The answer is simply this; for maximum and effortless performance on the road turbo it, on track N/A is the way to go.
That is the general rule as I see it, it’s black and white, but there is some grey in between and of course rules are there to be broken. Let’s break that blanket statement down.
I haven’t included supercharging in this article as basically I have very little experience with them, but some, if not all of the downfalls could be similar to turbocharging apart from fuel economy, a SC engine will be worse than a turbo and obviously an NA too in every day driving from an economy point of view, they drink fuel at a seriously rapid rate. It’s belt driven, so the engine has to be turning this thing over all the time, it takes energy (from fuel) to do this. If you’ve never tried to turn over a Supercharger over by hand you’ll be quite shocked when you do. From a packaging point of view the supercharger lends itself quite well and at least here in the UK you don’t tend to hear many bad experiences of SC packages.
Before we go too far let me tell you this; I’m not some old geezer who thinks N/A rules because it’s all i’ve ever known. Absolutely not, I was brought up on a diet of turbo engines, they’re like an old friend and i’ve built many from 300 to 600+ bhp so I know them inside out, I still own some too. You can’t beat that feeling as the turbo spools.
A turbo NC on the road is a great thing, that huge gob of mid range torque when you need it is not just useful, it’s addictive. On a decent stretch of road, drop a cog and boot it, take one, two or three slower cars in one shot with ease. Yet when you’re cruising down the motorway off-boost it’s reasonably economical and will pass an MOT emissions test, best of both worlds? Yes pretty much so, it’s like having two different engine modes.
Many turbo cars come undone when on track though, the reasons are:
The heat created is constant and cumulative, not occasional like it is on the road. When you’re pushing hard there everything gets heated up, but then gets chance to cool off, there simply is no chance whatsoever of holding a turbo car at full throttle everywhere unless at the track. The more power you put through an unmodified engine the more reliability issues you will have.
The intercooler blocks some or all of the flow to the radiator so the engine can’t cool down enough (as does the aircon rad if fitted), there is either no, or a very small oil cooler (as standard), the intercooler itself is often too small to enable it to fit into the nose of the car. The NC1 actually had two intakes at the front (upper and lower) whilst the late models only had one so have much less cooling area available. The intercooler on these models actually blocks the airflow to the rad completely.
The turbo is mounted high up which introduces heat into the engine bay and spoils the handling of the car due to a high centre of gravity, along with the substantial weight of the entire turbo kit*. Could you mount it lower? Well the steering column is in the way of a decent internally wastegated turbo and the manifold runners made from welded stainless steel are likely to distort and eventually crack. This is why left hand drive cars lend themselves to turbo conversions a little better.
I’ve seen a few turbo conversions and once you scratch beneath the glossy surface you’d be surprised how shoddy some are, if you buy one with a view to taking it on track you need to look at it as a probable project and be prepared to repair and improve the worst bits. The result of this will be that it starts to change from a road car to a track car.
*Weight, how much extra weight? When Jota were asked to prepare two cars for racing they produced an N/A at 850kg and a turbo at 1000kg. An extra 150kg (23.6 stones) in the front of a car is a heck of a lot. Not only do you have the weight of the turbo kit, but often also a heavier thicker radiator filled with more water. What was the original design criteria for the MX5? It had to be light and handle well with a 50/50 weight distribution, stick all that weight up front and it won’t that’s for sure. You won’t be going through the bends as quick, will need bigger brakes to stop and use more fuel, wear the tyres and brake pads out quicker.
The gearbox will only cope with a certain amount of torque before it starts to give major issues, (the later they are, the stronger they got however) although it has to be said that on the usual (N/A engines running turbo) low power outputs of our UK mainstream ones with an average 7″ wide wheel they seem to be coping ok, gearbox strength will only be an issue with really high outputs, aggressive and track use.
Thinking of buying a force fed car for track use? Ask the creator if they’ll warranty it for that, if they don’t, well…..
You can of course create your own turbo track car, but finding a way around all those issues is a big job taking a lot of dedication, time, money and patience so really you have to decide if it’s worth it. Personally I didn’t, life is too short and the older you get, the more you realise it, but hats off to those who do eventually make it work.
So a high powered N/A car for the road? Well the more power it gets, the worse the road manners get. Turbos are all about mid-range torque, N/As need to be revved out to make the best of them. It’s ok if you want to drive to the shops with your mum at between 3 and 8500rpm and sit outside with it ticking over at 1500, bellowing out 105Db via the performance exhaust, but um well, not really. You’ll lose some power at the bottom end and gain mid-range and at the top end of the rev range. The VVT will help here, also we can fit a bigger CC engine such as a long stroke 2.3 or 2.5 which will also help offset the loss of low down power. The stock 2 litre is what we call ‘over square’, whereby the bore diameter is less than the stroke. This means it’s lacking in torque compared to the others, but able to make good power at higher rpm than they will go to.
On track you will want something with a wide power band that will pull hard from mid range to the redline, something light, uncomplicated and reliable.
An N/A car is a lot easier to drive fast on the track than a turbo, especially for an amateur. The reason is down to the power delivery as it’s much more linear and controllable. When the turbo spools and a whole lot of torque goes to the rear wheels mid bend it will cause the car to go sideways very quickly. Admittedly this can be just as fun to some people as it is frightening to others!
This is where Duratec NC comes into it. Here we have a range of N/A options, you can have a mildly or heavily modded 2, 2.3 or 2.5 engine depending on what your requirements are. You just have to realise you can’t have everything, you either have a car which copes on track and road to a certain degree of success or one which does either of them exceptionally well. That’s the choice you have to make at the start of a project and stick with it.