Help me understand the capabilities of the stock 2.5L engine.

[Alex]

Hello, I’m working on a Mazda MX-5 NC project. I’ve already installed a 2.5L engine from a Ford Fusion, and now I want to add a TD04HL turbocharger from a Subaru WRX. I’m aiming for 0.5 bar of boost to start with; the engine is completely stock, as is everything else—including the automatic transmission. I’d like to know what the critical boost limit is for this stock engine and whether the pistons, connecting rods, and crankshaft bearings will hold up, as my goal is to build a reliable daily driver. I also plan to install an intercooler and coolers for both the engine oil and the automatic transmission fluid. Thanks in advance for your reply

[mituc]

Leave it at 0.5-0.6bar. The very long stroke and large bore of the 2.5 engine are a liability in the stock form of this engine, The engine itself would take a lot more boost in its stock form (1.1-1.2bar as long as you do not run out of injector) but it may not last. Actually it may not last very long as is... A way better deal is to use the 2.3MZR engine, or the 2.3DISI-T (MPS/Mazdaspeed/CX7) short block with a 2.5 head, and boost those to 1.6-1.7bar.

[Alex]

Leave it at 0.5-0.6bar. The very long stroke and large bore of the 2.5 engine are a liability in the stock form of this engine, The engine itself would take a lot more boost in its stock form (1.1-1.2bar as long as you do not run out of injector) but it may not last. Actually it may not last very long as is... A way better deal is to use the 2.3MZR engine, or the 2.3DISI-T (MPS/Mazdaspeed/CX7) short block with a 2.5 head, and boost those to 1.6-1.7bar.

Hello, Mituk. Thanks for your reply. You mentioned that the engine "might not last long" as it is—could you elaborate on exactly what you meant by that, even with 0.5 bar of boost? Perhaps I should lower the boost pressure? As for the injectors, I’ll be using 550cc ones; my goal is to build a reliable street-driven car.

[mituc]

Hello, Mituk.

I can't believe how hard must be to copy&paste someone's name when replying

You mentioned that the engine "might not last long" as it is—could you elaborate on exactly what you meant by that, even with 0.5 bar of boost? Perhaps I should lower the boost pressure? As for the injectors, I’ll be using 550cc ones; my goal is to build a reliable street-driven car.

The 2.5 engine doesn't like that much to rev. In an MX5 that's what you're going to do with it. You can level the boost so that it will not become a problem comparing to some other engine options, but it's just the geometry of the short block that works against it (longer stroke, larger bore, lower height pistons, more side thrust, thinner block/sleeves, less torsional rigidity). If the car will not be driven very often and too much then it should last indefinitely. But if you're going to drive more and spiritedly, like 10k kms/year or more - it's hard to tell how much it will last.
My recommendation is to either boost a 2.0 engine instead of the 2.5, or use the 2.5 head and a 2.3DISI-T/MPS short block.

[CrazyCanadian]

I'd say it's the opposite of what Mituc is saying.

Mazdaspeed people use the stock 2.5L bottom end with the stock 2.3L DISI head on it as a cheaper solution to replacing a blown up motor. Gap the rings for boost and most people have no issues running 1 bar or a little higher boost.

In the Miata world 300-400hp is pretty common. Once again gap the rings.. Don't bring in a lot of boost down low. Gotta watch cylinder pressures at lower rpm. How good the tuning is, will determine how long the motor lasts. That wrx turbo will want to spool early if your manifold is built right.

Your auto transmission will be the problem. There aren't tuning solutions for it.

[mituc]

Mazdaspeed people use the stock 2.5L bottom end with the stock 2.3L DISI head on it as a cheaper solution to replacing a blown up motor. Gap the rings for boost and most people have no issues running 1 bar or a little higher boost.

Yes, people do that, but those engines do not last. If you want to get the car back on the road and just sell it - ok, will work. But once you will start driving the car for what it is and not just as a commute car, things will not look so bright any more.

[CrazyCanadian]

Mazdaspeed people use the stock 2.5L bottom end with the stock 2.3L DISI head on it as a cheaper solution to replacing a blown up motor. Gap the rings for boost and most people have no issues running 1 bar or a little higher boost.

Yes, people do that, but those engines do not last. If you want to get the car back on the road and just sell it - ok, will work. But once you will start driving the car for what it is and not just as a commute car, things will not look so bright any more.

I did a deep dive on a 2.5L when I did my swap 4 years ago. I don't disagree the 2.5L is weaker, but it's not glass. I don't think the 2.5L bottom end is a good idea for the Mazdaspeed world. Piston shape doesn't work for direct injection. Port injection, boost control and tuning goes a long way in keeping the stock 2.5L bottom end alive.

If it was such a problem child I wouldn't get my ass kicked regularly by Miatas and RX8s with turbo 2.5L swaps on the track.

[mituc]

It's not glass and I'm not concerned about them breaking after demanding too much power from them. And they can make a lot of power well past the point on a MX5 you will need a lot of other mods to make it work.
I am talking strictly from a longevity/reliability perspective. Which is also the reason why I mentioned in my very first reply that if the car is driven a certain amount it's still a feasible approach.