We're spending today at Good-Win Racing in San Diego. Why? For years, they've been telling us how cheap and easy it is to swap an NC Miata's 2.0-liter engine for the bigger 2.5 from one of Mazda's bigger cars, so we're finally witnessing it happen. Here's the car, which Good-Win picked up for $5000 a few months ago:
We spent a few hours last night removing the old drivetrain, which meant we started the morning with a barren engine bay. Our goal is to drive the 2.5-swapped car to dinner tonight, and we'll be updating this thread and checking in on Facebook Live during the day as things progress. Think we can do it?
Let's answer a few questions:
How much weight does this add?
"Not much," says the shop's Brian Goodwin. "It's negligible."
How much power do you gain?
About 30 horsepower, though there is supposedly a big bump in mid-range torque too.
What special equipment or parts do you need?
None, though Brian Goodwin naturally suggests installing a Good-Win Racing intake and exhaust. You'll need to get your 2.0-liter ECU tuned to fuel the new 2.5-liter, too. The new engine is basically a bolt-in affair, though you'll need to do a little grinding on the 2.0-liter intake manifold.
How much does it cost?
If you don't want to do this yourself, Good-Win charges just over $1500 for the labor. They opted for a junkyard engine in order to fit this car's low-buck goal, but you can buy a crate engine from Mazda that includes a 12,000-mile warranty for less than $3000.
Here's what we'll need to swap over from the 2.0-liter to the 2.5-liter:
We'll also be re-using the 2.0-liter oil pan and oil pickup.
The biggest hurdle with this swap is the intake manifold. You have to use the 2.0-liter piece, but the 2.5 has bigger intake ports and the bolt holes don't line up. Forunately, it's not a tough fix. All you have to do is remove the locating pins and spend time time with an angle grinder and a drill to open up the bolt holes. Once you've given it room to slide up and down on the bolts a little bit, lining up the ports is easy (they're actually almost the same size, and match up fairly well once you've sorted out the bolt hole issue).
Then modify the gasket accordingly:
Enlarge and remove the shoulder of the lower holes:
Thanks for the walk through, guys! I've been thinking about picking up a NC for this swap..
Tom Suddard said:
We spent a few hours last night removing the old drivetrain, which meant we started the morning with a barren engine bay. Our goal is to drive the 2.5-swapped car to dinner tonight, and we'll be updating this thread and checking in on Facebook Live during the day as things progress. Think we can do it?
With three guys, prior experience and a shop like that? You should be able to do it with no issues. 
What's funny is that I got beat to the punch buying an NC Miata with rod knock for $2000 9 days ago. I was thinking of the 2.5 swap
And just like that, the ports match and the bolts line up:
I'll be following along. For your next trick, you should show us how easy it is to shove one in an NA.
Next up: Balance shafts. They're the big cast iron assembly that sits in the 2.5-liter's oil pan. The 2.5 pan won't clear the NC Miata's subframe, so it (and the balance shafts it houses) will have to go.
(And full disclosure, we pulled the balance shafts out of the 2.5 last night before dinner).
The balance shafts have an oil feed hole, and removing them causes quite a big internal oil leak. 
To fix it, we drilled and tapped the hole, then epoxied a plug into place:
buzzboy
HalfDork
1/20/19 12:17 p.m.
I've not driven a 2.0 but I drove a 2.5 swapped car and they're pretty fantastic. The good mid range makes for a really fun car.
The 2.5-liter oil pickup is on the left, while the 2.0-liter piece we're using is on the right:
We need more Tom getting dirty action photos.
Sorry, but you're going to be disappointed. Between me holding the camera and me being a guest in their shop, I'm not doing much this time. It's weird/guilt-inducing, but I can't argue with the pace things are moving.
Bolting up the 2.0-liter pickup and oil pan:
The 2.0-liter's oil filter adapter is transferred, too.
buzzboy said:
I've not driven a 2.0 but I drove a 2.5 swapped car and they're pretty fantastic. The good mid range makes for a really fun car.
I’ve not driven a 2.5 swapped one yet but after a 97 CSP, a 93 Monster, and a 99 ecotec, my 06 stock seems perfectly good. A bit more power is always good but NCs seem pretty darned good to me.
(full disclosure, one of the worst things about my ecotec swap was that it had balance shaft delete and that made it vibrate WAY too much)
Oh, and the Good-Win crew installed re-ground cams in the junkyard 2.5-liter before I arrived. Though it's not necessary for the swap, it's really easy to do before the engine is installed, and apparently the factory 2.5 cams are very truck-like. 
We transferred the 2.0-liter crank pulley over:
The swap uses the stock NC engine mounts, though we're upgrading with some stiffer Good-Win Racing pieces.
Add the valve cover and the 2.0-liter coolant plumbing and EGR, and voila, a 2.5-liter engine ready to bolt into the NC:
