The ideal power plant for an off-roader is able to produce its power at low RPM. Engines that do this can be driven in higher gear ratios in difficult terrain which is advantageous because the higher the gear ratio, the less chance of wheel-spin and the more delicately the driver can control the engine’s power output. Engines designed with long piston strokes tend to do this.
Good off-road driving technique calls for selecting the right gear for the conditions. If the gear ratio selected is too high, a more powerful engine may still have the torque to get through, but if the gear selected is too low, a big engine could, if not handled skillfully, cause excessive wheel-spin. For a novice driver therefore, high power is often a disadvantage but for the experienced it can be an advantage. For long distance travel, larger engines are more reliable because they rev slower with the penalty of higher fuel consumption.
Petrol Vs diesel engines
It is never an easy choice, but should be. What do you expect from your 4×4? Are you towing? Are you spending lots of time on road and then a short time off it? And when you are off it, what kind of terrain will be tackled? Once you have answered these questions, and more, you should come to the conclusion that petrol is the way to go, but for one thing: fuel consumption. This is the only significant advantage diesels have over petrols. Diesels are more expensive to buy, more expensive to service, require more frequent maintenance, are slower (a general observation), are noisier than petrols, the fuel smells more, diesel is often more prone to being contaminated with water or other chemicals – the list goes on and on.
If you want to go fast, rev high and pull a trailer, then a diesel is definitely not for you, I don’t care how many kilowatts is produced. Diesels are more prone to overheating and do not like sustained high-rev driving in hot climates – if they did, race cars would have them. So, if you are a more sedentary driver happy that on hot summer hill climbs the engine may lose some of its pulling power and that while idling, the clatter is sometimes intrusive and that if you spill a bit of fuel while pouring out of a Jerry can and you don’t mind the stink, then go ahead, buy a diesel. The additional range of a diesel vehicle, due totally because it uses less of the stuff, is another advantage, which, again, is about fuel consumption. So to summarize, get a petrol and put up with the consumption knowing that because its cheaper to service, at the end of the day, the cost-to-run difference is not all that much.
However, when driving off road, these engines do have their advantages, and as you will see, again, petrol comes out on top. A large capacity petrol engine is a good choice off-road, even if its power output is similar to that of a smaller turbo-diesel. They perform a lot better in the rough but are much thirstier than diesels, especially if the going is slow, through thick sand or on rutted tracks.
Turbo chargers boost power after the engine has, on average, reached 1200 RPM or more. A petrol engine will work at much lower revs, down to 600 rpm in some cases. The difference here is a mere 600 rpm, which doesn’t sound much, but low rev power is a distinct advantage off-road. Turbo-charged vehicles often have to take a tricky climb one gear lower than their petrol counterparts, because at some point in the climb, the revs drop to below where the boost is working, and the engine stalls. Other terrain where small-capacity turbo-charged diesels often struggle is on dunes, where momentum cannot be maintained because of turbo-lag.
Some turbo-diesels are fitted with an intercooler, a radiator which cools the hot air pumped by the turbocharger, which itself is powered by hot exhaust gases, before it enters the combustion chambers. They often increase power output by over 20%.
Like petrol engines, diesels are controlled by micro-processors. This is good and bad. It improves reliability and tuning accuracy. It also means that power output can be increased by the simple matter of adding a computer ‘tuning chip’ such as the Dastek Unichip, a remarkable device. Another advantage of a turbo-charged engine is that altitude has less effect on performance than it has with a normally aspirated engine.
For operations in Third World countries, diesel engines are the better choice for the reason that local truck transport relies on diesel and it is available more often and in more places. In these countries, the fuel is frequently contaminated with dirt and water with the result that fuel related problems cause more breakdowns than any other factor. Ideally, dual fuel filter systems should be fitted. At the very least, spare fuel filters should be carried.
But if you want to be the king off road, nothing beats a big petrol.
Occasionally one gets a brilliant vehicle without a good choice of engines. This is the fantastic Land Cruiser 105. It was built with a thirsty 4,5-litre 6-cylinder petrol or an underpowered 4,2-litre 6-cylinder diesel. Apart from both engine's reliability, neither engine did justice to the chassis. Toyota never built it with the turbo-charged version of diesel - I can't imagine why not - as it would have made a truly amazing wagon. The choice between diesel and petrol is often a difficult one.
Engine modifications
The four-wheel driver’s vehicle has two kinds of life: on and off road. However, modifications to improve on-road performance may have detrimental effects on the vehicle’s off-road abilities. Vehicle manufacturers always strive to increase engine power without increasing the engine’s size or weight. One of the ways of doing this is to improve the engine’s capacity to breathe. Increasing the amount of air that can be consumed by an engine during the combustion cycle increases engine power. Fitting free-flow exhaust systems or grinding and smoothing inlet and exhaust valve ports will increase air flow.
Modifications to engine components to increase performance are many and varied. With modern engines, a lot can be achieved with electronics.
Every vehicle design is a compromise. The better it is on-road, The worse it is likely to be off it. Sometimes the more the vehicle costs means that any compromises made have been dealt with by adding features that push up the price. Compromises include sophistication, noise levels, clearance, seating, ride comfort and load-carrying ability. one of the very best compromises is the rare Merc-G class. It's a bit noisy on the open road, but that's because it has no carpets to get spoiled.
Chipping the engine
Almost all currently available diesel engines, whether in a tractor or race car, are controlled by a microchip. By adding another piggy-back computer to override the manufacturer’s settings more performance can be taken out of an engine. Some chips are so clever that they can be mapped in several configurations, such as limit the revs when your 18 year old takes it out, or extra power at cruise speeds, set when towing. These are just two examples. Each preset is sent to the chip using your cell phone. The chip I am referring to is the Dastek Unichip. I have seen it work and it is phenomenal.
A breakdown in a remote area is no fun if your vehicle is over-complicated and requires special tools to complete basic functions. Unfortunately modern vehicles are built this way. I can see a future when vehicles will have notice warnings on the bonnet "No User-serviceable parts inside".
Free-flow exhaust systems
Free-flow exhaust systems consist of big bore pipes and free-flow silencers and are worth considering.
The advantages of free-flow exhausts are numerous:
• They improve fuel economy and thereby increase a vehicle’s range.
• They improve acceleration without negatively affecting the power and torque output rev-range.
• In many cases they are less expensive than a genuine factory part.
Although not spectacular, individually these improvements are noticeable. For example, when fitted to a Land Rover V8, fuel consumption improved by about 1.5 liters per 100 kms. I calculated at the time that for a new free-flow exhaust system to pay for itself in fuel savings, I would need to travel over ninety thousand kilometers!
If your existing exhaust system is due for replacement I recommend investigating fitting one of these systems. It is important to make sure that there are several mounting points and that the job is done well. Exhaust failures are common in rough country.
Choose a vehicle with an engine that you will be comfortable with. For example, if you know more about repairing petrol engines, is it wise to drive a diesel vehicle and take it into remote areas?
