V6 Calibra - C25XE Tuning

The V6 C25XE is a good reliable engine if well maintained. Regular and frequent oil changes are a must. In standard form the output is quite variable. The standard figure is quoted as 169bhp but some stock V6 Calibra's and Cavalier's have regularly clocked 180-190 bhp, even with a high mileage.

If you are serious about tuning It is important to get a 'baseline' Rolling Road test result. This gives you a valuable set of data against which you can measure future performance mods. This is prudent with the V6 as its' standard output is so unpredictable. It is possible to use rule of thumb methods such as timing your 0-60, 60-100, through the gears etc. GPS based accelerometers are becoming more readily available and offer the DIY user a standardised ball park accurate way to measure improvements.

C25XE isometric view

C25XE side view cross section

C25XE front view cross section


Before you consider tuning ...........

Old V6 Lambda Sensor
V6  lambda/O2 sensor

Give the V6 a good service (oil, plugs, all filters, etc).
When were the cam belts last changed ? (every 40k miles). If in any doubt change cambelt, rollers and the water pump.
Do you have any fault codes stored ? If so, resolve them.
Use a good fuel system cleaner .If you think the injectors are bad then send them off for ultrasonic cleaning (well worth it on a set of injectors with 50k+ miles).
Also consider renewing the following components if you suspect they are more than 5 years old:
Lambda (Oxygen or O2) sensor
Ignition plug leads
Cam cover gaskets & new bolts (stop potential oil leaks ruining new plugs and leads)

Check the operation of your throttle. Have someone sit in the car and depress the throttle peddle fully while you check the position of the throttle valve under the bonnet. Is the throttle valve fully open or can you push it further using you finger ? If there's extra movement then you need to adjust the throttle cable using the adjuster so full throttle pedal gives fully open throttle valve.

You need to know what you are starting out with so please obtain a rolling road result or a least go out and time yourself on a known stretch of road. How does it measure up to stock figures ? Bear in mind that a worn engine will produce less power than it used to.

V6 tuning potential
The main problem with the V6 C25XE engine is one of "breathing". That is getting the air/fuel mixture into the cylinders and evacuating the exhaust gasses after combustion. Addressing these shortcomings is not that easy and definitely not always cheap. The common V6 modlist usually features:
    Cold Air Feed
    Enlarged Throttle Body
    3.0 Litre Inlet Cams (Omega)
    Custom stainless steel Exhaust System

When done well these mods can take the V6 close to 200bhp.

However, It is important to realise that it is quite difficult and expensive to obtain a reliable V6 Calibra running more than 200 bhp; well, not without the use of Nitrous or a Supercharger.
If your aim is to go beyond 200bhp with the V6 then it's sensible to consider an engine transplant at the outset (3.0 or 3.2 V6 ). A fully modified, normally aspirated 3.0 V6  should see you close to the 250bhp mark.

Some C25XE V6 tuning limitations
V6 Stock Fuel Pump (in tank late type) : max 85 ltr/hr @ 3.5 bar
V6 Stock Bosch Fuel Injectors : max 204 bhp (peak 210 bhp) ---- requires 82 ltr/hr fuel @ 3.5 bar
V6 Stock F25 Gearbox/Std clutch : rated to a max 250 bhp (realistic 220 bhp) and low gearing


Cold Air Feed - Filters & Induction Kits

Cold Air Feed - removing the resonance box
This is usually where everyone starts. Removing the V6 resonance box (behind lower offside corner of bumper) and rerouting the air filter housing inlet hose directly to a hole cut in the towing eye cover.
This is cheap and very effective mod for the time and effort it takes. The idea is to get a free flowing supply of fresh, cold air to the air filter.
Cold air is more dense so you get more air molecules in the same volume the colder it is. The more air you can squeeze into the engine then the more fuel will be injected by the ECU to burn it up.
The ECU works out the incoming air volume using the Mass Air Flow Meter and compensates using the Air Intake Temperature. The ECU takes into account the engine operating conditions and then uses look-up tables (called MAPS) to determine how much fuel to inject and how to control ignition.
Therefore: the colder the air, the more of it there is = more fuel required to mix with it = stronger detonations = more power per ignition cycle.
Improvements of 1 - 5 bhp and smoother acceleration have been claimed from the cold air feed mod.
Try to use pipe with an inner diameter of at least 75 mm (the bigger the better). Keep the rerouted replacement pipe as direct and short as you can. Cut a small drain hole in the lowest section of your replacement air feed pipe if concerned about sucking in rain water. The front tow eye hook can get in the way when using the tow eye cover as a feed location.

Air Filters and Induction Kits
The air flow properties of stock paper filters vs. foam & mesh/cotton performance filters has been a hot topic of debate.
Paper filters filter more debris out of the air than other materials.
The difference in air flow properties between paper and foam is very small when taking into account the total flow restriction in the V6 inlet tract.
Unlike disposable stock paper filters, performance filters can be cleaned and reused.
Over application of the particle retention sprays, which must be applied to most performance filters, can cause coating of the hot ceramic tile in the Air Flow Meter. This results in a degraded response from the Air Flow Meter and  can cause a ECU fault light in extreme cases.
Foam filters produce a lower tone V6 air induction noise.
Mesh/cotton filters also sound more 'throaty' but can occasionally 'whistle' under high air flow demand.
Cone filters/Induction Kits - Usually slightly louder induction noise than replacement panel filters.
Open Induction Kits : do not use on the V6 engine. The V6 is designed to run warm. Sucking hot engine bay air into the inlet, through an open filter, is not going to give any kind of increase in performance.  Remember, colder air = more air in the same volume.
Sucking in hot air can cause poor idle but you tend to get away with this on the V6 more than you do with the 16v Calibra.
Sealed Induction Kits - Retail versions are expensive (Viper kit is ~ 250); DIY versions are cheaper (70) - using the original air box and a cone filter (see photo). Give good results on the V6 as you are forced to use the cold air feed mod with these designs.


Further Enhancing Air Intake Flow
Once you have optimised cold air flow to the filter you could pay some attention to other minor air intake restrictions.
The V6 Mass Air Flow Meter (Hot Film type) has an integral plastic mesh grill built into the air inlet side of the unit. Removal of this plastic grill helps lower the overall restriction of the air inlet tract. The improvement is very minor. Removing the grill also removes your last line of protection from foreign objects entering the air intake.
A larger volume, custom smooth stainless steel air intake elbow can be used to replace the 'concertinaed' black rubber stock version. This speeds up the air flow as the flow is less turbulent (not always a good thing) as it travels through the elbow and towards the throttle body.


Throttle Body and Inlet Manifold

Enlarged Throttle Body
Enlarged throttle body with an effective 20% increased internal bore (+6-8 bhp gain).

Expect mild improvement in response as well as power. A very highly recommended mod for the V6.

Inlet Manifold
An enlarged version was available from Mantzel tuning in Germany. Now quite difficult to obtain and expensive (550). Consider DIY internal smoothing of stock inlet manifold - to improve air flow.
Lower induction manifold and flange are easier to improve than the inlet manifold. These are more easily enlarged to provide better flow but should be matched with the Mantzel enlarged manifold.

Mantzel C25XE Inlet Manifold
Now difficult to obtain. 20% larger internal volume. 10 - 12% increased air flow rate - claimed 8-10 bhp increase

X25XE lower induction flange


Spark Plugs and Ignition Leads

Ignition leads degrade with age. Oil ingress destroys their conductive properties. Leads degrade quicker in hot temperatures.
The stock 8mm V6 leads are very capable leads.
Magnecor 8.5mm ignition lead sets  (or other performance lead manufacturers) are made specifically for the V6 calibra, fit well and are usually cheaper than a new set of stock leads from Vauxhall.

Iridium plugs - stronger spark at higher temps and last longer - more expensive. May need to be removed checked and cleaned at least once during plug life.
Standard twin electrode plugs are fine performers. May not last as long but perform just as well. Cheaper than iridium.



4 Bar Fuel Pressure Regulator
The stock V6 uses a 3.0 bar rated FPR. On a 5 yr old V6 this commonly reads 2.5 to 2.8 bar. An uprated 4 bar fuel pressure regulator can be installed to improve throttle response by increasing fuel pressure supplied to the fuel rail and injectors. This does not increase the amount of fuel delivered as the Motronic ECU indirectly detects the increased fuel pressure (over fuelling at the lambda/O2 sensor) and uses a shorter injection pulse cycle to deliver the required fuel dose instead.


V6 Chip Tuning
Chip tuning involves replacing all or part of the engine management software; either physically or just the software/firmware stored on the main chip. The key element in chip tuning is to modify the MAP tables.
The engines sensors take various operational readings which are compared with a reference table held within the ECU (this collection of reference tables are known as the MAPs). After using the maps as 'look-up tables' the ECU then sends the desired outputs to the engine's ignition and fuel circuits.


There have been three generations of V6 Calibra ECU's. The type of ECU is identified by a large two character code on the ECU case sticker. It is important to bear this in mind when considering a custom chip as most 'plug and play' type chips are based on the early PN generation ECU code. Fitting a custom chip based on a PN type may mean you loose immobiliser and secondary air injection functions.

PN First Generation (1993-1995).
HE Second Generation (1995-1996). Immobiliser function block added to ECU code
KC Third Generation (late 1996-1997). Immobiliser and Secondary Air Injection function blocks


SuperChips ( ~ 300)
SuperChips produce replacement main ECU chips. They are loaded with SuperChips own modified engine maps. It's a case of one size fits all V6's. Claimed improvement in power & torque across the board. They are not sold as DIY chips but must be installed at an approved agent but this does not involve a rolling road. They do a seven day guarantee but you'll loose the installation fee if you want to return it.
They Say : 10-12 bhp increase with more torque lower down the rev range - which sounds a little optimistic
Further info here > http://www.superchips.co.uk

The UNICHIP is a re-programmable chip/microcomputer, which works in a 'piggy-back' configuration with the stock V6 ECU. It is able to modify the fuel and the ignition timing by tricking the main ECU in to using modified ECU maps.
UNICHIP is fitted using a rolling road session to optimize the UNICHIP to your specific car. The UNICHIP software can be configured to work with any type of sensor and can adjust fuelling by up to +/- 50% and ignition by +/- 25 degrees. Other features include:

  • Variable valve timing adjustment
  • Rev limit adjustment & Idle speed stabilisation
  • Super Charger / Turbo control possible (w/ water inj)
  • Dual maps for use with different fuel grades & types
  • Nitrous oxide progressive injection control
  • Bigger/supplementary injector handling
  • Variable induction control
  • Full throttle gear changes w/ Shift light

The software allows a possible 238 adjustments sites (14 load x 17 speed). UNICHIP is re-programmable, so when further modifications are made to your car (i.e. exhaust, air filter, camshaft, etc.) the UNICHIP can be adjusted to take these alterations into account. This will involve a further rolling road session to once again optimize your engine's settings.
They Say : 7 - 12 bhp increase - should produce better results than a Superchip
Further info here > http://www.dastek.co.uk

Reprogrammed ECU
In mid 1996 Vauxhall upgraded most of their ECU's to use flash programmable EPROMs. New or modified control system software could now be uploaded using a Tech1a or Tech2 . As far as I know this does NOT include the late model V6 ECU. It was only done to the 2 litre XEV Simtec engine management system. Tweaking the V6 ECU therefore normally involves replacing the main program chip for a substitute chip which contains modified maps. It may be possible that some expert Opel tuners can reprogram all V6 ECU's via the ALDL bus. In the UK Courtenay Sport offer a re-map service for the C25XE.

Custom Replacement ECU (700-1500+)
New aftermarket/motorsport ECU management system (e.g. Bosch Motronic MS 2.8). Needs to be specifically set up for your car using rolling road tuning techniques. This kind of modification is recommended for heavily modified C25XE's

Ignition Timing Map

Fuel Factor Map

Motronic Map Dump to PC

Finally a word about ebay chips.
These are usually bogus chips which often consist of a simple resistor designed to fool the ECU into over fuelling. These are not good and are not a form of chip tuning. Avoid these on the C25XE engines, the ECU will simply compensate for the 'tricked' component or run poorly.
There have been genuine sightings of good chips on ebay; mainly second hand SuperChips and some custom German chips for the early V6 ECUs. If in any doubt check with the vendor what ECU part numbers the chip is designed for. Does it match exactly with what you have? has it got the same number of pins as your stock chip ? If they can't answer then you need to walk away.

Also be aware that the late model V6 ECU's feature Immobiliser support so if you are looking to purchase a backup ECU (in case things go wrong!) make sure you get the right type (there are three variants).


Uprated Fuel Pumps
If 85 litres per hour does not meet your needs then an uprated fuel pump is required. Generally only needed to support larger injectors with custom ECU and/or a hefty nitrous install. Also can be a consideration if you need to replace a failed stock unit anyway - and the extra expense doesn't bother you.

standard Bosch V6 injectors

Fuel Injectors
For C25XE tuning up to 200 bhp the standard V6 injectors are fine:

Bosch, Type 0-280-150-428
Flow = 196 cc/min @ 3 bar
Impedance = 16 ohm
BHP rating (each inj) = 26-34 bhp (39 peak)
6 injectors combined flow rate : 70.2 ltrs/hr

It is important to ensure that fuel system components are free from built up deposits. Regular use of an injector cleaner mixed in with the fuel is recommended (STP Total System is good).

If you have over 60,000 miles on a set of injectors it is definitely worthwhile having them ultrasonically cleaned as they probably perform like this set being tested before cleaning


uprated 252cc/min Bosch injectors

If you are considering larger injectors then you are probably in the market for an uprated FPR & Fuel Pump with 'Unichip' or custom ECU. There are higher flow compatible Bosch injectors available. These flow at 252cc/min (30% increase on standard) and use 12-16 ohms impedance.

Fitting 3 litre inlet cams (from Omega 3L X30XE V6 engine) is highly recommended, cost effective and can take a standard V6 close to 190 bhp when matched with a custom exhaust. Noticeable increase in power above 3500 rpm. Slight decrease in low end power. Some high performance 'racing' cams can result in lumpy idle and loss of refinement. Second hand Omega cams should cost you around 75-100. Late V6's with X25XE engine numbers still use C25XE heads so the cams are still the same (green inlets + green exhausts).

Stock Vauxhall/Opel Cams
C25XE : 4 x  'A' Green inlets & 'A' Green exhaust cams
: 4 x  'F' Brown inlets &  'E' White exhaust cams
X30XE : 4 x  'G' Blue inlets & 'A' Green exhaust cams

General Recommended setup :
Change for '
G' blue inlet cams and keep existing 'A' green exhaust cams

'MSD' recommended setup:
Change for '
G' blue inlet cams and change for 'E' white exhaust cams
Claimed to add approx 15 bhp

Courtney High Torque Cams : +15 bhp and costs 350

X30XE inlet cam

standard C25XE cams



Custom Exhaust Systems
Manifold (x2) - Down-pipe - Flexi Joint - Catalytic Converter - Cat back exhaust system

Stock V6 exhaust bore is 2.5". Custom stainless steel designs using the same bore will give improvements. A larger 2.75" bore will give greater improvements but only really effective when a larger bore is applied to the entire system (manifolds through to back box).

Exhaust Manifolds
The stock V6 exhaust manifolds are stainless steel but very restrictive. Considerable improvements can be made using custom built replacement manifolds. There are no off the shelf versions available. Expect to pay around 500 for a set of custom manifolds with connecting downpipe (Longlife, Powerflow, Regal).

Down pipe and flexi joint
The down pipe is a critical part of the system. It draws the outputs of both exhaust manifolds into one and houses the mounting for the lambda (oxygen) sensor. The end of the downpipe connects to the flexi-joint. The flexi-joint connects the downpipe to the catalytic converter and allows for some movement in the exhaust system without causing damage.

De-Cat Pipes & Sports Cats
De-cat pipes - Any good ? Why not ?
De-catting produces marginal gains in peak bhp BUT you loose a large amount of mid range torque. Hi-Flow Sports Cats are an improvement over the stock items and well worth considering. Sports cats need to be 'run in' correctly to ensure continued operation.

Cat Back Custom Exhaust Systems
Mono designs are more effective in enhancing performance; a single straight through design with a couple of absorption dampers appears to give a guaranteed improvement. Dual designs are very difficult and more expensive to get right and can be purely cosmetic with exhaust gasses favouring one side. The petrol tank really gets in the way for a decent dual-twin exit design

Lighter Flywheel

Considered a very worthwhile modification to the V6. The standard V6 flywheel weighs in at 16 kg. Replacing with a lighter flywheel (5-7kg) makes the V6 feel more 'free revving'. This does not increase engine power only the way in which the existing engine power is applied. Makes a big improvement when taking off in lower gears.
Beware drilling out of existing flywheel design to make it lighter - this tends to make it weaker.
Usually fitted in conjunction with 6 speed F28 gearbox (locked to 2 wheel drive mode).


Nitrous Oxide Supplementary Injection (N2O or NOS)
By far the best value bhp per modification you can make and all too often overlooked.  It's relatively straightforward to install yourself. Can be installed in a matter of hours. Gains of up to 100 bhp through use of 25 bhp increment jets. Can use wet (liquid form) or dry (gaseous form) N2O injection systems. Expect to pay between 500 - 750 for a good system with controller. Relatively easy for the amateur to install.

Calibra V6 GroupA 4x4 C25XE
with X30XE inlet manifold

"Progressive Wet" system is recommended - progressive meaning the gas is introduced in a precise computer controlled 'build-up' manner, using a programmable unit.
N2O and additional fuel are introduced in liquid form into the intake manifold causing a bonus cooling of the intake mix.
Using N2O will cause your engine to wear at an increased rate. It may be wise to renew or 'beef up' some of the internal engine components if planning 100 bhp N2O boost levels. A progressive system makes use of an electronic controller and allows you to program nitrous delivery to minimise the sudden mechanical shock of having additional fuel and oxygen dumped into the inlet causing more powerful detonations.
I have never met a disappointed N2O system owner. The downside is the necessity to refill the N2O supply bottles regularly at an average cost of 5 per llb of nitrous gas. It is possible to go through a 10llb bottle of nitrous on a busy day or if you are an occasional user it may last a week or four.
N2O is not directly explosive but just as it adds a huge amount of additional oxygen for engine combustion, it can also add huge amounts to an existing fire. Only open bottle valves when needed (remote electric valve openers are available). Mount a fire extinguisher somewhere accessible in the car.
Watch out for the V6 traction control interfering with Wide Open Throttle position when engaged. Best practice may be to disable traction control when using N2O - depending on your nitrous system configuration (more risky on direct port installs).


A engine belt driven centrifugal supercharger will produce a vast performance boost on a standard V6 engine. Unfortunately there are no aftermarket "off the shelf" solutions so the only answer is a custom design. Expect gains of 40-50 bhp and an increase in torque across the range. Expensive, expect to pay 1000 - 2000. Important to consider fitting enlarged throttle body, inlet manifold, exhaust manifold and custom ECU to complement the supercharger although it is possible to run a modest boost supercharger with the stock ECU.
Centrifugal type superchargers are the sensible choice as they can be installed at the side of the engine. Roots and Hybrid superchargers are usually mounted on top of the engine and the V6 just doesn't have the room.

Electric Supercharging
Not worth investigating - Avoid !. Electric supercharging fans, in general, do not provide enough air flow for the V6 - they act as a restriction instead. Some kits even try to use fans rated for computer cooling. Also consider that the amount of electrical power required to drive an expensive professional electric impeller (one which does provide adequate airflow) places far higher demands on the alternator and engine load. This tends to negate any potential small bhp gains this type of technology may offer.

There are rumours of a Vauxhall developed prototype Calibra V6 C25XE with twin turbochargers. This remains just a myth.

While it is theoretically possible to mount twin turbo's to the C25XE it is very difficult to make this arrangement fit into the Calibra engine bay. A big part of this problem is that the V6 engine is transversely mounted making room around both the front and rear exhaust manifolds very tight. Room is further limited at the front if your V6 has aircon fitted.

Saab low pressure turbo as fitted to 2.0L 4 cylinder engines. The Saab 9-3 used a single low pressure turbo (Garrett GT1752) - was used on four cylinder 2.0L engines to give 155 bhp. Some Saab 900 SE's and early 9-3 V6's used C25XE engines but these were never turbocharged. These are low pressure turbo units intended to provide very modest boost to the V6 engine. It may be possible to transplant these turbo units into a Calibra V6 but engine bay space may be an issue.

Saab 900 SE V6 (94-98) used the same engine as the V6 Calibra. The engine (designated B258i) was carried over and used in early Saab 9-3 production. The C25XE is a general GM European engine; German design, UK built. When used in Saabs the configuration was the familiar transverse mounting and even used the same F25 gearbox. Engine management was the same Bosch Motronic.
The early Saab 9-5 range also offered another GM V6 - the 3.0L X30XE from the Omega.


F28 6 Speed Manual Gearbox (from Calibra Turbo)

A popular transplant is the F28 six speed manual 4x4 gearbox from a Calibra Turbo. This is originally a four wheel drive gearbox but can be converted to 2WD using a popular conversion kit. The closer ratio's and extra gear change the characteristics of the C25XE power delivery and does away with the 'lazy' long gearing on the standard F25 V6 gearbox. Matched with a lightened V6 flywheel this mod changes the driving characteristics of the V6 considerably.


A quick mention of the suspension setup is important as proper uprated suspension/wheels/tyres can effectively increase your speed while cornering.
The stock V6 comes leaves the factory with special V6 rated springs (20mm lower than 8v/16v) and standard Calibra shock absorbers (same a 16v). This setup can be improved as stock V6's feel a bit 'woolly' and too refined in their ride.
The rare V6 DTM edition used matched irmscher lowered springs and shorter shocks. Springs were 25mm lower than the standard V6.
One of the highest regarded aftermarket setups is Eibach springs with Koni adjustable shocks. For a more subtle, less harsh ride, Eibach springs with fixed Bilstein shocks have also had good reports.

Notes on lowering - Dropping ride height below standard can cause/highlight the following issues:

Wheel camber misalignment - incorrect camber causes uneven tyre wear. A camber correction kit can be fitted to cure this (Irmscher or aftermarket) but long term results can be disappointing.
Fuel pipe rub - drastically lowering the V6 can force the under car fuel pipe to rub against suspension components. This can eventually lead to a ruptured fuel line.
Wheel Offset - Lowering your Calibra on aftermarket alloy wheels with the wrong offset can lead to increased tyre wear/camber problems. Always try to use wheels with an offset in the 45 to 49 range.


C25XE Rolling Road Results

power  mods
164 bhp @ 6380 rpm  Standard V6
173 bhp @ 6330 rpm  Standard V6
180 bhp @ 6066 rpm  Standard V6
182 bhp @ 6050 rpm  Open induction kit, custom stainless steel cat back exhaust
186 bhp @ 6040 rpm  Standard V6 with cold air feed
187 bhp @ 6080 rpm  Stainless Steel custom cat back exhaust (dual)
189 bhp @ 6028 rpm  Courtenay air box, Courtenay remap, lighter flywheel, 4 bar FPR, Magnecor leads, iridium plugs
190 bhp @ 6290 rpm  Stainless Steel custom cat back exhaust (mono), 3 litre cams
190 bhp @ ???? rpm  Enlarged throttle body, open induction kit with cold air feed, custom cat back exhaust (mono), Superchip
193 bhp @ 6074 rpm  Standard V6 with cold air feed
196 bhp @ ???? rpm  Enlarged throttle body, cold air feed, stainless steel cat back exhaust (mono)
200 bhp @ 6100 rpm  Cold air feed, de-cat pipe, stainless steel straight through exhaust

C25XE Rolling Road Test Graphs

stock V6

stock V6

V6 with custom induction and s/s cat back exhaust

factory V6 test graph



information gathered from various sources

< TOP >