Looking for TBI swap info

[wolf_walker]

I've only spent a few days reading and searching on this, you fellows seem to have your stuff together, so I thought I'd run it by you.

I'm planning on retrofitting a 2 barrel TBI setup to a 2.8L DOHC inline 6cyl Mercedes engine, 6500rpm redline, makes 150hp or so stock in low compression USA trim.
I'm not looking for power really, but I don't want it choked if I can help it.

This is a 73, so it's old school with no cat or other emissions equipment, auto trans.
It has a points distributor that I plan on using a petronix breakerless conversion kit with an MSD ignition box, which I'm told will supply TBI with an ignition signal it'll be happy with.

Couple questions.
The GM 2.8 TBI don't make a lot of power, but I figured it'd run OK with the stock maps till I get it settled out, being the displacement is the same. I wonder though, if even with some chip tuning if it's going to be restrictive on a motor turning so much RPM and making as much power as it does compared to the stock application? I could use the 4.3L T-Body, but it'd be rich at lower speeds I imagine. Thoughts?

Also, I can track down what years and models had what motor and TBI, but are there preferable years to look for? The VSS is going to be a bit of a pain in this swap, but do-able. Are the older one's going to expect a VSS signal too? If I could find a breakdown of ECM model numbers stating what they were from and the differences that would be great. I need somewhere to start hunting for parts basically.

Lastly, I have thought about MegaSquirt with a throttle body for this application, part of the attraction though is that this should, I hope, be a fairly bolt on and run setup. I don't really want to spend the time honestly tuning MS on this thing, it's not a very common application to say the least. Am I going to end up with more effort in GM TBI than with MS do you think?

Thanks so much for you time and any thoughts you have.

[Guest]

Your basics look good.

Using the 2.8L TBI set-up will get you up and running and shouldn't have too much issue doing so.

If you do find that the 2.8L TB is too restrictive, then going to the 4.3 TB would be a good way to open up some breathability.

I'm not familar with the Pertronix conversion, but if it will provide a magnetic hall effect type sensor, then using the GM ignition control module (ICM) would be a good way to both provide the ECM with the RPM reference and control spark timing. This is what I did with my Datsun, though I used a later Nissan dizzy that was magnetic trigger OEM.

For the ECM, you can use just about any TBI ECM, but would suggest one of two possibilities.

The 1227747, that was used from approximatly 1988 to 1992 in S-series and full size trucks. Should be fairly easy to find too.
To tune it, you will need to change the EPROM socket, to a regular DIP socket and then use an address adaptor to use the newer 28 pin EEPROM, since the early 24 pin EPROMs are hard to find and can be troublesome to use. The newer EEPROMs are easy and cheap to find, are also more quickly programmed, since they can be electrically erased. There's more options, but this is where I'll leave it for now.

There is also another possibility and that is a '427 (I'll get the full number later, if this is how you want to go). This is a newer ECM used in the 1994 to 1995 full size pick-ups, that also controlled the tranny, so you would need to start by changing some settings to disable tranny trouble codes and possibly some table changes. The reason I mention this one is that it is an 8192 baud ECM, the '7747 is a 160 baud ECM, this is the rate that the diagnostic serial port sends data at. The 160 baud is ok, and will likely be quick enough for you, especially if you only need minor changes. Sometimes the faster com rate of the newer ECM will help get the tune squared away quicker.

If you're new to swapping ECMs between different manufacturers, go with the '7747, this will eliminate any of the tranny control issues, and be a fairly simple swap.

For the VSS, you have a couple options.

If the tranny speedo cable output is the same as a GM, you can buy an adaptor from Painless (among other places), that will output the VSS signal.

There is also the posibility of using an Optical sensor that was originally used in the S-series (and likely plenty of of applications), attached directly to the speedo itself.

The speedo will need to be one with a reflective arm or similar in the back of the speedo, that spins with the speedo cable.

I'll add pictures of what I did in my Datsun later, when my computer doesn't give me fits.

[wolf_walker]

The 747 sounds like a good start.

How important is the VSS to operation? I can't get a straight answer on what it actually does, other than a few things I know I don't need, egr, etc. Idealy I'd like to get it up and running and figure out the VSS later. But whatever I have to do I guess.

Thanks for the info.

[Guest]

Here are the optical sensor pictures: