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Hybrid How To #17 - 2006 Honda Civic Si

Going Big Displacement - A K24A2 For The 2006 Civic Si

Tim Kelly
Dec 5, 2006
Photographer: Brian Gillespie
0604_sccp_01_z+2006_honda_sti+mods Photo 1/1   |   Hybrid How To #17 - 2006 Honda Civic Si

Wait, the 2006 Civic Si only just started appearing in dealerships and you're swapping engines already? You are if you're the self declared kings of Honda swaps-Hasport. Lucky for Hasport, Honda handed out some preproduction Si's to select aftermarket companies prior to the on-sale date. With that extra time (only about 3 weeks), Hasport was able to complete a swap for the SEMA show, and SCC is the first magazine to tell you about it.

Basically, there isn't a lot of aftermarket equipment for the new Si yet, but considering how similar the engine is to the RSX-Type S's, and the lead time Honda gave the aftermarket by handing out those pre-production cars, there will be a entire catalog of goodies by the time the car hits dealerships.

2017 Honda Civic
$21,500 Base Model (MSRP) 31/40 MPG Fuel Economy

What And WhyIts hard to top 197 bhp, a limited-slip differential and a 6-speed transmission as a starting point. But the new Si is more of a running back than a horse jockey when it comes to weight, and any extra off-the-line torque will make a difference. There's not much head room if you're not going to add forced induction, unless you add some displacement by using the 2.4-liter engine from a TSX.

We have a bone-stock '06 preproduction Civic Si that will lose its K20Z3 in exchange for a TSX K24A2. Stock horsepower on these two engines is nearly identical but torque on the K24 is 166-lb-ft, versus the K20's 139 lb-ft. It also comes lower in the rpm range.

Picking The ChassisThe Civic's chassis comes in a four-door version, too, which Hasport thinks will work just as well for the swap. Honda says the two- and four-door cars share the same engine bay dimensions, but the Si is the only car of the line offered with the K-series engine. Other Civics (except the Hybrid) have the new R series 1.8-liter mill. As soon as Hasport can get their hands on a vehicle with an R series engine, they'll try the swap and let everyone know, but they expect it will be no different.

Picking The EngineHonda's K-series has become their engine of the future. It's in the Civic, CR-V, Accord, RSX, TSX and Element. Hundreds of thousands are made every year with not too much variety between them. The K20A was the first generation of the 2.0-liter, and today we are on the K20Z. The K24s come in quite a few versions, but like the old B series, it's the head that makes the difference.

The K20A (JDM), K20A2, K20Z1, K20Z3 and K24A2 are the engines with the real DOHC VTEC heads. All the other versions are just poseurs. If there aren't three lobes per cylinder on the intake and exhaust cams, it's not real VTEC, no matter what Honda marketing tells you. For this swap, it was easiest to take a K24A2 from a TSX rather putting a VTEC head onto a K24 from an Accord or CR-V block.

Engine RemovalAt Hasport, these always seem to come out easy. But with the '06 Civic, Honda has done a great deal of improvement. The best new features are the new torque mounts, up top and down below. These new mounts limit engine rocking from hard or drag-style launches. The upper-right side engine mount is in the usual place but is directly connected to a stout aluminum arm on the strut tower. The lower mount, which is normally at the back of the block near the middle of the car, is now attached at the oil pan.

The mount on the oil pan connects to the engine subframe with a second aluminum arm to control movement. If you haven't already guessed, the oil pan from the K20Z3 will need to go on the K24A2.

It looks like the manufacturing engineers had a great deal of say when designing the newest Civic. With a minimum of nuts and bolts, the entire subframe, steering rack and lower suspension arms drop right out with the engine. After draining fluids and disconnecting the battery, begin disconnecting the steering inside the car at the joint closest to the firewall. Then unplug the electrical connections on the electric power steering rack.

Move on to the lower suspension points, disconnecting the steering arms and bolts that attach the lower A-arms to the hub and axle carriers. Remove the header (it hangs lower than the rear section of the subframe), disconnect the engine harness connections, including the ECU which is now under the hood near the left headlight, then go for the large bolts on the subframe.

Unless you have the ability to lower the entire subframe with the engine attached, drop the subframe at this point and then drop the engine. Even with a lift, this can be tough because of the large area of the subframe. Removing the engine without removing the subframe isn't possible. Because the cowl and windshield hang over the engine compartment so far, it is impossible to get the engine out from the top.

WiringThis is where the 2006 Civic throws us a curve ball. Unlike all the previous E-chassis, the ECU is under the hood. Likely this is a cost savings and assembly time move, but it makes the swap harder. Because Hondata doesn't have any reflashes available yet, we chose to step back to an ECU that's programmable.

That's the Hondata K-Pro. Now we're all set for any displacement, naturally aspirated or with forced induction.

Which brings up the next issue: where do we mount the K-Pro? It would take a very long harness to connect the plugs, which are now under the hood, to the usual location near the glovebox. To keep things simple, Hasport has used a modified RSX-S harness that plugs into the K24 engine. The harness was modified by adding an E plug for the K-pro ECU and key connections with the '06 Si body harness. The K-Pro was then mounted in the same location as the stock ECU. Since the K-Pro isn't weatherproof, there's no driving this car on anything but sunny days.

To make the connection to the body harness, Hasport spliced wires that are in the under hood portion of the chassis harness to wires from the modified engine harness. The important connections that had to be made with the cabin harness are ECU power, main and fuel pump relays, air-fuel sensor relay, radiator fan relay, coil power, and injectors. Honda's service manuals have the entire harness layout deciphered and labled.

Great, now you can run the engine, but you can't run the new double-decker dash. Honda has switched all the dash instrumentation to run on ECU CAN bus signals, which are basically an automotive digital protocol. The K-Pro isn't compatible, so you can deal with it or do what Hasport did and get an Xsport digital dash from Pi Technology. Pi is the standard for sensors and instrumentation used by most of the racing industry. Just feed it the signals for things like tach, speed, temp, etc., and you now have an even cooler dash.

Engine MountingThe K24 goes in as easily as the K20 came out. In fact, you can use all the stock mounts. The K20Z3 oil pan is an integral part of the '06 Si's engine mounting system. It bolts right onto the K24, so you can use the lower mount. Make sure you use the '06 Si oil pump too, the oil pan is deeper and the oil pump pickup extends farther to reach the bottom. The oil pump has to be modified also, so the housing will clear the longer stroked crank of the K24 engine.

Finally you'll need a CR-V right-side engine mount. It's a little shorter. Next, put the engine in from the bottom, install the left and right side motor-mounts, then attach the subframe, along with the rear mount, and install the front mount.

Fuel SystemNo real changes here. The K-series engines all utilize a returnless-style system. The injectors from the K24A2 are the same size (310cc/min) as the Si's and the fuel line is on the same side, so there's really nothing to do but connect the lines. If you're thinking ahead and want to install a big-horsepower engine down the road, you may want to do a return-style system.

Engine AccessorieS & CoolingUnlike older cars that were never designed with the K series engines in mind, all the accessories that came with the Si stay. Since the K24A2 has conventional belt driven power steering, you will need the idler pulley from the Si on your K24. If your K24 came as a stripped long block, all the accessories from the K20 will bolt right on to the K24.

When we had the engine out of the car, we mounted the thermostat housing from the K20Z3 on the TSX engine. The larger engine doesn't require additional cooling capacity beyond the Si's stock hardware and all the hoses from the K20 connect at the same places on the K24. Same for the heater hoses which warm the interior of the car.

Drivetrain HookupWe finally get a limited-slip differential in the Civic, so why use the TSX 6-speed? The TSX has taller (lower numerically) gears to take advantage of the increased torque, but ripping through the gear ratios of the improved Si transmission is too good to resist. Even if we wanted to use it, the K24 transmission mounts differently, making it impossible to use. Unbolt the K20Z3 transmission and add it to your K24.

ExhaustThe good news is that the factory exhaust manifold from the K20 bolts right up to the K24. The bad news that because of increased 2006 emissions restrictions and the Si's ULEV2 rating, that manifold has a close-coupled catalytic converter that keeps a lid on flow. A good 10 horsepower can be had on a K20A2 just by bolting up the JDM Integra header with its longer tube header, which puts the cat under the car instead of near the firewall.

Since the aftermarket got their hands on the new Si early, we were able to get a prototype DC Sports race header. It's not street legal because the catalytic converter was left behind, but what about this swap is CARB legal?

IntakeIn the usual OEM style, the stock intake system uses a series of black plastic boxes and snaking tubes in the engine bay and fender well. The surprising part is that it actually sounds really good. With the stock muffler, the intake is actually pretty burly, especially when the cams switch over to the big lobes.

But it's definitely a restriction and the longer manifold of the K24A2 puts the throttle body where it doesn't exactly match the K20's. On top of that, Honda has switched to a mass-airflow sensor (MAF) to deal with the new, tighter emission regulations. Since we've dumped the stock ECU for a Hondata unit that doesn't require a MAF, the sensor becomes an unnecessary restriction on the intake.

The swap does away with the entire factory intake system and uses a lightly modified AEM TSX short ram intake. Although you could bolt the K20's intake manifold on the TSX engine, you should also know that the intake manifold from the TSX dyno tests better than the one on the K20. Hondata has done extensive testing, and until you do major modifications like iVTEC advance adjustment, a race header, larger cams and more, the TSX manifold will outperform the one from the Si.

Wrap-UpThis crazy swap might not make sense now, but later down the line when Si owners get hungrier for torque this will be the first stepping stone to endless other hybrid combinations. It's a lot of work because of the number of parts that need to be swapped to fit the Si's unique engine mounts and work with the Hondata K-Pro ECU.

Soon Hondata will have a reflash for the Si, then they'll probably have a fully-programmable ECU like the K-Pro, which will alleviate the need to swap half of the parts that we've changed. But for now, we're all stuck with bolt-ons and we already know from years of dyno testing, the K series really comes alive with more than just bolt-ons and it takes a K-Pro to get that benefit.

If you're not Hasport, then just having a new Si will be cool, but having one that has incredible potential (Hasport is planning on turbocharging theirs to the tune of 550-wheel hp with a sleeved block) makes you nearly untouchable.

Swap Parts List* K24A2 or K24 with any A2 head* K20Z3 oil pan* K20Z3 oil pump and balance shaft assembly clearanced for longer K24 stroke* 2003-2005 Accord 4 cylinder throttle body on TSX intake manifold* 2003-2005 RSX-Type S throttle cable* K20Z3 idler pulley* 7 ribbed, 65.25-in. belt* Modified TSX aftermarket short ram intake tube* K20Z3 factory exhaust or '06 Si header* Pi Xsport digital dash* Hondata K-Pro

Brian and Scott of Hasport started this swap by removing the subframe. Make sure to disconnect the electric power steering.

The rear mount connects to a bracket on the oil pan and the subframe. It's a simple "dog-bone" style mount to limit engine movement and is great for preventing wheel hop on "sport-style" launches. The front and both sides are typical Honda affairs and disconnect easily.

If you're lucky, you have a lift and you just push a button to lift the Si away from the K20Z3 (once the subframe has been removed).

This is the stock Si right-hand mount. It has a built in torque arm or "dog bone" to prevent the main mount, which is filled with hydraulic fluid, from tearing under heavy acceleration.

We opted to use the '06 Si tranny for its limited slip and lower gearing. The stock shifter cables wouldn't work if we hadn't-something we found out after the fact.

It's already gone, but this is where the Honda ECU lived. On the new Si, Honda is mounting the ECU under the hood. This will prove tricky on our swap as the K-Pro is not weatherproof.

Although the stock mounts can be used for the K24 engine swap, the Hasport mount will be necessary for the 550-wheel hp turbo motor that will later find its way into the '06 Si. Notice how it removes the "dog bone" upper joint for even greater movement control.

The left-hand mount holds the transmission and is made of two parts. An aluminum bracket bolts to the transmission with a post that sticks upward, and the actual mount hangs off the frame.

Here is the Hasport left-hand mount and bracket. Quite a bit more substantial than the stock mount.

On top is the K20. Look close and you'll see four blue circles. They're from the #2 and #3 rod bolts. You're going to need to open clearance holes so the K20 assembly looks just like the K24 assembly.

This mutated mass is the stock Si intake system. The ironic thing is that it sounds pretty good.

Time to put some K20Z3 parts on your K24A2. On the right is the K20 oil pump assembly. Notice the deeper oil pickup snout. We have to use the K20 oil pan, which means we need the K20 pump. The snout is a press-fit piece.

The K20Z3 pan is on the floor and a K24A2 pan is on the engine. You can see how much deeper the K20 pan is. That's because it's also an engine mounting location this time. The K20Z3 oil pan has to go on the K24 because of this.

But this is the reason why it has to go-a mass-airflow sensor (MAF). A first for Honda, which has traditionally used a speed-density system (the '06 Si uses both). MAF sensors typically are a restriction in a manifold, so getting rid of them normally increases airflow. Besides, the K-Pro wouldn't know what to do with it.

Accord top, TSX bottom. Notice the TSX has a slightly bigger plenum, which makes for shorter runners, but they are no larger because the upper manifold (left) is the same for both. The TSX manifold has been dyno tested to give the best results on a TSX, even compared to the K20Z3 manifold, which is the same as the Euro Accord R. Leave the TSX manifold on.

The K20Z3 header is nearly identical to one from an RSX-S. Because of the '06 Si subframe, it has a slightly different turn to it and RSX headers won't fit on the Si. The header we used is a prototype '06 Si race header from DC Sports.

On the left is the TSX throttle body and the right is the Accord throttle body. They have the same diameter opening and bolt pattern. The TSX throttle body is almost identical to the '06 Si's. The only difference is the location of one of the water ports. Once Hondata comes up with a reflash for the '06 Si ECU, you should be able to use the TSX throttle body, since it is also drive-by-wire. Since we converted to cable operation we bolted on the Accord throttle body and throttle-cable bracket.

Drive by wire means there is no cable connecting the gaspedal and the engine. That function is controlled by the ECU, and with a K-Pro, we must switch back to a throttle cable. The original thought was to put in a conventional pedal instead of the Si pedal, which is on the right.

Instead of a different pedal, Hasport fashioned this very smart adaptor. A piece of bent sheet metal surrounds the pedal, while a couple of clamps attached to the original pedal arm pull on the RSX-S throttle cable. Difficult problem solved.

The wiring on this swap isn't as complex as you might think. The bottom plug is factory Si. Some of the wires in this plug were tapped to provide inputs for the K-Pro wires (three middle plugs). The top plug is a part of the RSX-S engine harness that's used instead of the Si's. See the wiring charts for the specific ins and outs.Caption25

Removal of the Si ECU means the majority of the instrument cluster won't work. That's because it uses the CAN bus system. The Hasport car has a Pi Xsport digital info center now. Shown here is the coolant-temp sensor mounted in a billet part Hasport sells for other K swaps. The sensor feeds directly to the Xsport cluster (it also needs to be grounded).

With some modification to its case, the Hondata K-Pro mounts in the stock computer's location. This is not a weatherproof solution and only temporary. Hopefully a Hondata reflash will be available for the stock ECU in the near future.

The K24A2 goes in just as easily as the K20Z3 came out, no tricks or special tips. Even though the K24 block is taller, it still fits under the extended cowl cover of the Si. Button it up and hide away your new red head.

By Tim Kelly
23 Articles

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