Sweet an innocent Stormy blue 1.8 NC gets a high powered 2.5…..

For sale now at: £7500.

Here at DuratecNC we love engines, engines is what we do best, engines are what people can’t do themselves as they don’t have the knowledge, nor the equipment. Some think they can, but then you drive past them on the straight, it blows up or it doesn’t meet expectations. We can do other things, but then hey, so can you, or at least, some of you can. You can scrape rust off bodyshells and paint them, fit roll over bars, coilovers, anti-roll bars, uprated brakes, wheels, polish bodywork etc. If you can’t, we’ll do that for you too, no problem.

So it made sense to me to put together a cheap car that had a highly modified, dyno proven great engine in it which you could buy, take it away and do what you wanted with it. The car could be test driven, bought, driven home. Developed further to make it into a proper track weapon or simply take the engine out and put it in your own car. Want just the engine? No problem, we’ll drop it out for you – take £1000 off the total price and it’s yours.

So here is such a car, it’s a bit more than a Star in a reasonably priced car.

It’s an 07 NC which originally had a 1.8 in it, 77600 on the clock. When the engine was changed the ECU and clocks were changed to those from a 2.0 and the mileage on those was 67200.

Underneath it’s solid, some surface rust here an there. The O/S cill and inner wheel arch area to the rear has been completely cut out and replaced (by me) so it’s been done properly. Painted and anti-stonechipped, wax injected in that area. Both cills were injected too. The rear and fronts of the cills had all surface rust ground off then coated with Rustbuster Epoxy (one of the best treatments around). It’ll be a few years before anything needs doing to the chassis.

Whilst the front subframe was off all the suspension bolts were removed, greased and put back in. Don’t underestimate the importance of this if it (the suspension) is to be overhauled at a later date as it’s a bugger of a job when the bolts are rusted in. It often includes the cutting off of suspension arms or complete removal of subframe and suspension. Not now, everything will come off like it did when new.

Rear sub is something else, never touched it so do with it what you will or pay us to sort it, but at least it’s easy to drop.

Exhaust is stock. It will need at least a manifold to bring the power back as it was mapped with an ILMotorsport one on. This is available by separate negotiation. A larger bore manifold (known as a 1.8″) is available elsewhere and would release a few more horses too.

Upper bodywork shows no rust apart from one rear arch which has a patch right at the top and a small part of the lip underneath has come off. Otherwise it’s what’s to be expected from a 2007 daily used car. Hood is fine, clean, no leaks.

Really the only bit of rust on the upper bodywork and an easy fix.

Suspension: Std dampers with H&R lowering springs.

Brakes work ok. Discs a bit worn, but calipers look they’ve had some attention or replacement recently, are all free and not seized.

Interior is stock, cloth seats, nothing much to report, but it does have some kind of Bluetooth fitted throughout. No leaks. Windblocker hinges have been snapped off, but if you want a replacement it’s no problem. Comes with a new and fully working oil pressure gauge, if you’re looking at that wondering what it means read the blog and the gauges for sale page.

So onto the exciting bit; the engine. It started off as a low mileage 2.5 which was stripped, cleaned and inspected and the following parts were thrown out: Pistons, conrods, bearings, balance shaft cartridge, sump.

Block bored with deckplate to suit new forged and lightened over-sized pistons, CR 12.5:1. New forged steel rods, DuratecNC St3 cams. Mild porting to head, valves and seats re-cut, lapped in. High lift, high RPM race valve springs.

All gaskets, seals, bearings replaced (including pulley bearings at front of engine) Crankshaft balanced and keyed, 550 injectors. Runs on 99 Ron, VVT is operational, fully and expertly remapped Mazda ECU.

Running bigger injectors gives you the headroom for other mods without finding bigger injectors. Putting both a bigger and completely different specification engine in with much larger injectors requires a calculation doing and applying to the ECU so it will run (this is in addition to the mapping which comes afterwards). This has been done which is why it drives at least as well as a standard road car.

A stock 1.8 makes 124 bhp and 123 ft/lbs. 2.0 produces 158bhp & 139 ft/lbs torque

This shows BHP @ the fly, WHP and torque in Nm

Output of this 2.5 is now at approx 100bhp and 82ft/lbs more than the car had originally at nearly 225bhp and 205 ft/lbs of torque:  Which is actually being held back by the original intake manifold. Stick some ITBs on to make approx 250 – 260 BHP (it’s dependent on how big the ITBs are) and you’ll have one of the quickest naturally aspirated NC in the UK. That’s more than a supercharged one! It’ll rev safely to 7500 and as it stands is devastatingly quick, 60 comes up in less than 6 seconds now yet it looks completely stock under the bonnet.

This is a bit odd as it shows BHP and torque @ the fly on the graph, but torque at the wheels is displayed top right. Red is power with the base map, green is after final mapping.

I built the engine with the intention of it being a full on track weapon which it is, but it’s also so easy to drive it would actually make a great road car. It’ll pootle along lazily at 1 – 2000 rpm in a high gear, but drop it down and it’ll pull hard until right off the end of the rev counter. It really does pull like a train, I really can’t say how much I am impressed with this engine and sure you will be too if you’re looking for a very capable N/A engine.

Comes with MOT until Dec 2021

Has not been declared to DVLA as an engine change as yet.

How the price breaks down:

Engine £6150

Car £1000

Removal and refitting of engines, mapping, development work, transport, welding, MOT: £2000

Total £9000

Total asking price £7500

Red trace is base map power, green trace is final WHP (187) the other two traces with dots are AFR Torque ATW top right (168)

So do with it what you will, improve the rest of the car to suit the engine or simply take the engine out and put it in another car, put your old engine back in this and sell it on. If you want the engine taking out and sending then there will be an extra charge for this.
This engine would benefit massively from ITBs as the original manifold is holding it back, estimate another 30 bhp on top of what it already has.

Because of our honesty don’t expect to turn up to haggle much as you’ll find you have no reason to, it is what it is.
Don’t just join in a track day to keep up with all the other MX-5s, set out to rub their noses into the tarmac and laugh like a lunatic 🙂

Want an engine to this exact spec supplied and built? £6150 including map. 

Associated Blog entries linked to this article:

More of a technical overview of what was done to the engine: http://www.duratecnc.co.uk/?p=823

Why an N/A?  http://www.duratecnc.co.uk/?p=214

Don’t know anything about the MX5 NC? http://www.duratecnc.co.uk/?p=52
Technical aspect of the 2.5: http://www.duratecnc.co.uk/?p=180
Cheap trackday car, why? http://www.duratecnc.co.uk/?p=369

Technical: Engines; why and what we do to them.

Quality, you get what you pay for. Sorry, not the most exciting of posts, but if you’re thinking of buying an engine from here then…

This is about why and what goes into a full professional build. Here is an explanation, starting at the top of the engine and working our way down.

Dipsticks. There is a lot of confusion over dipsticks, not only on stock 2.0 (see appropriate blog), but even more so when a swap is done. I either use a 2.3, 2.5 or a 2.0 cam cover, whichever I have an excess of or is easiest/best condition. The length of the new dipstick/camcover combo is checked against the original so the max level remains correct. Simple isn’t it? Yes. I don’t know why so many get their pants tied in a knot over it. If you buy an engine from me you can be sure it was considered and something done about it, no guesswork was involved or asking people on FB and getting 72 different replies, none of which can be substantiated. Or even worse, running low on oil on track!


If the engine is built with high power in mind then it really does pay to upgrade to larger injectors early on. What I mean by that is the originals will cope ok up to about 230bhp, but will be flat out at that with no room for power related upgrades (such as ITBs). We generally fit new Bosch 550s.

This is mainly due to the fact we are using the factory ECU. You need to tell the ECU that the engine size has gone up and also the injectors, once you’ve done that it’ll perform much better (the engine will act like a standard one at low rpm, idle, cruise etc), you need certain Ecutek permissions to do this so it will already have been done when you buy and engine & ECU combination from us.

Cylinder head.

Is completely stripped, cleaned and assessed. I have the tools and knowledge to check for wear in the various moving parts. If new valves aren’t fitted then the old ones are always cleaned up as new, seat face recut and an extra angle put on to aid flow.

What is done to the ports is obviously down to the spec of the whole build, but again I have the wherewithall to improve the flow quite easily so quite often a mild job is enough. Valve seats are always recut. EGR passageways are blocked off.

When cams are fitted I never rely on the slots at the end and always use a dial gauge to check lift at TDC. You really need to have some figures to work from and can never trust with 100% the cam manufacturer got it right.

Pistons. Pretty rare I use the orginals unless asked to do so or on a budget build. Mainly because the compression ratio is too low. Whilst putting a bigger cam in a std CR engine will see some kind of positive result, matching a big cam with high CR is a marriage made in heaven. This isn’t the time nor place to go into this extensively as it’s really an item all of it’s own, but doing it the proper way will not only make more power and torque (at every single RPM) it’ll also see better MPG.

Rods. For the price you can get a decent set of forged ones for it just is not worth using the originals. The engine is in bits in front of me so the labour factor is zero.

Crank: Gets cleaned, polished and oilways modified for improved oiling. The jury is out on the effects of removing the BS ring gear. The pulleys on an MZR/Duratec engine are of the floating variety, that is they have no keyways to locate them, you can spin them round and only the end bolt and often a diamond washer hold them from moving. This is ok for the cam sprockets, but the pulley on the crankshaft is known to move under duress. We therefore key the pulley to the crank to stop this. It also means taking it off (especially whilst in the car) is a lot easier. If it does move pistons will hit valves

The block gets a cleanand check like everything else and any unwanted holes blocked up. You can buy kits to do this, but as I have the equipment in the shop to do it quickly and properly there would be no point to buy something which is aimed at a DIYer with no tools.Some time and money saved there.

The bores are often re-bored and honed, if they are then a deckplate is used. This allows me to run the piston to bore clearance really tight. Less piston rock, no piston slap (a knocking noise when cold) and more power, less wear due to components fitting together properly.

The tumble flaps are removed and the housing gently blended in internally to aid air flow.

I think that about sums up a full build, it’s always good to know what you’re getting for your money.

Dynos and air filter testing

This has always been a hot topic with any marque of car, not least of all the MX5 market.

The other day we did a panel air filter test. By this I mean we swapped out just the filter element itself inside the original airbox. I’m going to call the original style of filter ‘Paper’ I’m not sure they are made of paper these days looking at them, but we’ll stick with that.

A little bit about chassis dynos

A chassis dyno is basically a tuning tool, it’s for tuning or remapping cars. You cannot take a car from one dyno to another and expect the same figure, end of story. Some dynos don’t even give the same power/curve shape as others, nothing like on some! You’ll see in the pics below that the same car run over and over again gave a different figure everytime. That’s because things are constantly changing within the car. The first run is always the lowest, the second will make more power after that it can be up and down and all over. The reason for this is that many items are heating up and cooling down. Friction (and also intake air if it’s a turbo car) therefore changes throughout. Changes which alter the power output (due to temperature) are tyres, diff (oil), gearbox (oil), driveshaft CV joints (grease), intercooler on a turbo car and of course engine temps – the temperature of the actual parts, the oil and the water and the intake air perhaps. Some things get more efficient as they get warmer, some not, all have a peak operating temperature which you can not reach, or exceed.

Did you know (all things being equal) that an engine with a plastic intake manifold will make more power than one with an aluminium one? It will and the reason is plastic doesn’t conduct heat very well so it doesn’t get hot from being bolted to the engine, therefore it doesn’t transfer unwanted heat to the intake air.

If the Dyno has ‘calculated’ power at the flywheel for the graph then things get even worse! Dynos guesstimate (sorry for our overseas readers there) the power at the flywheel using the figure taken from the wheels. They all use a different method to do this so whilst the Wheel HorsePower figure will be a little different from one dyno to another, the Flywheel Horse Power will be even less accurate, often some 5 to 10%. The Americans are better at this as they usually quote WHP not FWHP. The only issue with that is an engine will show a different figure at the wheels depending on whether it is FWD, RWD or 4WD, but then as we learned from above, you wouldn’t sensibly compare them anyhow.

The reason for that ^ little diversion is that you have to be very careful of what you read when people post up dyno results. If they’re from different dynos, or even the same dyno, but with some time lapsed between runs the figures will be less than accurate. Cheap bolt-ons like air filters show very small results anyhow so these results can be mixed up with, or even overshadowed by other changes which are happening all the time .

I have never tested an MX5 with one of those snorkel type replacement air intakes, you know the type. The airbox is completely removed and a length of pipe put in place leading to the front area of the car with a filter on the end. I am extremely suspicious of the figures bandied around as they seem to rewrite the laws of physics. Now if you were to do a test on such a type you would need to do what we did and do multiple runs, all one straight after the other. Therein lies a potential issue with testing these type, you would have to have pre-rehearsed how to change from one to another so you could do it really quickly. If a lot of time had gone by whilst you undid everything then some accuracy will be lost. I know that actually fitting them requires taking the front off the car to get it in.

You will have seen these K&N panel filter copies for sale here and there, mainly on FB. The vendors claim a power increase. Someone also incorrectly independently tested one. They took out a USED paper filter and replaced it with a NEW ‘uprated’ panel filter. They did one run and it showed a gain. Well no surprise, taking a dirty filter out and replacing it with a clean one gave a better result.

So anyway, onto our little test. We aquired one of these things and a brand new paper one too.

The car was a 2006 with a BBR 200 in it (for sale if you want it), power output as you can see was hovering around 200bhp.

6 runs were done overall, the first three were with the aftermarket filter in (Filter 1). The car was then stopped for no longer than a few short minutes whilst the paper one (Filter 2) was put in as quickly as possible. 3 more runs were done.

An average was then taken of the 3 runs to give wheel horsepower figures:

Paper averaged out at 149 WHP

Aftermarket 143.8 WHP

So an average loss of 5.2 WHP torque similarly lost 5.2 Nm

With a paper filter the car more or less matched BBRs figures.

Previous to that test we did another on a different dyno, it showed a loss too, It was done the other way around, paper in first then aftermarket after that. I honestly didn’t believe it hence us doing it again on a different dyno.

Don’t believe any of this tripe about the engine learning how to cope with this new found airflow, if there is more air available it’ll show as a power increase immediately. We don’t remap cars and wait a week for it to catch up and show a power increase.

I would guess that (or I would hope!) the aftermarket may outflow the paper after both have done 10 thousand miles or so and got some dirt in them. That would be a difficult test to do accurately. Mind you, with the size of the holes in an aftermarket filter it’ll let a lot of dirt through in the meantime.

Looking through a genuine K&N into the sun.
Looking through a paper filter into the sun.