Second only to the stock TBI ECM, the stock TBI fuel
delivery system leaves a lot to be desired. Starting at the tank
the fuel pump is a turbine pump. Quiet and long
lasting, but weak fuel delivery. The fuel pressure regulator is set for
approximately 12 psi (30 psi on '94 & '95 BBC trucks). With a
service range of 9 to 13 psi being considered good. This equates to a
17% to 20% change in injector flow rate over the service range.
Then the stock injectors are just
large enough for the engine they were on top of.
From all of this, it is obvious that GM never put a TBI unit on
anything but a low performance engine. The largest GM TBI unit was
provided on the BBC 7.4l trucks. This is a 2-bbl unit with 2"
Even though in stock form TBI setups were never used on a performance
engine, it doesn't mean that they can't be used on a high performance
All is Not Lost:
There are several ways to increase the flow capacity of the TBI unit
and the injectors. The TBI units can be bored out larger for more
airflow. Two 2-bbl units may be placed on a dual-quad intake manifold.
Holley and Accel make 4-bbl TBI units. For additional fuel larger
injectors and/or high fuel pressure can be used.
The first thing to do is to start with a decent estimation of engine
horsepower. For the best results be reasonable in this estimate. From
this we calculate the required fuel for that horsepower. Then work
required injector flow rating. Selecting the injector and fuel
pressure that delivers the required amount of fuel as the final step.
You knew there had to be some math. Can either use your
Windows calculator or open a spreadsheet, then follow along.
An important parameter in engine performance is known as Brake Specific
Fuel Consumption (BSFC). It is the pounds of fuel required to generate
one (1) crankshaft HP for one hour. The lower the number the more
efficient the engine. Engines using gasoline with forced induction
(turbo/supercharger) run between .55 and .60 BSFC. That is it takes
between .55 and .60 pounds of fuel to generate one horsepower for one
engine with moderate compression ratio and so-so heads the BSFC will be
about .50. An engine with higher compression ratio and a
decent chamber runs about .45 BSFC. With ultra high
running .35 BSFC and lower. If you have the engine dyno'd (not the car)
the BSFC can be measured.
For your basic high performance street engine with decent heads a value
of .45 is typical. Let's say that the engine is making 400 HP peak. How
much injector is required?
First calculate the amount of fuel required to produce 400 HP at a BSFC
400 * .45 = 180 pounds of fuel per hour (#/hr).
Then factor in a maximum of 85% duty cycle for the injectors:
180 / .85 = 212 #/hr
The duty cycle of the injector is the ratio of on time to
the available time.
Note that this is not the best method. The available 'off' time is
really tied to RPM, not a duty cycle. With the injector firing
synchronized to spark plug firings, the available time is less as
the RPM increases.
OK, now we know that the engine requires 212 pounds of fuel per hour to
produce the rated 400 HP. To do this with two injectors each needs to
provide half that amount.
212 / 2 = 106 #/hr per injector.
As of writing this there are no 106 #/hr TBI injectors
available. However, by using BBC 81 #/hr injectors and
fuel pressure the 106 #/hr delivery can be achieved. To
calculate the required fuel
pressure we will use the square of the ratio of the injector flow
is the stock fuel pressure in psi for the rated 81 #/hr):
SQ(106 / 81) * 13 = 22.3 psi
(To do this with the Windows calc put it into
scientific mode: View->Scientific. Then use the x^2 function for
By using two 81 #/hr injectors at 22.3 psi they will provide 106
pounds of fuel per hour per injector. This is enough for the 400 HP.
And by running the fuel pressure above 18 psi it is a perfect candidate
for a Vacuum Referenced Fuel Pressure Regulator (VRFPR). See further
along for information on the VRFPR.
So now how do we get 22.3 psi of fuel pressure. First thing is
to install a high pressure pump. Second is to install an
external FPR or modify the stock one.
For a fuel pump be careful to not purchase the biggest baddest pump
there is. High pressure pumps move a lot of fuel at 43 psi (port fuel
pressures). And deliver even more
fuel at 22.5 psi. The TBI unit, regulator, and return fuel line need to
pass back to the tank any fuel that isn't used by the engine. A pump that is too large can
cause the fuel pressure to increase above the set point.
Once the fuel pressure is over 18 psi a decent upgrade is to use a fuel
pressure regulator (FPR) that is referenced to the intake manifold
vacuum. This vacuum referenced fuel pressure regulator (VRFPR) changes
the fuel pressure as the engine load changes. A higher engine load
equates to a higher fuel pressure along with a higher injector flow
rate. And with a lower engine load a lower
fuel pressure along with the lower injector flow rate. This not only
makes tuning easier, the
engine also runs better.
To take advantage of a VRFPR you need
to use an ECM that is set up to handle the varying injector flow rate.
Both the EBL Classic and EBL Flash have
the required calibration parameters for this.
Note that an external port style bypass FPR can be used for a TBI
setup. Normally a TBI setup runs a consistent fuel pressure. To do this with an
external regulator leave the vacuum port open to the air. To use the
regulator as a VRFPR connect the vacuum port to intake manifold vacuum.
If using a port on the TBI unit for manifold vacuum double check that
it is manifold vacuum and not 'ported' vacuum.
about MPFI FPR's when connected to manifold vacuum. This has the actual
fuel pressure changing with manifold vacuum. However, it is done to
provide a consistent fuel pressure across the injector. With the spray
end of the injector exposed to manifold vacuum, the fuel pressure needs
to change in order to have the same fuel pressure differential between the inlet and
outlet of the injector.
On a TBI setup the injector spray end
(outlet) is basically open to the air and is not exposed to manifold
vacuum. So by vacuum referencing the FPR the actual fuel flow of the
injector is changed. We take advantage of this trait when using increased fuel pressure and a VRFPR on a TBI setup.
Back to our calculating fuel requirements 400 HP engine example, use a
2-bbl BBC TBI unit with the 75 or 81 #/hr injectors. Then use a VRFPR
set at 23 psi (26 psi for the 75 #/hr) with the vacuum line
This setup will easily support 400 HP with good driveability.
& TBI Unit Information
GM produced TBI units in both 1-bbl and 2-bbl configurations.
Single 1-bbl units were used on 4 cylinder engines. Dual 1-bbl units
were used on the Corvette and Camaro Crossfire injection setups.
In the 2-bbl TBI series the popular units are found in three sizes:
2.8l and 3.1l engines:
have 1-3/8" bores and flow about ?? CFM
4.3l, 5.0l, and 5.7l engines:
have 1-11/16" bores and flow about 490 CFM
have 2" bores and flow about 645 CFM
The flow ratings are approximate and are at 1.5" Hg. This is the same
pressure that 4-bbl carburetors are rated at.
For injectors they come in various flow rates. The common injectors
that are found in the above TBI units are:
2.8l and 3.1l engines: 33 #/hr
4.3l engine: 45 #/hr (note: some units have
different flow rate
injectors. This is done to help with fuel distribution.)
5.0l engine: 55 #/hr
5.7l engine: 61, 65, & 68 #/hr
7.4l engine: 46*, 75 & 81 #/hr.
watch for the '94 - '95 BBC truck units as the
injectors only flow 46 #/hr at 13 psi. Which is 74 #/hr at the 30 psi
these TBI units use. On the up side they come with a 30 psi FPR. The
down side is that the injectors are small.