• XPAG tuning information

Tuning your MG engine and performance modifications.

Care should be given to selecting your internal engine modifications.

While it is tempting to fit every performance up grade and modification that you can, it is worth noting that not all mods work well together.


For example, a polished and flowed MG XPAG cylinder head fitted with a sports camshaft produces less power than it would with just one of the modifications, if the proportions of the inlet and exhaust ports are not compatible with the lift and overlap of the camshaft being fitted.


However if some modifications are done together correctly it will achieve more power than the sum total of the individual parts.


What you don’t  want to end up with is wonderfully high power or BHP reading that is produced with a high peak.  This will be great to tell you MG buddies at the local natter but will provide an unusable narrow torque band.

Max power or peak power is only part of the ultimate engine.  You should really concentrate on the torque curve and build an engine to give a torque curve that is as flat and as wide as possible.

Some basics first......

How to make your MG engine go faster?


Push the pedal harder is one way, but there are some other basic things that can be done.

The internal combustion engine works on the principle of suck, squash, bang and blow.  The more efficiently it can carry this out and burn the fuel, the better the power output will be.


Suck                      Air is sucked into the engine by the downward motion of the piston and mixed with the fuel.

Squash               The mixed fuel and air is compressed by the upward motion of the piston.

Bang                     A spark from the ignition/spark plug then ignites the mixture and the compressed energy is released, forcing the piston down.

Blow                     The piston then comes back up, forcing the residue out through the exhaust valve and into the exhaust header and down the pipe.

External modifications.

The easiest and most common engine enhancement is through the breathing of the engine.

The original factory XPAG engine inlet and exhaust systems are quite frankly diabolical, noticeably restricting the breathing . The MGA and MGB are better but still leave a lot to be desired in the tuning department.

Changing these factory items for ‘performance’ items such as bigger bore exhausts, flowed inlets or Webber carbs etc can greatly improve the power of the engine.  However as I mentioned at the start, if these items are not correctly shaped or sized for their required purpose, they will actually decrease the driveable power of the engine.

Internal engine modifications.

There are lots of modifications that can be made to the internal XPAG and B series engine.

Increasing the bore size, changing the compression ratio in the cylinder head, altering the gas flow and gas speed to and from the cylinders, sports camshafts with varying lift and overlap, lightened flywheels and so on.


Let’s start with the top end and look at the cylinder head.

This is the most miss understood performance item within the engine.

Most commonly people talk of fitting massive valves and polishing the head to extremes. 

This will not work!

If you look at what is being done to modern engines you will see that we have made massive advances from the early days of the XPAG engine.

We have learnt that fuel mixture ratios, gas speed and volume are critical in producing different power graphs.

In an engine’s ideal world it wants high levels of fuel and air to ignite and it wants it to be supplied to the chamber as quickly as possible.  This is why so many people have incorrectly gone down the route of ‘gas flowing’ everything!  What then happens is quite remarkable, the gas speed is allowed to increase, which is great but as there is now little turbulence the fuel that has been mixed with the air simply falls out and by the time it reaches the chamber, the mixture is wrong, as there is too much air and not enough fuel.  Lack of performance.

With the original factory XPAG cylinder head, the gas speed is very poor and to compound the problem further, the Siamese port design creates too much turbulence and slows the gas speed down even further allowing the fuel to once again drop out of the mixture.


So you say....what can be done.   Well, the original XPAG head can be greatly improved by correct gas flowing, shaping, polishing and by using the correct valves.  The shape within the cylinder heads ignition chamber is also critical to gaining maximum power and must change depending on your engines specification, camshaft etc and especially when fitting super chargers.


The size of the valves being used must correspond with the size of the bore, the stroke of the engine and the port size for both inlet and exhaust.  However the inlet and exhaust ports can usually be changed to get the correct ratio.


For example, a 1350cc XPAG engine running standard stroke on the crank can run a maximum of 40mm inlet valve, this in turn must be run with an inlet port of 75% of the valve head volume.  Are you with me so far?

The exhaust valve size is dependant on the inlet valve size and is worked out by a percentage of the flow rate of the inlet valve.

The exhaust port as it exits the cylinder head and enters the exhaust pipe must be 95% of the exhaust valve head size.

Creating a step within the port or cutting in turbulence swirls can greatly improve the engines ability to keep the fuel mixed within the air and give the required back pressure at the nesersary points along the way.

Basically this keeps the flow, gas speed and back pressure correct and in turn will give your engine what it requires and you the power and drivability that you require.


Hopefully, it is now easy to see, that if you simply change one component in the cylinder head, it will have a knock on effect and usually detrimental.

All things need balance to work effectively.  Sorry if i sound like a Buddest Monk.

This information may seem technical, and it is but it is quite simple and easily applied to every engine whether a standard road engine or full out race unit.

A lot of customers say to me when I talk of these facts that they are not intending to race the car, they just want a good engine.  Putting these simple calculations into practise on the engines that I rebuild does not produce a race engine, it simply creates ‘ a good engine’  exactly what the customer required.






I shall be looking at the Bottom end improvements in February,  So come back and see more then.


In the mean time, you are always welcome to come and see me at my premises and have a look at what is going on. 


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XPAG tuning information

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