The Xenocron Tuning HP Fuel System Calculator     


This is a simple Excel document, so first you need a program that can open an Excel Document.  The purpose of this calculator is a way for people to see, compare and calculate your fuel system components based on a variety of parameters including using various fuels.  Most internet calculators focus on using pump gasoline fuel and as you may see after playing with this calculator, the Specific Gravity and the Stoich Value of the fuel you are using play major roles in the components you may want to select when sizing your system.

This document can also be used as a “sanity check” for the novice tuner where you can compare measured power numbers that an engine is producing on the dyno and you can back figure your BSFC using your current fuel system capacity and the actual HP figures that are being measured by your engine or chassis dyno.  This can help you understand possible fuel system problems or efficiency issues in your setup.

To understand Brake Specific Fuel Consumption a little better, please read the Wikipedia Link along with doing your own basic research:

For most production engines, you can generally guess on a BSFC range of .40-50 for Naturally Aspirated 4 Stroke setup and roughly .5-.6 for a Turbocharged 4 Stroke setup.  The lower the number means the engine is more efficient and uses less fuel to produce each engine HP so when you are guessing or calculating, I suggest you use a higher BSFC value and that will make you oversize your fuel system by choosing a value that may actually be less efficient than reality.

Next you need to input your individual injector flow rate and at what fuel pressure differential your engine will see in its fuel rail.  We have included a tab with ALL current sizes of Injector Dynamics injectors because frankly they provide the best public data and they make it easy to use in this way.  Decide on your fuel pressure differential, go the to the Injector Dynamics tab and choose the Injector Slope (cc/min) that lines up with the fuel pressure you will run.  Input that value into the calculator along with the fuel pressure.  The pressure here does not play a role in the calculation, but reminds you if you go playing with the figures what your flow rate is at that pressure.  If you can get this data from the fuel injector company that you will be using, input their data here as well.

Also, choose the max injector duty cycle you would want to see while running your engine at max power.  The general suggestion is to keep the injectors to a max of 80-90% and of course at 100% duty cycle, you really have no control over your injectors so avoid running them that high in the real world.  This is one of the basic reasons I made this calculator, is to help customers understand what size injectors that they need to purchase when designing their setup and also to keep the injector duty cycles to a safe level.

Next, input the number of injectors that you will be using on this engine.  If you will be using one injector per cylinder, then this number is the same, but if you are using multiple injectors per cylinder, you will want to put the total number of injectors being used.  If you by chance are using fewer injectors than number of cylinders, make sure you input the number of injectors being used.

After this we need to make a choice about what fuel we are running.  I have put some approximate or typically accepted values for most common pump fuels, ethanols and a few race gas blends.  These values are readily available online or by calling the fuel manufacturer of your choice to get accurate values if you do not see them listed here.  I find it interesting to use this to see if my current fuel system setup will support trying a different fuel or if I may need to change out components. Make sure you accurately input both Specific Gravity and the Stoich Point of the fuel you are going to use.

If you are ONLY using this calculator to choose injectors, you are DONE!  The calculator will convert your volume flow rate to mass flow rate of your fuel injectors and give you an approximate maximum engine power capable given the variables you fill in.  REMEMBER, this is Engine Horsepower NOT Wheel Horsepower that we are calculating here.  Drivetrain losses, inertia, transmission gearing, wheels, tires, traction and strapping will affect your Horsepower results on a chassis dyno.

If you also need to look at your fuel system on the pump side, you will need to fill in your Fuel Pump Volume Rating and the max duty cycle or capacity of the pump that you will want to use.  Again, remember the more you push any component in your system to its limit, the closer it is to failing all together or simply just not being able to produce the results it is “supposed” to.  All fuel pumps are not created equal and they may not actually all flow the same in the real world so please give yourself some cushion here.

To find these fuel pump ratings is probably a touch more difficult than what is out there for injectors.  You can contact the pump manufacturer, you can find someone else’s testing or results on the internet, you can have a pump tested yourself or you can just use the pump manufacturers general “rating” that they name when marketing the fuel pump.  A Walbro 255 pump flows approximately 255 liters per hour maximum.  Here are a few charts I found with a simple Google Search, take them for what they are worth.  Also added some links to the pumps we carry in stock directly to our website.

Walbro 416
Bosch 044
Walbro 255

Make sure to look at the flow rating based on the rail pressures you will be demanding when running your vehicle.  My calculator asks for ratings in Liters/Hour (Volume) and one of these charts shows Lbs/Hr (Mass). Understand the MASS FLOW shown in this chart is based on a particular fuel or test fuel so these values don’t work perfectly when using other fuels which is why it might be considered better for starting with a Volume Flow Rating.  Some Fuel Pump manufacturers like Weldon market their pumps in Gallons Per Hour which is why I put a way for you to Convert between LPH and GPH and vice versa in the calculator as well.

Essentially this is where I leave you to play and explore.  If you have any constructive criticism about this calculator, please email me as I would love to make it better. Please remember however that I am providing this as a public service for free and that my goal is to help educate the enthusiast tuner that wants to really learn and understand how their fuel system works.  I am not interested getting to engineering level work here, I will leave that to the engineers!

A few other details to remember…
- Your fuel pump and injectors work on VOLTAGE and if the voltage in your system is LOWER or HIGHER than it should be or the ratings you are looking at, your actual results will vary.  Lower voltage to your fuel pump will make it flow LESS, and higher voltage can make it flow more, IF the wiring is sufficient enough to move the current that the pump will be requesting.  For your fuel injectors, if your voltage is off and your Fuel Injector Dead Time (latency) values are correct for your ECU then their flow will be the same BUT you will see higher actual pulse widths and duty cycles at lower system voltage.
- Fuel Specific Gravity values are rated at a specific temperature and therefore if your fuel temperatures increase as your engine runs, the values here will change as well.  As temperature increases, DENSITY of the fuel will decrease and therefore your VOLUME remains the same, but the MASS of the fuel you will be moving will be LESS.  Most fuels do not vary all that much but some will vary greatly based on fuel temperature.
- With some fuel injectors, there are points of diminishing returns when increasing fuel pressure in the system.  Some injectors I have experienced personally will flow LESS at higher fuel differential pressures or possible lock completely open or closed at very high pressures.  Most “street” fuel pumps have an internal fuel pressure relief valve built into them which effectively means that no matter how hard you crank down on your adjustable fuel pressure regulator, you will not be able to increase the pressure in the rail or move more volume.  How do you move more volume here?  Add a second pump or replace it with one with a higher internal relief or a single pump that flows a higher volume.