A/F ratio and fuel volume

[Jason_Leone at amat.com]

Damon pleads:

<<I am trying to calculate a rough value for the amount of fuel a turbo
2.2 dodge engine will use at around 19 psi of boost. I want it too be
rich for starters so I don't need to be right on. All I want is a formula
or something I can work with to get me there.>>


Damon,

 Okay, now I can help you better (since I know what you're trying to do). It's
pretty fundamental to calculate fuel requirements. What threw me off was the
mathmatical approach you had.

OK, what you're trying to calculate is called B.S.F.C. (Brake Specific Fuel
Consumption), in one form or another. You want to know how much fuel your 2.2L
turbo engine will need @ 19psi boost. Right? Most atmo engines run about .45-.50
BSFC, while forced induction equipped engines run about .50-.60 BSFC. To run a
little "rich" at first, let's assume .60 BSFC. We can always dial the fuel back
later.

Let's attack it from a different angle. Let's set a power output goal first,
then use that as our guideline. Say, you want a healthy 300hp. Using that
number, we can calculate all the fuel needs and component requirements. Let's
look at the formula below:

Fuel requirement (lbs/hr) = (HP * BSFC) / (# of injectors * duty cycle)

Example for a 4 cyl w/ 300hp using a turbo and 80% duty cyle max:
Fuel = (300 * .60) / (4 * .80)

Fuel = (180) / (3.2)

Fuel = 56.25 lbs/hr per injector (at that injector's rated pressure)

Convert lbs/hr to CC/min by multiplying by a constant of 10.515. So, using our
example: 56.25 * 10.515 = 591.46 CC/min

So, each injector must flow 591.46 CCs/min to achieve 300hp @ 80% duty cycle @
the the rated pressure of the injector (usually ~43.5psi). Looks like a 600
CC/min injector will work fine.

Then, you have to determine what the system pressure is, in regard to the test
(rated) pressure of your injectors. If you use an adjustable FPR and set it to
43.5psi, no problem. What if the system is more or less though? Well, we need to
calculate the difference in flow vs. pressure Delta.
To calculate this:

(new fuel pressure/old fuel pressure) =x
square root of x * old flow = new flow

Say you're running about 58psi, but the injectors are rated at 43.5psi.

(58psi/43.5psi) = 1.33
(1.33) square rooted = 1.153
(1.153 * 591.46 CC/min) = 681.95 CC/min

Hmm, now your injectors flow over 680 CC/min. Now you can downsize the
injectors, reduce the system pressure, or reduce duty cycle to match your 300hp
goal.

We need to know how much fuel the pump can supply now. We know how much the
engine needs, now we need to figure out the pump. Let's say you have an in-tank
unit and it's stock. Use a graduated cylinder or a Buret to measure CCs of fuel
delivered and at what pressure vs. time. I usually jumper the pump on and fill
up my container for 30 seconds. Measure the amount of CCs and multiply it by two
(to get CC/min delivered). Have a friend measuer the actual voltage AT THE PUMP,
while you do this. You can now use some graph paper to make a linear fuel
delivery vs. voltage chart. More voltage at the pump, more fuel delivery. Kenne
Bell offers a device called the "Boost-a-pump" that does just that. It's an
adjustable voltage regulator that's boost referenced...so more voltage gives
more fuel as you come on boost.

Once you have a little chart together (or maybe your factory service manual will
have a chart already...Bosch equipped cars usually do), then you know how many
CCs the pump can deliver @ a certain voltage. Divide that number by the number
of injectors to know the theoretical flow maximum to each injector. If you know
the factory pump isn't going to work from the start, simply add the sum total
flow of all injectors and look for a pump that's rated in that range. Using our
43.5psi example, we know that we'll need a 225 lb/hr single pump (56.25 lb/hr
per injector * 4 injectors). Convert lbs/hr to gal/hr by dividing by 6. So, 225
lbs/hr is about a 37.5 gal/hr pump (@43.5psi). Convert gal.hr to L/hr by
multiplying it by 3.785. That means a 145L/hr pump @ 43.5psi will work.

Using a little secondary pump works well too. For example, leaving the in-tank
pump to feed a larger frame mounted pump is fine. By having a low pressure unit
feed a high pressure...or vice-versa...even an accumulator reservoir in between
the two pumps is great. Vortech makes a unit called the "T-Rex", SX makes a few
pumps, Paxton makes a few, NOS does, Kinsler has a few, Bosch has millions of
them that all these companies call their own.....

Hope that helps. Knowing the fuel requirements will help you size everything
properly. Tuning is another story. A/Fs of about 12.5-13 are common in the turbo
world (while at max boost). If using auxiliary injectors to supplement factory
ones...the above formulas can still help. If the software/engine management you
have can't allow for large injectors...piggy back ones are your answer. I can't
stand doing it that way though, I refuse to add injectors because my intake
manifold isn't designed to flow fuel (one cyl runs lean, and KABOOM!). Split
Second makes a good A/F meter and fine tuning unit. Haltech makes decent
injector controllers, and such. FWIW, I use the Electromotive TEC II, and it's
great for turbo cars.

Cheers,

Jason
'93 SLC