giving it less throttle.

Gearing changes have much the same effect. When you gear a bike shorter, you'll find that less throttle is generally needed to go a given speed, but the motor is turning more rpm. You're taking smaller gulps of fuel, but taking them more often, but in the end you're still making 15 horsepower, i.e. the torque times the rpm. Likewise gearing taller results in more throttle being needed at a lower engine rpm to go 60mph. You're taking larger gulps of fuel but you're taking them less often, i.e. you're making 15hp with more torque and less rpm. But in the end you're burning about the same amount of fuel. These things have a way of balancing themselves out.

Ultimately, fuel mileage comes down to the efficiency of the engine at turning the fuel into power. How complete is the burn, and how good of a job does the engine do at turning the burn into usable power? How much is lost due to heat, and how much is lost due to frictional losses? Those are the things that determine the fuel mileage.

To the extent the bigger motor has higher frictional losses (and it probably does), fuel mileage will be reduced. But when performance mods are made, often things are done to extract more power out of the burn itself, and in many cases the fuel mileage will go up overall. We've done, for example, XL's that can break 100hp on the dyno and yet get 55mpg in everyday riding. You do not have to sacrifice fuel mileage for power.

Is the 1250 worth the extra money?

A 1250 is about 4% more displacement than a 1200. Assuming displacement is limiting the power of the motor, 4% more displacement would yield 4% more power. In reality, it generally comes out a little more than that, on the order of 6-7%, because there's an increase in compression that comes with it when comparing to a stock 1200. Also, the larger bore unshrouds the valves a bit more and enhances breathing.

So is 6-7% more power worth the cost of the kit? On an otherwise stock 1200, probably not. You can find more power than that with simple pipe and air cleaner changes. But once you've picked all the low hanging fruit, you'll quickly find that additional horsepower starts getting very expensive. At that point, a 1250 kit will be the cheapest horsepower you can buy.

On an 883 that's being converted, though, the picture is quite a bit different. The incremental cost of the 1250 kit over a 1200 conversion kit is only a couple hundred dollars or less. That makes the decision quite a bit easier. Toss in the fact that Sledge Hammer cylinders and pistons are much higher quality pieces than the stock type conversion pieces and it becomes a no-brainer.

Does more horsepower mean that I have to lose bottom end power?

NO! This is a myth that needs to be debunked. There are many, many combinations that yield more power across the entire rpm range, or more top end power with no loss in bottom end power. We've done it hundreds of times.

Now of course, an engine optimized for top end power may suffer a loss of power on the bottom end, and we intentionally do that in race motors, but that's not typically the case in a street motor.

One thing we've noticed is that people tend to give the cams far too much credit for the shape of the powerband, while totally ignoring the dramatic effect the pipe has. The two play together and matching them is critical. More overlap in the cams (i.e. the sum of the intake open and exhaust close timing figures) essentially gives the pipe more control. If the pipe pushes back at low rpm (or any other rpm for that matter), and the cams have lots of overlap, sure, you'll lose power there. But it's also entirely possible to gain power at low rpm with long overlap cams, if the pipe is pulling good at low rpm.

The best running engines match the cams and the pipe to put the power where the tuner wants it. Street riders usually like a pipe that diffuses such that it pulls over a wide rpm range, matched to cams with enough overlap to take advantage of it, and have moderate intake close timing. Racers generally only care about higher rpm's and will choose (or build) a pipe that pulls at high rpm and use cams with later intake close timing to optimize cylinder fill there.

Do I need adjustable pushrods?

Many times when doing performance mods, the overall height of the engine will get changed, specifically it will get lowered due to head milling and/or the use of thin gaskets to optimize squish clearance. Likewise, it's not uncommon for the valve stem protrusion to change (due to a valve job, clearancing work, or longer valve stems) or for the rocker arm geometry to change, or for the cam base circle to change. Any or all of these things can individually or cumulatively create a need for different pushrod lengths.

As with many things when it comes to motors, done in moderation these type of changes cause no issues. The intake manifold will still fit fine, the exhaust system will still fit fine, all the braceketry will go on fine, etc. What's more, because the bike has hydraulic self-adjusting lifters, moderate reductions in the overall height or stem protrusion etc don't cause any issues with pushrod length. Stock lifters have about +/- .050" of self-adjustment range, where the lifter still works properly. It's only when you get outside this range that you need to worry about correcting the pushrod length.

Now, suppose your heads have been milled .035" and you're going to be using gaskets that are .020" thinner than the factory gaskets. You'll be dropping the stack by .055". Now you need to be concerned about pushrod length.

Many shops will advise you at that point to change to adjustable pushrods. Not only will they ask you to fork over $100 for a set of adjustable pushrods, but they'll also inform you that you need a set of collapsible pushrod tubes to be able to make the adjustments, to the tune of another $150.

That's not only expensive, but it's also not a good thing to be doing, for a couple of reasons. Number one, an adjustable pushrod is by it's very design weaker at a given weight level than a fixed length pushrod. Number two, using collapsible pushrod covers creates more opportunities for leaks. Nevermind the hassle of adjusting them, or the possibility of one coming loose.

If you're using standard hydraulic lifters, you're far better off to simply buy a set of fixed length pushrods that are shorter than stock. They'll be stronger, less expensive, won't require adjustments, and won't create new risks. Hammer Performance offers some of the best pushrods on the market.

Really, the only case where adjustable pushrods make sense is when a travel limited lifter is being used, i.e. a solid or a Hydrosolid or stock lifters with travel limiting washers. These lifter designs require a precise setting of the pushrod length that can only be achieved with an adjustable. If you're using standard hydraulic lifters, save your money for things that matter.

Want more information? Read this for a much more in-depth discussion of the topic, complete with pictures and drawings.

Check back for more FAQ's, we'll be adding to this page regularly