View Full Version : ?'s about modifying factory GM TBI

10-04-2005, 07:12 AM
This is for the 86 Cavalier, TBI, four-cylinder in my trike. It is a temporary stand-in until I build an Ecotec to replace it. The engine is completely stock right now, with the exception of the air cleaner.

First Question
It has a single oxygen sensor, located about six inches from the head, in the collector area (factory "header" with short tubular primaries). I want to split it into true duals with dual collectors.

Will the oxygen sensor provide the correct signal if it is exposed to only two cylinders worth of burnt gases, or do I need some kind custom setup with dual sensors?

Second Question
The throttle body is mounted like a traditional carburetor. The injectors fire right down the pipe into the throttle body. At idle, the butterfly is closed and the fuel kind of bounces off of it, and is sucked in around it. I want to fabricate a new intake with the throttle body mounted almost horizontally.

Will the engine vacuum be enough to pull the fuel spray into the engine, or will it puddle and stumble?


Fuelie Fan
10-05-2005, 12:43 PM
The answer to both questions is pretty much that it's not ideal, but it will work. We have a similar setup on an mgb, and it runs well, but we did have to play with the minimum throttle settings to eliminate a minor stumble, just as you predicted. As long as your engine is healthy, reading only two cylinders is ok. Many manufacturers used to put only one EGO on v8's, for example. If for whatever reason one the cylinders not being monitored developed a localized intake leak and leaned, the system would be unable to correct, but that's pretty rare all things considered. Especially since this is temporary, i'd say you're fine

10-05-2005, 07:32 PM
Thanks Fuelie! That's all I wanted to know - that I could get away with it for a while. When I finally get around to building the Ecotec for it I hope to run individual throttle bodies, canted up a little with stacks, so this will be the precursor.

I can't wait to see what it sounds like with duals! I guess I'll group 1-4 and 2-3 together. That's how they're usually grouped in a tri-Y header for inline fours.