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Discussion Starter · #1 ·
Hi all, I have a k24a2 with an RBB intake manifold I'm looking to swap into a 2001 NB Miata. I was wondering if I could get advice on building a k24 to make 240-250 hp N/A. I do want to build it to be ready for boost, but still be competent while I'm saving for cash the turbo, etc.

Also, I'm going to be building my motor in my friend's car shop

Thanks!
 

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I would not chase an NA hp goal if your ultimate desire is to go turbo. Get parts that will support your end game. That said getting 220+ hp out of a K24a2 isn’t that hard NA. Go 3in exhaust for sure.

the issue of chasing NA goals is it could cause you to spend money on parts that may not be the best for your future for turbo plans. some cams are better for NA while others are better for turbo. same goes for intake manifolds, larger plenum volumes are better for turbos but not the best for NA.

I hope this makes sense.
 

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Discussion Starter · #3 ·
Thanks, what would you recommend for 220hp and ability for future turbo upgrades? I’ve heard k20 oil pump, drag cartel cam, rods and pistons is a good setup. Just not sure what to really choose specifically as there are a lot of options out there.
 

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Lotus Elise K20A2
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K20 oil pump is only needed if you want to rev the K24 beyond 8k. This is a bad idea anyhow as you get excessive piston speeds. Stick to 7200 or 7400 and it will last.
Rev it to 8500 and you will join the untold number of engines that fail in short order.
Not even NASCAR and F1 reach piston speeds that these 8000 rpm K24 reach.

Cams for turbo and NA are not that different as there is a low pressure differential across the engine if a proper manifold and turbo is used.
For a K20 or k24 turbo build, the stock cams work extraordinarily well. Don’t bother with other cams. Invent the money in a proper (turbo) exhaust manifold and 3” system. Turbos HATE back pressure.
Same for intake manifold. The RBB is fine and torquey for a DD, NA and turbo.
If you want those 8500 rpm on boost or NA, you’re better of with a K20. They were designed for these rpm with their shorter strokes.

The OEM pistons and rods of the k24 are perfectly fine for a 400-450HP turbo daily driver. If you do sustained WOT such as circuit track work, open up the ring gaps. Past 400HP, the gearbox becomes an issue. if you like brutal gear changes, it might break 4th even before that.

There are a lot of options, but for a 220HP NA that gets later turbocharged, you don’t have to do anything to the engine.
 

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Discussion Starter · #5 ·
Thanks for the advice, the k20 oil pump is more for the size than anything else and is required in the kmiata kit I'm getting. Trans also shouldn't be an issue as I'm using a BMW Getrag trans.

I have set aside a budget to lightly build the motor, as I have already bought the 3" exhaust and a slew of other parts. With this in mind would a performance cam or other components be worth the extra power they provide? I have roughly 1600-2000 USD budgeted aside for the motor.
 

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I would suggest you get the engine into the Miata, mapped and running. You need an other intake manifold anyhow.
Once it is all sorted you may add a turbo if you want to add that later anyhow. But you would then start with a working conversion.
For a turbo, you do not need other cams. But you need suffiecnet fuel flow and bigger injectors. For boost, no stock Honda injectors will be sufficient.

If you definitely know, you will TC it, you may open the ring gaps.

An other way would be to fit 4032 alloy forged pistons and steel rods.
At a 10:1 CR, pretty much stock, this would be OK for NA, but would also take decent boost to well over 400HP.
Necessary, no.
 

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Stock cylinders work well with forged pistons. Those building 500+HP turbos usually sleeve the engines, though.
Usually you need to bore the cylinders to the correct diameter to provide a suitable piston to wall clearance for your application.
 

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I would keep the stock intake manifold, especially if you're going turbo later. Speaking to the end game, what are you building the car to do? I have seen dozens of boosted K24s (et sim.) making far more power than necessary and far beyond driver skill. IMO 350whp+ in any generation Miata is a lot to handle and practically useless on street unless your goals are burnouts. A 220-250whp NA build in a Miata is a far better driver's car IMO. Then again, I have never seen a boosted K-series making less than 300whp, but presumably this is easily achievable via low boost.
 

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Discussion Starter · #12 ·
End game for me is to have a car that is fast when I want it to be, I’m trying to build more for straight line races than cornering. I’ve had prior experience with a boosted s2000 so I was hoping to have a similar experience. I’m only looking to make 200-250 NA horsepower as of now to get used to it in a Miata, but I just wanted to make a flexible build that would take turbo power(350-400) without having to tear down and upgrade the motor again. I’m not planning to turbo the motor anytime soon, more of a making it easier for the future type of situation. If I do go turbo it’d be in at least a year.
 

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you cannot use the stock intake manifold anyhow, as the TB would point rearwards.
The suitable aftermarket IM will bump top end power a tad anyhow over the RBB trading it off for a little torque.
If you find some aftermarket cams, fit a 50° VTC actuator and install a suitable pin to block it advancing too much.
How much depends on cam, available data for your engine, piston & cam combo or you need to clay the motor to be sure.

An other approach is the “do it once, do it right” approach :
Looking at the overall cost of this conversion, adding a GT30 type/class of turbo right away won’t increase the budget that much anymore. You get your 350-400HP at 10-14PSI and it flys with decent shove off-boost thx to the stock K24. No aftermarket cams or pistons needed. The engine can remain as is.
For a few straight line bursts on the road, you won’t even have to bother opening the ring gaps.
 
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