Harley Davidson Performance




Looking for the easy way to increase the horsepower of your big twin motorcycle? We can improve your horsepower and torque. Twin Cam engines with fuel injection have become a favorite performance platform over the past couple of years. MPH Ohio offers you information for a variety of upgrade paths.

Building the Perfect Street Engine

Here a MPH we get hundreds of requests for engine configurations.

Riders want the simplest engine to build, the highest power they can get, an engine to beat their friends, an engine to go racing with. In short, every rider wants the ideal engine. They want big power increases and instant throttle response. They have describe a fire breathing race engine sitting in a street bike. What many riders don’t want to discuss or hear are the issues associated with these “over the top” engines such as:

  • Exceptionally loud exhaust tone(do you really want 118 dB drone in your ears on your FLH when riding down the interstate?)

  • A strong tendency to overheat when exposed to parade duty (called cruising around town)

  • High detonation potential when running on pump fuel (can you really find race fuel every 200 miles and if you can are you paying $18-$28/gal)

  • High maintenance usually requiring decarbonizing the cylinder heads and pistons on a frequent basis.

  • Reduced Reliability (do you really expect an engine that has huge power and ridden very hard to last as long as a stock engine?)

And these are just a portion of the issues you have to face with these “monster” engines. Unfortunately, few riders think about this until their dream engine turns into a nightmare 9 months down the road. – more

  • EFI Basics

    In its simplest terms, an electronic fuel injection (EFI) system is a computer-controlled fuel delivery system. This electronic control unit (ECU) reads various sensors located on the vehicle and makes the determination of how much fuel to allow the engine to have based on this information. The computer will open and close the injectors allowing gasoline into the engine based on the sensor inputs and the fuel map programmed into the computer. The various sensors (RPM, engine temperature, air temperature, throttle position, manifold pressure, crankshaft position) provide information on operating conditions and load on the engine. Figure 1 is a detailed drawing of a typical fuel injection system and the sensors that might be in place. The EFI Components table provides a detailed description of each of the major components.

    We will limit our discussion to the types of fuel injection systems that are likely to be encountered on motorcycles. One of the major differences between fuel injection systems is how the sense the amount of load that is placed on the engine. The load can be sensed by how far the rider has twisted the throttle and MAP (manifold absolute pressure). EFI systems that sense engine load by the use of the throttle or TPS (throttle position sensor) are referred to as an Alpha-N system. EFI systems that sense engine load by the use of a MAP sensor are referred to as a Speed-Density system. Harley-Davidson has used both types of EFI systems on its vehicles. While many of the sensors can be the same in both types of EFI systems, there is a significant difference in the way that each system determines what the load on the engine actually is. The Magneti-Marelli system used on EVO and Twin Cam FLH’s until 2001 and Buell XL engines is an Alpha-N EFI system. On these bikes, load sensing is determined by the throttle position. The newer Delphi EFI system used on current Twin Cam engines is a Speed Density system. Speed Density EFI systems determine the engine load based on the intake manifold vacuum.

  • Fuel System Diagram

    The fuel system diagram provides a simplified view on how fuel flows within an EFI system. The fuel tank supplies gasoline to the fuel pump. The fuel pump can be located in the fuel tank or external to the tank. The fuel pump provides a high-pressure feed to the fuel rail that contains the fuel injectors. On the fuel rail there is a pressure regulator that limits the fuel pressure to 39-45 PSI and maintains a constant pressure. The pressure regulator has a vacuum line that connects it to the intake manifold. Any excess fuel leaves the pressure regulator and is returned to the fuel tank.

    Why is EFI better than a carburetor?

    Choosing a carburetor and tuning it is always a series of compromises. A carburetor is a demand device dependent upon the velocity of the air entering the venturi to create the air/fuel mixture that feeds the engine. A carburetor maintains a series of fuel circuits to help maintain the best possible fuel mixture. The idle circuit, intermediate and main jetting circuits are used to tune the mixture across the operating RPM range of the engine. These different fuel circuits can interact with each other creating rich and lean spots in the fuel curve. Some riders will go as far as to add one or more additional fuel circuits (Thunderjet) in an effort to improve the fuel curve. The interactions of these additional circuits further complicate the tuning efforts. A change in jetting to one fuel circuit can impact the required jetting in another circuit. Sound complicated? It certainly can be.

    Lets simplify matters and assume a carburetor with nothing more than an idle circuit and main circuit. To optimize low RPM performance a small diameter carburetor provides the best performance, but at high RPMs a large volume of air is required to feed the engine. This requires a much larger diameter carburetor. The ability of a carburetor to provide a good air/fuel mixture is very dependent upon the velocity of the air going through the carburetor. This “signal” must be present to maintain good throttle response. If the diameter of the carburetor is too large for the engine, low RPM performance can be very poor. Most HD mechanics and riders are aware that the “large carburetor” syndrome creates a poor running engine. This situation does not occur with an EFI engine.

    With an electronic fuel injection system, the required fuel amount for each RPM and engine load condition is located in the fuel map located in the ECU(or addition on a powercommander). Once this primary fuel amount is known, then the ECU further adjusts the fuel mixture for the engine and air intake temperatures. In many cases, the mixture will even be adjusted for the barometric pressure and altitude. Based on the various sensor inputs to EFI ECU, there is only one fuel value that is generated. The correct fuel amount is fed into the engine at all times. Because airflow does not have to pass through a venturi to provide an air/fuel mixture, the throttle plate diameter can be quite large. This engine is allowed to draw all the air it wants. Since more air equates to higher horsepower potential, all the EFI has to do is provide the correct amount of fuel for the increase in airflow.

    In conclusion this is why any modification needs to have the most critical part of any build performed, DYNO TUNING. This holds true to every time you ask “Do you think my bike needs tuned?” By now you should be able to answer that. “But I only put on slip-ons!” Again the answer is and will always be “YES!” I would take a perfectly tuned stock bike over any monster poorly tuned bike any day until you bring that monster into MPH and let us make it breath fire than all bets are off.


How do I get more power out of my Harley?

“How to get more power” is one of the most frequently asked question received about improving a Harley’s performance. Some people are looking for more “get up and go” from their stock engines, while others are looking for higher dyno numbers.
For those who have a stock or mildly modified engine the answer is to move up to the next “stage”… or a Stage 1 kit to be more exact. Before dragging out the catalogs and bolting on every gizmo claiming to make your ride go faster let’s start with the basics of a Stage 1 upgrade.
You’ve probably heard terms such as Stage 1, Stage 2, etc. Now depending on who you ask, a Stage 1 upgrade could be anything from an exhaust upgrade to a new carburetor. I’m going to define the term as we use it:


Stage One - Upgrade Consists Of The Following:

  • Free-breathing air cleaner.
  • Free-breathing exhaust.
  • Re-jetted carburetor or remapped EFI.

There is probably no greater beginning upgrade you can make than Stage One which is entirely a bolt-on procedure. If you want you can take this upgrade one step at a time however, I recommend going in the order listed above if not all at once.

Stage Two - Consists Of The Following:

  • Engines do not involve any internal changes to the engine other than replacing the camshaft and camshaft bearing. All modifications are made by bolt-in changes. Some special tools and training is required to perform these modifications. Unless you are a very good mechanic, it is recommended you bring in your ride to MPH to install the camshaft and bearing. For those of you that want to install the camshaft yourself, use extreme caution. Over the past several years we have installed and tested many manufacturers components. The list of parts we use in building the ultimate Stage 2 engine is the end result of years of testing. The best of the individual components were selected and tested as a individual part as well as a package. The fact that virtually all the parts used for the Stage 2 are based on everything we have dyno tested from years is no coincidence.  As always we recommend the best components for the application, budget, and riding requirements.

  • All the parts used are readily available through our parts department if you wish to tackle this on your own. The components were installed and tested on several models from FLHX toFXD Dyna Super Glide. Custom tuning was performed to achieve the results. All components were installed according to the manufacturer instructions supplied.
  • The results are due to a well-matched set of parts. MPH feel that years of experience allows use to develop these packages. I have seen many bike come into the shop with brand x cam, brand y heads, and z tuning system with very poor results. All of our packages are based on the riding experience and not the ending number on the dyno sheet. I feel the best packages a designed for the rider and not the dyno!
  • Still not enough power for your needs? We decided to find out what happens when you take the above stages and add it all to a

Stage Three – Big Bore Kits

Is all about cubic inches. “Big Bore Kits”. With the addition of displacement to your motor MPH considers this the biggest bang for the buck. This goes back to the argument in the automotive muscle cars. And the question on everyones mind is “small block” or “BIG Block”. The same is true here. Whether you have an 88″, 95″ 103″ there is a big bore package out there for your application. When setting up this stage it will take years of MPH’s knowledge and blueprinting motor building practices to achieve the results you looking for. Yes any back yard mechanic can throw on a set of jugs but thats not what we do. There are several steps involved in building a long-lasting, fire breathing, window rattling, stump puller Harley other than just bolting on a set of jugs. Things like ring end gap, piston to wall clearances, piston to deck, piston to head, and piston to valve to name a few. So the next time you want your hard-earned money spent on some beer for your buddy to assemble your thousands of dollars of parts so he can turn your pistons into ashtrays good luck. This Stage is for qualified mechanics ONLY.

Stage Four - Icing On The Cake

  • Plain and simple, a set of CNC ported heads. Many options are available along with porting like oversize valves, updated valve springs, compression releases. The list goes on...
  • Regardless of STAGE of your build MPH take pride in servicing your needs and budget. We take an approach to engine building from a customers point of view. How much is it going to cost and how much am I going to get for that hard earned money. If you trust our methods the best way to build a motor of your dreams is to sit down with me “Doug” and with your realistic budget in mind, I will build you the best, most reliable motor for the money. Plain and Simple!

Long Block Crate Motor

“Want an all in one package?” A few companies are make Long Block Crate motors 143 cubic inch, 136 cubic inch, 124 cubic inch, and 120 cubic inch. By far these are the best bang for the buck. Direct fitment in stock frame, can use stock air cleaner or aftermarket air cleaners. Many use stock throttle body depending on application. Come with a cam and fully billet upgraded oil pump and cam support plate. All motors come with manufacturers 1 year warranty and all made right here in the USA.


Proof is in the Pudding

    This is on a 2003 Harley Dyna Low that has a 120 cubic inch crate motor. Custom 2:1 exhaust, 90 degree high-flow air cleaner and many more accessory upgrades. Making 105 peak hp and 132 peak torque.