Extreme Engine Tech: Building the Ultimate K24 Part 3 - The Finished Goods!

by Mike Kojima

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.  

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.
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Friday, June 12, 2015 8:22 AM
Hoping to see this engine squeezed into an NA Miata engine bay!
Friday, June 12, 2015 8:39 AM
Mike, with the issues (minor as they may be) you've had putting a K20 head on a K24 engine, would you say in hindsight it would be easier to start with a K20 engine and install a K24 crank to get the displacement? Or is there some advantage to the K24 bottom end?
Friday, June 12, 2015 9:08 AM
@sethulrich, my guess is the K24 block is taller to allow for the longer stroke crank. So putting the K24 crank in the K20 block would maybe require shorter rods which would make the rod:stroke ratio worse and/or shorter pistons which would push the pin bore up in the piston. That would maybe reduce the strength of the piston or cause issues with the piston rings and oil grooves.
Mike Kojima
Mike Kojimalink
Friday, June 12, 2015 10:52 AM
Yes to above. Another thing is the customer wanted to start with a late model engine. We were happy for the challenge.
Friday, June 12, 2015 12:49 PM
is it hitting the dyno on the next installment? or are you done with your part and we won't see the power curve till someone puts it in a car?

And since you mentioned putting it into an RSX, it won't fit with the oil pan, just an FIY
Mike Kojima
Mike Kojimalink
Friday, June 12, 2015 9:49 PM
The motor will be off to the winners of the contest. We are sad to see it go.
Saturday, June 13, 2015 11:05 AM
The EFR 7670 does not have an aluminum bearing housing
Mike Kojima
Mike Kojimalink
Saturday, June 13, 2015 10:46 PM
The Borg Warner web site says it does but you are right, when actually looking at it it isn't. I wrote some of this before we actually had the turbo.
Monday, June 15, 2015 1:33 AM
I'd put it in an S13 (but then the oil pan has to go).

Haters welcome.
Wednesday, June 17, 2015 1:25 AM
I wonder how it'd work in a miata.
Wednesday, June 17, 2015 6:19 PM
The 7670 does not have a mixed flow turbine. The 7163 is the one and only EFR turbo with that feature.
Mike Kojima
Mike Kojimalink
Wednesday, June 17, 2015 10:20 PM
There spec sheets are full of mistakes then.
Thursday, June 18, 2015 8:55 AM
When putting on the K20 oil pump, Did you have to Plug the oil hole? "Like on a K24a Block."
Also with the drain in the top of the block you could have ground out the hole a little to help more.
Another thought- "oil filter" I believe I saw it on a LS1 block, They sell a cover that loops the oil passages. I'm sure no one makes one for that K24. Yet.
Wednesday, December 30, 2015 9:12 PM
Missing dyno graph?
Thursday, May 26, 2016 1:19 PM
If you were going to build the ultimate K20/24 as an NA motor what would you change?
Mike Kojima
Mike Kojimalink
Friday, May 27, 2016 2:00 PM
We are going to do an NA build, coming soon!
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