*INFO* Tuning your SPI 8v Engine
I have to say, i had these mods on my old 1.2i, and they are well worth doing :thumb:
Thanks to Aragorn for the guide
This guide is aimed at tuning the 1.2i engine as opposed to the 1.2 tuning guide posted by MC, although a lot of his stuff will apply he simply suggests changing to carbs for the 1.2i which isn't really and option for some people (k plate cars) and others may simply not want the hassle of carbs and prefer the reliability side of injected engines. I have performed all the mods in the first section myself and have had good gains from it (I'm awaiting RR readouts but have had an 18.8 sec 1/4 mile in my bros car with quite a high 60ft (2.9secs) his car has no rear seats and cobra Monaco's tho so some weight saving going on)
The 1.2i (C12NZ) makes 10HP less than a 1.2 carb as standard, however it
makes identical torque due to the different design of cylinder head.
The engine also has a number of factors which restrict it compared to the
carbed engine. The Cylinder head is very restrictive compared to the
1.2carb version. It has very small square ports with bad flow characteristics
whereas the 1.2 carb head has much larger cathedral ports and bigger valves
which flows much better, its these restrictive ports which help the 1.2i
make the same torque as the carbed engine even tho it makes much less
power, however that torque is available over a shorter band which is why
the 1.2is are slower. Unfortunately the carb head will not fit streight
onto the 1.2i becuase the inlet manifold wont line up, i give some
instructions later in the guide about swapping heads but as i mention
there this only works if your fitting the SRi injection system or twin
40's and is not what i'd designate a cheap mod. I am going to look into
mounting the SPi throttle body on a 1.2carb inlet manifold which will
enable you to fit the carb head but i havent got round to this yet. The
other restrictions are the cam which has lower valve lift and the engine
has a restrictive cat fitted, thats what were going to work on here mainly.
Exhaust:
The first mod i would suggest is to fit a better exhaust system right from the manifold back, there are two approaches here depending on the age of your car and how much hassle you want come MOT time, if your car is a H or J plate then you don't need a cat at all, if its a K plate you will need a CAT for the MOT unless YOU can prove to the tester that the engine was built before august 92 (depends on the tester, only real date stamp on the engine is the cam).
If you don't need a cat (or don't mind refitting the complete original system come MOT time) then the first step is to find an SRi (or late 1993 GSi) exhaust manifold with lambda probe port - the engine needs this sensor to run properly so you need a hole on the manifold for it. Then you want either an SRi or GTE/GSi downpipe and a full system for a GTE (Ashley do a nice 2" system for ?130, or you can use a std system from scrappy)
If you do require a cat for mot then you can do it a different way, if you find a SRi in the scrappy and get entire exhaust from it, then get a decat pipe for the sri system and that way you can refit your cat for mot and put the decat on at other times. I personally would take the first approach and if your car needs a cat then modify the cat of the original 1.2i exhaust to fit the GTE system, that way you can fit the middle section of the 1.2i system for mot and return to the GTE/Aftermarket system afterwards. Or you can find a friendly MOT tester ;)
Cam:
The standard cam on this engine is very restrictive, fitting a cam from a higher spec model will give great gains (I'd recommend getting one from an injection model(SRi, GTE, GSI) although an SR one would work too. If you want to fit it yourself the head needs to come off and the gasket and headbolts will need changed while your there
Air Filter:
The standard airbox looks ****e but does the job fine to a point, just remove the handle section off the front of it and throw it in the bin for extra noise - if you want an upgrade there are a few cheap ways of doing it...
You can use my guide to making a cheap induction kit here: http://www.novaload.net/forums/viewtopic.php?t=90933
Or you can get a 1.2i Corsa airbox and fit then then either shove a cone on the end or use an SRi airbox with performance element
don't bother wasting money on a green induction kit as its ?70 you can spend elsewhere - the Corsa top box shouldn't cost more than a 5er and neither should the sri airbox
These are the main cheap mods for this engine before you start hitting big money, and the majority of it is cheap stuff (air filter shouldn't cost more than ?30 if you follow my guide, cam is probably about ?30 then another ?30 for the bits to fit it (unless you pay a garage), exhaust is a bit more but the advantage is that its usable on a larger engine you may fit in the future
Weight:
If you want to go fast your main enemy in a small engined car is weight, lots you can do in a nova to lose it but it reduces flexibility, removing the rear seats and rear belts will save you about 20-30kgs and the car can still look cool, if you want slag then you can remove all the door panels, fit Plexiglas windows and ditch the dash board, you could probably shave off 70kgs in total if you did all that and thats gonna be similar to the difference of driving alone and driving with a passenger which will be very noticeable in a 1.2i nova :p
Suspension and Brakes:
Acceleration is one thing but if you car handles better and stops faster you can drive faster as you wont lose as much speed through corners and can leave braking later, a good quality matched spring and shock kit is what you need - springs alone wont help handling much (they will help but just not very much) and some Astra GTE16v (or Cav 2.0) 256x24mm disks and callipers will help stopping power greatly Do not underestimate tyres either, ****e tires will make a huge difference to the cars performace and handling and stopping characteristics, best for the nova would be some 6x15" wheels with ET49 offset and 195/45R15 tires, Uniroyal Rainsports and Goodyear EagleF1's are good grippy tires and are decent in the wet too. Don't go sticking ET38 or 7" wide alloys on and expect the car to handle well, it wont.
Service:
I'd recommend giving the engine a good service while your here, new rotor arm and dizzy cap, new plugs and leads (don't buy performance crap, just standard NGK plugs and normal Bosch or whatever leads, also do an oilchange with a new filter and replace the thermostat if your taking the head off to fit the cam
Gearbox:
A point often overlooked in tuning smaller engines - the standard 5 speed 1.2 boxes are very wide ratio, you ain't got a lot of power so theres no point having gearing that can manage 150mph when the car will only do 105 an F10CR box from an SR or F13CR box from a GTE would be a good choice and fitting a 418final drive to either of those boxes would give an excellent gearing for acceleration
Taking it further:
At this point your not gonna get any major power hikes without spending lots of money - its arguable wether its worth it at this point but as long as you only buy stuff thats transplantable onto a new engine you should be alright in terms of wasted investments.
The head is very restrictive so fitting either a 1.2 carb head or a 1.4SRi head(skimmed to sort compression) would be the next step
You could then either fit the complete 1.4SRi injection system (arguably a waste of money) or fit some twin 40's (these can be fitted to a larger engine in the future if required)
if you used the 1.4sri system you might need a remap but it does have a lambda so it might work out whats happening by itself
fast road and rally cams are available with varying attributes - a 285 cam should be alright in the std engine and you should be able to push to a 295 if you fit the SRi stuff or twin40's
fully modified with worked sri head, 40's 295 cam full exhaust etc you could see the friendly side of 100HP
So there we are - hopefully that will answer most of your questions and provide a solid base to work on
Aragorn
