Two-Stage Dry Nitrous Install and KPro Wiring for a K-Series Honda

This build runs two independent dry stages that fire sequentially rather than at the same time. Stage one is a single-fogger nozzle mounted in the intake pipe and fed by a 10 lb bottle; stage two is a direct-port layout, one nozzle per cylinder, fed by a 15 lb bottle. Sequencing the stages keeps each bottle under load for less time, which reduces the mid-run pressure drop that otherwise leans and weakens a single-bottle kit near the top of a pull. Because both stages are dry, all supplemental fuel is delivered by the port injectors under KPro control rather than through the nitrous plumbing. The goal was a safe, repeatable 350 whp on an otherwise bolt-on K20A2, up from 315 whp / 258 tq on the earlier single-fogger setup.

Each of the four runners is drilled and threaded to accept a spray nozzle. Locate the bung as far toward the plenum as hood clearance permits, giving the charge more runner length to mix before it reaches the valves. Use the tapered tap from the kit: because it opens the threads further the deeper it is driven, you can sneak up on the fit by taking successive light passes and test-threading the nozzle between them. The target is a nozzle that snugs up right as it points straight down the runner. Spend the extra time getting that fit correct on all four.

Drilling and tapping leaves aluminium swarf inside the runners that would be pulled straight into the engine on the first start. Flush the manifold thoroughly with water, then blow compressed air through every runner and passage from several directions until nothing more comes out. Inspect the runners visually before moving on — any fragment left behind is a direct path to engine damage.

Run a hard line from the distribution block to each nozzle. Use a proper tube bender and form one bend at a time, checking the fit against the manifold after each bend rather than trying to shape a whole tube at once. Be aware that distribution blocks differ: a standard block tends to feed the cylinders unequally, so an equal-flow block is worth fitting even though switching to one means bending a fresh set of tubes.

Apply thread sealant to the NPT fittings and the nozzles before final assembly. For fittings this small a thin band around the leading thread is enough — more than that just squeezes out. Thread each nozzle and fitting home, keeping the nozzles aimed down their runners as set during tapping.

On a direct-port kit the pressure regulator (if used) belongs close to the solenoid, and the solenoid belongs close to the distribution block, so packaging takes some planning. Route the feed line into the regulator, under the manifold, past the purge solenoid, through the main nitrous solenoid, through a port for a pressure sender, then up into the distribution block. Building in that sender port between the solenoid and the block lets you data-log nitrous pressure at the block later. Both stage feed lines land together near the single-fogger solenoid.

Split the electrics into two circuits — the first serving the solenoids, the arming and stage switches and KPro, the second dedicated to the bottle warmers. No solenoid should be able to energise unless the car is running. Drive a master relay from an ignition-switched source (the black/yellow wire at the under-hood fuse block) and take the solenoids' actual power as heavy-gauge wire straight from the battery/charge harness — never from thin ignition wiring, which can overheat under that load. Ground the solenoids to the chassis. Give each nitrous solenoid its own relay; with the master arm switch on, both sides of those relays are hot, and grounding the coil trips the relay. Route the stage-2 ground to KPro pin E21 and the stage-1 ground through the steering-wheel button to KPro pin E20. The master arm switch grounds KPro pin E29 (which arms the system in KPro), trips the master relay that powers everything else, and grounds the purge button.

Keep the warmers on their own circuit that is not tied to the ignition, since they sometimes need to run while the car is off. Control each warmer with its own relay that depends on both a dash switch and a bottle-mounted pressure switch. With the dash switch on, the relay trips and the warmer heats only while bottle pressure is below the switch setpoint (for example 1000 psi); once that pressure is reached the warmer shuts off. Wire an LED into each switch so it lights only while that warmer is actually heating — when the light goes out, that bottle is up to pressure and ready.

In KManager, map the arm input to pin E29, stage 1 to pin E20 and stage 2 to pin E21. KPro governs when each stage may fire based on throttle position and rpm; for stage 1 both KPro and the steering-wheel button must agree before the solenoid activates. Set up the supplemental fuel delivered through the ID 1000 injectors and the ignition retard for each stage within KPro rather than adding fuel through the dry nitrous plumbing. Note that controlling both stages requires an added pin on the ECU connector.

Install an inline nitrous filter in each feed line in the engine bay to keep debris out of the solenoids and nozzles. Add a pressure sender on each bottle and another on the distribution block; logging both makes a starved shot obvious immediately and shows how a full bottle behaves versus a half-empty one. Secure the -4AN feed lines along the car with 3/8 in cushioned clamps.

Begin conservative and step up jet/pill size while watching torque gain against expectation, air/fuel and knock. As a rough guide, small .019 pills add little, .025s more, and .029s land around 150 hp at 900 psi. If a larger jet fails to add the expected power and the trace turns jagged even though air/fuel and knock look safe, suspect nitrous delivery — a low bottle or a regulator clamping flow — rather than detonation. Purge the line before making an adjustment. Read the spark plugs between pulls to check heat range and whether the cylinders are getting an even charge; drag and V8 communities have good references on reading nitrous plugs. For reference, a .046 pill on the single fogger is a 100 shot.

The pressure regulator is passive: it caps flow to a fixed amount, but its output pressure tracks its input, so as the bottle empties the input falls, the output falls, and line pressure can drop low enough that nitrous vaporises just before the nozzles — worst during back-to-back hot-lap pulls. Several pressure combinations were tried (around 1200 psi in the bottle with 1050 psi out, and 1050 in with 950 out) and none held consistency once the bottle was part-empty, because too many variables drive the pressure on the input side. The unit itself also failed in three ways: a blown internal o-ring, a leaking output fitting, and a gauge face that detached. Removing the regulator gave more consistent, more explainable results. Final numbers, regulator gone: the single fogger delivered 291 whp on a .046 pill (a 100 shot), while the direct port produced 337 whp on four .029 pills (a 150 shot) — strong given a dyno room above 100°F.