The first thing is to decide what you are wanting to do with the bike. You will build the engine different depending if you are wanting to win dyno shootouts compared to if you are wanting a lot of take off power. This article will lean towards high torque, stop light to stop light power.
Think of this. Harley engines are capable of making tons of TQ or HP. One of the ways to increase horse power is to increase TQ in the higher rpm range. Horse power will increase if you increase torque. Torque does not increase when you increase horse power. The HP is calculated on a dynamometer by the computer using a formula. The formula that dynojet normally uses is torque multiplied by RPM divided by 5252 multiplied by 1.20.
We will use this dyno for an example. The figures are not exact but close enough for you to understand this theory.
It shows that there is approx. 95 tq at 3,000 rpm. 95 x 3000 = 285,000. 285,000 divided by 5252 = 54.265. 54.265 multiplied by 1.2 = 65.118.
So the theory is that if you increase TQ, the HP will increase also. Let’s say that you are not able to increase TQ anymore but you want more horsepower to win a dyno shootout. Using this theory, you can move the TQ peak RPM future up the RPM band by retarding the cam timing. This graph shows that the HP stops around 5,500 RPM. It is making approx. 100 TQ at 5,500 RPM and approx. 110 TQ at 5,000 RPM. If you could move the TQ peak up 500 RPMs you would then have a calculation like this:
110 TQ x 5,500 RPM / 5252 x 1.2 = 138.233 HP at 5,500 RPM.
This is why Sport Bikes make so much HP. The TQ is located far up the RPM range. Even though the TQ on sport bikes is much lower than on a HD, it’s located so far up the RPM band that it still makes a lot of HP. This brings us to the next decision.
Should you increase TQ on HD engines to increase HP or increase RPMs to increase HP?
Keep this in mind, The HD was manufactured for cruising. That is why it is designed for high torque, not high RPM and high speeds. One proof of this is the tire rating. They are rated for a max. speed of 130 MPH. They may be able to go faster than that but, the rating says that they are unsafe above that speed. The engine was designed with a max. RPM of 5,200. This means that you have to make a lot of modifications to increase power in the upper RPMs such as larger valves, higher lift cams, stronger valve springs, stronger push rods…
Also keep in mind that when you increase the valve lift a lot, then it compresses the valve spring more causing stress on the cams, push rods and rockers. When you increase power in the upper RPMs, you will need stronger valve springs to help keep the valves from floating. The stronger spring again increases stress on the cams, push rods, rockers and the valve itself. You may have to increase the valve size depending on the cubic inches and max RPM. Remember, the more stress that it takes to turn the engine, the more power that it robs from the engine and the less fuel economy you will have. Just like turning the air condition on in an automobile. You will feel a drop in engine power because the A/C compressor is putting more strain on the engine and robbing power. Also remember that these components will always have increased stress on them even if you are just cruising through town during normal riding. Stress affects the life of the engine and the fuel mileage. This is why that I prefer to increase the TQ in the lower RPM range and keep the max. RPM under 6,000 RPMs.
Another Thing to keep in mind if you are wanting good street fighter, is not only matching the engine parts but, also matching the engine to the gearing of the bike. Before purchasing any parts, take you bike out for a spin and watch the tachometer to see where the RPM falls to after it has run to max RPM (red line or where you plan to make the max RPM). Example: My 2007 Ultra drops approx. 1,000 RPMs between gear changes.
You want to target your RPM drop to be as close as possible to the “peak torque”.
Using the Dyno graph below, the bike stops making power at approx 5,500 RPM so, when it changes gears the RPM will drop down to 4,500 RPM which is very close to peak TQ. This is keeping the engine in the power.
The little spike @ 4,200 RPM is possibly caused by clutch slippage. Peak TQ is about 4,600 Rpm.
This is one of the reasons why the bike is so quick. In racing or dyno shootouts, there is no reason to run the engine above 5,500 RPMs because it is no longer making HP. Since the max RPM is that low, I can use lighter valve springs which will have less stress on the engine, let the engine turn up quicker and help with fuel mileage (I’m using light “Bee Hive” springs). Since I’m using 2.02” valves with the SE 103+ heads, I can use less valve lift like the Woods TW-7H which is has a .575” lift. Smaller lift equals less stress which again has less stress on the cam, bearings, rockers, springs and makes it easier to spin the engine up. The easier the engine spins, the more power it will make, the better fuel mileage it gets and longer engine life. This is a another good reason for the use of nitrous. You can have a mild engine build with little stress and good fuel mileage and still have a lot more power when you need it at full throttle. Please read my other articles on the website to get a better understanding of theories to increase power including NITROUS. Nitrous will give you the extreme power that you need, when you need it without continual stress on the engine.
Another thing to consider is the exhaust that you are planning to use.
The cam and the exhaust plays the most important part of the engine build. The closing degree of the exhaust and the type of pipe will either increase or kill the power. For more detailed information on this subject,
please read the exhaust article on this site.
This should give you some things to consider before building your engine.