Dynamic EFI

Bringing TBI and Multi Port Fuel Injection to a New Level.

TBI Fueling


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" bores. 

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 engine.

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 toward the required injector flow rating. Selecting the injector and fuel pressure that delivers the required amount of fuel as the final step.

Calculating Fuel Requirements:

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 hour. An 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 performance engines 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 of .45:

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 increasing the 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 rates (the 13 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 SQ).

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.

Fuel Delivery

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.

Vacuum Referenced Fuel Pressure Regulator (VRFPR):

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.

A note 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 off. This setup will easily support 400 HP with good driveability.

Injector & 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:

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:
*Just 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.

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