The brief leading to the design of the DB4 engine was finally agreed in 1956, having initially started as a requirement for a 3 litre engine, it was gradually enlarged over the years to 3.7 litre capacity.
Today, it can be made to run reliably at up to 4.7 litre capacity, with a longer stroke crankshaft and increased bore size.
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Fast road engine enhancements
The main elements of a fast road engine involve:
An increase in engine capacity to a maximum of 4.5 litre, that is the limit on the safe cylinder bore size that can be reliably tolerated. It is more normal to increase to 4.2 litre, that has become the default specification for any overhauled DB4, DB5 and DB6 engine. While the 4.2 litre engine leads only to a very minor increase in power, there is some useful mid range improvement in torque, but to achieve any significant increase in performance, other changes to camshafts etc will be required.
Carburation – The original standard engine configuration involved a variety of different carburation arrangements, ranging from twin SU HD8s on the DB4, triple SU HD8s on the DB4 Vantage specification, and with the DB4 GT, triple twin choke Weber DCO-E9s. Later, with the introduction of the 4 litre DB5, the triple SU HD8s became the default with the Vantage tune standardising with triple twin choke Weber DCO-E9s.
Fast Road Camshafts – there are several candidate camshafts to choose from, depending on the initial state of tune and the camshafts fitted have evolved over the production of DB4, DB5 and DB6 engines. One aspect of fitting “bigger” cams will be an increase in tappet noise, resulting from the steeper cam profiles that are used. Today, the choice of cams for the fast road engine are several, but we would recommend:
Aston Workshop Fast Road– these retain the flexibility and smoothness of the standard engine, but provide a general improvement in mid and top range torque. These suit best those who want a more lively car, but do not wish to compromise flexibility in traffic
Aston Workshop High Lift – best suited to use with the DB5 and DB6 Vantage with triple Webers, in which they provide a very noticeable and consistent increase in engine torque of the region of 25% or so over the standard spec. The car will feel noticeably livelier and from 2 to 2500 rpm, it will pull particularly strongly all the way to the red line. The downside is a slightly lumpy tickover and some loss of flexibility below 2000 rpm.
Lightweight Forged Steel Crankshaft – While this component is not an essential addition to the fast road engine specification, if the engine is being given a complete overhaul, the additional cost arising from the fitment of this new crank compared to the cost of regrinding and polishing the original is marginal and with the added assurance of standard sized journals as opposed to reground undersize journals.
Lightweight forged Cosworth pistons and new cylinder liners – a rebuild of an engine involving conversion from the 3.7 or 4 litre to 4.2 or 4.7 litres will involve fitting new liners and pistons.
Compression ratio – This is set for the requirments of the engine.
Ignition – As part of the default specification will be the fitment of a full solid state electronic ignition using the 123 Distributor. This will not only control the advance curve for the ignition more accurately, but will also provide a higher intensity spark with commensurately cleaner exhaust emmisions.
Reliability, Flexibility & Refinement
The design of the engine provides limited scope for major refinement improvements, but the following would be of benefit in any engine of this type. These are:
New "high flow" water pump to increase circulation through the cylinder head and block
Improved engine cooling – aluminium oil and cooling system radiators and electric fan
The Ultimate (the 4.7 litre engine)
It is possible to safely increase engine size by fitting the enlarged liners of a 4.2 litre engine and by fitting a crankshaft with a slightly larger stroke to give 4.7 litres. Fitted with the fast road camshafts and with the triple Weber DCOe9 carburettors, the resulting increase in engine torque is of the order of 35 to 40% and a reliable power output to better than 340 to 350 bhp. Any engine fitted to this specification will be very noticeably livelier, particularly mid range torque and the ability to pull a very high ratio final drive will be significantly enhanced. Commensurate requirements for a 4.7 litre engine are:
A new cylinder block with stiffer main bearing webs and main bearing housings which are additionally cross bolted to add to the structural stiffness of the cylinder block
A new increased stroke crankshaft
4.2 litre sized cylinder liners
New forged Cosworth pistons
Fast Road Cams
Reworked cylinder head to cater for the increase in engine bore size
A twin plate racing clutch to absorb the increase in engine torque
Aspects of engine development also include gas flowing and blue printing of the cylinder head and block assemblies. Each project really needs to be discussed in detail before a final specification can be finalised.The GT Engine
A further development is to fit the GT Cylinder head. This differs from the standard by having twin spark plugs for each cylinder. Originally this cylinder head was developed for the DB4 GT. Around 120 DB4 GTs were manufactured during 1960 and 1961. The GT engine provides a further small but significant increase in power, particularly at 4,500 rpm and above. This GT cylinder head can be offered on any of the fast road or ultimate engine specifications.
Some significant modifications are required to the engine bulkhead and heater assembly, to provide adequate clearance for the second distributor. The cylinder head comes with some significant improvement in inlet and exhaust porting over the standard head, which further aids the breathing.
A fast road engine will be quite capable of pulling a higher final drive ratio. The recommended change will be from 3.73 to 3.54 to 1 giving an extra 2 mph per 1000 rpm in fifth gear. For engines fitted with auto transmission, the standard final drive ratio is 3.54 to 1 and the recommended change there will be to 3.31. This would also be the recommended change for manual DB4 cars fitted as standard with a 3.54 to 1 final drive.
An additional option is the fitment of a 6 speed manual. Currently not a standard transmission option, it could be offered if desired and would potentially enable a further reduction in the final drive ratio for manual cars from 3.54 to 3.31, with 6th gear giving a road speed in the region of 35mph per 1000 rpm. Such an option could realise a very useful improvements in fuel economy.
In summary the options are:
Final drive ratio change – 3.73 to 3.54 to 1
Final drive ratio change – 3.54 to 3.31 to 1
With other changes:
6 speed manual plus change of final drive from 3.73 to 3.31 to 1
5 speed auto with 3.54 to 1 final drive
A general refit and upgrade of the suspension is strongly recommended when moving to a fast engine specification. The key options that are available are:
The Monte Carlo handling kit. This brings:
Stiffer front roll bar
Fast road front and rear springs
Up-rated Koni front dampers
An additional change highly recommended will be the fitment of telescopic rear dampers in place of the standard lever arm shock absorbersComplimentary Brake & Wheel Upgrades
The brakes fitted as standard on DB5 and 6 cars, which is a 4 wheel all disc duplex Girling brake system has proven to be effective and capable of adequate retardation form very high speeds consistently and safely. With DB4 cars, the early simplex all disc brake Dunlop system lacks the same capacity to absorb heavy heat loss arising from very high speed speeds. The recommended changes to the standard brake systems are:
DB4 – Fit the standard duplex Girling Brake system upgraded as below.
DB5 & DB6 – Improve brake system resilience as follows:
Fit harder Pads
Upgraded Brake Servo assistance
Wheels : Default to move to 6.J rim size wheel and to fit 205/70 section radial tyres with an added option to fit 225/70 radial tyres on 15 wheels.
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