TBI questions (still researching conversion)

[nvrenuf]

I'm trying to piece together a TBI set up for my K5 and still ironing out what I need / don't need. I'm looking at buying a harness from Howell but need help with the options.


I'm currently running a complete MSD ignition, if I go with an OE TBI type distributor can I keep my MSD 6AL box? Can I keep the coil?

Knock sensor is an option, get it? This is for a 454, I thought I read somewhere that some don't use a knock sensor?

Heated O2? Seems like it's what everybody uses but I have to ask.

I'm sure Howell can answer these questions but I'd rather not depend on the salesman.

Fwiw, this is for a minimal street driven, wheeler with a mild 454.

 

[dyeager535]

So the obvious question is why aftermarket? I know there have been threads on this, not sure if yours was one of them, sorry if redundant, but you've got a '91, the '87-91 stuff will bolt right in. If it's a factory duplicate, fine.

I'd sell the MSD stuff. You are talking TBI, so I don't see that it's going to be needed for the high RPM capacity it has over stock. Yes, I think MSD is a waste of time and money (plus adds a failure mode) on a street rig, so I'm biased.

I'd run the knock sensor myself, there are different schools of thought on them. Personally I think it doesn't hurt, if for some reason you get a bad tank of gas (very unlikely, but still) or something else happens, it will help prevent engine damage. Not sure what year 454's started getting knock sensors, but you are right, it wasn't all years/applications with TBI.

Heated O2 is nice, but IMO not necessary. If you are running headers it's "more nice". Even far downstream they go closed loop very easily (often will idle open loop, even if programmed for closed loop idle as the O2 goes cold) in headers, so it's up to you. If you get a free/used of either, use it. It's just two more wires for a heated sensor, and they aren't tied into the TBI system, it's just a power and ground that can be added at any time.

EagleMark may know, not sure if using a small block TBI harness from '87-91 on a BBC is possible (length) but that would give you knock and ESC. If that will work with the later/greater ECM's, that is probably the way to go if starting from scratch.

Not trying to make this complex, but since you aren't starting with anything, you've got time to research and decide what is best for you, just throwing out ideas.

 

[nvrenuf]

Why the aftermarket harness or Howell in general? As for the harness, mostly because it's simple (ie; engine only), clean and all new (no broken connectors, etc). Imo, it's not exactly cheap but really not that expensive considering it's new. As for Howell, really just a source for the harness. I already have a nice OE throttle body w/ OE adapter plus my K5 still has the electric fuel pump and (unused) 7747 computer.

* knock sensor: I know it'd be best for the truck but I really want to go as bare minimum as possible for simplicity sake (just a wheeler) BUT not at the expensive of being foolish or creating a system that doesn't work well.

* MSD: I'm amazed at how many folks say to scrap it. I've had great service from my stuff. I just keep thinking that since I have it, use it. It's also this line of thought that makes me hesitate to go back to an OE style distributor.

Ultimately I just want as simple yet smart of a system as possible because it's a bare bones wheeler.

 

[dyeager535]

If you don't know me, I can't keep my opinions to myself, so take this with a grain of salt. If you are ok with keeping an MSD, which adds unnecessary failure modes, and TBI, which adds a fair number more, then you should be perfectly fine adding the one wire necessary for a knock sensor, right?

IMO the knock sensor is essentially a safety device. If something goes wrong (IE bad fuel) you won't destroy the motor. If the tune is off a bit, you won't kill the motor. It's not something that is going to intrude on space or your time. AFAIK the knock sensors are near bulletproof, I can't recall anyone having one fail. Although obviously they must, for as many are out there, I can't remember a single case of one actually being the culprit.

The '7747 really kind of sucks from what I understand. There are later ECM's that are (again AFAIK) interchangeable, which have more functionality. EagleMark will be able to comment more on that. Perhaps with the manual trans, you won't need that additional functionality, but the '7747 is a sloth, even compared to other OBD1 GM ECM's.

You mention the TBI only portion of an aftermarket harness, has yours been destroyed? Having the fuses and what not in the stock locations would seem simpler, but thats just me. How are you going to handle VSS? Has your cluster already been changed over to something else, since a '91 would require the VSS 241/electric speedometer, or is it still in place?

 

[nvrenuf]

Good points on the knock sensor, I hadn't thought about it like that.

That's interesting info on the computer, I'd love to know if there's a better unit to use. I really assumed that all of the early OBD1's would perform the same.

As for the vss, all of that stuff is gone. The cluster is a combo of autometer gauges and an earlier speedo and fuel gauge. The original tbi harness was chopped up too so I stripped all the unnecessary leads out and just have a few wires remaining for the starter, alt and such.

 

[dyeager535]

VSS is going to have to come back into the picture. So factor that in now. If money is no object, should be able to get the right setup pretty easily. Without money, you are talking about using factory speedo/cable that is 1982-1989 (VSS/buffer bolts to back of speedometer) or an NP241 (or the very rare 205) with VSS tailshaft housing already, but with doubler, I assume that is out?

The newer ECM's they have more capability, are a bit more refined (as I understand it) in how they can be tuned, and offer an exponentially faster data feed, which really only helps for tuning, but you will be going there anyway, so might as well not handicap yourself if another option is out there and won't kill you $$ wise.

 

[nvrenuf]

There's got to be a work around for the vss, it seems too broad for everybody with TBI to have converted. Maybe a tach signal?

 

[samsterman99]

Vss is good to have, it keeps the truck happy at low speeds off road and keeps it from dying I have noticed. I had it without the Vss for a while, and it works fine, just not as refined as it could be. Since adding the Vss I have never had my tbi stall out.

 

[dyeager535]

You are running factory cable drive speedometer, right? Not a big deal to add it in that case.

I suppose you could fix the stalling issue by increasing the idle speed way up, but that would introduce its own problems.

People say they run without it, and it "runs fine", but guaranteed it doesn't, not in all instances. Some of the later TBI ECM's might be even more dependent on it than the '7747, as the '7747 is the one I hear people talk about not noticing a missing VSS nearly as much as other OBD1 GM setups.