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Thread: Fuel System Calculator
08-11-2015, 07:31 PM #1
Fuel System Calculator
This calculator is based on original work done by Black2003Cobra on the site here over the years and original work I did in the early 70's working with 71 series GMC blowers and mechanical fuel injection. It is tailored to PD blown engines but can be used for turbocharged engines also. If you use it for turbocharged engines increase blower size and overdrive to reach your target boost levels and the calculator will model your fuel and fuel injector size requirements. The calculator runs in MS Excel and has four different screens. Each screen is associated with a tab at the bottom of the Excel window. The calculator will open with the following screen which is tab#1, Engine Config Pg. 1.
Most of the information is fairly self explanatory. Local altitude is not. You can find your local altitude by using Google Maps Find Altitude <= Clickable. Do not use density altitude, it is temperature corrected. The calculator does its own temperature corrections based on the IAT1 and IAT2 data you enter.
Metco lower pulley sizes are listed in the chart so you don't have to go hunting for them. Depending on the pulley sizes you choose and the Max Engine RPM you choose you can see the blower's rotor pack RPM displayed for you. If your target engine speed and/or pulley sizes push the rotor pack RPM above 18,000 RPM the cell will turn red to warn you. Some versions of Excel will not support the conditional formatting used to change cell color so always pay attention to the Blower Rotor Pack RPM.
Intake vacuum at WOT is a measure of how restricted or unrestricted the inlet tract to your blower is. Small changes in this metric can have significant impact on your engine's power. The only way to get an accurate number is to measure it with a sensor while the engine is under power ar WOT. To do the job correctly you need to measure after the air filter and before the MAF, after the MAF and before the throttle and finally in the plenum after the throttle and before the rotor pack.
Wherever the lowest reading occurs the item just in front of it is the bad guy that needs to be upgraded. If the pressure drop is in front of the MAF then the air filter is the choke point. If it is between the MAF and the throttle then the MAF is the choke point and if it is in behind the throttle then the throttle is the choke point.
When you choke off airflow to the blower it is the functional equivalent of lifting on the throttle. Your engine's power will immediately start to nose over and fall down. You should try to maintain the same inlet pressure drop from midrange engine speeds all the way to your max engine speed.
Injector and Fuel Config Pg. 2 looks like this;
You get to specify your target AFR, Injector Duty Cycle, IAT1 and IAT2 Temps along with your Fuel Specific Gravity (see page 3). The calculator will size your injectors at 3 BAR and tell you how much fuel the engine wants at the conditions you have specified. The Fuel Volume at WOT is not your required pump size it is how much fuel your engine demands for the target AFR you have specified. Your fuel pumps need to be larger than this.
The fuel injection industry rates all injector flows at 3 BAR on gasoline with a Specific Gravity of 0.72. The Fuel system calculator calculates the required injector size based on mass of air and mass of fuel. It represents the injector required for those specifics at 3 BAR and using a gasoline Specific Gravity of 0.72. This approach allows you to select injectors without having to normalize them for the specific gravity of the fuel you have chosen to use.
The lower box on the right of page 2 allows you to increase base fuel system pressure to see what base fuel system pressure (delta pressure) you can get by with if your injectors are not quite large enough. Raising base fuel system pressure will increase the fuel flow through the injector making smaller injectors look bigger in actual operation.
When you raise base fuel system pressure you need to exercise caution because both the return and returnless style fuel systems which we use are boost referenced. This can cause the actual pressure in the fuel lines to become deceptively high. In tank pumps frequently have an internal pressure relief valve to prevent damage to the pump. That relief is usually set somewhere in the middle seventy psi range. When it blows open your fuel system pressure drops precipitously creating a sudden massive lean condition. When that happens you can kill an engine.
If the combination of your manifold boost and your base system fuel pressure hit the middle seventies range the New Fuel System Target ΔP cell will turn red. Again this does not work in all versions of Excel so pay attention to your New Fuel System Target ΔP plus your boost number form page 1. If the two are over 70 psi you are on thin ice. For external gear type pumps this is usually not an issue because they have no internal pressure relief valve.
Page 3 of the calculator provides a table of fuels and AFRs to help you. It looks like this;
This page will provide you a quick reference to AFR's by fuel type and referenced to Lambda (λ). Again, fairly obvious. The only changeable cells are the yellow cells. If you run race gas your race gas provider can tell you the correct specific gravity and stoichiometric point for the fuel you have purchased from them. Enter it into the yellow cell at the top of the Gas column. Similarly if you run an ethanol blend other than E85 ask your provider for the Stoichiometric point and the Specific Gravity.
Many times we have to convert from one unit system to another or from liters per hour to cc’s per minute or pounds per hour. Page 4 of the calculator gathers together many of the more common conversions we have to work with when we begin to modify our fuel systems. There is no road map although the various conversions sort of fit together. If there is a conversion you want but do not see send me a PM and I will update the conversions page for everyone.
This is what page 4 currently looks like;
Some conversions are fuel specific gravity sensitive. They tend to be the gravimetric style conversions. As a good rule of thumb always plug your fuel Specific Gravity into the SpGr. Field at the top of the page.
Because of differences in fonts, font appearances and screen resolutions I have prepared the calculators in both a Mac and Windows specific format. They are distinguished by their file names;
Fuel Sys Calc R2.11.Mac.xlsx — is the Mac Version
Fuel Sys Calc R2.11.Win.xls — is the Windows Version
The Instruction File for the calculators is a .PDF version of this posting named;
Fuel Sys Calc R2.11.Instr.pdf
Most everything in this calculator, except the BSAC fuel system sizing logic, can be found on the net if you are determined and diligent enough. I have only seen the BSAC approach to fuel system design / management used by either black2003cobra or myself albeit on different occasions, different engines and at different times. Enjoy the tools and use them to create more and importantly more reliable horsepower.
Last edited by eschaider; 08-12-2015 at 10:51 AM. Reason: Spelling & Grammar
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08-12-2015, 09:11 PM #2
- Join Date
- Jan 2014
- Barnhart, MO
I was waiting for this to be posted again, Ed. Awesome tool here.
08-13-2015, 10:43 AM #3
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