The allure of swapping to a newer engine with more power is attractive, especially with the FD3S utilizing twin, sequential turbos and producing 255hp stock (over 55hp more than an Kouki FC3S).  In reality, swapping a 13B-REW twin-turbo engine from an FD3S into an FC3S is not easy.  There are two known methods of addressing the engine mounting problem, and we well cover both options.

The most popular method is to adapt the stock 13B-REW engine mounts to the FC3S chassis.  Trying to adapt the stock 13B-REW engine mounts to the stock FC3S engine mount locations on the chassis is a difficult proposition, and it's not a recommended design.  The stock 13B-REW engine mounts are both located on the rear, side housing; the stock FC3S 13BT engine mounts are located on the center, intermediate housing.  This makes for a sharp angle to connect both "points".  When researching engine mounts in general, the weight load vector is universally in a vertical plane; the engine (above) loads the mounts (below) straight up and down.  Thus, we should try and keep this design structure in mind.  The easiest way to accomplish this is to keep the stock 13B-REW engine mounts and weld in a new crossmember toward the rear of the FC3S engine "cradle".  There is just enough space to weld in a 3" crossmember, and getting the 13B-REW engine mounts to mount nicely is pretty easy.  This allows you to keep almost all the 13B-REW components.

The optional method to welding in a crossmember is to drill and tap the 13B-REW center housing to accept the stock FC3S 13BT passenger engine mount; three M14 x 1.50 holes need to be drilled and tapped into the underside of the center, intermediate housing of the 13B-REW engine to accept the three 17mm bolts that secure the bottom of the engine to the passenger side engine mount.  This requires a custom oil pan or the stock FC3S 13BT oil pan.  The stock FC3S driver's side engine mount is secured by two nuts on already existing studs on the 13B-REW.  The passenger side engine mount is mounted in the middle of the engine, necessitating the oil pan change.  If you decide on using the FC3S 13BT oil pan, you need to seal the front edge of the 13BT oil pan, as it's longer than the stock 13B-REW oil pan by about 1/4".

Once you take care of the mounting problem, the rest is pretty easy.  An FC3S clutch can be bolted onto the FD3S stock flywheel if on a budget.  The stock FC3S transmission should mate with the 13B-REW with very few problems; if the 13B-REW came off an automatic transmission equipped chassis, there is an extra bolt hole in the bell housing, which will not mate up with anything on a stock, 5-speed FC3S transmission.  The FC3S starter should fit with little fuss.

If you're going to run a stock FD3S stock ECU, you'll need the engine harness, ignition system harness, ignitor, and all coils from the FD3S, minimally.  Going to aftermarket EMS fuel and ignition control gives you more flexibility but may require additional ignition components.  Choosing a Haltech EMS for control will allow you to use the stock 13B-REW ignition input (trigger wheel and magnetic pick-ups) and the stock FC3S coils/ignitors!

Due to the 13B-REW already using a front-mounted intercooler, this will pretty much require you to do the same.  This would probably be one of the most expensive aftermarket components to get this conversion completed.  A popular option is to run the GReddy elbow into the throttle body, as this is a polished aluminum piece versus the stock plastic pipe.

An upgrade fuel pump is almost a necessity, as the 13B-REW uses 2x550 + 2x850 versus the stock 4x550 on a FC3S 13BT.  A Walbro 255lph is more than adequate for this application.

The radiator hoses are slightly different for the 13B-REW, so mating up with the stock FC3S 13BT radiator will take a little custom work.  "Universal" ribbed, radiator hoses can be used as a cheap alternative.  It's highly recommended to get an upgrade radiator if you still have the stock unit.  The GReddy pulley kit is another option to slow down the radiator and alternator a bit and comes with it's own special size belt.

Stock twin turbos will easily fit into the FC3S engine bay, but a custom downpipe is required to fit with the rest of the FC3S exhaust system.  You can stick with the stock intake system on the twin turbos which will minimally require a pair of cone filters or equivalent.

Oil lines will need to be custom made to fit the 13B-REW oil fittings to the stock FC3S oil cooler up front.  The front oil cooler lines is close to the 13BT set-up, but the rear oil cooler line mounts higher up on the engine by the oil filter.

I have mirrored Paul Ko's infamous 13BT versus 13B-REW write-up here:
This page goes into detail in the differences between the two engines.

13B-REW components price breakdown
Component: Price: Comments:
13B-REW engine $2,000 Prices vary, but on average they range between $1,500 - $2,500.
Clutch kit $350 ACT Heavy Duty clutch is only good for ~450lb-ft of torque.
Downpipe $100 Whether you're going single turbo or stock twin turbos, this exhaust pipe is needed to mate with the rest of the exhaust system.
Front-mount intercooler $1,200 The GReddy V-spec front-mount intercooler kit is the standard for which all front-mount intercoolers are judged for the FC3S.
Oil cooler lines $200 Typical cost to custom fab a pair of oil cooler lines from any name brand SS AN manufacturer (i.e. Earls, Russells, XRP).
Walbro 255lph upgrade fuel pump $150 You can use a pair of Walbro 255lph fuel pumps for a little cheaper.
Haltech E6K {$1,500} Optional - Stock FD3S 13B-REW ECU, engine harness, ignition components can be substituted.
Upgrade radiator {$500} Cost of an upgrade radiator and custom radiator hoses to fit the 13B-REW inlet/outlets.
GReddy elbow {$200} Optional
GReddy pulley kit 13B-REW {$120} Optional
Just on those estimations, we're looking at a cost of just the components.
The prices do not take into consideration on which route for engine mounting, as both require some special tools and labor for fabrication.
Prices are typical retail, so it's possible to get most of it discounted slightly.

The prices look pretty cheap to do this swap, but keep in mind the special labor it takes to get the engine mounted.  If you have access to welding resources, fabricating a crossmember should not be that bad.  For those who need to pay someone else to do the work, be prepared for a large bill!  A Kouki 13BT engine comes very close to what the 13B-REW offers, and power potential is very close at this point.  I wouldn't normally recommend this swap, as a Cosmo engine swap is a little easier to do over the FD3S 13B-REW.  The Cosmo offers their own variation on a 2-rotor, sequential twin-turbo 13B; with Cosmo engine mounts being offered to make this conversion a drop-in affair, I would normally recommend going with the Cosmo engine swap.

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