GM TBI on 258

Anyone done a junkyard conversion for TBI? I know you need to get an adapter plate, but does it matter if you get a setup off of a v8 or v6? And how do you take care of the computer controlled timing in the distributor?
Any info would be great
Thanks, Brian

 

What year is your 258. Specifically is it a single barrel or 2 barrel intake?
If its a single barrel intake then the only TB I know of that will fit is Holly's FI system for a singel barrel intake.
If its a 2 barrel then you can use a rochester 2-barrel to GM TBI conversion plate and use the Junkyard parts.

Ideally you want the computer, wiring harness and throttle Body from a 4.3L V-6. This can be used with out re-programming the computer.

The V8 computers and throttle bodies flow more air and the injectors are larger. You would have to have the computer reprogrammed to meet the needs of the engine.

 

What about the distributor? Can you install the ICM in a different distributor or do I need to use the one off the donor?

 

You have a couple of options. One is to use a GM HEI distributor adapted to the 258. This is a fairly easy conversion, and you can use an ICM in the HEI distributor that provides spark control. The other option is to use a Duraspark distributor from a '78 or later 258 and mount the ICM externally. I know this can be done a few different ways with at least 3 different ICMs with the Megasquirt, and I presume the GM computer could be configured similarly.

HudsonHawk has a lot of experience with the Megasquirt - you could ask him about that if you are interested. Personally I wouldn't mess with the GM computer - I'd use Megasquirt.

 

I broke down an got a howell..
same concept.. GM TBI ona 258.. works great no complaints..

only a few suggestions..

I rebuilt my 258 with a cam and more compression. (a different story in itself)
Took a while a couple hundred dollars to get the right chip burned for the GM CPU..

the Howell works great on a stock spec motor.. if you do any internal or perf. mods be prepared to tune, tune, tune..

 

You will want to pull a complete setup out of a chevy truck,van,suv. It needs to have the '7747 ecu, v6 or v8 will work but the v6 will have injectors better suited to the 258, plus the proper initial programing to get you started.

I used a HEI with the mechanical locked out, i converted the 4 pin module in it to a 7 pin which is controlled by the ecu.

Lots of tuning is required.
go over to binderplanet.com under there efi fourm they have lots and lots of info. billusn1 on there helped me with my tuning.
Pic of my setup.

 

It will run this way but wont have the performance you want. It ends up running like a poorly tuned carb.

You will need a chip burning setup. If you get that far i can email my tune for you to use or modify.

 

I took a look at the megasquirt setup and the adjustablity looks nice but whats the cost of that compared to a reprogramer

 

The engine components are the same price as for the GM TBI. Or less, since you don't have to use the popular GM hardware. You have to build or buy the Megasquirt - most people go with a kit. The assembly instructions are verrry thorough, but you'll have to read a lot to complete the system. IMO you'll have to do a similar amount of research to assemble and tune a GM system, so I don't think that's much of an obstacle in your case.

Here's an article on the IHC site about installing a complete TBI system with Megasquirt for less than $300 http://www.binderplanet.com/forums/showthread.php?t=53524

 

A gm tuner costs about 200bucks to buy. Giving what i know now, i would go with the megasquirt setup from the begining.

 

If you want to find some really good info on swapping gm fuel injection go to binder planet they have great info listed on there site.I am shore it could be linked here but I am not that computer literate. If you go with a howell set up or any other system that uses the tac signal to fire the injectors CHANGE TO A HEI again CHANGE TO A HEI the reason is when the stock jeep ignition starts to go bad it will give off a false high RPM reading to the computer causing the injectors to dump raw fuel into the engine as soon as you turn the key . witch inturn fuel washes the cylinders taking out your newly rebuilt 258 before you figure out what is happening. Ask me how I know if you haven t already figured it out

 

Also if you run an HEI you only have to map fuel curves in the Megasquirt. Makes tuning much easier then fuel and spark.

 

There is a BIG advantage to CC spark. I don't know if Mega Squirt does automatic timing retard, but the gm setup will allow you to run right up to the point of detonation with out danger providing you have the knock sensor installed.

The 258 has a VERY conservative timing curve stock.
I am running up to 58* of advance.