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Magazine Feature

Gm throttle body injection article from Trucks magazine


GM Tbi - Fuel Pressure Tuning

GM Tbi Fuel Pressure Mods

A Secret To Tuning in More Horsepower and Drivability

       

     The first performance step is  to address is the fuel pressure. GM did not set the fuel pressure exactly the same on each vehicle - they just made sure they where set between 11-13psi. A 1-2 psi difference in fuel pressure makes a drastic difference in performance! Believe us! In fact, 13psi is just enough fuel pressure for a stock tbi engine to accept one significant airflow improver (ie: our Tbi Power Plate, ported throttle body, headers, or etc.) without running into a significant lean condition.  We have found that 100% stock tbi engines enjoy 13-13.5psi and those with some modifications enjoy 14-14.25psi.


NEW!!!

as of 12/26/03

CFM-Tech Fuel Psi Adapter (pat. pending) for the GM Tbi.

CFM-Tech adapter shown with optional 15psi fuel pressure gauge.

Our fitting installs into the GM Tbi unit by replacing the OE injector pod to fuel line inlet adapter.  It features two 1/8" NPT fittings and one screw in plug. Imagine being able to install a mechanical gauge under the hood in the engine compartment and/or an electrical gauge in the driver's cockpit - all without having to cut fuel lines, bypass the fuel filter, or climbing under your vehicle as is needed with  most other tbi fuel psi testers.

We don't just sell great products - we make them.


   Q:  "Why should I add fuel pressure? It's running fine."   
 
 A: Is it? Do you know what your 02 sensor is reading for an A/F ratio?  [See below for tips on tuning with the 02 sensor.] Do you know if your detonation sensor is decreasing timing to compensate?  Just because there is no drivability problem does not mean that there is no significant power increase to be had from richening the A/F ratio somewhat.  Horsepower is not always what you bolt on - the whole system must be tuned to work with each other!

   Q:   Why does my vehicle pick up speed when I back off from full throttle?
 
 A: Your probably running lean! One, when you let off the gas you are cutting off the air supply and thus the A/F ratio returns closer to ideal. Two, the computer reads the 02 sensor under .75 throttle and thus adds / subtracts fuel to maintain the A/F ratio.  Over .75 throttle the computer does not read the 02 sensor! It uses it's programmed "fuel enrichment" mode! This mode is typically programmed for engine reliability not maximum power, thus, it is just rich enough that the head gaskets and pistons are not in risk of catastrophic failure. 

   Q:  When it is cold out my vehicle starts okay but stumbles when I apply the gas. Why is it okay once it warms up?    A:  Your computer relies on programmed values until the 02 sensor heats up enough to start reacting to the A/F ratio. Once it does, the computer reads the signal and adjusts the fuel trim to make the A/F ratio correct.  Q: Is this okay? Do I have to live with it?   A: No, it is not okay. Plus, it is very bothersome!  Again, this is telling you that the factory tbi system is set up very lean.   Raising fuel pressure will alleviate this!

   Q:  Okay, it seems pretty obvious that I need to raise my fuel pressure but  how much do I raise it? And, how do I know how far to go?     
   A:  Get yourself a fuel pressure gauge! You can either be creative with fittings and "T" one inline on the inlet side or you can purchase a gauge kit made for the GM Tbi.  We have them and so do some parts stores.  Take notice of the stock reading before you add the adjustable pressure regulator so you have a known value and running condition. Install the Adjustable Fuel Pressure Regulator and set to 13.5psi.  Take the vehicle for a test drive and note it's performance, especially at WOT. Raise the pressure in increments of .25. When the performance drops off and/or the vehicle runs too rich, lower fuel pressure to the best setting.  Checking your 0-60 times with a G-Tech or 1/4 times at the drag strip will be your final indicator of which gives max performance.

GM Tbi Flow Rates Measured at 13psi

Application

Color Code

Flow Rate lbs/hr at 13psi

5.0L

Green/White

40

5.7L

Orange/Black

55

5.7L Police package

Yellow/Brown

68

*7.4L

Blue/Black

75

*7.4L

Dark Red/ Dark Blue

85

* Note:  These 7.4L injectors are pre'94 and used in typical GM Tbi low pressure applications. '94+'95 7.4's use a small injector with much higher fuel pressures of around 26-32psi. Do not use these'94-'95 injectors for any other application than their intended application.

Examples of How Fuel Pressure Effects Injector Fuel Flow

Fuel Pressure

11psi

12psi

13psi

14psi

 15psi

Injector

 

 

 

 

 

5.0L in lbs/hr

36.8

38.4

40

41.5

43

5.7L in lbs/hr

50.6

52.8

55

57

59

Our Observances

  • GM's fuel pressure specification for the 5.0/5.7  Tbi's is 9-13psi.  We have found most are set between 11-13psi.

  • GM's computer programming is quite lean for all Tbi 5.0/5.7's.  Emission's and reliability prevailed over power!

  • We have found that raising the fuel pressure to 13.5-14psi on stock 5.0/5.7L improves overall power.

  • We have found that raising the fuel pressure to 14-14.5psi on slightly mod'd 5.0/5.7L is of major benefit.

  • Trucks with 11-12 (or lower) psi will experience a drop in power, detonation, and/or drivability problems when any airflow inducing modification is made. Typically the product added is blamed, however, it is actually the lack of fuel that is the real problem! We can't stress how common this is!

  • Trucks with 13psi-13.5psi can usually handle one or two airflow inducing mods - like our Tbi Power Plate and an air induction system. I stress usually , meaning not always! A too lean condition is that close by! Anyways,- the fuel pressure should really be adjusted to 14 psi at this point.

  • The stock tbi fuel pump typically has a stall pressure of 15-16psi, meaning, you cannot adjust the pressure higher than this because the pump will no longer push fuel at any higher psi. We've seen a few stock pumps push 18-20psi but this is not the norm.

  • If the pressure needs to be pushed past 14.5 psi the computer should really be re-programmed for this means that the stock programming is unable to keep up with the demands of this particular engine.  Remember, adding fuel pressure is linear -- 1 dimensional. Computer programs are typically 2 dimensional and 3 dimensional.  Use the adjustable fuel pressure regulator to fine tune the fuel trim only - not to "cover up" a mismatch between programming and the engine components.

 

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