ALTERNATIVE ENGINE TUNING

Forthcoming, and indeed existing, emission control regulations are making it more and more difficult for people to run modified engines on the road. Sometime ago, I said that I would be looking at alternative ways to make a more interesting road car to the normal route - bigger everything (pistons, carbs, cams, etc.). I must confess to being a supporter of milder states of tune for road use cars for sometime. I have done the hairy beast and fire-breathing monster motors and short stroke screaming banshee thing in the fourteen years I've been messing around with Mini's of all sorts. These incredibly rapid variations were fun for about a week - but soon became a complete pain when used everyday. Horrendous lumpy idle speeds, cooling problems in traffic, driving at slow traffic speeds, fuel consumption, etc. all blighted the desire to have a stupendously quick Mini for every day use. I soon found that a much more mildly tuned car more fun - apart from the drastic difference in servicing required!

Most Mini or Metro owners that want more speed from their bollide have read the propriety books on the subject, the "big yellow tome" being by far and away the most popular. However, such books are really only interested in maximum horsepower, and the injection of large amounts of beer-vouchers. Claims are made that this multi-mega horsepower endowed tyre stomping beast can give 35 mpg. What isn't made clear is that this was only achieved by several weeks of dynoing to get EXACTLY the right fuel mixture at all load conditions. At a current price of £300 a day dyno time - the likes of you and I could not possibly afford this. The noise generated by such a conversion is nothing short of awful for everyday use. And it's not just induction roar either. Consequently I intend to show what is achievable on a sensible (read-minimal) budget to provide a reasonably quick, useable engine that can be relatively economical (dependent on weight of right foot) yet remain civil and unobtrusive.

One of the most over-looked, yet most important aspects of everyday vehicle usage is establishing just what rpm band you use. You are more likely lo spend most of your time in the 2000 to 5000 bracket than in the 4000 to 7000 bracket - where most monster motors are developing their usable power. Consequently an engine developing maximum horsepower at 5000 to 5500 rpm is far better than one developing it at 7000 to 7500 rpm. And when you get down it, the actual difference in maximum power outputs are not going to be THAT different.

I won't cover the 998 based engines initially, although much of what is used will apply - but will appear to give a slightly "hotter" engine. Having said that, the effect will not be as marked as when using the hotter cams necessary to develop maximum horse power from an over-bored 1275. So the starting point will be the 1275 engine as this is the most commonly used engine to boost power output over the 998cc engine - being a direct swap in most instances.

I would like to establish that all results are the findings of driving my car every day in all traffic conditions and a variety of road types. Actual power figures are (as always) not gospel. Different readings can be obtained from different dynos - be they engine or chassis dynos. To this end I will be quoting figures ascertained from the Marshalls of Cambridge rolling road facility run by the venerable Peter Baldwin. From experience, it isn't always figures that impress, but the way the car feels and drives generally - so this will also be an aspect of the reports. The figures will be horse power readings, conversion to torque is possible but not particularly accurate - so I won't cloud the issue.

Apart from the Metro Turbo, the MG Metro is the most developed A series engine from the factory and is normally aspirated. Using this as a bench mark, a good one on Peter's rollers gives 70 bhp at 5500 to 5750 rpm. My MG Metro in standard trim gave an average of 40 mpg, a worst of 32 and a best of 44 mpg (very careful driving!). The MG Metro has a special concession to get it through MOT's, so we will look to better this if possible.

The MG engine comes factory fitted with alloy inlet manifold that is pretty efficient, and a cast iron "LCB" type exhaust manifold. Now this exhaust manifold performs pretty well, and helps keep the noise level down. Steel LCB's are OK, but "ping" with exhaust pulsing and are therefore more noisy. It is possible to fit these to a Mini (the latest ones have them standard), Mini Spares sell a pair of downpipes to fit this to the RC40 system. In the event that the 1275 engine does not have any manifolding, use the Mini Spares inlet manifold C-AHT770 (even more efficient than the MG one) and an LCB exhaust manifold. This should be coupled to an RC40 exhaust system (RC50 for the stainless steel option) for maximum potential without loads of noise or booming. In the further interests of civility, the standard air cleaner set-up is retained using a K & N replacement filter to minimise induction roar. Power suffers very little, and makes mixture connections much easier. Full K & N pancake or sports filters cause all kinds of fuelling problems - not insurmountable, but .generally cured by compromises unless you have a fortune to spend on an extensive rolling road session and a number of modified and discarded needles. If possible use an HI F6 1.75" SU as fitted to 1300 Metros. It is a very efficient carb, and has the capacity to deal with engines developing up to around 120 bhp so is ample for our needs. If a 1.5" SU is used, it is not a drastic problem, but will be lagging at the top end.

The first move was to look at replacing the camshaft. After all, an engine's power output is largely governed by the cam and cylinder head, and the MG Metro head is capable of doing the job initially. It has 1.401 " inlets and 1.15" exhausts standard, and flows sufficiently to give in the region of 90 bhp. This is on the basis that around 1 CFM of induction flow is needed for each 1 HP required. The restriction is usually the inlet manifold and carb to a certain point. Rather than put in what people construe as a fast road cam, I opted to start at the bottom and work up. Properly modified heads cost money and will not reap as much benefit when fitted on their own. Fitting a head once the engine is in the car is easy, but a cam change is a nightmare.

An M0256 cam was installed with new followers, and timed in at Kent's recommended 106°. I took advantage here of fitting a set of our 1.5 roller tip rockers. As I intend to alter their specification slightly, some long term testing was needed. Apart from that no other changes were made, and after a short breaking-in period for the cam, it was off to the rollers again. The results were as follows:

that this was definitely a big improvement, and a good start.

The next stage was to try an M0266 for direct comparison. Unfortunately a friend of mine sowed a seed in my mind regarding con rod lengths. My curiosity got the better of me here. I got a crank ground down to 1.625" big ends, and installed a set of 970 'S' rods. These are .125" longer than standard, so I machined the tops of the pistons to bring them .010" down the bore and increased the dish size to bring compression ratio down a bit to that of the standard MG Metro. I had heard all sorts of reasons for using longer rods, so I needed to find out what effect they had in an A series engine. I know what they do in a full race engine when correctly ratioed - but what about a road car?

This coincided with the arrival of my five speed gearbox, so they both went in together. Five hundred miles later, back to Peter's rollers.

The difference over the standard output is quite large considering the financial input. The extra power was immediately noticeable of course, but it was also far better to drive, pulled much smoother, very good over-taking performance in top gear, and in fact idled better too. When you consider what I said earlier about where you spend most of your time driving in rpm terms, there is a very good power band available peaking at just at the right point. Cruising at 3500 rpm equates to around 70 mph, with an extra 14 bhp available within 1000 rpm overtaking is made relatively relaxed. OK, maximum power is not barn-storming, but when compared to a full race 1275 with all the right bits on it on Peter's rollers there is only a 28 bhp difference.

Fuel consumption hardly altered. Not surprising as a needle change wasn't necessary - the standard BOL needle of the MG Metro being very versatile. The worst went down to 30 mpg on account of being able to drive the car much, erm, more "progressively". Average was 42 mpg and best still 44 mpg, but not quite so carefully driven. The overall impression was

NOTE: All power readings were taken in 4th gear. The results were much as predicted, but the outstanding difference is how much the nature of the engine had changed. It was more willing to rev, ran smoother, quieter, idled nicely, pulled well with even further improved "overtaking" performance. Somewhat surprising when you compare the power figures, as it had less power until 5500 rpm! This is one of those situations where driving impressions bely the actual power outputs. Once it hits 2400 rpm it really gets going, pulling strongly all the way to 6000 rpm. Very pleasurable to drive with a broader top end power band.

Once again the needle did not need changing, although it was running ever so slightly leaner than the last engine, but not where it would cause a problem. Fuel consumption just using the four gears was monitored over several days. Worst went up to 33 mpg, average 43 mpg and best up to 47 mpg. In the next issue I will cover fuel consumption using the five speed box to maximum effect and look more closely at the reasons behind using the longer rods and why they made the differences they did.

Keith Calver