Holley ProJection set up

Terry I've run the Holley ProJection (#502-12, 2-injector, analog) for about 5 years now. I installed the closed loop kit very shortly after installing the open-loop TBI unit, but in the ensuing five years, I have run it both ways often enough to have a pretty good sense of the difference. The closed-loop portion of the kit can be "disabled" or "bypassed" simply by grounding one wire, by the way. And I've actually wired this to a switch, so that I can quickly and easily opt for either mode. I've been told that the closed loop kit will only make very minor corrections and is not worth the trouble or expense. I've also been told that the system without the closed loop kit is junk but the closed loop kit elevates it to godliness and it becomes the perfect system in every regard. I would like to think that my opinion is somewhere between these two extremes. As I understand the design and how it is supposed to work, this is what you can expect from the closed loop kit. The feedback from the oxy-sensor will have NO EFFECT whatever on how the vehicle runs when cold, or how long it runs in the "high-idle" or "choke emulation" modes (my terms, not Holley's). The air/fuel mixture during this phase, and how long this phase lasts (until what engine temperature) are determined by the dialed-in settings of the "Mid-Range", "Idle" and "Choke" potentiometers on the ECM. Tangent: The "Mid-Range" dial appears to function as something of a "global level set", and impacts the other ranges as well. It is a mistake to think of it exclusively as a "Mid-RPM-range" control, or as something that will affect the motor ONLY "above idle", but "less than full throttle". It's setting very much appears to "spill over" into the "Idle", "Accel", "Choke" and "Power" settings. I believe that John Hofstetter -- another long-time ProJection user -- will concur in this assessment. The feedback from the oxy-sensor will likewise have NO EFFECT whatever on the air/fuel mix delivered to the motor in the extreme "Power" operations --which I believe to be above approximately 2/3 throttle (as determined by the throttle-position sensor). Allow me another small tangent: My throttle-position sensor was grossly mis-calibrated right out of the box. Check this with an ohmmeter; adjust if needed; it has stayed reliably where I adjusted it for five years now. What remains is that the feedback from the oxy sensor WILL modulate the air/fuel ratio:1) AFTER warm-up has completed, and2) during most normal street driving, and certainly almost all of your trail driving Moreover, the LED on the closed-loop circuit box will still *indicate* whether your air/fuel mix is above or below the theoretically "ideal" stochiometric ratio (14.7 to 1, I think), even when it is not *controlling* the mixture. This is also true when the above-mentioned wire is grounded, forcing you into open-loop mode. The closed loop circuitry continues to act as an "indicator" if nothing else. It will indicate that the mix is somewhat richer than "ideal" during warm up, for example -- which is as it *should* be. Practical observations: In closed loop mode, I can begin the day at my home (+/- 5,000 feet above sea level), drive on the street to a trailhead at 8,000 - 9,000 feet, run at rail that takes me up to 12,000 feet and back down -- and all the while not have to fiddle with the various potentiometers to keep my air/fuel mix at or near 14.7 to 1. No carb in the world can adjust its jets "on the fly" like this. In "open loop" mode under the same circumstances, I will observe the same things that I would observe with a well-tuned carburetor. My mixture will become somewhat richer than ideal as elevation increases. It will lean back out as I come back down. Overall, performance will be no worse, however, than a carb subjected to the same operational extremes, and "base-tuned" for the same elevation. Though I *may* manually fiddle with the dials throughout the day, I am certainly not *obliged* to or forced to -- no more than I would be "forced" to re-jet my carb every couple of thousand feet. If I am going to run at max throttle in a truck pull in Tulare, CA (+/- 600feet above sea level?), where I know that my ECM will revert to the manually dialed-in "Power" settings, I will surely take a moment to re-tune. Again, no different than if I expected maximum performance from a carburetor – and certainly easier than re-jetting or swapping metering rods. Air/fuel mixture is not the *only* problem encountered when subjecting your motor to extreme variations in altitude. Adjustments of air/fuel mix (whether automatic or otherwise) will not solve all problems. For example, I've foolishly trailered my rig from Boulder, CO to Death Valley, CA (282feet BELOW sea level) filled to the brim with 33 gallons of 85-octanehigh-altitude "regular-grade" gasoline. No amount of mixture or timing fiddling would keep my engine from knocking/pinging under load. The closed-loop kit is not a magic-bullet, and did nothing for me in this instance.>this from someone with>the resource $ to install any darn thing he wants and not settle for less. This surely doesn't apply to me (the ex-wife has seen to that), but my opinion is nevertheless that the closed-loop option was a worthy investment of the $180 or so that it cost me. It seemed like only a marginal addition to the $550 or so that the base system had cost. The benefits under *most* driving conditions are real; and in those circumstances where it is NOT of benefit, it still performs a "diagnostic" function that I value.::: looking up ::: My, this has become quite a dissertation. I apologize if this is a $2answer to a 25-cent question. But it's a frequent one, and I thought I'd share my experiences and observations.

Regards,

Bill Thebert

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