Miata Stuff - 230whp has been fun, but now I want 300whp...options?

Hey everyone. My NB is currently turbo’d at 230whp/230wtq, and it is definitely fun to drive. However, I have always wanted to have a 300whp Miata. I know the stock engine cannot handle this and it will need to be built. I am just starting to research all that is needed, so I’m not 100% sure on all that is involved when building up the BP, but I’m trying to learn all I can.

However, I started thinking, what about a turbo K-swap? Kpower Industries makes what looks like a pretty decent kit for a K24Z3 engine to drop into the Miata, which is then also capable of boost. With my 300whp goal, that would be considered a mild build for a stock K24Z3, whereas the BP I would need to get forged internals. The BP would, however, take less time and money to get up and running, whereas the K swap would take much more time and money to get started.

There is another thing that I’m not entirely sure about. Looking at engines only, a forged BP would cost a bit more money than buying a K24, as I would have to outsource the actual building and machining part of the process (I can pull the engine and drop it off with the needed parts, but I do not trust myself to put a whole engine together). Forged internals also aren’t the cheapest thing in the world. If something was to blow on the built BP, I would be out thousands, and would have to repeat the expensive process to rebuild the engine. Of course, I have no idea how likely that is at 300whp. With the K24, however, it would be much cheaper to source another K24Z3 and just drop it in again if something were to blow. The problem with the K-swap though, is that it would cost a whole lot more upfront, and it would also be a much more long-term project for me.

So, I’m having a bit of a dilemma. Do I pony up the money upfront, swap in a K24, and then boost it for 300whp, or do I just build a forged bp and drop it in? What would you Miata guys do in this instance? If I went the forged BP route, it would be quite simple to hook everything back up and get it re-tuned since I already have the ECU and turbo, but my biggest concern with that is the long term “what if” of blowing the forged engine and then having to do it all over again for thousands of dollars. Again, not sure how much of a concern that is at 300whp, but it is a thought I have.

Any help, comments, or words of wisdom are appreciated!

TL;DR: NB2 Miata, do I do a built BP for a little less money since I already have a turbo setup and ECU, or do I pony up the cash, swap in a K24Z3 engine, and then boost that for my 300whp goal. Kind of leaning towards built BP right now.

Whats the transmission plan?  I understand with track work going above your power level, lifespan beings to fall off dramatically.  Does including that in the build significantly change things?  I imagine a K24 to Miata trans adapter is the same price as a K24 to BMW trans adapter, while the BP to BMW trans adapter is quite pricey.

In reply to ProDarwin :

I was thinking of a 6 speed Miata transmission, since my power goals are 300whp it seems that the 6 speeds can generally handle that. I still have a lot of research to do, but that is what I am understanding when it comes to the transmission side of things. It seems things get hairy with the 6 speed above 350whp. I do know the K24 to Miata transmission adapter plate comes with the K Power Industries kit, so that would be included in the price of the kit.

The car will be a mostly street car with occasional (maybe 1, max 3) times on the track per year. I actually have not been to a track before, but I would like to try to get to one eventually just for some fun and try it out. Otherwise, I love me some fast street cars.

Oh I also plan on swapping in a torsen diff whether I go built BP or K24.

With everything you already have (and I have a similar setup) building a bottom end capable of what you want is under $5K and retains the feel of a Mazda.

A K24 swap is likely to be at least $10K and might be as much as $15k with a turbo setup.  

ProDarwin is right, somewhere around 300hp is where transmissions get quasi-expendable.  The 6 speed is sturdier.

KyAllroad said:

With everything you already have (and I have a similar setup) building a bottom end capable of what you want is under $5K and retains the feel of a Mazda.

A K24 swap is likely to be at least $10K and might be as much as $15k with a turbo setup.  

ProDarwin is right, somewhere around 300hp is where transmissions get quasi-expendable.  The 6 speed is sturdier.

Thank you! Thats honestly what I was thinking, and as much as I love Honda K engines, I figure for what I want the built BP may be better. How much risk is there of blowing a built BP with 300whp? I know, thats a tough question to get a definitive answer on, but just something that concerns me with the cost of a forged engine.

In reply to Japanspec :

It should be sturdier at 300 hp than the factory motor at 230.

Doesn't your car already have a torsen diff?

This can be done with the mazda BPT 1.8, near certain is a bolt in for that chassis.  Play games with exhaust and bigger turbo, ie give Keith Tanner your money, boom, done. 

In reply to KyAllroad :

I do not believe it does. I will have to double check, but I'm pretty sure its an open diff.

Japanspec said:

KyAllroad said:

With everything you already have (and I have a similar setup) building a bottom end capable of what you want is under $5K and retains the feel of a Mazda.

A K24 swap is likely to be at least $10K and might be as much as $15k with a turbo setup.  

ProDarwin is right, somewhere around 300hp is where transmissions get quasi-expendable.  The 6 speed is sturdier.

Thank you! Thats honestly what I was thinking, and as much as I love Honda K engines, I figure for what I want the built BP may be better. How much risk is there of blowing a built BP with 300whp? I know, thats a tough question to get a definitive answer on, but just something that concerns me with the cost of a forged engine.

300 rwhp built BP shold be pretty reliable for street/autox use, assuming you tune it properly.  A 6-speed should also last reasonably well assuming "mechanical empathy".  Hard core track use may well change that.

The jump from 230 to 300 is a significant one in terms of cost.

A well-built BP with quality internals and a good tune shouldn't have any trouble at that power level. Build it once and it's good. We've got engines out there that have been running 400+ for years. And remember that if you build the BP, you'll have a new engine instead of a junkyard unit, which is always nice. 

The transmission will be less sanguine. 6 speed gives you the best chance but it's no guarantee. And yes, mechanical empathy is what separates the endurance racers from the sprinters and track day "winners" :)

In reply to codrus (Forum Supporter) :

Thanks! Yeah, once its built I plan on getting it over to the tuner who tuned my current setup. He tunes many turbo Miatas (and Hondas) so I trust that he would tune it well.

In reply to Keith Tanner :

Thats good to hear that you guys have engines with 400+ running for years! And knowing Flyin Miata, I'm sure those cars are flogged at the track regularly. Sounds like my 300whp is at the lower end of a built BP's power ability. You're right that at least with a built engine, it will be new whereas the K will be a junkyard swap. I won't be doing any full throttle clutch dumps with the car, so hopefully the transmission should last a pretty long time. I do tend to drive the car hard, and do plan on taking it to the track sometimes (if you can consider 1-3 track days as "sometimes" lol), but it isn't going to be an all-out track rat, so hopefully I should be okay.

A deliberately bad turbo K24 will beat 300hp I don't know Miata drivetrain parts but be mindful that a $100 Chinese T40e on a $54 iron manifold will easily be 30-40hp stronger than your goal. If that power is the edge of your envelope take it into account. 

How does the cost of GM LS drivetrain swap compare to all the options above? Might be a way to a reliable 300 horsepower with a gearbox and rear diff that can handle it. 

In reply to Type Q :

Not sure if easier or not but LFX options are out there too, and engine/trans aren't terrible cost.

I figure the LS swap starts to make financial sense at about 400 hp, and that's assuming you're starting with a stock car. If you've already invested money into a turbo and ECU and all the supporting mods to run 230 reliably, it's pretty hard to make the case for the LS financially unless you have a customer for the take-out parts. There's no question that a 430 hp LS3 swap is a complete anvil from a drivetrain standpoint and will also run cooler when worked hard. It's just loafing around waiting for some real work to do.

The initial purchase price of the LFX is lower but the cost to do the swap isn't radically different. If you decide to stick with the stock diff, you can save a bit more but I'm not sure how that gearing math works out.

The stock transmissions don't seem to explode on clutch drops, they just shear the teeth off the gears under power. Keeping the fluid fresh and cool would probably help a lot, as would taking an extra half second to shift. The Torsen rear ends are good in the sub-400 range as long as you avoid wheel hop. Not indestructible, but failures are rare in my experience.

As cool as the LS swaps are, I'm a bit of a snob for Japanese 4 cylinder engines. Just my personal preference, nothing wrong with the LS. But yeah, it seems it would be quite a bit more expensive considering I have the ECU and all related turbo parts already on my BP.

Keith, what fluid would you recommend for a 300whp Miata? I've been using Redline but I'm planning to switch over to the Ford Motorcraft fluid since I've heard its one of the best options for Miata transmissions. I'm not sure what gear ratio diff is in my car now, but I heard the 6 speed has quite a short gear ratio as is so I was thinking of upgrading to a Torsen 3.9 if I can even find one.

I don't know what year your car is so I can't tell you the current ratio. The 6 speed does have a shorter high gear than the 5, so the 3.9 or even a 3.6 (not factory fit in the US) is preferable. 

I've always used Redline in my miata transmissions. 

I would really look hard at what everything is going to cost and if you can reasonably afford it.

And anytime you do something like this, there ARE going to be unforseen costs. Whether it's things you missed or things you find you need to do once you start tearing everything apart.

My OBD-1 S52 swap in my E30 years ago, ballooned in cost because I'm "while I might as well do X, Y, and Z while in there."

Ford 5.0, T5, Mazdaspeed diff.  Cheaper than LS and just as much fun,  Oh, that's what I built....

In reply to z31maniac :

I can afford to do the built BP, just not all upfront. It will be a process of collecting the parts little by little, then saving up some money before sending the engine off to the shop to be machined/built. It definitely won't be a few weeks build, but a longer term build, although I'm not too worried about that.

@Keith, my NB is a 2001, base model I think. As long as the 6 speed can hold up reasonably well at 300whp, then I'm fine with it. I just don't want to be swapping in a new transmission every month. Same goes for the diff as well.

I know, with more power comes the higher risk of things breaking...but its always been a dream of mine to have a 300whp Miata.

Your 2001 5-speed has a 4.3. You'll definitely want a new rear end with that trans.

Keith Tanner said:

The stock transmissions don't seem to explode on clutch drops, they just shear the teeth off the gears under power. Keeping the fluid fresh and cool would probably help a lot, as would taking an extra half second to shift. The Torsen rear ends are good in the sub-400 range as long as you avoid wheel hop. Not indestructible, but failures are rare in my experience.

Yeah, I think the transmission acts as a fuse to prevent the rear ends from blowing up.

My 5-speed failed the way you say (sheared all the teeth off 3rd gear).  The 6-speed that I blew up was the input shaft bearing, probably due to excessive heat from lots of track days.  Were I still running a turbo Miata at the track I would be looking for transmission oil coolers.

We ran one on the Track Dog back in the day. And the temperature numbers I've logged on the ND on track were  pretty shocking. Maybe I should start working on that.