The XPAG is not a powerful engine by modern standards, but it responds predictably to the classic tuning ladder: more compression, better breathing, and — for the ambitious — forced induction. This page sets out the period supercharger kits, the factory-style stage-tuning routes, and the mechanical condition each route assumes.
The honest premise
A standard XPAG makes roughly 50 bhp at the crank in healthy condition. That is enough for the character of the car, but not enough for confident modern traffic. The two ways to add meaningful power are tuning the engine normally — raising compression and improving breathing — or adding a supercharger. Both work. Both demand that the engine is right first.
Forced induction does not fix a tired engine. It finds the weakest part and breaks it.
The stage figures below are taken directly from Eric Blower’s period article "Tuning the XPAG Engines", which condensed the factory tuning publications for the TB, TC and TD. The supercharger material is drawn from the MG T Society’s long-running survey of period and modern kits. Both sources are linked at the foot of the page.
Period supercharger kits
Supercharging the XPAG is not a modern idea. When these cars were current, more than half a dozen manufacturers offered kits. British makers included Shorrock, Marshall-Nordec, Wade and Arnott; Italian options included SCoT and Itel-Meccancia; the American market had Judson. Many survive and are traded today, though parts and rebuild expertise are now scarce.
| Kit | Origin | Type | Notes |
|---|---|---|---|
| Shorrock | UK | Concentric eccentric-drum compressor | High adiabatic efficiency, low power absorption, very quiet |
| Judson | USA | Concentric compressor | Common American aftermarket kit; MG TD-specific variants known |
| Marshall-Nordec | UK | Roots-type pump | Supplied by North Downs Engineering for TA-TD and Y-Type |
| Wade | UK | Roots-type pump | One of the earliest period kits documented for the XPAG |
| Arnott | UK | Concentric compressor | Carburettors Ltd; low-pressure supercharger aimed at touring use |
| SCoT / Itel-Meccancia | Italy | Roots / concentric | European aftermarket options in period |
The Shorrock is usually described as the most refined of the period kits: an eccentric-drum compressor with high adiabatic efficiency, low parasitic drag and almost no mechanical noise. The Judson was the common US aftermarket route and is still discussed on T-Series forums; MG TD-specific variants exist, so a Sprite or Midget unit should not be assumed to bolt straight on.
Roots vs concentric compressors
The period kits fall into two families. Roots-type blowers — Marshall-Nordec, Wade and SCoT — are air pumps. They move mixture into the inlet faster than the engine can consume it, and the pressure builds in the manifold rather than inside the blower itself.
Concentric compressors — Shorrock, Arnott and Judson — trap a volume of air and fuel internally and compress it before releasing it into the inlet tract. The design is quieter and generally more efficient at the low boost pressures suited to a touring engine.
For a road XPAG, low boost is the right boost. The MG T Society’s testing suggests a realistic 40–45% power increase from 5–7 psi of boost, with the engine staying reliable if the installation and fuelling are correct.
Stage-tuning without a blower
Eric Blower’s article lays out three progressive stages. They are a ladder, not a menu: each stage assumes the previous one and adds more compression, more valve area, or special pistons. The head-machining dimensions are exact and must not be exceeded.
| Stage | CR | Head work | Output | Fuel | Notes |
|---|---|---|---|---|---|
| Standard | 7.25 : 1 | 76.75 mm face depth | ~50 bhp (T Society estimate at crank) | 74 octane minimum, 82 for maximum power | Twin SU 1⅛ in, 0.090 in jets, ES needles, 8 mm valve lift |
| Stage 1 | 8.6 : 1 | Remove ⅛ in; finish 74.37 mm | 60 bhp at 5,500 rpm | No. 1 pump fuel (high octane) | Standard head gasket with ½ in washers under nuts; ¼ in rocker packing; LA11 plugs |
| Stage 2 | 9.3 : 1 | Remove ½ in; finish 73.575 mm (maximum safe) | 66.5 bhp at 5,500 rpm; 68 bhp with 1½ in carbs and EL needle | No. 1 pump fuel | 36 mm inlet / 34 mm exhaust valves, 150 lb open competition springs |
| Stage 3 | 12 : 1 | Standard 76.75 mm with special pistons | Racing specification | Methanol-based blend (80/10/10 + castor oil) | 100 jets, GK/RC/RV needles, 40° flywheel advance, twin SU pumps |
Stage 1 in detail
The head face is skimmed by ⅛ in to give a finished depth of 74.37 mm and a compression ratio of 8.6 : 1. The standard head gasket is retained, with ½ in thick washers under the head stud nuts to compensate for the lost thickness. Rocker shaft packing pieces ¼ in thick restore the rocker geometry. Ports can be tidied and polished, but the valve choke diameter must not be enlarged. Champion LA11 or Lodge R49 plugs replace the standard items, tappet clearance is opened to 0.022 in on engines up to number 24115, and ignition timing stays at TDC. The result is 60 bhp at 5,500 rpm.
Stage 2 in detail
Stage 2 takes the compression ratio to 9.3 : 1 by removing ½ in from the head face — the absolute maximum Blower recommends. Larger 36 mm inlet and 34 mm exhaust valves are fitted, which means machining the combustion chamber wall for clearance, opening the valve choke to 33 mm inlet / 29 mm exhaust, and recutting the seats. Competition valve springs with 150 lb open tension are fitted with the closed coils toward the head. Output is 66.5 bhp at 5,500 rpm, or 68 bhp if 1½ in competition carburettors and EL needles are used.
Stage 3 — racing only
Stage 3 uses special high-compression pistons in an unskimmed head to reach 12 : 1. It is not a road specification: it requires a methanol-based fuel blend, 100-size carburettor jets, GK/RC/RV needles, an ignition setting of 40° advance on the flywheel, and twin SU fuel pumps with duplicate lines. This is the territory of racing, not fast road use.
What forced induction demands
A supercharger makes the engine work harder on every stroke. The extra load is not evenly distributed — it lands hardest on the crankshaft, bearings, pistons and head gasket. Before fitting any blower, the bottom end should be inspected as if for a rebuild.
- Crankshaft: check for cracks, scoring and out-of-roundness greater than 0.001 in. Any of these calls for a regrind and matching shells.
- Bearings and oil clearance: mains should be 0.0008–0.002 in, big-ends 0.0005–0.001 in. Verify with Plastigage on assembly; do not trust shell markings alone.
- Pistons and compression: the standard 7.25 : 1 CR is well suited to low boost. Higher boost on a high-compression engine will detonate on pump fuel.
- Head gasket and studs: boost tries to lift the head. A fresh gasket, even torque, and sound studs matter more than on an atmospheric engine.
- Ignition: timing must be conservative for the boost level. A distributor with a known advance curve is essential.
- Fuel: supercharging raises the effective compression. Use the highest octane fuel available and jet richer than you think.
- Oil temperature: sustained boost generates more heat. An oil temperature gauge is more useful than an extra oil pressure gauge.
The exact machining limits and shell sizes are on the companion XPAG & XPEG rebuild tolerances page. If the engine is not already within those limits, it is not ready for a supercharger.
Modern supercharger options
New kits are available that avoid the parts-scarcity problem of period blowers. The MG T Society describes modern Eaton-based systems — the Mirage Garage kit being one example — that deliver 5–7 psi for about a 45% increase in rear-wheel horsepower, with an 8–10 psi option for those who have built the engine to take it. These are self-contained units with no external oil lines, reversible with no chassis cutting, and use a modified SU H4 (1½ in) carburettor.
The same rules apply: a healthy bottom end, conservative ignition timing, good fuel, and a realistic boost target. The modern kits are more forgiving than period iron, but they will still break a worn engine.
Sources
- Eric Blower, 'Tuning the XPAG Engines' (Motoring, c.1950s; PDF via mg-cars.org.uk)— Primary source for stage-tuning figures
- The MG T Society, 'Supercharging the XPAG'— Modern and period blower overview
- The MG T Society, 'XPAG Engine Camshaft'— Camshaft timing and lift data