s2000Junky

Thats right, your ecu runs leaner. There is a good chance you would want to have an adjustable rate fpr to increase your fuel pressure, to have enough fuel in that modified vote transition point. Its not a big deal to run one, it insures you have plenty of fuel in open loop tune to work with. I feel its smart no matter what MY to go to the dyno with one to be prepared just incase. Its an easy 5min bolt on if you do a run and find out you do need it. If not, return it in the package.

Apex1.0

OK, so 00-01 ecus run richer across the board? that means if I lower vtec to 4500 without any other changes, the af ratio from the new vtec engagement to the old will be? richer, leaner? Acceptable?

I am looking at an original vafc. that will give me everything the newer ones have except, more points of adjustment, the ability to vtec based on throttle position and a fancy display, right?

isn't the vafc vmatch/unmatch function supposed to correct the a/f to the new vtec engagement?

does the vafc and vafc2 use th same pnp harness?

User 121020

They don't run all that rich across the whole RPM range. Here's a plot I dug up from the Jan 2011 era. The "quasi-stock" data was using an APEXi NEO with vtec set at 5600 RPM. I didn't mess with fueling much (if at all? who knows at this point). The "STR trim" data is not using a VAFC or any other piggyback... just the stock ECU controlling fuel and timing.

User 121020

Another reasonable option that you could consider is the Greddy E-Manage Ultimate. They're about $500 used with a PnP harness. A few of us spent a fair amount of time researching the harness configuration(s). They're similar to most other piggy-backs in that you typically only tune in the open-loop range of operation, but there is capability to control when the stock ECU transitions into open-loop. This means you can trigger this event at lower RPM and lower load, allowing for tuning of a larger portion of the operating range.

You also get individual control of fueling, timing, VTEC engagement and quite a few other parameters. You can also connect a wideband controller 0-5v output directly to the unit and be able to log AFR along with the other engine parameters that can be logged. It's pretty decent for a piggy-back. There's a bunch of info in this thread: STR ECU tuning thread. It's a lengthy thread...