3.0   Modifications

3.1   General

3.1.1    Suggested literature

In addition to the workshop manuals listed in the Maintenance & Repair section:

• Competition Preparation Manual, GT-6 and 2000, British Leyland Motors, by R. W. Kastner (2nd edition 1977 revised by Mike Barratt
• Kas Kastner's Spitfire, GT-6, TR-3, TR-4, TR-250, TR-6 Triumph Preparation Handbook, by Kas Kastner, purchase online at
http://www.kaskastner.com/kasbooks.html
• Tuning Standard Triumphs Over 1300 c.c., by David Vizard, Speed & Sports Publications Ltd., ISBN 85113-029-1
• Tuning Manual for Standard Triumph Cars Produced Between 1959 and 1980, by G. Thomas

3.2   Engine

The GT6 did not receive drastic & difficult-to-reverse modifications to its engine to meet US emissions regulations, especially when compared to other cars like the later Spitfires with their air pumps, etc. See "Differences between GT-6 Mk2 & GT-6+", above.

Some changes had no direct bearing on performance after the car was warmed up or off idle:

• Evaporative loss control equipment: no effect on performance at all
• CDSE features differing from CD features: effective only before car is warm or during deceleration
• Vacuum retard feature of distributor: Functions only at idle

In fact, the only changes affecting performance were:

• Lowered compression, via thicker head casting or addition of a "sandwich plate" between the block & cylinder head
• Slightly different-spec carburettor needles
• Different cam for lower idle

So by adjusting compression via changing or shaving the head, optimizing ignition timing from spec, & (if desired) adjusting the 'non-adjustable' jets with the special CDSE tool or trying other carburettor needles, home market performance is achieved.

It is illegal to operate a car on the road in the US with its emissions equipment disabled, so modifying these cars (except for off-road or track use) so that they no longer meet the emissions requirements effective at the time of their original sale is illegal. Also, in states which require regular testing for emissions to operate a car on the road, these changes may increase emissions past an allowable level to pass the test.

Cars in states which mandate emissions testing have sometimes experienced difficulty passing even unmodified, due to wear & difficulty of tune. It is VERY likely that in these cases the addition of a small, modern catalytic converter will enable these cars to easily meet or exceed requirements. Modern catalytic converters are much more free-flowing than the first examples, & have a negligible effect on performance.

Below is excerpted a February 25, 1980 letter regarding the GT6, from Michael J. Barratt, Competition Manager East for Jaguar Rover Triumph (USA):

... compression ratios range all the way from 9.5:1 to 7.4:1 on later emission engines. However, the datum point is the 8.5:1 compression ratio with a cylinder head of 3.460. The only modifications that were made to the engine on later models, to achieve a lower compression ratio, was to cut the head casting thickness. Therefore, regardless of the original thickness of your cylinder head, to achieve a compression ratio of 10.25:1, it is still necessary to mill for a final thickness of 3.375.

The JRT Competition department offered three competition camshafts for the GT6:

Camshaft	Part #	Application
Type S-2	V-532	Street/Race
Type S-4	V-579	Race
Type S-5	V-688	Race

However, as of February 25, 1980, the supply of factory competition camshafts was drying up, & the JRT Competition Department was offering other recommendations:

Turning to the camshaft problem; the S2 camshaft as quoted is no longer available due to the fact that we are experiencing difficulty in getting the cam blanks from the factory... Meanwhile, we would suggest that you contact Iskenderian Racing Cams, 16020 S. Broadway, Gardena, CA 90248, 213-770-0930, who can regrind your existing cam to a specification which is very close to the S2, which they call Z19 grind; which is suitable for competition and street use. The S5 camshaft is an all out racing cam and is in no way suitable for street use. Michael J. Barratt, Competition Manager East (USA)

As of April, 2007, TS Imported Automotive in Pandora, OH (USA) are offering cams based on the Competition Department's S2 & S4 cam specs. http://www.tsimportedautomotive.com/

The OEM rockers have a rocker ratio of 1.5, so the valve lift of a cam using the OEM rockers is 1.5 times the cam's lobe lift.

Notes:

Triumph manufactured a number of cylinder heads to fit their 2.0/2.5 litre engine block. The cars, like the GT6's, used them in different configurations with various camshafts & piston types (domed or flat). The variations in these heads include thickness (affecting compression ratio), stud size, & overall design (some flow better than others).

Not all heads & blocks are directly interchangeable, but many are. When swapping heads, it's important to check among other things the deck of the block & surface of the head to ascertain the final compression ratio, whether the water passages mate up, & whether the intake & exhaust ports mate to the manifolds.

Chris Witor researched several Triumph 6 cylinder heads to gauge their flow characteristics. His conclusion was that the 219016 head flowed best. As the 219016 head was originally fitted to the 2.5 engine, it will require skimming if installed on a 2.0 engine to achieve a reasonable compression ratio.

Identification: The Part Number is the number by which the head was identified by Triumph, the Head Number is a concave number stamped into the cylinder head; the Casting Number is the raised number on the cylinder head.

3.3   Fuel System

A low-pressure electric fuel pump is required for the GT6 if one is used. A typical replacement is manufactured by Facet, model 40105.

It should be fitted as near the tank as possible, lower than the fuel outlet, the mechanical fuel pump should be removed, & the opening in the engine block covered with a plate & gasket.

It is essential that the fuel pressure of any replacement fuel pump be only 1.5 to 2.5 psi if used with the Stromberg carburettors.

3.4   Ignition

Lucas Sports Coil #DLB105 can be fitted in place of the original coil for hotter spark.

For cars with the ceramic ballast resistor bolted to the side of the engine to the left of the distributor, the resistor must be bypassed. Disconnect the wires from both ends of the resistor, connect them together with a male double-ended spade connector, & wrap the connection in electrical tape. (If it is decided later to go back to the OEM coil, the wires can be easily reconnected to the ballast resistor.)

After the Sports Coil is installed, Lucas advises the spark plug gap can be increased to .035". This gap may need to be reduced to as little as .030" if neccesary to assure good firing for a car's individual tune.

3.5   Rear axle

The non-overdrive GT6 came with a 3.27:1 differential; overdrive cars came with a 3.89:1 differential. These differentials are directly interchangeable with each other, with simple differences sometimes of flange size & number of holes for studs on top (6 for rotoflex cars, 4 for swing-spring rear suspensions. Flanges can be swapped, extra holes can be filled with grub screws or studs sawn off flush, additional holes can be drilled & tapped if needed.

These units are to the same extent interchangeable with the differentials of various ratios fitted to Vitesse & Spitfire cars. In all, the available ratios are 3.27, 3.63, 3.89, & 4.11 from the various cars.

The following are calculated based on the OEM spec of 155R13 tires:

A well-liked modification for an overdrive-equipped GT6 is installation of a 3.63 differential. The 3.63 differential was a common fitment on late Spitfires in the home market, but few 3.63 differentials arrived in the USA. However, it is possible to assemble a differential with a 3.63 ratio from other Triumph parts:

To create a 3.63 differential, you would need a 3.89 Spitfire/GT6 differential as a basis, and the gearset or differential from a TR7 4-speed. Basically, the TR7 gears are a swap for the 3.89 gears. If you have the TR7 carrier, you can use that too -- it will probably be stronger than the 3.89 one. (The 3.27 differential has a different-size carrier, but the casing is the same.)

--Nick Jones

3.6   Suspension & steering

Triumph successively installed three different types of rear suspension in the GT6 during the production run.

GT6 rear suspension types
Suspension type	Used in GT6 model:
swing axle	Mk1
rotoflex	MK2/+ & early Mk3 (up to commission # KE/KF 20,000)
swing spring	later Mk3 (beginning at commission # KE/KF 20,001)

The US Triumph competition department issued a memorandum describing the fairly simple conversion of the Mk1 cars with swing axle to the swing spring suspension of the later Mk3's.

Basically, it is simply the replacement of the original transverse leaf spring & the plate which clamps it to the differential with the same items from the swing-spring cars, and plugging the two unused mounting holes on the differential.

The part number for the replacement spring assembly (159640) is for the Spitfire Mk IV spring assembly, which can be fitted to earlier Spitfires & the GT6 Mk 1.

The Competition Department bulletin below describes the conversion of the Mk1 swing axle to the later Mk3-type swing spring setup.

Swing Rear Spring Conversion Owners of early Spitfire and GT6 models interested in improving the handling will find that the best investment is the installation of the rear "swing spring", as fitted to the Mk IV & 1500 Spitfire models. The arrangement of swing axles and transverse leaf spring, used on early GT6 & Spitfire models, has an extremely high roll center which under high speed cornering conditions results in the wheels becoming cambered, with a consequent loss of adhesion. The way to minimize this is to reduce the roll stiffness of the suspension. On the later Spitfire (MkIV and 1500), the Engineering Department modified the Spitfire's transverse spring to cut its roll resistance by 75%. Only the main leaf is clamped to the differential, so it can contribute to roll stiffness. The other leaves are free to rock on their centers and simply provide bump stiffness. The wheel spring assembly also has a higher two-wheel bump rate, so the camber range under braking and acceleration is considerably reduced. To compensate for loss of roll stiffness at the rear, the front anti-roll bar was enlarged from 11/16" to 7/8". To install the new spring setup you will require: spring assembly 159640, 4 studs 131008, and 2 plugs PU 0804. The swing spring is mounted on 4 studs as per the drawing on the next page, and the 2 remaining stud holes in the axle casing are plugged to ensure that neither differential lubricant is lost nor foreign matter enters the axle casing. These plugs should be installed with the appropriate Loctite sealing compound. Alternatively, the 2 center studs can be tightened fully home in the axle casing and then cut off flush with the casing. If, after installation of the swing spring, you car has an oversteer characteristic, it is suggested that you replace the original 11/16" front anti-roll bar with the 7/8" bar, 217033. We are sure that you will be delighted with the results of this modification as it will greatly improve the handling of your vehicle and enhance the subsequent pleasure you will derive from driving it.

Note: The above details only the conversion of the swing axle suspension to swing spring. Conversion of the swing axle or swing spring suspension to rotoflex, or rotoflex to either of the other systems, is more complicated (requires many more parts).

3.7   Brakes

Performance brake pads for the GT-6 front disc brakes are available from a number of vendors. Also, other cars used the same pads, & interchangeable pads are often easier to locate.

Perhaps easiest of all to find (Stateside anyway) is the Datsun 240Z. Lots of high-performance options in the same-sized pad. - Steve Smith

3.8   Electrical

The OEM alternator has an output of 43 amps. A direct replacement alternator utilizing the same mounts & electrical connectors but yielding an increased 55 amps output can be sourced from the Ford Fiesta. Additionally, sources for these alternators sometimes offer better warranty terms than suppliers of the standard replacement alternator.

The description for ordering the replacement alternator is
1978 - 1980 Ford Fiesta, with air conditioning, Bosch style, 55 amp alternator.

3.9   Chassis & body

All US-spec cars were equipped with seatbelts. In other markets, seatbelts may have been an option or otherwise may not have been fitted.

If it's desired to equip a non-US car with seatbelts, the best & easiest way is to just create the factory's US-spec seatbelt installation in the car. The part numbers can be found in the factory parts catalog, & in the case of the MK3, an installation diagram is provided.