Compression is by far the most important consideration if you want to start tuning this engine. The compression ratio will determine the extent to which this engine can be tuned. It responds dramatically to changes in compression (both static and dynamic).The XU9J4 doesn’t achieve 84hp/litre through accident. It’s a very well developed package. All the various components of the engine work together to make an engine that has a very broad power range that is both economical and a pleasure to drive. There is basically no “bolt-on” item that will improve this engine. So I’m not going to talk about pod filters and big exhausts. The first step is to ensure the standard engine is healthy. If you haven't got 185+ psi per cylinder on a compression test there's no point in proceeding with other modifications.

Liner wear is the biggest problem with the XU9J4. Disassembling an engine that has done 200-250 000 Km will most likely reveal pistons in excellent condition, but worn liners. They wear badly on the thrust side, approx. 30mm from the top. The amount of wear is a function of servicing, but they’re basically all much the same. Typically you’ll find anything up to 0.003” wear at this point. All is not lost however, particularly for track cars. It’s possible to hone the liners a thou. or so, then fit new rings. A slight increase in piston-bore clearance is not an issue for street cars. Up to 0.003” clearance is fine for track cars, even though they use strutted pistons. You may not be able to completely remove the wear mark but the resultant rebuild will be a great improvement on how it was.

Be very careful when choosing a place to bore and hone these liners. An inexperienced operator will mostly give you back oval shaped liners. The liners must be clamped in a fixture which simulates the axial load placed on the liner when the head is torqued down. The machine shop I use has lots of experience doing exotic wet liner engines and is Aircraft Certified.

When looking to increase the CR, the first thing to consider is the compression height, which determines the quench distance. Regardless of compression ratio, engines respond well to quench distances (the distance from the piston crown to the cylinder head). To illustrate this consider the following. The early 1.9L 205 GTi released in Australia was fitted with the XU9J1/Z DFZ engine and had just 8.4:1 CR. The piston in this engine has a 9cc dish, but the compression height is 37.50mm, some 0.38mm higher than the XU9J4 and 1.22mm higher than the XU9J4Z. The piston sits just 0.02mm down the bore. Despite the lower compression, this engine has fantastic throttle response and low end torque.

The XU9J4Z has comparatively poor bottom end and midrange torque, with a distinctive peak at 4000 RPM as it starts to get going. The untrained think that this “rush” at 4000 is fantastic, but it’s only a result of the flat spot prior. The XU9J4 in comparison, has a nice flat torque curve and is two to three car lengths in front at 4000.

The quality of the petrol in your local area will determine the maximum compression you can use, but keep in mind that the minimum octane rating for the XU9J4 was 97 RON. With 98 RON being available in many countries, increasing the CR towards 11:1 (and beyond) should not be an issue.

There are four methods of increasing the CR, all with their own advantages and disadvantages.

1. Copper Head Gaskets
By far the cheapest method, but also the potentially the deadliest. Copper + Aluminium in an electrolyte makes a battery, which equals corrosion. The aluminium. head or block being the anode and the bit which corrodes away. So maximum assembled time is 2-3 years when using a solid copper gasket.

The standard head gasket is 1.32mm thick. Solid copper gaskets are available in 0.5, 0.7, 1.0, 1.6 and 2.0mm thicknesses. A 0.5mm head gasket will take the CR from 9.7:1 to 10.4:1 on the XU9J4Z, and a 0.7mm from 10.4:1 to 11:1 on an XU9J4. Because the head and the piston are now closer, quench distance is also improved, so you can expect significant gains in midrange torque and throttle response. Approx. AUD$120.


2. Piston/Liner Kits
A piston/liner kit is great way to upgrade an XU9J4Z to 10.4:1. Peugeot’s kit also contains all the gaskets and seals you need to complete the job. In addition to more compression, you get a fresh, as new engine. Unfortunately these are no longer available. You might find a set sitting on the shelf somewhere gathering dust. Approx. AUD$1500.


3. Block/Liner Decking
Whether you’ve got an XU9J4 or Z, material can be removed from the block and liners to optimize the compression and quench height. Aim to get the piston flush with the top of the liner. This means approx. 1.3mm needs to be removed from the deck of an XU9J4Z. Remember what I said about the DFZ 205 GTi? Decking the liners is a specialist job however. The liners need to be held on a mandrel to keep them square to the long axis of the bore. If you get it wrong it will be very ugly as the liners won’t seal properly.

Done correctly, with honed liners and new rings, this upgrade makes for a very sweet engine. You end up with approx. 10.8:1 and more torque than you ever thought possible from a normally aspirated 1.9L engine. Allow approx. AUD$400 + honing + rings.


4. Forged Pistons
Forged pistons are of course the ultimate way to get more compression. Whilst unnecessary for street engines, if you’re looking for big hp and high rpm, they’re the best. Forged pistons also allow several liner “lives”. Depending on wear, you might be able to use 83.50mm and 84.00mm sizes on consecutive rebuilds. There are various companies around the world that make pistons for the XU9J4. Beware of paying for what you don’t need. It’s possible to buy pistons with very large domes and corresponding deep valve pockets to accept the wildest of cam lifts. Deep valve pockets are an unnecessary reduction in compression ratio that has to be overcome with a high dome, which causes a barrier for the flame front. Approx. AUD$1200 for custom pistons + liner boring.

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