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.
Before you consider tuning ...........
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.
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.
IDENT ECU 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
|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
V6 with custom induction and s/s cat back exhaust
information gathered from various sources
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