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Take great care when making alterations to modern 4WDs.


Modifying a recent-model 4WD isn’t the simple operation it was with previous-generation vehicles. Electronics confer greater safety and efficiency, but changes to standard specifications can upset electronics’ programming and behaviour.


When the 4WD modification business began in the 1970s vehicles were basic and relatively slow, so no-one in authority cared very much what people did to improve the performance and off-road ability of their machines. Now that many 4WDs are sophisticated, high-performance vehicles, things have changed.

You need to modify with great care and, fortunately, there’s a document that can guide you.

In early 2006 a draft National Code of Practice (NCOP) for Light Vehicle Construction and Modification was released. Since then, it acquired status as a Vehicle Standards Bulletin, VSB14, which brings its contents closer to becoming legislation. However, no State or Territory has adopted it fully yet.

In the meantime Australian Automotive Aftermarket Association’s 4WD Industry Council is lobbying to promote national adoption, after further industry input.

(See also our story on modification legalities)


Vehicle lifts


By far the most critical area of modification to modern 4WDs is in the suspension, wheel and tyre area. VSB14 allows the rolling diameter of any tyre fitted to an off-road passenger vehicle or a commercial vehicle to be 50mm larger than any tyre designated by the vehicle manufacturer for that model. However, VSB14 allows a suspension-plus-tyre upgrade with a combined lift of only 50mm in total and it specifically excludes modifications to any vehicle that’s equipped with electronic stability control (ESC). Nearly every new 4WD comes with ESC.

The Australian 4WD Industry Council believes a 75mm overall lift – a 50mm suspension lift plus a 25mm rolling radius tyre lift – is a safe upgrade for most 4WDs and has lobbied lawmakers to have this accepted.

In the meantime, most police and traffic authorities are turning a ‘blind eye’ to modern, ESC-equipped 4WDs that have a total 50mm lift over standard – that’s a combination of slightly larger-diameter tyres, plus a suspension height increase.

Vehicle modifiers have told OTA that they believe a 50mm lift has little or no effect on ESC system operation and some have done testing on standard and modified vehicles to prove this. ARB has video recordings of vehicle testing that the company says illustrates correct ESC action after suspension modification.


Replacement Tyres

Australian Design Rule 24 requires that all vehicles under 4.5 tonnes gross mass rating be fitted with a tyre placard that contains information on original and optional tyres and rims for that vehicle model.

A motor vehicle which is required to comply with ADR 24 may be equipped with tyres other than those listed on the tyre placard provided that: the load rating of the tyres is not less than the lowest load rating listed on the tyre placard of the vehicle or equivalent variant of that model vehicle; the speed rating of the tyres fitted to a passenger vehicle or soft-roader is at least 180 km/h (‘S’) when the tyre placard requires a higher speed
rating than ‘S’; the speed rating of the tyres fitted to vehicles with special features for off-road use of at least 140 km/h (‘N’) when the tyre placard requires a higher speed rating than ‘N’.

In special circumstances, the speed rating may be less than the ratings specified above if the speed rating of the tyre is more than the vehicle’s maximum speed.

Where a vehicle has its GVM re-rated, the tyre load capacity must be capable of the carrying the revised GVM, both in total and across individual axles.


Non-Standard Tyres and Rims

Replacement aluminium alloy rims must comply with one of the following standards: Wheel Industries Association (Australia) (WIA); Standards Association of Australia (SAA); Technischer Uberwachugen Verein (TUV); Japanese Industrial Standards (JIS).

The wheels must be contained within the mudguards, including ‘flares’, when the wheels are in the straight-ahead position.

The wheel track of 4WDs must not be increased by more than 25mm beyond the maximum specified by the vehicle manufacturer for the particular model, except for vehicles fitted with front and rear beam axles, where a maximum wheel track increase of 50mm is allowed.

Paradoxically, the fully-floating rear axle wheel bearings of the 76, 78 and 79 Series look more than capable of carrying the additional load inputs from spacer-mounted wheels, but current and future law forbids the use of wheel spacers to make the rear track match the front.


GVM increases

As modern 4WDs have become heavier – particularly large wagons – their payload capacity has decreased. The typical large wagon used to have a payload capacity of up to one tonne, but that’s diminished as equipment levels have gone up, dropping payload to around 500-600kg in most cases.

This situation has prompted some after-market modifiers to come up with packages to increase the gross mass rating of some vehicles, above the manufacturers’ GVM ratings.

Provided this situation isn’t abused by owners anxious to go even higher in payload there shouldn’t be any issues, but it does put owners of such vehicles in a grey area should a warranty or insurance claim situation arise.

We’ve seen 4WD makers deny warranty claims on vehicles that have been modified and taken ‘outback’. We’ve also heard of signs of sand and mud around chassis and mechanical components called ‘driver abuse’ by 4WD dealers.

On the subject of GVM, it’s important to note that any weight the caravan or camper imposes on the towbar is part of the payload. It’s possible to overload the vehicle’s back axle if this isn’t taken into account and police and road authorities are taking a lot more notice of overloaded 4WDs and vans these days.


Performance upgrades

Electronically-injected 4WDs are designed to comply with emission ADRs; to balance performance with powertrain life and to give optimum economy. It’s a delicate balancing act that costs 4WD manufacturers a great deal of R&D time and money.

Because fuel delivery is governed by an electronic control unit (ECU) it’s not too difficult for after-market specialists to come up with performance enhancements by altering the original fuel ‘map’. I’ve asked several of these modifiers to confirm that compliance with emission ADRs isn’t affected, but I’ve had no written reply to that effect.

A ‘chipped’ 4WD can have considerably more performance than a stock vehicle, but be aware that the original powertrain warranty is void. Some vehicle computers also have a ‘trace’ that indicates the one-time presence of an engine enhancement ‘chip’ even if it has been removed.

Upsized exhausts are popular and there’s no doubt they can make a difference to even a standard engine. Improved gas flow is almost imperative if a chipped engine is to gain maximum benefit.


Filtration issues

Filtering out grit from engine air, oil and fuel has always been vitally important, but fuel filtration is even more critical with today’s high-pressure injection systems.

Be careful if you decide to use an after-market filter in place of genuine, because there are many cheap look-alikes that just don’t do the job well enough. Also be aware that there are cheap rip-offs in what look like genuine parts boxes.

The modern 4WD’s fuel system is calibrated for delivery pressure to the main fuel pump and excessive restriction – caused by an inappropriate, faulty or blocked filter – can damage expensive fuel system components. Putting an additional filter in the line can also cause back-pressure issues.

Very few genuine and aftermarket fuel filters can actually block water in more than small quantities, so if there’s a risk of getting watery fuel it’s best to fit a centrifugal water-stop – a housing containing a ‘spinner’ that flings water and particles to its outer wall, where they drop into a bowl. The lighter fuel continues through the unit to the original fuel filter.

Modern turbocharged engines with extended oil-drain intervals need the right oil and the right filter. Contaminated oil won’t lube turbocharger bearings adequately.


Dual battery systems

The traditional voltage-sensitive relay that has worked well in charging an auxiliary battery, while isolating the starting battery from unwanted discharge, just isn’t up to the task in the case of many electronically-equipped 4WDs. Many modern 4WDs have variable-voltage alternators that are controlled by the engine ECU to vary alternator load in the interests of improved fuel economy. A simple voltage-sensitive relay won’t charge the auxiliary battery in sch an system and what’s needed is a specifically-designed battery charger that overcomes the intermittent and low voltages generated by some modern


As we said at the beginning: modify with great care.




























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