Injector Math

Mon Mar 22 12:43:29 GMT 1999

Greetings, as most of you know by now, I'm building an
efi-based mopar stroker engine using a turned down 440 crank
combined with a .030 over 383 block, for a total
displacement of 431 cubes.  With the below math, I'm trying
to caluclate several things.

Approximate HP/Torque to be expected from the motor, both
N/A as well as boosted (10 lbs of boost max), the
appropriate injector size for a sequential-based injection
system, as well as cfm of air required at 5000 RPM naturally
aspirated as well as with 10lbs of boost.  Yes, I'm having
fun.  Anyhoo:

Using several different sources (boogs, mags, websites) I
once again found this formula:

        fuel flow (lb/hr) = HP x BSFC
                            ---------
                              cyls

BSFC would be .45 for a N/A engine, .50 for a Nitrous
engine, and .55 for a forced induction motor, and 1.1 for an
alcohol motor, and 1.76 for a nitromethane motor.

Cyls is easy, I have eight.

So I have flow (lb/hr) = HP x .55
                         --------
                            8

Now, I have two variables left.  Flow, which I'm trying to
calculate (gives me injector size), and HP.  How does one
calculate the math for the HP without testing the engine?

The engine has these stats:  1969 383 cid stroked to 431 cid
using a 1970 forged hipo crank turned down for the
appropriate journal sizes of the 383 block.

Bore: 4.25" + .030 overbore (4.28" bore)
Crank Stroke: 3.75" (1970 forged 440 crank)
Rods: 6.358" (B rods) or 6.768" (RB rods)

I have chevy rods but I'm prolly going with Mpar 440YJ rods
and having Ross or Wiseco make pistons with the pin higher
into the head of the piston, with a final compression ratio
targeted at 9.0:1 before turbocharging, which I think I have
sized appropriately for 10lbs of boost as a maximum with a
water-based intercooler system.

Also, the cam will be an RV cam once I find something
appropriate, as this engine will be in a low RPM Torque
motor for my Dodge truck, for towing purposes.  The goal is
to make all the power on the low end for towing in the
1500-3000 RPM range, with a 5000 RPM maximum redline
determined arbitrarily.

As far as the intake goes, I am using an aluminum Edelbrock
383 Streetmaster intake, which I have modified by welding on
injector bungs so the fuel injectors are flush with the end
of the runners right at the heads, and where the carb
normally would have gone, I've welded together a bolt-on
plenum that is 10" wide, 15" long, and 12" high, minus a 3"
transmission cooler core configured to become a water-based
intercooler.  The volume of the plenum therefore is 1575
cubic inches including the volume of space taken up by the
intercooler, and should be more than adequate.  The front of
the plenum has two Ford throttle bodies that I have to
interconnect mechanically so they are effectively ganged,
each connected to a turbo. TE-34 would be my first guess for
a turbo.

For horse power calculations, I used Corky Bell's PLAN
formula, which is Power = bmep x stroke x bore area x
putts.  Breaking this down further, I ended up with this:

power = bmep x stroke x area x putts (cyl * maxrpm/2)
power = bmep x 3.75" x 57.5489 x (8 cyl * 2500)
power = bmep x 4316167.5

How do I resolve this with two variables?  Same problem as
before.  I could merge the above two formulas, however then
I'd end up with this:

So I have flow (lb/hr) = HP x .55   and HP = bmep x
4316167.5
                         --------
                            8

Merged I have flow = bmep x 4216167.5 * .55
                     ----------------------
                              8

flow = bmep x 2318892.125
       ------------------
               8

Still, I have two variables, and can't resolve either of
them.  So, I ventured onto more simplistic formulas.  Corky
has a range formula like this:

powerlow = .052 x 431cid x (10lbs of boost + 14.7) =
553.5764 hp
powerhigh = .077 x 431cid x (10lbs of boost + 14.7) =
819.7189 hp

Then, I opted to extrapolate the same formulas to work
without boost, and ended up with the following:

powerNAlow = .052 x 431cid x 14.7 = 329.4564 hp
powerNAhigh = .077 x 431cid x 14.7 = 487.8489 hp

Now I am truly confused.  However, I continued the math for
giggles, and took the original formula using the highest
value I ended up with (820 hp):

        fuel flow (lb/hr) = HP x BSFC
                            ---------
                              cyls

and substituted seemingly random numbers like so:

        fuel flow (lb/hr) = 819.82 HP x .55
                            ---------------
                                   8

        fuel flow (lb/hr) = 56.362625 lb/hr

Bendix/Siemans has injectors that flow 55 lbs/hr as well as
62 lbs/hr.  MSD has 50 lb/hrs (2012) that work with stock
ECMs (high impedence) and fuel rail pressure can be
increased to compensate I would imagine so there is more
flow.  Injectors are rated based on certain fuel rail
pressures, so increasing the rail pressure would increase
the flow proportionally, within reason.  50 lb/hr injectors
when I need 56.x lb/hr injectors could be close enough if
the rail pressure goes up 13%, which I can do easily enough.

How off am I in my math?  Any ideas?
-- 
Frederic Breitwieser
Bridgeport, CT 06606

http://www.xephic.dynip.com
1993 Superchaged Lincoln Continental
1989 500cid Turbocharged HWMMV
1975 Dodge D200 Club Cab (soon to be twin turbo 440)
2000 Buick GTP (twin turbo V6)