Frank Kelly on stage during a visit to Belgium in 2020 (Courtesy of Bruno Cornet)
For those who prefer to rally on tarmac as opposed to on loose ground, the challenge presented by the roads of rural Ireland is unsurpassed. Established in 1978, the globally renowned Irish Tarmac Rally Championship (ITRC) annually embraces half-a-dozen or so events in the Republic of Ireland and Northern Ireland. Those revered rallies are run on closed public roads and cater for machines right up to World Rally Car level, although these days World Rally Cars are not allowed to score points towards the overall championship.
Within the ITRC, one of the most intriguing arenas outside four-wheel drive is Class 14, which accepts naturally aspirated two-wheel-drive cars. This caters for traditional – many would say ‘proper’ – rally cars with a longitudinally mounted front engine and rear-wheel drive, and it allows freedom of development for up to 2.5 litre engines. The archetypal such vehicle – predating the transverse-engine, front-wheel-drive roadcar revolution – is the Escort MkII fitted with a competition version of a Ford I4 or a derivative of it.
Millington Racing Engines, based in Bridgnorth in England, is a supplier of one of those derivatives, and its managing director Julian Millington says, “These [ITRC Class 14] cars are pretty advanced compared to the Escorts of the 1970s and ’80s, having an engine management system using a high-end ECU running sequential [port] injection and coil-on-plug ignition. The cars have traction and launch control, and a paddle shift that allows shifting without lifting. Quite a lot of technology goes into them.”
One of the ITRC’s best known and most successful Escort MkII competitors is Frank Kelly. He campaigns a Group 4-specification Class 14 car with a six-speed sequential gearbox fed by a 2.5 litre Millington Diamond Series II engine. Its 350 bhp makes for a snarling, tail-snaking, tyre-smoking spectacle much loved by spectators along the sinuous Irish lanes. It is the sort of drama that four-wheel drive and front-wheel drive cannot hope to emulate.
Background
Across a number of issues of RET (most recently 130, March/April 2021) we have explored Cosworth BD-series rally engines, which are based on the 1959-launched Ford Kent pushrod I4. A more recent Ford I4 design is the Pinto, which was manufactured from 1970 through 2001. Likewise having two valves per cylinder as stock, it used a belt-driven single overhead camshaft. Cosworth again produced a 16-valve double overhead camshaft conversion, which it called the YB.
A key difference between the original BDA and the YB was that the latter was developed specifically as a turbo-supercharged unit; Cosworth even beefed up the production iron block. By the late 1980s BD-series engine refurbishment was becoming problematic; the BD replica parts industry we know now was still some way off. This prompted builders of naturally aspirated rally engines to turn instead to the YB, stripping off its turbocharging system.
A Millington Diamond Series II tarmac rally engine
While a naturally aspirated YB conversion offered a more modern cylinder head than its BD alternative, its major drawback was the weight of its strengthened iron block, particularly compared to the aluminium alloy BD block originally developed by Brian Hart in 1971. To overcome this, Roy – father of Julian – Millington designed and produced his own aluminium alloy replacement for the YB block. Thus in 1990 was born the Millington Diamond engine.
Subsequently, specifically to overcome the compromise of a cylinder head designed many years earlier for a high-power boosted engine, in 2005 Millington designed and produced his own replacement. Thus was now born the Diamond Series II that married the existing purpose-designed block to the new purpose-designed head.
Having been devised to take the YB head, the Millington block retained the same Pinto-derived bore spacing and head bolt pattern. A Diamond block can therefore have an eight-valve Pinto head or a 16-valve YB head, or – for Series II specification – a 16-valve Diamond head fitted. Its bore spacing and crankcase design allow displacements ranging from 2.0 to 2.8 litres, and Series I and II engines have been supplied by Millington for tarmac and gravel rallying and circuit racing and hillclimbing.
Frank Kelly uses the tarmac rally 2.5 litre Diamond Series II discussed here. A 2.5 litre Diamond Series II engine weighs only 83 kg ‘undressed’ – that is without the ECU, the loom, starter and alternator.
The Diamond block
In designing the bespoke Diamond block, Millington provided additional mounting lugs to allow its use in a wide range of applications, including single-seaters. The original 1990 block specification is still used across many competition applications.
The only major constraint for the design of the Diamond block was having to retain the Pinto’s 102 mm bore spacing and head bolt pattern. However, where the YB followed the Pinto with a wet sump, the Diamond block was designed from the outset for a dry sump. There have been optional wet-sump versions but they are few and far between. Kelly’s rally engine is dry sump, as per the original recipe.
CAD showing the bottom end
A significant difference compared to the YB’s modified Pinto iron block was the integration of the main bearing caps into the lower crankcase for additional strength. Millington (Jnr) reports that initially the crankcase was fully compartmentalised, with each section having its own scavenge. However, it was found that there was a danger of residual oil in a cylinder imparting sufficient hydraulic force upon start-up to actually bend a con rod. A ventilated crankcase was therefore deemed prudent for customer use.
The concept of two bolt main caps was retained, with peripheral bolts marrying the lower and upper sections of the crankcase as well; likewise, the concept of having a closed deck and dry liners within support towers. However, the cooling provision for the liner-carrying towers was enhanced, Millington notes.
Another key difference was a thicker deck. The deck height of the Kelly engine is 219.6 mm. In fact, the Diamond block is used with deck heights from 200 mm through 245 mm. 225 mm is the nominal dimension, with machining down to lower heights, while a deck plate is used to obtain higher deck heights to suit larger displacement engines up to 2.8 litres. The plate is attached before it, and the towers are machined together to insert the liners.
The main caps are each attached by a pair of M12 bolts
Given the 102 mm bore spacing, a 96 mm bore as used by Kelly is the largest considered feasible using cast iron liners; using nickel silicon carbide coated aluminium liners 97 mm instead would be deemed safe. For rallying, cast iron liners are preferred in view of consideration of serviceability, Millington explains. An interference fit, the dry liner is located on a step at its base. A top flange is not considered necessary, other than for more extreme applications.
The Diamond block is cast at nearby Grainger & Worrall and is machined in-house. The same is true of the lower crankcase, of the Diamond head and of the cam cover. All of these are aluminium alloy LM25 castings. Ideally, Millington would like customers to run a carbon fibre front timing cover, but the complication of access that item represents and the regular checking of belt tension invariably means one is not used.
The intake manifold is also machined in-house, and is made from a casting supplied by Jenvey. Other in-house productions include the valve guides and tappets.
Both the head and the lower crankcase are attached by 10 M12 bolts, the latter having M8 peripheral bolts. The head gasket is a triple-layer steel production.
The Diamond head
Roy Millington took a clean-sheet approach to the design of the Diamond head, being constrained only by bore spacing and head bolt pattern. Where the YB 16 valve head had been designed by Cosworth specifically for boosted applications, the Diamond head was focused primarily on naturally aspirated use. A key difference was reducing the included valve from 45º to 34.5º.
The Diamond cylinder head differs from both the Pinto and the YB in having an integral tappet block with the camshafts secured by individual caps. The Holbay Warrior has a Pinto-derived head, and consequently that is another option for the Diamond block; like the Pinto it has a detachable cam carrier.
Interestingly, when Millington at one stage developed a high boost turbo-supercharged version of the Diamond Series II, it attached the cylinder head by bolts that travelled right through the block into the lower crankcase. This ‘belt and braces’ approach was not considered necessary for subsequent naturally aspirated versions.
The introduction of the bespoke head gave the Series II an instant 25 bhp gain over a YB head-equipped Series I, reports Millington Jnr. There has been ongoing development of the Diamond head in the 17 years since it was introduced, and Millington notes that since his father did the original design this has been assisted by the availability of more sophisticated CAD modelling and by the introduction of five-axis CNC machining at the Millington facility.
CAD of a cylinder head machining operation
Conceptually the porting is the same now without tumble generated on the intake side, Millington reports. However, the inlet port geometry is now in its eighth iteration, the exhaust its fourth. In essence, the intake porting now has a larger volume than originally, while the exhaust is of comparable volume, having grown then shrunk back again. The exhaust porting is notable for a figure-of-eight style rather than oval exit, Millington adds.
The valve stems have been reduced from 7 mm diameter to 6 mm. Meanwhile, the exhaust valve head has grown from 32.0 to 32.5 mm, the intake from 37.5 to 38.0 mm, so the intake has gone from 30.4% to 31.2% of bore area. A greater percentage has not been found beneficial. Valve lift is 530 thou (13.5 mm) intake, 500 thou exhaust, while the valve timing is undisclosed.
With the intake valve area as 31.2% of bore area, the intake valve head-based mean gas velocity at the engine’s 8300 rpm peak power speed is 76.2 m/s. In respect of the so-called Lovell Factor, this is considered a high figure; 70 m/s is more typical of a mature race engine.
The 2.5 litre Diamond Series II con rod used by Kelly is 148 mm eye to eye. It follows that mean piston speed is a comfortable 23.8 m/s at 8300 rpm, rising to 28.6 m/s at the 10,000 rpm seen on occasion (9000 rpm is the nominal redline).
Millington reports that the 14.4:1 compression ratio pent roof combustion chamber has been developed into a clover leaf shape as additional squish has been introduced.
The rotating and reciprocating assembly
The 96 mm bore of Kelly’s ITRC engine is combined with a stroke of 86.3 mm to keep within 2.5 litres. The scope exists for a stroke of 98 mm, which combined with a 96 mm bore would give a displacement of 2837 cc. As noted, Millington does offer up-to-2.8 litre displacements for other disciplines.
The crankshaft was originally fully counterbalanced but now counterweights are only one at each end and one either side of the central main bearing. This approach has been empirically proven as the best, Millington says.
No vibration damping is required, while the flywheel is notably light. A recent 3 kg crankshaft weight saving has come from a central gun-drilling and a scalloping of the crankshaft webs, “removing unnecessary material”, Millington explains.
Millington uses H-section con rods
The camshafts and oil pump are belt-driven from a pulley on the nose of the crankshaft. An optional power steering pump is driven off the other end of the oil pump. The water pump and alternator are driven by a second front belt, while the fuel pump is electric. The drive belts are Gates 1.0 in-wide items. The timing drive has a trapezoidal tooth profile, while the ancillary drive uses a multi-ribbed belt.
The con rod is an H-section steel production with a two-bolt cap, conventional in all respects aside from lightening pockets on its flats. The small end is bronze-bushed and accepts a 21 mm-diameter uncoated steel piston pin, which is through-drilled and is located by a pair of round wire clips each running in a semi-circular groove.
The slipper-type piston is a 2618 aluminium alloy forging that carries three rings of undisclosed axial heights. Running against the cast iron bore, the rings are uncoated. The top ring is chrome steel and has a flat face, while the second ring is of the same form but is nitrided steel. A conventional three-piece oil control ring is used, this having chrome steel rails.
The Series II’s supporting cast
The valves are solid uncoated steel productions running in bronze guides and against sintered iron seats. Only a single steel coil spring is used for valve return; Millington wishes to keep its design and its producer confidential, simply saying it is “unique”.
Each steel camshaft directly operates nitrided steel inverted bucket tappets, likewise uncoated. No DLC is used in the engine. The camshafts run directly in the head, and have individual two-bolt caps.
Two barrel throttle bodies are used, each providing an individual barrel for its respective cylinder. The barrels are linked by a steel key between the two bodies such that they can all be operated from one end while there is a rotary potentiometer to read throttle position at the other end. The four barrels feed from a common plenum, which in turn is fed through an air filter.
For tarmac rallying, the exhaust is four into two into one.
Mounted at the front of the engine, the water pump feeds coolant through the block and then up at the rear of the block to the head, from where it travels to the front of the engine.
The current oil pump is three-stage, having one pressure and two scavenge sections with one scavenge pick up located at each end of the lower crankcase. The crankshaft oiling is conventional, via individual main bearings feeds, with the main gallery also feeding a single oil spray jet for the underside of each piston.
The Diamond Series II head provides a 34.5º included valve angle
Millington explains that there is a feed from the main gallery up into a gallery running along one side of the head. From there, the oil also feeds via a drilling to a parallel gallery, so that each camshaft has its own lubrication channel. That oil feeds via a hole in each follower to provide a supply to the respective spring.
“The oil drains back from the head to the sump in such a fashion that it doesn't land on the crankshaft. It's taken away in a special channel, to the benefit of hydrodynamics,” reports Millington.
As standard, Millington specifies a DTA engine management system, whereas Kelly opts to use a Link system. The ECU incorporates a driver for each of the four solenoid injectors. It also directly controls the four ignition coils, each of which is mounted on its respective 10 mm spark plug.
Millington engines are sometimes drive by wire, but Millington says Kelly prefers the simplicity of a traditional mechanical linkage. It follows that his ECU controls only the injection and ignition, aside from functions such as switching the fuel pump on and off. It does provide full data logging though, while its functionality includes traction and launch control and integration with the gear selection system, all via ignition cuts.
Contesting the Irish Tarmac Rally Championship
The ITRC does not constrain the development of petrol-fed, naturally aspirated 2.5 litre engines, although the block has to be a production-based I4. That rule was brought in for 2005 to stop the trend to more radical solutions, such as motorcycle I4-based V8s. It stymied Millington for a while, given that the Diamond block is a custom design.
However, so widely was the Diamond block used by 2005 that the organiser quickly relented to pressure from many competitors. It granted the Diamond block ‘production’ status on the basis that specialist sportscar constructor Davrian had made it the mainstay of one of its models.
Since the Diamond Series I block was introduced in 1990, Millington has so far produced 530 examples. To date it has also produced 350 Series II engines .
Aside from the block having to be approved, the ITRC engine formula is open – there are no restrictions on charge airflow, fuel flow or engine speed or other constraints. Millington says, “If we wanted to, we could even use beryllium components, but our customers couldn’t support the cost. We could use titanium con rods but then we would probably need shorter service intervals, adding to the cost of running the engine, which is why we use steel rods.”
Although engine development is so open for ITRC Class 14, the nature of the series’ events puts an emphasis on the spread of power and driveability rather than top-end output. This is due to the organiser prioritising safety over speed. The Irish lanes tend to be twisty, and where there are more open sections chicanes are invariably used to curb car speed. The upshot is, in Formula One terms, Monaco rather than Monza.
The Millington Series II is belt-driven
The top-end power of an ITRC-specification 2.5 litre Diamond Series II is 350 bhp at 8300 rpm, which represents a very respectable peak power speed BMEP of 15.1 bar. This is using a 102 RON octane competition fuel, helping allow the compression ratio of 14.4:1. Peak torque is 318 Nm/235 lb-ft (at 5600 rpm, representing a BMEP of 16 bar). Torque in excess of 300 Nm is maintained throughout the spread of 5000 to 7000 rpm.
Millington notes, “We could shift the whole torque curve higher up if we wanted to, but then we would be inviting people to rev the engine harder.” Moreover, while in theory peak power could be pushed to a higher figure using higher rpm, that would not be consistent with the performance required to cope with the Irish lanes. Plus, as Millington points out, it rains a lot in Ireland – the car has to perform as impressively in wet and greasy conditions as in the dry!
“As it is, we are using the torque to maximise driveability,” Millington emphasises. “For tarmac rallying we don’t want the engine to be like an on/off switch, so we’ve had to compromise a bit on the top-end horsepower figure.”
The engine excels in terms of driveability in particular, notes Millington, owing to the instant response afforded by its barrel throttles. “We don’t use a variable cam timing system; that is not really necessary.”
Interestingly, the single injector per cylinder is situated upstream of the respective barrel throttle within the common plenum, pointing in the direction of the flow into the respective trumpet. Millington also explains that the recent development of a 3 kg lighter crankshaft has been intended to make the engine more lively.
Although the redline is set at 9000 rpm, Millington reports that the engine can be run to 10,000 rpm without harm. “Going to 10,000 rpm can help with gearing. Our [ITRC] customers tend not to change the gearing for different events, and if sometimes they find they are caught in the wrong gear, they can just let the engine rev on a bit more. We don’t like them doing that much, because it could lead to premature component fatigue, but the engine will withstand it.”
Aside from the requirement for driveability, the engine needs to be 100% dependable, Millington says. Competitors such as Kelly are not geared up for engine work; they need an engine that is straightforward to run and that can amass a lot of stage miles between rebuilds.
Experience has proven that an issue with a Diamond engine is unthinkable, provided the competitor has installed it correctly and runs it within factory guidelines. The engine is mounted via the car’s bellhousing and via one mount on each side, although additional side and front mounts are available should a customer wish to exploit them.
An engine will be supplied from Bridgnorth already run-in and will just need its mapping fine-tuned on a rolling road to account for the nuances of its installation in the given car. Class 14 engines are not required to use a catalytic converter, but a silencer is mandatory.
Millington says, “The engine is very straightforward to run.” (See sidebar: Engine operation). Engine control isn’t closed loop, and knock sensing isn’t used on events: “that isn’t necessary.” Once mapped on the rolling road, the ECU should take care of ignition and injection settings across all regular stage scenarios.
He adds that running on 102 octane fuel detonation is never an issue, provided the engine has been properly installed and properly mapped. Around 6º BTDC at low throttle, representative ignition timing at peak power is 28º. The firing order is 1-3-4-2.
A freshly installed engine will be good for 1000 stage miles (plus a few hundred miles between stages) before it needs to go back to Bridgnorth for a rebuild. It takes around 27 hours to strip and rebuild a Diamond Series II engine.
“It is very, very important that our engines are 100% reliable,” Millington emphasises. “That means we have to be very careful when changing the specification, as the ultimate test has to be on the stages. It also means our con rods are extremely strong, our valvetrain too.
“We could in theory go from a 6 mm to a 5 mm valve stem, and from a steel to a titanium retainer, but we think such lightening would be too risky. For the same reason, we use a three-ring piston, where other engine builders will use just two rings. We want to avoid premature wear.
“It's difficult to compare [the ITRC] to other forms of motorsport because we've got a lot of compromises [in terms of how the car is used]. They do crash the car regularly, so that restricts some of the materials it is feasible for us to use.
“Also, for example, we cannot use a carbon fibre inlet manifold, as the manifold has to be strong enough to be used for hoisting the engine in and out of the car. Although we put nice lifting eyes on the engine, they'll still put a chain on the manifold. Our engine has to be strong!
“We use a big Denso alternator; when these cars jump over tarmac humps that can break an alternator bracket if it isn’t strong enough. So we have to make our alternator brackets really strong; they are aluminium alloy 6062 CNC machined from solid. We use rubber bushings too, and even the adjuster strap is made from nylon so there's no vibration to break the alternator or its brackets.”
The competition
In ITRC Class 14 the main competition to Kelly and other Millington-powered Escorts comes from YB engines, Ford Duratec I4s and Hondas and Vauxhalls. The Hondas are the K20 I4 in 2.5 litre guise, the Vauxhalls the C20XE I4 with its Cosworth-developed ‘Red Top’ head and in 2.4 litre guise.
Millington notes that the Duratec, Honda and Vauxhall I4s were designed as compact units for transverse front-wheel-drive installation, so bore spacing is less generous than that of the Pinto, which was designed for a traditional rear-wheel-drive car. It follows that the Duratec and Honda engines have a 94 mm bore, the Vauxhall a 93 mm bore.
Millington observes that a smaller bore not only restricts valve sizes and causes more valve shrouding, it also adversely affects stroke-to-rod ratio. “With the longer stroke you end up with a shorter con rod,” he says, adding that the cumulative effect of these factors, “is that they don’t seem to be able to make the same power as we do.”
DATASHEET
Millington Diamond Series II
Frank Kelly - Irish Tarmac Rally Championship 2022
I4
96.0 x 86.3 mm = 2498.6 cc
Naturally aspirated
102 octane gasoline fuel
Aluminium block and head
Dry iron liners
Five main bearings, plain
Steel crankshaft, four pins
Steel con rods
Light alloy pistons; three rings
Belt-driven twin overhead camshafts
Four valves/cylinder, one plug
34.5º included valve angle
38mm intake valves 32.5 mm exhausts
Electronic ignition
Sequential port injection
Engine management system
14.4:1 compression ratio
Maximum rpm, 9000
SOME KEY SUPPLIERS TO THIS ENGINE
Head and block casting: Grainger & Worrall
Inlet manifold casting: Jenvey
Crankshaft: Arrow
Crankshaft: Walford/Angstrom Engineering
Camshafts: Kent
Drive belts: Gates
Piston assemblies: Omega
Con rods: Robson Engineering
Big-end bearings: Mahle-Clevite
Main bearings: Mahle-Clevite
Fasteners: ARP
Valves: undisclosed
Valve springs: undisclosed
Head gasket: Cometic
Ignition system: Denso
Spark plugs: NGK
Fuel injectors: Siemens
Engine management system: Link
Engine management system DTA
Sensors: Bosch
Water pump: Mark Water Pumps
Oil pump: AT Power
Air filter: ITG
Exhaust: Simpson Race Exhausts
Wiring loom: Bremax Electronics
Oil: Millers
Machine tools: DMG Mori
ENGINE OPERATION
The Millington Diamond Series II tarmac rally engine is very straightforward to operate. As befits the privateer nature of series such as the Irish Tarmac Rally Championship, it is designed to be run by a driver/mechanic rather than require the input of an engine engineer.
The company’s managing director Julian Millington notes it is important that the engine is run on 102 octane fuel – Carless Racing Fuels’ Hiperflow 300 is recommended. A lower octane fuel can cause misfire or even piston failure.
The fuel pressure regulator is fixed at 2.5 bar to avoid customers playing with its setting. This factory unit should be the one used, as Millington has found that some other regulators lose pressure at higher flow rates.
The installed engine must be run with sensors matching those it was mapped with at the factory. “Each specific sensor has its own parameters within the ECU,” Millington explains. “When a sensor sends a false signal to the ECU, the engine will run in accordance with the incorrect or non-existent sensor values,”. If the crank sensor has had to be removed, or has been moved during installation, the sensor gap has to be reset.
Whenever the engine is started from cold it needs initially to be run on a 'fast idle' – 1600-2000 rpm – with attention paid to the oil pressure, fuel pressure and so on. It then needs to be run gently under light load, keeping the engine revolutions under 2800 rpm until the oil temperature is at least 40 C. It then needs to be kept below 4200 rpm until the oil temperature is at least 50 C and then below 6500 rpm until it is at least at 60 C.
Millington explains that the engine should only be driven hard if the water temperature is above 50 C and the oil temperature is above 60 C. He notes that modern oils can be run continuously over 110 C but does not advocate doing this.
Once the water temperature has reached 72 C the thermostat will open. This should ensure the water temperature stays at around 60-80 C. Millington advises that the engine should not be run if the water temperature exceeds 100 C. He says: “We would rather run the engine hotter than we do but we want to keep a safety threshold from boiling. Mind you, sometimes our engines do run over 100 C and it doesn’t seem to hurt them too much!”
It is recommended not to exceed 40 C air temperature under high levels of load for long periods of time. Millington recommends that the air filter is serviced after every event. In fact in more extreme climates it is recommended that the air filter is serviced throughout the event.
Millington advises checking the engine oil level every time the engine is used, by revving the engine to 3000 rpm for 15 seconds, switching off and immediately checking the dry-sump tank. Allowing the tank level to drop below two-thirds full can adversely affect the deaeration of the oil and will reduce the life of the engine, Millington warns.
Customers are also recommended to ensure that the tank is full before using the engine again. It is important as well to check the condition of the oil and make sure the engine is not breathing heavily out of any of the breathers – light smoke out of the breathers can be normal. Millington recommends a high-quality competition oil such as Morris Oils’ Multivis MLR 10W-50.