Mappable Ignition - New levels of performance and efficiency from the Rover V8

Many people seeking greater power and/or efficiency from the Rover V8 are aware of the importance of a correct air-fuel mix and may seek improvements with a re-map or performance-fuelling chip. Such tuning can only give moderate gains, when the existing air-fuel ratio is less than optimal. Often little thought is given to the ignition and resulting combustion of this fuel. The ignition timing and fuelling requirements are inter-related and both are essentially linked to producing optimum engine performance.

Most four stroke engines produce optimum torque and power when maximum piston thrust (combustion pressure) occurs, approximately 15 degrees (ATDC) after the piston has fully compressed the fuel air charge (TDC). At low engine speeds the flame front inside the cylinder has plenty of time to ignite the air-fuel charge fully but as the engine speed increases there is a smaller time frame in which to get the mixture ignited so we must spark the mixture earlier (advanced) to ensure that maximum combustion pressure is still generated when the piston is approximately 15 degrees ATDC.

However, engine speed is not the only factor that alters timing requirements. Imagine one engine is under full load (full throttle) at say 3000rpm, this engine will be filling it's cylinders to it's maximum efficiency (VE) so that the flame front will burn faster producing peak combustion pressures earlier in the engine cycle. Imagine a second identical engine running at the same 3000rpm but this time under light load (part throttle). This second engine is only part filling its throttled cylinders and is operating under a reduced dynamic compression ratio so the flame front burns much more slowly requiring much earlier (advanced) spark timing than the first engine.

Many older engines have no means of altering ignition advance in relation to engine load and simply provide a fixed timing for any given engine speed, resulting in efficiency and power losses everywhere below peak power.

Ignition timing alterations often require fuelling alterations or vice versa, for example a leaner air/fuel mix burns more slowly and therefore may require an earlier (advanced) spark to ensure the best power is extracted from this particular air-fuel mix or conversely may even require less advance because of a lean fuel mix’s reduced resistance to pinking. Any modification that improves an engine's volumetric efficiency (ability to fill its cylinders) often requires both different ignition and fuelling settings. Re-mapping / chip tuning of the 14CUX injection system (as fitted to all TVR variants of this engine) only alters the fuelling and cannot alter the ignition timing under different load /rpm combinations, as this is still controlled by a mechanical distributor with its fixed ignition advance curve. As a result, whilst gains are often made they are not what they could be if a full engine management system is fitted that can easily optimise both fuel quantity and spark timing using a dynamometer.

Whilst it is true to say that these engines are very tolerant of ignition errors, a lot of potential power and efficiency is lost. It is often wrongly assumed that little can be done to improve this overlooked, yet vital, aspect of the Rover V8 besides fitting good leads and setting the base timing correctly. We commonly find we can make some of our greatest gains by optimising the spark quality and timing simply because this is the area that is most lacking on these particular engines. 

Firing the cylinders with a strong spark at the correct time, combined with the optimum air-fuel ratio is absolutely crucial to getting peak power and torque.

All TVR’s utilising the Rover V8 engine, including the later interim serpentine engines, were fitted with a mechanical distributor as standard. These basic electronic distributors use a mechanical mechanism of rotating bob weights and springs along with a vacuum canister to cater for the engine’s ignition advance requirements.

The vacuum advance canister is designed to have very little effect on the timing at wider throttle openings and only advances the spark under high vacuum levels e.g. throttle barely open, slow cruising. High performance camshafts with a large amount of overlap (e.g. TVR units) serve to significantly reduce vacuum at idle / low rpm rendering the unit virtually ineffectual. Owners of high performance Rover V8 engined vehicles such as the TVR are often advised to remove the unit altogether! The reasoning behind this is that if it were to stick in the advanced position (rare) it could lead to a destructive amount of ignition advance when the engine is next placed under high load. Please note that removal of the vacuum advance will reduce the fuel efficiency for no gain in performance. Under wide throttle openings it is the centrifugal bob weights and springs alone that control the ignition advance. These bob weights control the ignition advance over a fixed advance curve. The distributor's advance curve can be advanced or retarded but the relationship between rpm and ignition timing is essentially fixed.

This mechanical advance curve is often at odds with the engine’s advance requirements, particularly with tuned engines such as those installed in the various TVR models. This is clearly illustrated in the graph shown (below right).

Clearly this mechanical system is unable to provide optimum timing control. Whilst it is possible to modify the advance curve of the distributor to give closer to optimum timing, it is impossible to achieve the optimum amount of ignition advance at all loads and engine speeds as this mechanical system advances timing purely as a function of rpm. Due to the linear nature of the ignition advance curve this mechanical system can only ever get the ignition advance correct at certain RPM and engine loads, whilst under other RPM’s and engine loadings we have to compromise. Optimising the ignition advance in one area can result in an efficiency loss – or even worse, engine destruction through detonation, in another. When working with these systems we attempt to give the best compromise possible between reliability and performance, although power and efficiency are never what they could be with the ignition advance optimised under all conditions.

Other disadvantages with the distributor includes poor general timing accuracy. The distributor is run from a helical gear that in turn runs from the camshaft, this in turn, is driven by a cam chain from the crankshaft. Harmonics and vibration in the valve train, particularly when combined with a little wear, disturbs the motion of the distributor itself. This is a well known problem that we cannot eliminate when using the distributor, we can only attempt to limit this motion by ensuring minimum play in distributor drive mechanism i.e. the timing chain and distributor drive gears.

Another weakness with the distributor is the use of a mechanical rotor arm to distribute the sparks to the correct cylinders. The mechanical distribution of the HT current certainly works although it can often lead to incorrect ignition timing particularly as RPM rises as the easiest path to ground may not always be the correct cylinder! 

It is not unknown for sparks to track across plug lead contacts direct to ground creating a momentary misfire, or even worse, fire the adjacent cylinder at the wrong time. These problems become inevitable when the ignition is exposed to moisture or water. The Rover V8’s well known Achilles heel in off-road competition is water. Rover V8’s with a distributor don’t like water! Even the Rover V8 powered TVR’s are not immune to moisture related misfires.

All of these problems can be solved at one stroke with a 3D programmable engine management controller that has both fuel and spark control. This dispenses entirely with the inaccurate and archaic mechanical distributor and replaces this with a system that has no wearing or moving contact parts.

This engine management system (EMS) can be simply programmed to give the optimal air-fuel ratio and ignition timing combination at all RPM / load combinations, as we noted earlier these are not separate and should be considered intrinsically linked in our quest for optimal power and efficiency.

The crankshaft position and engine speed is fed to the engine management system direct from a crank sensor that is triggered by a custom triggering wheel. Taking the crank position reference direct from the crankshaft eliminates any timing inaccuracy caused by harmonics and wear in the valve train. The EMS also receives information relating to MAP (mass air pressure), coolant temperature, air temperature and throttle position to help calculate the correct amount of spark advance. Once the correct ignition advance figure is calculated, the EMS fires the relevant coil pack at precisely that ignition angle. The coil packs are triggered direct via low power switching therefore making the entire distributor/ distributor cap and rotor arm redundant. This completely solves all our waterproofing issues and significantly improves general timing accuracy.