Registered: 13th Aug 01
Location: Romford, Essex Drives:Honda S2000
User status: Offline
Cylinder Head Tuning
The x1.2xe and the z1.2xe engines are part of a new breed of xe engines and are probably of closer relation to the 1.8xe that any others in the range. The camshafts in these engines are all chain driven, this is both a good and a bad thing, the fact that they are chain driven means that the drive mechanism is stronger and more durable allowing the engine to even longer between services. On the down side this makes it alot more difficult for the engine to be tinkered with.
Belt driven cams can be used with vernier pulleys to make minute adjustments in the timing to achieve the optimum, but these are fixed and much more difficult to get to. But lets face it, when GM are designing a new engine the last thing on the priority list is the engines potential for tuning. In the past when GM wanted an engine that could be tuned the trusted its development to someone who had considrable experience in that sort of thing. The 1.6 and 1.4 xe engines were developed by Lotus and the show stopping 2.0 xe is the handy work of Cosworth engeneering. To the best of my knowledge no such partnership was involved in the development of the 1.2 xe.
Having said all that, the format is not a bad one, many superbikes have chaindriven valve gear and the 1.2 has the shortest stroke of all the Corsa B engines, meaning that the crank should be able to spin up faster and suffer less vibrational stresses.
The head is always one of the most effective tuning areas, and the 1.2xe is no exception gains of up to 10% can be had through careful reworking of the ports and chamber, but to have a real winner you need to have performance included in the plan from the drawing board stage, so don't expect miracles. What can be expected is about a 10% increase in top end power, and to get the most of this the rev limiter would almost certainly have to be reset to about 7500 / 8000 rpm. Camshafts are hard to find for this engine, partly due to the low valve clearance at TDC and the fact that its chain driven, but Dbilas dynamic do several durations to pick from. These engines have most of their power in the top end of the rev range and only come into their own after 4000rpm so too much of an overlap in the valve timing may make worse the already poor torque and render the car quite undrivable in everyday use.
Engine Management Tuning
There are two types of 1.2 16v engine that have been used in the corsa range, the first was the x1.2xe as fitted to the Corsa B, sadly no relation to the 1.4 and 1.6xe engines, and the z1.2xe as fitted to the Corsa C. The x1.2xe is distinguishable by its unpainted natural aluminium cam cover, the z1.2xe has a black one.
The outputs of these basicaly similar engines differs by 10bhp with the z1.2xe having the edge. This being due, in part, to some refinements in the ECU program. There are gains to be had from tuning the ECU in these engines but it is unlikely to yield more than about 5 to 7 bhp this is the quoted performance figures from Superchips website. this suggests that there are other things limiting the performance of this engine.
The X1.2xe was the first of the 1.2 16v engines as found in the late Corsa B. In its original format it produced 65bhp but then with the release of the Corsa C for which the engine was intended the engine was refined to produce an extra 10bhp in the form of the Z1.2XE. The changes that gave the extra power were mainly fueling and timing based with the new engine seeing better cams, a reprogramed ECU, new injectors and an electronicaly controlled throttle.
The induction system on the 1.2 is alot more direct compared to the 1.4 and 1.6 engines, with the throttle body mounted on top of the compactly designed induction manifold/plenym. From the throttle, it is only a short trip to the airbox via the air flow meter and a top hat similar that no the the 2.0XE engines. In the case of the 2.0XE some gains can be made in the redesign of this top hat, but in the case of the 1.2 I would doubt if the flow is fast enough for this to have an effect, the only way of telling would be to flow test it on a test rig. A flow testing rig is a very useful bit of kit, more commonly connected to head work but if you were planning on gas flowing your head with the use of a test rig then I would suggest testing all the induction components seperatly and together as they would be fitted to the engine. A well flowed head is little or no use if it was the induction system that was the limiting factor in the first place.
The cam cover breather pipes ae connected to the induction system of the X and Z 1.2XE engines via the top hat, and under hard driving conditions oil sludge can be drawn up this pipe and into the induction system. The engine was designed like this so that any oil vapours released by the engine would be combusted, but the down side is oil collecting in the throttle and induction system. This can lead to fouling of the injectors, giving a poor spray pattern and gumming up of the passageways in the throttle. The manifold is designed so that any oil that might collect in the plenym stays there and does not trickle down the header pipes to the ports. This could be prevented by fitting a breather filter by pipercross, the connection on the top hat would have to be blanked off to stop unfiltered air entering the induction system. This may or may not have a slight effect on the flow meter depending on how much air was drawn through the breather line.
One of the easiest and more prefered ways (in insurance terms) to increase the output of the X1.2XE is to raise it to Z1.2XE status by fitting the induction systen and throttle from the Z1.2XE. I know that the throttle is electronicaly controled but I am unsure if it is done via the ECU if it is then hings will be more complicated than just switching over a few bits. But a Corsa C ECU would be the best option to fit with the new throttle.
if on the other hand you wnt to throw the rule book right out the window then go right ahead, SBD will sell a raw kit that should be able to be modified to fit the 1.2 cylinder head either by using an unmachined manifold that was used for their 1.6 8v kits or by contacting their component suppliers Jenvey for four small single manifolds. The main disadvantage with this kit is the lack if the off the shelf ECU program that is available with the throttle body kits for the 1.6, 1.4 and 2.0 XE engines. The MBE management would have to be set up on a rolling road in order to get it to perform.
The standard exhaust as fitted to the 1.2 16v engine is one of the most grim pieces of iron mongery I have ever seen, which I am sure yields great fuel economy and is nice and kind to all those bunnys sitting in the hedge rows quietly munching away as you tootle by listening to classic fm.
I'm sure GM are very proud of what they have created, but its just not suited to the job in hand when it comes to performance. For a start its got a catalytic converter the size of a KFC bargan bucket bolted directly to the manifold which heats the engine bay like a pot belly stove, it also interferes with the front anti roll bar position meaning that the SXi requires a specially shaped one that cannot be uprated to improve the handling.
The cat thing is a real pain, but take it away and the Department Of The Environment will grind your bones to make their bread. So heres a novel idea, move it. The 1.4 and 1.6 have theirs under the car aft of the front wheels, infact why not use the same type in the same position?
The original cat on the 1.2 and 1.0 engines is in the position it is because it is a resonator type and also needs the heat to allow the cataytic reaction to take place efficently, the other type works well on the 1.2 8v. By law the car needs a cat as long as it has one thats enough for me. The car if correctly set up will pass emission tests without a cat so it only has to be present, or at least apear to be.
Piper will be bringing out a 4 into 2 into 1 manifold for the 1.2 soon and with a but of nip and tuck this could be made to fit around a 1.4 /1.6 cat if you want to stay legal or on its own if you don't worry about that sort of thing.
If you were planning on fitting a cracking 2" stainless system, you would be best to finish the job properly and go the whole hog to get the most out of it and other modifications to the engine.
Submitted by AgentBlue