We have been working with Motovicity for the last few weeks to demonstrate the building of a potent Honda K24Z7 motor, built completely from in stock and off the shelf parts available from Motovicity themselves. We chose the K24Z7 as it is currently the OEM engine for the Civic Si and is relatively difficult to modify due to it’s emissions bound cylinder head. Our goal is not to build a dyno queen or a drag motor but to build a strong K motor all from off the shelf parts with the intention of getting the most power possible on pump gas with the widest most useable powerband.
In the last two editions of our series we focused on the cylinder head and bottom end of our K24Z7. We swapped to an earlier model K20Z3 head to get Vtec on the exhaust side and a removable exhaust manifold. Installed Skunk2 camshafts, Kelford valvesprings and Supertech valves better suited for turbocharged use. For the bottom end we added lower compression JE pistons and stronger K1 rods, removed the problematic balance shafts while we were in there.
Part 1: Cylinder Head
Part 2: Bottom End
Now it’s time to finish off our motor.
Starting off with the good stuff, we chose a Borg Warner
EFR 76/70 Turbo with a divided .94 A/R T4 flange exhaust housing. The Borg Warner EFR is a highly advanced turbo with a long list of features that a turbo geek will drool over. The turbine is super lightweight titanium aluminide that greatly reduces turbo lag. The turbine wheel is a high flowing mixed flow design which takes some of the efficiency benefits of a radial flow turbine and combines that with the high flow capacity of an axial flow turbine. The turbine spins on dual ceramic ball bearings which reduce lag and transient response. The turbine housing is made of investment cast stainless steel which is both lightweight and helps efficiency due to its low thermal conductivity (about half that of cast iron) The compressor wheel is a highly efficient forged billet extended tip design that also tends to reduce lag. To top it all off, the compressor housing is a ported design that gives the turbo a large degree of surge resistance. The Borg Warner EFR has about every trick feature you could think of or want.
A twin scroll turbine housing uses pulse conversion to help speed spool. Cylinders 1-4 and 2-3 are paired together on ether side of a divided turbine housing. This hits the turbine with 4 discrete power pulses which helps spin it up faster, up to 20% faster. A divided housing can also have Volumetric Efficiency advantages as it separates adjacent cylinders so cross contamination of cylinders by reversion is more difficult on overlap.
Borg Warner makes medium frame turbos with large internal wastegates. We like this because it simplifies packaging by eliminating the need for an external wastegate. This is particularly cool on a twin scroll turbo as these often have to have dual wastegates. Keeping things simple makes for greater reliability. The exhaust housing has good wastegate port placement and good diffusion on the exducer. The housing also uses a single bolt V-band to attach to the downpipe.
You can see the extended tip billet compressor wheel here. Extended tip wheels tend to make boost at a lower shaft speed and are a little less laggy. Billet wheels are more efficient because their superior strength means that the blades and the center hub area can be made thinner for better aerodynamics, improving flow and efficiency. The annular groove just behind the compressor wheel inducer is the recirculation port for the port shroud compressor housing. When the compressor approaches surge, the air that is starting to back up in the compressor flows through the slot and recirculates out the big vents just in front of the inducer. This bleeds off the excess mass flow and mitigates surge.