Junkyard TBI

If you're doing it for the 225 count me in too.:v6:

 

You can use 2 adapters to make this work and both can be had at summit racing. 1st is a rochester to square adapter. Then use a square to tbi. I had to do this when I went TBI on my 258. Make sure you seal all the mounting holes well or you will get vacuum leaks.

 

I am very interested. However, before I commit, I'm interested in hearing how involved the conversion is. Would going to TBI require an eletric fuel pump capable of higher line pressures, say around 40 psi? What other modifications would I need to make to the fuel tank? I still run the stock fuel tank under the seat with two lines going to it (one pick up and one return). The tank is vented through the cap, so it is always at atmospheric pressure.

 

I would be interested in this as well. Let me know when funds would be needed.

 

The TBI runs at 13 PSI. So you need a pump that can maintain at least that preasure at WOT. So you you would have to replace the stock pump with something else. The pick up and return lines can be reused for the TBI so no mods need to be made to your fuel tank.

In order for Fuel injection to work you also have to source the sensors that are needed by the computer. And mount them in the appropriate areas of the engine.

1) AIT - Air intake temp
2) MAP - Manifold presure (load)
3) TPS - should be on Throttle body
4) CTS - Coolant temp sensor
5) Tach - Engine speed for fuel flow calculations.
6) O2 - oxygen sensor if you want to run in closed loop (more efficent)

The first 5 are menadatory the O2 is optional but worth it. If you are sourcing your TBI from a junk yard make sure to get the computer and as much of the origional harness/sensors as possible from the doner vehicle.

Or if you are feeling frisky you can always go the way I did and build your own computer. http://www.megasquirt.info/

 

Sign me up for the TBI <--> Rochester 2 barrel intake (stock 225 intake). I'm trying to stay away from stacking adapters. Send me a PM or email me at pcharlton5@comcast.net about the funds, this will kick me in the butt to finally get this done. THANKS !

 

Guys, I will break out my carb gasket kit and start making a rough model of this. I need to head to the Junkyard to get my TBI unit to compare the 2 and get the adapter started. I' ll keep you poseted.

 

Mike, if it's a 225 intake to TBI adapter, you can also count me in. Will the larger 670 CFM of the 4.3 TBI reduce torque as a larger CFM carb might?

 

I think so, yeah. IIRC the torque issue has to do with the volumetric efficiency of the cylinders and valve timing overlap. Bigger hole = less pressure difference inside to outside = more charge per cylinder. Having lower VE is like having a cam with a shorter duration; less overlap and more compressing in each stroke at low speed. Ideally you'd like to adjust the valve duration for engine speed, but I don't know any way to do that.

I'd think you could balance it put by using big throttle body and a short duration cam. Not possible with a carb because the vacuum signal will be too low at low speed (I think). With injectors, you don't need the vacuum signal to modulate the fuel. AFAIK TBs are routinely sized very large compared to carb venturis because TBs don't need to control the fuel flow via vacuum signals.

 

Sounds interesting! Please sign me up too.

 

As far as I know the throttle bodies are the same but the injectors are different between the 4.3 and 5.0, 5.7. The wiring is quite easy, it just looks hard because of the amount of them. The ECM (computer) to have is the 7747 model, found in late 80's GM's. It's popular because aftermarket companies support it if you need anything changed. Whatever computer you get with the TBI will be fine, mine is not a 7747. I had a '91 4.3 and blew it this winter. I put a '94 engine in and kept the '91 computer.

I would say use the sensors that your computer already had. For example my '94 engine didn't have a knock sensor,(that I could find) but my old '91 did, so I transferred the sensor over to the new engine, because my computer had one before. There are wiring diagrams online to make the wiring easy.

Hudsonhawk was right about the fuel pressure, I've heard it said 15 lbs. I ran mine with an external pump from a Ford F150 (50 lbs) for a while, and now have an internal XJ fuel pump (30-40 lbs) mounted externally. I know a guy who has done this for years with no ill effects. I have 5/16 feed and return fuel lines and the gas just circulates. The XJ pumps are easy to find, where for me an aftermarket 15 lb pump is expensive and hard to find, especially if you keep a spare in the tool box. The donor GM harness has the fuel pump relay and wiring.

 

I use a 30 PSI holley inline pump on my TBI. The pressure regulator will just allow any excess fuel to go down the bypass back to the tank.

 

I figured people would think I was crazy running twice the pressure, glad to hear it from someone else. Works great!

 

OK so I have been looking at TBI's from the junkyard and they want $350 + for the whole set up. Anyone running Holley 670 cfm?

 

You can buy them all day long here for $100-$125 the last time I looked.

 

At that price I would gladly pay you to buy a system, box it up and ship it to me.

 

Wouldn't the TBI to 225 V-6 2 barrel manifold adapter be the same as the Howell TBI to Jeep Carter BBD 2 bbl adapter? I know that the Rochester 2 barrel as used on the 225 V-6 and the Carter BBD as used on the 258 have the same bolt patterns. In fact, I ran a Rochester 2G on my 80 CJ-5 with 258 for a couple years.

 

What all do you need? Wiring harness, ECM, and throttle body... anything more? You'd probably want to go with new sensors. Ebay is said to be a good source for this stuff.

 

So does anyone have a complete stock GM 3.8l or 4.3l system working on an odd fire 225? I thought in our previous discussions on this topic that the odd fire pulses confused the stock ecm as to the true RPM? I was thinking of going with Megasquirt as it is supposedly able to handle the odd pulse period, but if there is a stock system that would work, that may be cheaper.

 

That was my understanding too. Megasquirt can work with an oddfire, but I don't know of a way to adapt the GM ECM to the uneven oddfire triggering. You'd still need the rest of the GM TBI bits though - throttle body, sensors, etc.