I was there : Tickford’s M-Series turbo development

Few people realise it, but there’s a great deal of history between Tickford and Rover, especially when it comes to engine development. As the Technical Manager – Engines at Tickford, it was my job to work on these and liaise with Longbridge to ensure we were delivering what the company required from us.

One of our most satisfying projects was the creation of a new, sporting version of the Rover 800 during 1990. The rules concerning the equivalent amount of taxable income that is represented by a company car have changed since the system was introduced. In the late 1980s the amount of benefit in kind (BIK) was based on the swept volume of the engine and it seems there must have been some break point at 2.0-litres.

Even if this was not the case in the UK then it may have been in other countries, it certainly was in Italy. Around this time Rover’s Marketing Department sought to take advantage of the then current rules by providing a turbocharged 2.0-litre version of the Rover 800 as an alternative to the Rover 825 which was fitted with the 2.5-litre Honda V6 engine.

The Marketing Department wanted the 2.0-litre turbo vehicle as quickly as possible but the mainstream engineering groups with Austin Rover, as it was at the time, were already very busy working on future developments, including a turbocharged version of the 2.0-litre T-Series engine which was derived from but superseded the M16 engine. The solution was to outsource the engineering and thus Tickford at Milton Keynes were contracted to do so.

Rover calls in Tickford

The mainstream engineering teams were busy with their own developments aimed at later introductions and, although not unfriendly, could only afford to provide a small amount of occasional advice. Nearly the whole of the modification of the M16 into the M16T was carried out by Tickford.

The speed at which the car had to be developed meant that nearly all the different components had to be in production for other vehicles or easily adapted from them with minimal change to the production tooling.

To suit turbocharging the compression ratio was lowered from its standard of around 10 to a conservative 8.0:1 by obtaining suitable pistons from Mahle. These were made from the same blanks as used for the Montego Turbo pistons but with different machining.

Delivering the torque…

Some early baseline tests in this lower CR configuration, but without a turbocharger, showed an increase in torque at low engine speeds as the ignition did not have to be retarded to avoid detonation. A fairly suitable turbocharger, made up of standard parts, came from Garrett and it was only much later that Tickford learned that a better-matched unit was being supplied to Rover Engines for the forthcoming T16 but that Garrett had been told not to supply it to Tickford so that the T16 could be better when it was released.

The turbine housing was designed for some other vehicle and the only option was to accept it as it was. The outlet had to be coupled to the exhaust downpipe with a cast elbow but there were continual problems with thermal expansion that stretched the three bolts which then lost their tension.

A test with special temperature sensitive exhaust valves showed that sodium filled valves were necessary, so they were specified. Some modifications were made to the crankcase ventilation system so that there was the required depression applied, even when on boost with a new depression limiting valve to prevent excessive vacuum in the crankcase.

Making it different to the Montego Turbo

The Montego Turbo had its turbocharger installed with the shaft axis inclined. This inevitably results in a higher axial load and requires a thrust bearing with more friction. Garrett never liked this compromise as it was driven by the installation so the exhaust manifold that Tickford designed for the M16T mounted the turbocharger shaft horizontally.

The manifold coupled cylinders one and four together and also two and three together before all four flows joined just upstream of the joint with the turbine housing. This method of keeping the exhaust pulses separate had been shown to give better low speed boost and faster response than a simple log manifold.

It was cast by Duport Harper Foundries in D5, a cast iron with 35% Nickel content to resist the expected temperature. Unfortunately, back in those days, thermo-mechanical finite element analysis was not so competent and available to analyse the stresses and fatigue caused by thermal cycling and exhaust manifold cracks occurred on some vehicles after a few years’ service.

Cooling solutions

The central bearing housing of the turbocharger needs to be fed with pressurised engine oil and, unfortunately, the turbocharger was on the opposite side to the only available feed point, the oil pressure switch. Tickford tried to avoid having a long oil pipe running round the end of the engine and schemed a modified oil pump in which a boss was added to supply oil via a very short pipe on the exhaust side to feed the turbo.

Sand cast prototypes were made and Hobourn-SU (as it was called then) who made the standard pump, were exceptionally helpful in trying to machine the sand cast prototypes in the evenings when the production line was closed for the night. However, in the end, it was too complicated and expensive to have a special pump, so it reverted to a long turbo oil supply pipe.

The oil drain from the turbo was achieved with a special sump that had a rigid elbow welded into the side and was coupled with a short length of special rubber hose.

During the development phase it became apparent that an oil cooler was desirable, and provision was made by using parts that had already been designed for the Montego Turbo.

Oil concerns

In an ideal system the oil should not flow through the cooler until it reaches a defined temperature, or possibly a reduced viscosity. This implies that some sort of thermostatic or pressure relief valve should be incorporated. This must have been too complicated or expensive for the Montego Turbo because the only feature to control oil cooler flow was a fixed washer placed in the pump unit as a partial restrictor for the flow to the engine.

The oil cooler was deleted from the programme at a very late stage when it was realised that to provide sufficient air flow would mean the deletion of the extra lamps beneath the front bumper. Marketing was insistent that these create additional sales and Tickford had to prove that the car could just scrape through the uphill towing test without exceeding the oil temperature limit.

A neat coolant supply was arranged by making a special adapter that was fitted into the side of the cylinder block in place of one of the core plugs. This was not without problems as the tolerance on the holes, although quite adequate for the standard slightly deformable cup plugs, was not suitable for the rigid adapters, which sometimes came loose. When Audi adopted a similar layout many years later, they used a cup plug with a stub tube welded into it.

Making use of the parts bin

The cooling pack was taken from the VM Diesel engine version which was turbocharged and therefore incorporated an air-to-air charge air cooler. Apart from the differences mentioned above, the main engine was very similar to the naturally aspirated unit. However, there were some significant developments concerning the engine management system.

The M16T used the Lucas 14CUX fuel control that was fitted to the V8 engine in the Range Rover. The drivers for one bank of injectors were deleted but most of the hardware was common although extra software and new mapping was required.

The first issue concerned the mass airflow meter which fitted directly into the bottom of the air filter. This was the unit already in production for the Montego 2.0 EFI among others and was sized to not quite reach its maximum output signal of 5v with the airflow of that engine. The M16T would have more power and therefore more air flow so logically a larger air meter should have been specified. This would have been an expensive option as it would require re-tooling of a large plastic part so the simple and cheap solution of dropping the output voltage with a resistor bridge was adopted.

Lucas delivers the goods

Generally, the 14CUX did most things asked of it and Lucas were very cooperative in designing and installing the software for the novel boost control system that was conceived by Tickford.

The major cause of driveability complaints towards the end of the development was the sudden dip in idle speed when the radiator cooling fan cut in and suddenly increased the alternator load. The idle speed control valve could not respond quickly enough and there was no spare feed-forward system as used for air-con. It was hoped to find a soft start controller for the fan, but no such unit was readily available.

The other driveability issue was not a functional problem but was caused by the unusual constant torque over most of the speed range. This gave drivers the impression that the car was not accelerating as fast as it should be. It can be likened to the very constant acceleration one senses in a jet aircraft during take-off in that the initial acceleration is sensed but then the body rapidly becomes accustomed to the constant acceleration force. The result was that several drivers did not sense how much they had accelerated and arrived at corners much faster than intended.

Slow processor means compromises

The 14CUX does not control the ignition timing. That function is performed on the normal M16 naturally aspirated engine by a unit known as AB17. It receives input signals from a pick-up that senses the reluctor ring in the front face of the flywheel and provides the spark at a given time.

The issue with this unit was that it only used a single processor and, in the brief period it was doing its calculations of required spark timing, it could not trigger the spark. Thus, there were certain crankshaft angles at which it was impossible to fire the spark plugs and these dead windows became larger in angle terms as the speed increased.

Fortunately, the naturally aspirated M16 did not require spark timing within these dead angle zones so it was satisfactory. The turbo version of the M16 did require sparks in the dead zones so a solution had to be found. The idea of offsetting the reluctor ring or the pick-up was investigated but it was an expensive solution and could have led to problems if the two similar parts had been mixed up in production.

Montego lessons

The adopted solution was to use one function of another electronic control box named ERIC (Electronically Regulated Ignition and Carburetion) which was normally used to modify the fuelling of an SU carburettor as well as providing the ignition signal. This unit had the capability of providing the spark at any crank angle. The next hurdle had great parallels with those encountered with the lashed-together K16 Turbo.

The Rover 800 with the 2.7-litre V6 engine was quoted as having a power of 177bhp and the power target was set to be close but not exceed this at 175bhp. The gearbox used with the M16 engine was the PG1 which was made by part of the Rover Group, but the design control was still with Honda. The torque limit given to Tickford was 216Nm and not a Nm more as this would require an expensive re-validation for which there was not time.

In addition to this it was desirable that the torque should be as high as possible, within the above limit, across the speed range.

We also developed a novel boost control system which enabled the engine to produce the same performance irrespective of some production tolerances and ambient conditions. I’ll go into that at a later date.

This solved a problem but created another when it came to homologation. The rules of the witnessed power test specified that the output had to be corrected for temperature and pressure as is quite normal. There was no provision for an engine that self-adjusted to produce the same power when atmospheric pressure or temperature varied. The problem was solved by waiting for a day when the conditions were as per the standard, so no correction factor was required.

Issues with the clutch

One other issue that came to light very late in the development programme was that the standard clutch, although adequate for most conditions, did not tolerate large amounts of slipping when moving off up a very steep hill. It transpired that the very worst situation occurred when loading cars onto the top deck of a car transporter, so each production vehicle had a sticker on the windscreen stating ‘lower deck only’ to avoid the clutch being damaged before the customer received his new car.

With these problems solved in what was a speedy development programme, we had the engine ready for the Rover 820 Turbo 16V – later colloquially known as the 800 Tickford – within the timeframes set by Rover Special Products.