I think you should wait a year before spending a lot of money on "upgrades". Get to know it first and then act as you will. You will find you have changed your mind as this car is really pretty good right out of the box.

 

@cosmomiller oh I’ve definitely been giving it a good shakedown

I’ve been to 3 years of HPDE events and am loving it. It’s a lot of fun to hold your own against GT3RS and GT4’s in a stock S2K and 200TW rubber (at least until they hit the straights)…

I was back and forth with swapping to a Miata platform because of affordability vs the S2K and not feeling bad about taking the Miata down a focused track car route, but recently decided that I want to keep a car that I can drive on the weekend and to the track, and drive home again, so I’m going to be sticking with the S2K.

My current (and always changing) plan is to change things slowly over time, starting with cooling, then better grip, then better braking, then more power.

Or something along those lines

 

Well, as far as better grip, I recommend Michelin 4s summer extreme tires!

 

@cosmomiller I’m currently running Falken Azenis RT660 in stock size, but the next upgrade will be to a square setup on wider wheels and rolled fenders that have room for a BBK

 

Consider replacing the coolant overflow tank. The plastic can fail, which shouldn't cause a catastrophic failure, but still an annoying mess.

When you say stock Miata, do you mean a Mazda OEM radiator cap fits on the Koyo rad? From which generation, NA/NB/NC/ND? I'm planning on the Koyo and haven't looked into caps yet.

Honda OEM hoses only. I'm planning to replace mine. If nothing else, replace the one under the VTEC solenoid and TCT. It's a small hose, easy to access, and chances are some oil has dripped on it over the years.

 

@Save the Manual Wagons! Good point! I forgot to mention I already replaced the overflow tank due to a stress crack.

My google searching led me to sakebomb's site that shows the 98-05 Miata cap (KL0115205) as a direct fit for the Koyo Rad....I'm going to call my local mazda dealer to see if they have one and I'll test fit it on the Koyorad when it comes in to confirm!

Just checked with the local dealer and they wanted $200 plus tax for upper and lower radiator hoses...so that's going to be a "order them if I break them" item....unless I order them online for $100 and wait another 2 weeks to tackle the project....

 

Let us know if that cap fits.

Oil cooler hose, this is the one I would definitely replace, $9: https://www.hondapartsnow.com/genuin...2-pcx-000.html

$200 for the upper and lower hoses is nuts. $45 each online.
- Upper radiator hose: https://www.hondapartsnow.com/genuin...1-pcx-010.html
- Lower radiator hose: https://www.hondapartsnow.com/genuin...2-pcx-010.html

 

I copied this from user Kyle. You can use the Miata cap, and then buy a Honda stocker to put on it. OEM look. The Honda sticker psi spec is same, so it all aligns with truth.

These are the respective dealership part numbers to source:

Honda radiator cap label:
19046 PAA A00

Mazda cap:
KL01-15-205

 

Just finished the upgrade and it was really simple!

The stock was simple to drain and pull with only a few bolts, and the Koyo was a perfect fit replacement.

I did paint the top of the Koyo black with SEM 49143 Trim Black Ultra, and added some foam weatherstripping as well, painted the foam black on the top piece and left it gray on the side pieces.

For my AP2, I had the smaller temp sensor, so I used the supplied adapter from Koyo. I didn’t replace the gasket on the sensor as it was in great shape, but that’s something that I should have purchased anyway. It was hard to tell from the parts online which sensor size I had which is why I didn’t buy it.

I ordered a Mazda 1.1bar radiator cap (to match the factory pressure) from sakebomb and it worked perfect! The part number from Mazda is a couple posts up if you want to get it local.

I used the knock off Lisle funnel from Amazon to fill and burp the system and it worked flawlessly with the car angled up a little bit in the driveway.

The Koyo is so efficient that the radiator fan never kicked on unless I rev’d the engine to 3500 and kept it there for a couple of minutes…and then as soon as I let it idle the fan would turn off again. It’s 70F today in Houston and genuinely after running the car for 30minutes the fan still didn’t come on and I had 8 bars on the temp gauge.

The system has no air bubbles and is blowing 175F at the vents, so I’m super happy.



Easy to pull the stock radiator! First make sure that the car is on a slight incline so that you can properly burp it later. Start by hopping in the car and turning it to acc power, adjust the air temp to full hot to open the connection to the heater core and turn off the car. Then drain the radiator by opening the plastic drain plug underneath the car, and opening up the top vent port (located in the yellow circle area) as well as removing the cap. While it’s draining, remove the air cleaner box by removing the 3 bolts and taking it out. Careful that there’s a wire harness connected to the box so you may need to cut or otherwise remove the zip tie holding it on. No need to remove the air cleaner tube connected to the throttle body, as you can maneuver the box out with it in place.
Next remove the plastic overflow tank by removing the black rubber tube from the radiator and then pulling the plastic tank upwards and directly out. Dump out the fluid.
Next remove the 2 fan connectors on either side of the radiator, very easy to reach.
Finally, remove the two bolts holding the radiator clamps on the top of the radiator.
now you can lift it out. It’s light and easy to maeuver.


Time to remove the 5 bolts to transfer fans, along with the thermostat and bottom bump stops. Pretty straightforward. The thermostat on the left has an o ring that I would have replaced but it was in good shape. The Koyo includes new hardware for the fans, and a drain plug, as well as a thermostat adapter in case you need it.


The new Koyo, top painted black for a more stock look. You can see the new foam painted black with the same paint on the top, and the grey foam on the sides. I used Frost King polyurethane foam air conditioner weather seal kit 99” from Home Depot. It’ll take some trimming to fit, but you can figure it out
The L shape foam goes on the side, and is trimmed down to slide in. The rectangular piece is cut in half lengthwise and then trimmed to fit the top and bottom of the radiator. I used 3M spray glue to attach the foam to the radiator after I painted it black.


The stock on the left is about half as thick as the Koyo on the right. The foam is blocking the view of the Koyo, but the radiator is the full width of the foam 1.5”.


The new Koyo with everything mounted and ready to go back in.

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All installed, filled, and burped! Now ready for some engine cleanup

Installation is straightforward and the reverse of install. Just make sure the drain plug is snug

To fill, install a lisle funnel or equivalent from Amazon. You’ll have to use an assortment of gaskets to make it fit snug with the special cap in place.

Make sure you’ve closed the vent port on top of the engine (you can keep it open and use it as a visual for when the radiator is full, but it will spew fluid out if you’re not paying attention. Make sure to close it once fluid is coming out).

Slowly pour fluid (Honda Type 2) into the funnel avoiding huge bubbles if you can. You’ll use about 1.75 jugs of coolant…after a while the coolant will start to accumulate in the funnel and you’ll want to let it fill about 1/3 of the way up.

Hop in the car and turn the car on and set the fan to full blast. Keep an eye on the temp gauge and the funnel. You don’t want the fluid to drop below the bottom of the funnel and you don’t want the temp to go past 8-9 bars. If either happens, turn the car off and reevaluate your life choices lol.

Let the car run for a while…hopefully the fan will kick on…(drivers side fan). If not, you may have to rev and hold the engine at 3500RPM for a few minutes. You can occasionally blip the throttle as well to get air bubbles out. Also, squeeze the upper radiator hose gently occasionally. Careful as it’ll get hot!

Keep making sure the fluid level stays up and keep watching for bubbles. After 30 min or so you shouldn’t see any more big bubbles coming out, and the air blowing out of the vents should be stupid hot…175F or so.

Once those two things happen you should be good to go. Turn off the car, make sure the fluid level is around 1/3 of the funnel, and let it cool off for a while like 20min.

Insert the plunger in the funnel and remove it from the car. Put on the radiator cap and let the car come to full cool temp.

Once cool, fill the overflow tank to the min level and admire your handiwork!

 

Very nice job! Thanks for all the photos.

Do you have plans to paint that sheet metal panel black, too?

Thanks!