I have a fuel qusetion about the fuel system on my 94 chevy 1500 i upgraded my motor 30 over bigger cam hedders and the works the truck is runnin lean i was wondering about these biggere tbi's i am seein on E-bay now with these units do i need bigger injectors or can i get away witht the stock ones and the bigger pods or should i go bigger all the way around if so what size would you recommend? Also i wanted to know if the computer chip i have which is a super-chip that i had cut to handle hedders and spacers and remove the govener or do i need a new custom one this project has been goin on too long and i want to finish it. If you know if anyone with a holley unit (tbi) for sale let me know and thanx in advance  

This thing drives me nuts a write a length reply, post it and it disappears. So here goes again.

You've exceeded the limits of the computer's understanding of sensor inputs to the fuel map. It sounds like you applied headers and spacers, blew a new chip and then did additional work. Is this correct? If so what additional work?

While Manifold Absolute Pressure (MAP) injection systems aren't the most flexible to changes that modify mass flow thru the engine, changing to a .030 overbore and headers shouldn't have gone off the map to where a new chip/program was required. However, changes to the cam, heads, compression or total gearing would require a new chip and any change that moves the air flow thru the engine more than 10% will require a chip.

If you're running lean, it would appear that the changes in total are greater than the programming is able to correlate sensor data with the fuel and timing maps. The down side of these set ups is that when modifying a MAP engine whether it TBI or TPI, everything has to be done once and a new chip installed for all those changes otherwise each change requires a chip as it's incorporated and that gets pretty expensive as you probably already know.

There's a couple ways to go from where you’re at, the first is an adjustable regulator put into the TBI body replacing the OEM fixed relief. However, this will just richen up the mixture everywhere and in all probability will prove to be too rich in some operational situations and not in others. The second thing is to revisit the chip. If it was reprogrammed with all the changes accounted for, then it may have an error, or fuel pressure may be too low. If there were changes made to the engine beyond headers and spacers that the programming doesn't know about, it can't resolve sensor input to the fuel map and a new chip with a new program needs to be installed.

I'm not a big fan of spacers; they really can upset the volume of the plenum in negative ways. People selling these things make some pretty high claims but seldom are forth coming about other changes made to the engine that allow them to optimize their function, especially those spacers that use one large hole as they then let any cylinder breath like the entire throttle body capacity is available to it. Without programming changes, the fuel map isn't designed to feed this situation. A similar situation exists with those kits that increase air flow by raising the injectors higher. The fuel map doesn't know about this unless it's reprogrammed.

Changes increase flow thru the TBI, heads that flow better like a Vortec for Swirl Port swap, changes in compression ratio, changes in the cam or rockers that net more lift and duration at the valve, or to overall gear ratio all singly or in combination work to change the relationship of engine vacuum and mass flow for any given RPM and throttle position. This causes a disparity between what the sensors are telling the program and how the engine is trying to operate. The fuel and ignition map is not self correcting; it integrates the sensor data and any given moment and pulls a fuel mixture and timing position from the maps that correspond to the sensor readings regardless of what the engine is trying to do.

A larger TBI or larger injectors won't solve this problem. However, the opportunity that presents itself is that the stock TBI for a 305 or 350 uses a 1 and 5/8s bore and flows about 420 CFM which is rather too small for a 350 without a governor and other performance mods. The 454 engine uses a TBI with a 2 inch throttle bore and Holley sells a 2 inch bore replacement unit, both of these are rated at 670 CFM and include larger injectors to keep up with the capacity without having to run the injectors beyond 80% duty cycle. Turbo City also will take your stock TBI and bore it to 2 inches, rebuild all the parts and stuff in higher flowing injectors for similar result to the 454 or Holley 502-7 units.

Just remember that you want to roll all these changes into one new chip and be done with it as a piecemeal approach will require the chip be programmed more than once and that ain't cheap to do.

A cheap trick you can do to test your current mixture's leaness is to put in a 160 degree thermostat, unless you new chip already subverts the cold engine protocall, a cold thermostat should make the computer thing the motor's still cold and it will throw mixture enrichment fuel at it, like it's on the choke of a carbureted engine.

I don't recommend this or simple fuel pressure adjustmens as a solution as constantly running the engine rich is hard on upper cylinder wall lube and will reduce piston, ring and wall life. It's just a test.

Bogie