[Diy_efi] Base Fuel Calculations..

[Tlsalt at aol.com]

Gregory Croft wrote,

I am starting from scratch.
I have a full stand alone system, and am figuring out both the fuel map, and
the ignition map.

My unit doesn't do VE calcs, Just a spreadsheet like excel with pulse width.
It's a batch fire system.

Unfortunately, I don't have any of the factory stuff. Daimler is pretty
tight with their fuel maps for the neon...
I have datalogged with an OBD-II scanner, but that doesn't report
pulsewidth.

I don't have any literature or formulas..

Hello Greg, 

I suggest you get a copy of Jeff Hartman's book, it's a good starting point. 
The formula he suggests uses cylinder displacement and estimated VE (at peak 
torque) to calculate the weight of air in the cylinder and then computing the 
weight of fuel needed using a target air-fuel ratio (12.5:1) and converting 
to injector puslewidth based on the injector rating.  His method is a very 
rough guide, but I have used it and it will get you up and running so that 
you can start tuning the engine. You will have to estimate VE at the torque 
peak rpm to calculate the highest pulse width and extrapolate the rest of the 
fuel curve.  Try to get an accurate copy of the factory hp and torque curve.  
If the injectors are properly sized, the idle pulsewidth will be somewhere 
around 2ms.

Convert static displacement (cu in) to displacement at VE (est 80-90% street 
engines).
Divide cu in by cubic in foot (1728).
Multiple by lbs air per cu foot (0.07651).
Divide by number of cylinders.
Divide by target air -fuel ratio.
Divide by number of injections per power stroke.

Equals= lbs fuel per injection(A)

Divide injector rating (lbs/hr) by 3,600,000 (lbs fuel per ms) (B)

A/B = pulsewidth at peak torque

Example:

454ci *90 VE=408ci
408/1728=0.2364......
0.2364......*0.07651=0.01809........
0.01809...../8=0.00226......
0.00226..../12.5=0.0001809.....
0.0001809/2=0.0000904.....(A)

30lb/hr injector/3,600,000=0.000008333....(B)

(A)/(B)= 10.85ms pulsewidth

Paul Saltwick

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