6-speed K-series manual transmission (K20Z3, from a 2006+ Civic Si FA5/FG2)
for the Civic Si
Yes: the 6-speed K-series manual transmission (K20Z3, from a 2006+ Civic Si FA5/FG2) fits the Civic Si (2002-2005) — difficult difficulty, modifications required.
Installation notes
Required modifications
The 5-speed-only K20A3 engine harness fitted to EP3 Civic Si (and base RSX) cars has no connector for the 6-speed's reverse-lockout solenoid, so you need a 6-speed-capable engine harness (Hybrid Racing's own harness-selection guide calls out the K20A2 harness as the one that works with either a 5-speed or 6-speed box) and a K-series ECU reflash. On Hondata K-Pro, the reverse-lockout solenoid is normally grounded through pin B7 on the 02-04 K20A/A2 engine harness; when that circuit isn't present on a converted car, Hybrid Racing's guide documents rerouting the solenoid wire to a spare, otherwise-unused ECU output such as E21 (the EVAP vent VSV pin) on the K-series conversion harness, then setting KManager's Nitrous 1 tab to "Reverse Lockout" with that output and a minimum-speed threshold. Separately, a 5-speed car's differential has no speedometer drive gear, since the stock VSS setup is different; a Hondata forum thread with a Honda EP3 owner moving a 2006 Si 6-speed into his 5-speed EP3 covers adding that gear (roughly $10 from Honda) to the 6-speed's differential, or alternatively running a speedometer-signal converter (e.g. Dakota Digital) instead of swapping internals, though several owners reported losing working cruise control with the converter route. The same thread states input and output shafts can be swapped between 5-speed and 6-speed cases without pressing the gear clusters off, but the input-shaft and differential clearances/shims should be checked against the factory service manual rather than assumed.
Important caveats
- The K20A3 harness used on 5-speed EP3s and base RSXs has no connector for the 6-speed's reverse-lockout solenoid; per Hybrid Racing's own harness-selection documentation you need a 6-speed-spec engine harness (e.g. K20A2) and ECU to get that function.
- Without the lockout wired up, the shifter can be pushed past 5th into the reverse gate while driving - Hondata's own support forum confirms this is exactly what happens when a 6-speed runs on a 5-speed-era ECU/harness.
- A 5-speed car's differential lacks the 6-speed's speedometer drive gear from the factory; you either add that gear (~$10 part per Hondata's forum) or fit a speed-signal converter, and multiple owners reported cruise control stopped working with the converter route.
- One owner on the Hondata forum reported a mechanic damaged gears attempting a similar EP3-to-base-RSX internals swap, so check input-shaft and differential shim clearances against the factory service manual rather than assuming a straight press-fit between cases.
- Final-drive and gear ratios differ between 6-speed K-series variants (RSX Type-S, 06+ Civic Si, Integra Type R DC5); confirm which 6-speed unit you're sourcing actually has the ratios you want before buying one.
- Connecting a K-series conversion harness to the wrong engine harness can damage the ECU, sensors, and wiring per Hybrid Racing's own warning - double check harness compatibility before powering anything up.
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Reference
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