I'll take a quick stab at this and if you have other questions, just ask.
Timers are a much nicer, more consistent and repeatable way to control lean out/enrichment than check valves which are dependent on system pressure. Anytime you change the main pill, the system pressure on a pass changes and everything will be different for those check valves. Timers don't care...they do their thing when the time comes regardless of RPM, pressure, pill, tide, phase of the moon...
If you partially (or completely) plug a nozzle on a pass, all your check valves will be wonky and come in WAY too early for example.
"Normally open" and "Normally closed" valves can each be used to bypass fuel (lean) or stop bypassing fuel (enrich) when they are activated.
A normally closed electric valve can be activated with the transbrake to lean a system out when on the converter or against a launch rev limiter. It slams shut and the system goes rich and ready to make torque when the brake is released. A clutch car can lean the system when the clutch pedal is depressed and go rich when released (maybe with a switch on low gear so it only happens at the starting line).
It's pretty common that a blown system in a heavy car wants to eat more fuel for the first 100' or so to get the car moving and once rolling along, fights better a bit leaner. That might be your 2nd stage leanout valve...what I call a "low speed bypass" that kicks in 1.5 - 2 seconds out.
When things are motivating along pretty well and HP (not torque) is needed for the top end charge, another bypass can take fuel away at the top end 1.5 - 2 seconds from the finish line.
In a nitro situation, an electronic valve can be used for a high-side/low-side setup. You could even make it go to the high-side automatically when the throttle is even slightly depressed or the clutch pedal comes all the way out.
Anywhere you can route a wire and mount a switch, logic could be added. Using a series of micro-switches and/or RPM switches, a lean out could be setup to only work after a certain number of seconds, only if in high gear, and only if engine RPM is above a certain value.
Someone wanting to control the staging volume, the low speed and high speed will need at least three different valves.
Because most folks don't want to spend $1,000 for simple on/off control of fuel and timing changes, FIE offers a 4 channel timer that's worth looking at:
https://fuelinjectionent.myshopify.com/collections/tuning-parts/products/4-channel-timer-controller It'll change the state of each channel twice per pass if desired. Here's the valve we like to use for these purposes:
https://fuelinjectionent.myshopify.com/collections/tuning-parts/products/electronic-lean-out-valve-assembly It comes in an enrichment version as well.
People use a channel to activate a timing controller to retard the timing at the starting line for the first second or so. Or shift the car (one channel will shift twice for you). If you have a channel left over, you could use it to activate a shift light by time.
Some classes don't allow electronic valves or even simple timers. For some reason, air controlled valves are usually considered ok in those cases.
I'd prefer to use time vs. RPM to change fuel or timing. If traction is an issue or converter temp/slippage is a bit different from pass to pass, the RPM will differ a little each time causing your events to be triggered differently - which in my mind only amplifies the error and inconsistency on the lap.
Time waits for no one!
Spud
