The comprehensive guide to the RB25 Engine

"All you need to know about RB25DET Engine Tuning"

With the help of TMS Motorsport we are pleased to provide an all you need to know guide to tuning the popular RB25 engine.

This has to rate as one of best engines to base a tuning project on, with massive tolerance of power gains, and many tuning part options.

Please note that cars fitted with the Tiptronic require a totally different approach to tuning, as the gearbox can't handle major power gains. (Triptronic are R34 only. R33 are Normal 4 speed Autobox with snow, power and economy mode)

6 Cylinder 2,498cc 24v valve turbo engine.

RB25DE - NON-TURBO twin-cam 180 - 200 hp @ 6000 rpm,

RB25DET - twin-cam TURBO (T3 Turbo) 245 to 250 hp

The NEO head fitted from 1998 helped with emissions, it also had solid lifters and used a different cam profile.

RB25DE NEO- NON-TURBO twin-cam 219 hp @ 6000 rpm

RB25DET NEO - twin-cam TURBO 276 hp @ 6400 rpm

  • Spec 1 – Has an ignition amplifier. Coil packs are marked I + E, coil pack loom is different, therefore engine loom is slightly different. There were from early 1993 to 1994, but doesn’t seem to be a hard and fast rule. The S1 also had an aluminum compressor wheel, later editions were ceramic.
  • Spec 2 – Engine doesn’t have ignition amplifier. Coil packs are marked IB G +, coil pack look is different, which makes engine loom slightly different. Camshaft angle sensors were revised for a Mitsubishi CAS (early ones suffered from broken positioning teeth though so the Black CAS was introduced.)
  • Spec 3 - Engine on very late R33, very few about. From late 1997 onwards. They run different camshafts, the Crank Angle sensor which went on to be used in the R34 RB25DET Neo engine.

Before you start, please have a goal in mind for your RB25 engine. TorqueCars hate to see the pick and mix approach to tuning, where various parts are just added to see what the power gains are then they are swapped out or other parts added.

So set a power goal and then look at the components you'll need to achieve this.

We would suggest that the following bhp targets of 300, 350 and 450 are good goals, as they have fairly clear upgrade paths.

Anything over 500 requires quite extensive work and a totally different range of parts so if this is your ultimate goal bear this in mind when fitting and sourcing parts.

Engine tuning

Fast road camshafts usually bump the power through the rpm band, you could sacrifice a little low end torque but your top end will be higher.

Race camshafts, bump the top end band but as a result the car will not idle smoothly and low end power nearly always suffers.

On a car used daily, it should ideally be matched your engines power to your typical driving style.

There are tuning limits for every engine, with some being over specified and some only able to handle stock power, thankfully the RB25 block is build to handle lots of power.

However there are still power limits for stock internals, and as a few different versions of the RB25 were made it is possible to swap out parts to get the best of all worlds for your project.

We recommend you find these restrictions and upgrade to forged components to cope with the power.

Standard Boost is 2 stage. 0.5 bar until NVCS (about 4.5k revs), then 0.7 bar until 7k rev standard rev limit. This helps maintain fuel economy and keeps it drivable in slow traffic, the stage 2 boost option is usually the first thing an RB25 tuner will do and makes an easy power gain.

The mod involved quite simply cutting the black wire on the solenoid and connecting this to ground on the car body, forcing the car to stay in stage 2 boost all the time.

There are plenty of detailed step by step instructions around in our forum, and many popular skyline forums out there.

Another way to achieve higher boost at low rpm is by fitting a boost controller. You can usually set this to 0.85 bar on a standard turbo.

The heads were pretty much the same on the R33. The NASP (naturally aspirated, non turbo) engines don't have water and oil lines to the turbo (obviously).

The map and fuel pump and injectors also have an effect on the power gains you'll make.

Extending exhaust or intake durations can alter the power band and on most engines the exhaust and intake durations do not need to match, although most cams and DIY tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

Stage 1 mods: Fast road camshaft, Panel air filters, Drilled & smoothed airbox, Intake headers, Remaps/piggy back ECU, Sports exhaust manifold.

Stage 2 mods: Ported and polished head, Fast road cam, Sports catalyst & performance exhaust, fuel pump upgrades, induction kit, high flow fuel injectors.

Stage 3 mods: Competition cam, Internal engine upgrades (head flowing porting/bigger valves), Crank and Piston upgrades to alter compression, Engine balancing & blueprinting, Adding or Upgrading forced induction (turbo/supercharger), Twin charging conversions.

A remap should help to to establish the full potential of all the mods you've done to your RB25.

It will usually give around 30% more power on turbocharged vehicles and 15% on NASP engines, but your mileage will vary depending on the mods you've done and the condition of your engine.

Forcing fuel and air into the RB25 engine is the aim to any car tuning project.

Intake headers take the air from the air filter and allow it to be fed into the engine and mixed with fuel.

Design and flow characteristics of the Intake headers can make a big effect on to fuel engine efficiency on the RB25.

Weak spots/problems with the RB25 engine?

Running more than 0.9 bar boost on standard ceramic turbo causes premature turbo failure.

Oil cooling is required and highly recommended on spirited use, as these run quite hot so keep an eye on the oil temps.

Oil surge on high speed cornering leading to bearing failure (People often overfill standard engine by about ½ litre or more when on track, up to the H on the dipstick which is about 10mm over the full line)

  • After 300 bhp you'll need an upgraded ECU.
  • At 320-340bhp and greater you should uprate the turbo, head gasket, clutch, airflow meter and injectors.
  • Pistons are generally good for 500bhp.
  • The RB25 block can be rebored from the standard 86mm to 86.5mm or 87mm.

Fuelling

The R33 GTR Fuel pump is a straight drop in and good for 500 odd BHP so becomes and obvious choice for the tuner, with the added benefit that these are quite easy to source in the used markets.

As a rule of thumb add 20% capacity when fitting an injector, this accounts for injector deterioration and affords you some spare capacity should the engine need more fuel.

The standard fuel injectors usually hit the maximum at around 350BHP. GTR ones don’t fit, unless modified and you use GTR injector ballast (different feed and impedance) so most people go to 55cc injectors, similar to those on the better Subaru Impreza engines.

RB25 Turbo upgrades

We've seen tuners spending a lot of money on turbocharger upgrades on the RB25 only to see the engine throw a rod on it's first outing after it's been enthusiastically driven.

Bigger turbochargers commonly suffer no power at low rpm, and small turbochargers spool up more quickly but won't have the peak end engines power gains.

We are pleased that the choice of turbo chargers is always evolving and we now see variable vane turbo chargers, allowing the vane profile is altered according to speed to lower lag and increase top end torque.

Twin scroll turbo chargers divert the exhaust flow into 2 channels and feed these at differently designed vanes in the turbo. They also improve the scavenging effect of the engine.

It is not unusual that there's a restriction in the air flow sensor (AFM/MAF/MAP) on these engines when considerably more air is being drawn into the engine.

You'll see that 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor sapped bhp and torque at a much lower level.

What are the best turbo upgrades and what power figures do these support, mention a bolt on swap and also list the more complex turbo upgrades and hybrid options.

R34 GTT turbo is a little bit bigger. My preference would be 3037 garret or HS 2835 for around 400 ish bhp, more than that get a bit pointless on the road !

You may need to improve your exhaust if the current exhaust is actually creating a restriction in flow.

Exhausts

On most factory exhausts you'll see your flow rate is ok even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Sports exhausts generally help improve air flow out of the engine but avoid an exhaust that is too large or you could will reduce the flow rate. Stick to 1.5 to 2.5 inches as a rule of thumb.

Common exhaust restrictions are in the pollution control filters/catalysts installed, so adding a faster flowing performance alternative will help avoid this restriction.

Looking over the exhaust you could fit a 75mm downpipe which improves Torque and BHP by upto 35lbft and 25bhp in some cases. Then taking off the catalyst (an off road mod in most countries) will give another 10 bhp.

Please help us to complete this article, use the feedback box below if you spot any errors or have any tips or comments to pass on to other RB25 engine tuners.

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One Response to “Tuning the RB25DET engine”

  1. Ivan says:

    Hi im doing a RB25DET build looking to make 450 hp, ive made my minde on 750cc injectors and making it a topfeed to 11mm fuel rail, but i dont know what the bottom size of the injectors should be and WHAT injectors to order? Please help me with this tune,



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