K20/K24 Frankenstein: K20 Head Swap on K24 Block

While this is an old tired topic, I wanted to have this information here for ya’ll since I’m a nice guy.

Coming from a stock block NA build point of view, and for a quick back history on this topic: stock vs stock – a performance K20A head CFM out flows any performance variant K24 RBB head. Plain and simple. Its been dyno proven over and over. K20 head swap alone is an approximate 19whp gain under the curve when bolted on a K24A2/JDM K24A RBB/or K24A3 block. The K20 head unlocks midrange and upper top end power breathing whereas the small port K24A RBB head shines more-so in the low to mid range for that city driving powerband but suffers up top.  With a worthy camshaft upgrade they compliment the K20 head even more.

The CFM graph below was provided yeaaaars ago on the K20A.org forum by DRAG (aka 4piston Luke). They bench flowed all of the common K-series cylinder heads showing obvious flow differences. Check out that RBB blue line – yikes!

Stock-vs-stock CFM flow bench results are as follows from best to meh:

RSP head / JDM K20A & UK/EU K20Z4 (FD2/FN2) – 1st best flowing oem factory head. Found on the JDM FD2 K20A CTR and UK/EU FN2 CTR engines. Rare af to find in the states. Head casting style looks similar to that of a K20Z3 (RBC) & K24A2 (RBB) heads w/side mounted water neck housing and 45° hose barb. This head received special treatment from Honda versus any other K-series performance head. The port casting surface is much smoother than typical K-series head, same treatment that the NSX cylinder heads received. Its not hand ported, just casted with more care with smoother surfacing. Uses 3-lobe camshafts on both sides of the head with single spring valvetrain. Runs typical valve sizing of 35mm intake / 30mm exhaust. These heads are easily identified by their “RSP-x” part number stamping around the head casting.

PRB head / JDM K20A, K20A2/K20Z1 – 2nd best flowing oem factory head. Most common head used for K20/24 builds. Found on the 02-06 RSX Type-S K20A2 & K20Z1 engines and JDM EP3/DC5 K20A Type-R engines. Has front mounted/facing water neck with 90° hose barb. Runs typical valve sizing of 35mm intake/30mm exhaust. Uses 3-lobe camshafts on both sides of the head. Early 02-mid ’03 K20A2 PRB heads came with dual valve spring valvetrain, then later ’03 onwards switched to single valve spring valvetrain (this carried over to the JDM Type-R head variants as well). These heads are easily identified by their “PRB-x” part number stamping around the head casting.

PPA/PPL head / K24A1 – 3rd best flowing oem ECO head, very similar flow to a PRB head but in an ECO cylinder head cast department. While there’s a handful of K-series ECO heads, the PPA/PPL head is best of that bunch in its stock form (and ported form as well). Found in the 02-06 CRV K24A1 engine. This head is most popular in the drag racer scene for building VTEC killer engine since it does not utilize the typical 3-lobe camshaft rocker assembly nor have the caste lost motion assembly bosses. Typically stamped with “PPA-x” or “PPL-x” around the head casting. 4piston has done some port job wonders with this head and increased CFM flow substantially.

RBC head / K20Z3, JDM K20A CL7 Accord Euro-R, CAN K20Z2 – 4th best flowing oem factory head, but 2nd most used head for K20/K24 franks in the US. Found on the 06-11 Civic Si, Canadian CSX*, and JDM 03-08 Accord CL7 Euro-R. Exterior head casting style looks similar to K24A2 (RBB) head w/side mounted water neck housing and 45° hose barb.  Uses 3-lobe camshafts on both sides of the head with single spring valvetrain. Runs typical valve sizing of 35mm intake / 30mm exhaust. These heads are easily identified by their “RBC-x” part number stamping around the head casting.
*The CSX K20Z2 engine has a detuned ECO version RBC head. It basically has K20A3 valvetrain, rockers, camshafts but has dormant lost motion assembly bosses, so you can actually convert this head into a proper 3-lobe camshafted RBC head with proper valvetrain components pulled from a performance K-series head.

RBB head / JDM K24A, USDM K24A2, UK K24A3 – 5th best but poorest flowing oem performance factory head. Found on the 04-08 TSX, JDM 03-08 Odyssey and Stepwagon vehicles, and UK CL9 Accord Type-S. Exterior head casting style looks similar to RSP/RBC heads w/side mounted water neck housing and 45° hose barb.  Uses 3-lobe camshafts on both sides of the head with single spring valvetrain. Runs typical valve sizing of 35mm intake / 30mm exhaust. These heads are easily identified by their “RBB-x” part number stamping around the head casting. Honda gave special treatment to the USDM 06-08 K24A2 engines receiving 1mm larger intake valves (36mm) and proved to be a non-power adder (so chill tf out you RBB head nerds), larger profile intake camshaft (unchanged exhaust cam), slightly beefier rods, piston pockets were opened up a little to compensate for the 36mm intake valves but retained the same 10.5 compression ratio, intake manifold plenum enlarged along with slightly larger bore throttle body. There’s certain internet nerd drama surrounding the suffix numbering when it comes to K24A RBB engines ala: ‘RBB-1/RBB-2/RBB-3/RBB-4’. I’ve made an article about this topic here if you’d like to delve into the stupidity. While the RBB head is one of the poorest flowing K-series performance heads due to its small ports, its a GREAT candidate for having port work on it due to having meaty ports. Port this head instead of ruining a perfectly fine RSP/PRB/or RBC head…actually an RBC head is a borderline porting candidate too.

PRB Type-S head to K24 block required parts:

• CRV / K24A1 head gasket – Honda P/N 12251-PPA-004
  This gasket matches a K24 87mm bore size but also shares the same exact coolant passaging patterns as the a PRB Type-S head gasket + head. You don’t want to use K20A2/Z1 head gasket because of its 1mm smaller (86mm) bore size and the gasket inner bore edging may hang over the K24 cylinder bore that can create hot spots during running operation.
  Alternatively, you can also go Cometic headgasket if you wish, but they don’t offer a specific CRV gasket last I checked, they only offer one type of K24 gasket with optional bore sizing and thicknesses.
• REMOVE/OMIT COOLANT DIVERTER IF PRESENT!
  If left in place, may cause overheating issues. The PRB head isn’t designed nor requires the use of a coolant diverter.
• Use factory K24A head studs or you can go ARP headstuds if you wish
• Use factory oem K24A timing chain, timing chain guides, and PRB/RAA timing chain tensioner

RBC / RSP Type-R head to K24 block required parts:

• TSX / K24A2 headgasket – Acura P/N 12251-RBB-004
  
• Accord/Element / K24A4/A8 headgasket – Honda P/N 12251-RAA-A02
  This gasket matches a K24 87mm bore size but also shares the same exact coolant passaging patterns as the a RBC/RBB/RSP head gasket + head. You don’t want to use K20Z3 head gasket because of its 1mm smaller (86mm) bore size and the gasket inner bore edging may hang over the K24 cylinder bore that can create hot spots during running operation.
  Alternatively, you can also go Cometic headgasket if you wish, they only offer one type of K24 gasket with optional bore sizing and thicknesses.
• USE A K24 COOLANT DIVERTER!
  not sure if you can use a diverter with a Cometic gasket
• Use factory K24A head studs or you can go ARP headstuds if you wish
• Use factory oem K24A timing chain, timing chain guides, and PRB/RAA timing chain tensioner

Some final thoughts..

Not to shat too hard on the RBB head, you can actually build a sweet torque street monster if top end power isn’t your goal. With an RBB head used on a K20 for instance, the right intake manifold choice, worthy 4-2-1 header, 50° VTC gear,  cams, and tuning, you can build yourself a dank torque beast!

When it comes to a boost build, its not even worth swapping to a K20 head. CFM becomes a moot point as you can just turn up the boost.

Credits to K20A.org forum & members for some of the information pulled for this article.