MX5 Miata 4 Bent Rods VVT

The Mazda MX5’s engine started out life in a 323 GTX which was not only turbocharged from the factory, it was using a homologation car for the efforts in rally competitions all over the world.

Suffice to say, the core engine design is well suited to having a reasonable amount of boost thrown at it, especially in the case of the B6 engine (B6T in the 323GTX). With a factory forged crankshaft, somewhat stout rods and factory oil squirters to keep the pistons cool – it’s certainly far more equipped than your average 80s engine.

Now, this doesn’t mean by any stretch of the imagination that the engines are ‘bulletproof’ and don’t require a considerable amount of attention to keep them alive above a certain horsepower rating and over the years there have been a few ‘golden rules’ of how much power stock parts will take before either exiting the block sideways or finding some other method of embarrassing you on the racetrack.

Here’s a list of the most power the stock engine of the MX5 / Miata can take, reliability depends a lot on the health of the engine and how it’s used, launches or high rpm will kill rods and oil pumps quickly, as well as gearboxes. A lot of people ask what the maximum horsepower or torque an MX5/Miata engine can take – while there is a somewhat agreed number it is more to do with how the power is applied in the curve that prematurely kills stock engines.

For example, here’s what flat shift can do when you’re making just that little bit too much power.

All of this assumes that for at least the most part, the car is ran on a reasonable oil that has a film strength that can support the additional load in the system and it has a stock rubber damper that isn’t falling to bits.

Maximum Horsepower and Torque the MX5 / Miata Engine can take;

(WTRQ = Wheel Torque, ETRQ = Engine Torque, WHP = Wheel Horsepower, BHP = Engine Horsepower) – BHP is a calculated value if your tuner only provides this value air on the side of caution (the lower value).

Part WTRQ ETRQ WHP BHP Notes
Block 400+ 400+ 400+ 400+ The block is a closed deck design made of Iron, it’s incredibly strong and have been known to take over 500BHP. The 1.8VVT Block is stronger.
Pistons 300~ 330~ 330-350 350-380 Keep EGTs down to ensure the pistons don’t melt as this is typically how they fail.
Rods 220-240 240-260 280-300 300-330 Rev limiter, high torque below 4krpm, knock and a weak oil film causes the rods to bend – be kind to them at low engine speeds and they’ll last longer than expected – however, this is like walking on a tight rope.
Oil Pump 280-300 300-330 320-360 350-400 Shock loading, rev limiter, boost cut, high rpm (7k+) causes the gears to shatter. A balanced assembly driven respectfully will live, again like walking on a tight rope.
Crank 400+ 400+ 400+ 400+ The cranks are incredibly strong considering the displacement, however starting with a straight and round crank is important, line boring with ARP main studs to ensure everything is straight. Shock loading/launching will kill them at higher levels especially without an upgraded damper.
Engine Damper 200-250 230-280 230-260 260-300 Technically the stock damper is overwhelmed under 200 WHP and should be replaced with an upgraded unit, doing this will ensure reliability to the crank, bearings, oil pump and the rest of the system by extension.
5 Speed Gearbox 250~ 260-270 270-300 300~ Launches, flat shift and more will kill these units. A combination of Lightweight Shockproof and MT90 can keep them alive for longer as the wide tolerances of the gearbox are taken up by the thicker lubricant. The theory is also that the heat from downpipes makes the case of the gearbox flex, heat management is key here.
6 Speed Gearbox 330-350 340-380 400+ 400+ Launches, flat shift and more will kill these units. A combination of Lightweight Shockproof and MT90 can keep them alive for longer as the wide tolerances of the gearbox are taken up by the thicker lubricant. The theory is also that the heat from downpipes makes the case of the gearbox flex, heat management is key here.

Using a conventional sprung clutch helps to soak up the shock and vibration.
VLSD (1.6 Diff) 180-200 200-220 200-220 250 These differentials use a 6” ring gear, they’re simply undersized for turbocharging and can’t cope with over doubling the torque capacity.

1.8 differentials bolt in place, using 1.8 prop shafts and half shafts.
TorSen Differential 300+ 300+ 400+ 400+ Quite uncommon to break a TorSen differential, certainly possible but far less heard off. Even more unlikely if you aren’t launching at the drag strip.
Clutch 140-160 180 180 200~ How new the clutch is, what it has previously been replaced with can all impact how much the stock unit will hold – the 1.8 clutches are larger than the 1.6s, the 1.6s will hold less than shown here.

 

That’s a rough guide to base your build around, its also aimed at the upper limits – not safe limits. A typical street oriented ‘200WHP’ MX5/Miata won’t require much in the way of anything too trick, a stock engine with upgraded injectors and a clutch for good measure will be happy at that power level all day long.

However, this gives us time to talk about the way torque curves work and how with ‘200WHP’ the car could be tuned in several ways to make the experience feel very different and at the same time offering very different levels of protection.

The goal here is to keep the torque peak at a reasonable level, allowing the boost to run away uncontrolled can lead to a torque peak at around 4.5k rpm far above the maximum you’d like to make – for example, with the right turbo without the boost being controlled properly, seeing 250-270ftlbs at the engine – or even more depending on the size of the compressor.

Unsafe MX5 Miata Power Torque Graph Example

Unsafe MX5 Miata Power Torque Graph Example


This spike causes the car to feel super fast, but also unrewarding. As it’s all over by 5k rpm with the torque falling off and the rate of acceleration falling off too.

A properly set up stock block engine should be running around 200-220ftlbs, ideally at the wheels or the hubs in a 1:1 gear removing any ambiguity.

If your turbo has enough airflow to maintain 220ftlbs all the way to 7000RPM you’ll be making 293WHP. The calculation for this is TORQUE X RPM / 5252. If you can’t maintain that level of torque to the end, and hopefully it doesn’t fall off a cliff – try and set your maximum torque to this level.

For example, if you produce 180ftlbs at 7krpm, but you’re able to make 200ftlbs at 6500rpm, that’d likely be your shift point and there is little to optimize. However, if your torque is falling off past 6000rpm then the car won’t reward you for revving it out, say it falls to 180ftlbs, achieve that number as soon as you can (say, 3500rpm or less) and hold it there all the way to redline – 180ftlbs at 7000rpm is 240WHP, see how a small amount of torque high in the rev range makes a HUGE difference to power output?

Example Safe Linear MX5 Miata Turbo Dyno Power Chart

Example Safe Linear MX5 Miata Turbo Dyno Power Chart

Note the angle of the horsepower curve, this is what makes the car feel fast as you build through the revs, the steeper the angle the faster the acceleration, or rather the rate of change from one moment to the next.

How to make a stock engine turbo MX5 / Miata reliable.

If you’re keeping the engine stock and not putting rods in it – not opening it up at all then you’re limiting yourself to the curve above, much more than 225ETRQ starts to become a little like walking on thin ice. However, if you have the car set up with Closed Loop boost control and a reasonable rev and boost limiter that’ll go a long way to keeping it alive in the winter months when the turbo will be far more efficient and will make more power / boost for the same throttle position when compared to the summer.

Having some form of oil pressure and temperature monitoring will go a long way to keeping the rotating assembly happy, we prefer to run Oil Coolers on our cars but this can cause issues of overcooling in the winter months if not shielded from airflow – cold oil is as bad, if not worse, than oil that’s too hot – running around 90-110c is the sweet spot.

Using a good quality oil with a high film strength to take the additional load is also important, we use and recommend the Millers CSF 10W-40 in most cases.

Ensuring your cooling system is up to the task and your injectors are also distributing fuel evenly to each cylinder will ensure that you don’t have one cylinder running leaner – potentially making considerably more power than the others, or running particularly high EGTs. All of which won’t be picked up by your Wideband sensor as it’s an average of all 4. We recommend coolant re-routes as they maximise the efficiency of whatever radiator you have installed and provides equal cooling to all cylinders.

Keeping the heat in the engine bay under control and the heat of the downpipe, there is a theory that the additional heat pouring onto the gearbox causes the case to expand contributing to premature failure. Wrapping your downpipe, especially if its a 3″ unit and the car is being used on track, is more than recommended.

We’ve found success using the redline lightweight shockproof and Mt90 (1 of each) in the gearbox and 1 Lightweight Shockproof in the differential to provide reliable shifting at all temperatures. Considering the tolerances of the gearbox being somewhat ‘tolerant’ the thicker nature of the lightweight seems to take up some of the slack, providing that cushion when power is applied violently.

Keep the RPM to the factory limit, 7-7.2k with the stock oil pump and damper. Engines making little over stock power have been known to explode the sintered oil pump gears when pushed much higher than this limit, notably 7800-8000 being more or less a death sentence. Our advice is to keep the RPM conservative while trying to stay shy of the limiter, as that creates considerable load on the gears and driveline in general. Replacement gears and Dampers are available to allow you to spin the engine to the moon with confidence.

Putting rods and pistons into the engine will allow you to shoot for whatever power level your turbo can put out, keeping in mind the fragility of the gearbox much past 330-350WTRQ.

Now the engine won’t be stock, but at this point, you’re basically limited by the turbo itself – many cars are running 450WHP+ on the street using 1.8VVT blocks and EFR6758s for example, with usable responsive powerbands too. The Mazda engine platform while capped at 1.8L unless bored / stroked supports an incredibly surprising range of power and with the right chassis supporting modifications it doesn’t have an issue putting it to the ground, take the V8 Swapped cars (monster miata, flyin’ miata) for example.

Hopefully that fills in some of the gaps, keep in mind  that your tuner requires a level of restraint compared with the numbers above, no one wants an engine to blow up, don’t be surprised if on a stock engine you’re capped around the 200WHP mark – as frankly, that’s the proven generally reliable all day number.

Happy boosting.

 

 

 

 

 

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