The K24A2 06-08 TSX valve springs are the same as the 05-06 K20Z1 RSX-S single springs. They can rev to 8600 all day long with a stock cam, or even a drop-in aftermarket replacement (typically Stage 1). Hondata uses 8600 in all of its base tunes for stock RSX-S motors. Aftermarket cams, with their commensurate steeper ramps, need bigger springs and better retainers as directed by the manufacturer.

The real issue on rpm for a K24 is the bottom end. With the longer stroke (vs K20), the piston speeds get astronomical really quick. And that stroke puts a lot of side-load on the piston. The stock rod bolts also cannot handle big rpm. For a car that's going to see a lot of track use, 8000 is a solid practical limit for a stock bottom end. In fact, Hondata puts 7600 in all of its K24 base tunes.

To go higher, the minimum is forged pistons and rods...which also allows you to get the compression where you want it. 12.5 is a solid practical limit for pump premium with a good tune.

PS: All of the above is from lessons learned the hard way, as well as lots of research with real people, not just the internet.

Adding to my post above, here's where I see the sweet spots for K24 builds:

1) Completely stock 06-08 K24A2, with bolt-ons, 50 VTC and tune on pump premium...typically makes 235 Dynojet in FWD applications (less for RWD). RPM limited to 8000. Massive torque!

2) Same as #1, but with mild cams/springs added. Makes 250 Dynojet at 8000 rpm. Gains are from 5000 rpm and up.

3) Same as #2, but with aftermarket rods and pistons, running on E85, 13:1 compression. 270 hp at 7600 and then drops, revs to 8500. Gains everywhere vs #2.

4) Same as #4 but with K20 head (or ported TSX head). 285 hp at 8000 and holds power to rev limit. Progressive gains from 6500 and up.

For reference, the pic shows what a K20A2 (RSX-S) head does versus a K24A2 (TSX) head.

PS: If you want reliable 300 hp, sleeved bored K25 is the way. But $$$.

I agree with everything you stated above, hence my plan for a mild cam and valve spring/retainer setup. FWIW, the mean piston speed of a K24 at 8000 RPM is 26.4 m/s, which is N-U-T-S.

For reference, the most recent Formula 1 V8 mean piston speeds were 24 m/s at 18,000 RPM.

Some good info there Andy. I'd like to see 250whp over the winter with a valve train and cams in my 04-05 but I know that's reaching. Might do some 09+ or aftermarket pistons too we'll see.

04-05 rods are not as good as 06-08. Best to rev limit those at 7600.

If you do 09+ TSX pistons with anything other than an 09+ head, you need to grind down the nubs on the one side to get proper piston-to-head clearance. Search on K20a.org for my posts on that topic.

Racked the car today to knock out a few things. We got about half the exhaust built with a Vibrant flex and 3” cat. Waiting on a couple of tight-radius 90s to build the rear half of the exhaust.

Attachment 231862

FM frame rails installed:

Attachment 231863

Intake cut down to size and installed (missing one hose clamp)

Attachment 231864

It will come back over later this week to finish up the exhaust and get the sways/endlinks installed and set up properly. Once that’s done, it’s just the interior that’s left.

Hey guys, any final build pictures? I know we are still playing around with a couple little things, but I think we'd all love to see the 99% finished product. Such a pretty and well-rounded build.

Ive convinced myself to commit to the K swap in my 01 LS Miata. I can piss off my B series friends, And I know I'm not financially responsible for the LS3. My friend who has a Miata as well has been convinced to K swap instead of turbocharging the iron slug BP. Is 1600$ too much to spend on an 08 TSX engine ? Thats with no intake mani or exhaust and no trans obviously. Just moved back to NC so I dont have to worry about CA rules. If no one's routed the intake towards the windshield (cowl ?) I'll have to consider that

Sounds like fun. $1600 is a bit high, but it depends on where you're located. I'm in Chicago and $1200 has been the highest I've paid. Check car-part.com and be sure you look for 06-08 engines. Also, don't be afraid of looking in a several hundred mile radius for the right deal. I can connect you to our freight company that can get it to you very easily.

Depends on mileage too, I paid 1600 for a 06 motor with 60k full long block with manifolds and wiring, minus ecu. Was 20min away from me.

I live in NC and have sold a number of k20 and k24 engines. The last '06 K24A2 I sold for $1200, that was with ~130k miles but very good leak down numbers. That was with manifolds, injectors, coils, etc.

$1600 seems a bit high to me, unless it has extremely low mileage.

You can buy a whole wrecked TSX for $1600...sometimes

I just bought a running driving 03 RSX type-S last week for $2500 that was hit in the front. Radiator support, passengers fender, and hood is trashed, but it runs and drives just fine. It has a 05-06 motor in though...K20Z1, not the 03-04 K20A2. Detailed threads to follow from me when I have time to post a write up and photos.
Craigslist, man. Dime a dozen.

Good find, and the K20Z1 will work just fine too. You can sell that trans, axles, shifter box and cables, half shaft, etc for over $1000 alone.

Just bite bullet and go for this: https://www.miataturbo.net/miata-par...ts-sale-90011/

:)

Thanks, I actually spoke to you last week I think it was. I look forward to giving you my business.

I can't wait to see more build pics and some final thoughts once it's road tested. This choice of engine swap is getting more attractive all the time.

Didn't you have a K24 before? That's a good pick up on the RSX.

At MATG right? Sounds good, we're here whenever you're ready.

Post 6:

Final update, just buttoning up loose ends.

We got about half the exhaust built with a Vibrant flex and 3” cat.

Attachment 231844

The other half is built out with a few tight-radius 90* bends and a Magnaflow 12579 (3" in/out, twin 2.5" internal chambers). It is OEM quiet at idle, near-OEM at cruise, and rowdy in VTEC.

Attachment 231845

Attachment 231846

Attachment 231847

Nice and centered.

Attachment 231848

FM frame rails installed:

Attachment 231849

Intake cut down to size and installed (missing one hose clamp)

Attachment 231850

On to the interior. This car was pretty ratty inside, even for 130k miles, so Andrew elected to go all out.

Starting with a Blackbird Fabworx GT3 bar. This is the only NB bar that is glass-window compatible AND has a welded harness bar, so it's the obvious choice.

Attachment 231851

All the HVAC goes back into place after the wiring is complete.

Attachment 231852

New Pioneer touch-screen head unit with SiriusXM support, Spotify control, Bluetooth audio, and a ton more. I will bug Andrew to post more about the audio system if there's interest.

Attachment 231853

We did a carpet kit from a company in the UK that included a full under-felt kit:

Attachment 231854

And way nicer carpet than what originally came in the car:

Attachment 231855

Tan Probax Elise seats round out the upgrades here.

Attachment 231856

Outside the car, a fresh Robbins canvas top replaces the old worn-out vinyl.

Attachment 231857

The new 8,000rpm rev limiter spawned a gauge upgrade from Revlimiter. Rx8-style faces with realigned needles, new 8,000rpm limiter, and the tach range expanded to 9,000rpm. Very OEM+ to match the overall build.

Attachment 231858

After a complete cut/seal/polish/wax job, the car is now ~98% complete. There are still a few details to be ironed out (idle valve, A/C support, dyno tuning, possibly cams/valvetrain later on) but this is essentially the finished product. Pretty clean car, but that's what you get when you strip a car from the rear bulkhead forward and replace 80% of it :)

PS: Props to those of you who can figure out what the license plate means :)

Attachment 231859

Attachment 231860

Attachment 231861

3 lobes? It's a VTEC joke. My old D16Y8 had 3 lobes on the intake (or was it exhaust? stupid cheap SOHC head), 2 for normal mode and 1 shared lobe for VTEC mode.