The Guide to the Nissan RB25 Engine. Though not as well-known as some of its contemporaries, Nissan’s RB25DET was one of the best tuner engines of the 1990s. Nissan first introduced the normally aspirated version, the RB25DE, in 1991, followed by the turbocharged variant in 1993. Since its introduction, the RB25DET has been praised for its ability to provide a large amount of power in a consistent manner. The RB25 is frequently compared to its older brother, the RB26 twin-turbo (RB26DETT).
This tutorial will teach you all there is to know about Nissan’s RB25 engine. We’ll start with a brief history of the engine before delving into the technical specs, engine specs, typical problems, and, most significantly, performance and mods.
History of the Nissan RB25 Engine
Nissan first offered the RB25DE in the Skyline 2.5 GTS-25 Type X and Laurel Medalist in 1991. The new RB25 replaced the previous and frequently used RB20. The RB25 was also used in the Cefiro in 1992 and the Stagea 20s in 1996. Nissan introduced the turbocharged variant with the Skyline 2.5 GTS-25t in 1993. Nissan eventually used the RB25DET in the Cedric/Gloria 2.5 Gran Turismo, Laurel 2.5 Club S, and Leopard 2.5 XJ Four.
The RB25DE is rated at 180-197 horsepower and 170-188 pound-feet of torque by Nissan. The RB25DET increases power slightly, producing between 235-276 horsepower and 200-246 lb-ft of torque. Though Nissan discontinued production of the engine in 2004, in 2019, the NISMO Heritage Programme began making parts for older engines, such as the RB25, available again.
Nissan released Nissan Ecology Oriented (NEO) models in addition to conventional RB25s. These RB25s were outfitted with NEO heads for improved emissions, allowing them to be classified as Low Emissions Vehicles (LEV).
RB25DET Engine Swaps
While it was out of production for many years, the RB25 became an extremely popular swap engine. Many people have installed RB25s in their 200sx (S12) and 240sx (S13/S14) to replace the stock SR20DET and KA24DE engines.
The RB25 isn’t as well-known or celebrated as the RB26, but it still has a devoted following. It also has a large aftermarket community, both in Japan and in the United States, which is somewhat remarkable given that it was never available in the United States.
Nissan exclusively sold Skylines in Japan, as well as temporarily in Europe and Australia, and never in North America. However, RB25s have been legal for import into the US since 2014 (RB25DE) and 2018 (RB25DET), so they have finally begun to make their way over.
The RB25 was eventually succeeded by Nissan’s VR and VQ engine series in the early 2000s.
Technical Specifications for the RB25DE and RB25DET Engines
Nissan RB25 Automobile Applications
RB25DE Vehicle Applications
- Nissan Laurel (C33, C34, C35) 1991-2002
- Nissan Skyline GTS (R32, R33, R34) 1991-2001
- Nissan Cefiro (A31) 1992-1994
- Nissan Stagea 20 (WC34) 1996-2001
RB25DET Vehicle Applications
- Nissan Skyline GTS (R32, R33, R34) 1993-2001
- Nissan Laurel (C34, C35) 1994-2002
- Nissan Stagea 25 (WC34) 1996-2001
- Nissan Cedric/Gloria (Y33, Y34) 1997-2004
- Nissan Leopard (Y33) 1997-1999
Basics of Nissan RB25DE and RB25DET Engine Design
With the exception of the turbocharger, the RB25DE and RB25DET engines are very identical. The engine code is as follows: RB – “RB” engine series; 25 litre displacement; D – DOHC; E – Electronic Fuel Injection; T – Turbocharged. Due to the lack of boost, the compression ratio on the RB25 turbo is 8.5:1, whereas the naturally aspirated version is 10.0:1.
Both engines are straight-sixes with cast iron blocks and aluminium heads. They also have DOHC twin-cam configurations with four valves per cylinder, for a total of 24 valves. The bore and stroke of the RB25 block are 86mm and 71.7mm, respectively, and it is based on the previous RB20 block. The pistons and rods were borrowed from the RB20 as well, but they were stronger and larger.
Originally, the RB25DE did not have the Nissan Valve Timing Control System (NVTCS), but Nissan began employing it on the intake cams in 1993. Since its introduction in 1993, the RB25DET has had NVTCS on the intake cams. The RB25DET turbo is a Hitachi 45V1 with a ceramic wheel and hybrid T3/T28 housing/internals.
Nissan updated the RB25 series and made several improvements in 1995. The turbo was significantly larger (45V2), but the reported horsepower remained unchanged – a likely indication that it produced more than the 276 horsepower advertised. New electrical systems, ignition coils with built-in ignitors, a new airflow metre, mass airflow sensor, throttle position sensor, camshaft position sensor and ECU were also included.
The NEO RB25 Variants
Nissan released the NEO version of the RB25DE and RB25DET in 1998. Nissan Ecology Oriented (NEO) technology was utilised on the cylinder heads in these models to reduce emissions and boost gas efficiency. The NEO heads used solid lifters rather than hydraulic lifters like the non-NEO heads, as well as redesigned cams. The NVTCS remained, but it now had an on/off solenoid. Nissan also enhanced the flow of the intake manifold.
The RB25DET turbo was also increased in size, and the ceramic disc was replaced with a more durable steel version. The turbo version’s compression ratio was raised from 8.5:1 to 9.0:1. Nissan provided forged connecting rods from the RB26DETT and an oil pump from the N1 standard RB26 to the RB25DET NEO.
While it wasn’t a major change, it did result in improved flow and performance.
Despite the improved pollution technology, the power of the NEO engines increased. The RB25DET increased its horsepower from 250 to 276 – albeit it is actually closer to 300 hp.
RB25DET versus RB26DETT Nissan
Since Nissan released the RB25DET in 1993, enthusiasts have been comparing it to the somewhat larger RB26DETT. Both engines have some similarities, but they are vastly different. The RB26 block is essentially a stroked version of the RB25, having a 73.7mm stroke vs 71.7mm. It has a total displacement of 2.6 litres, which is just 0.1 litre more than the RB25.
The RB26 has twin turbos rather than the RB25’s single turbo, however they are significantly smaller (T25). The RB26 lacks NVTCS as well, though it can be added as an aftermarket upgrade using HKS’ V-Cam system. Except for the RB25 NEO, which has RB26 forged steel connecting rods, the internals are different and stronger in the RB26.
In general, the RB26DETT produces more horsepower than the RB25DET. It has reduced lag and stronger internals, which is why the NEO version gets the RB26 rods. Nonetheless, the two are fairly comparable, and the RB25 is no slouch. Consider the RB25 vs. RB26 discussion to be analogous to the 1JZ vs. 2JZ debate. The RB25/1JZ is a little smaller and less well-known than the RB26/2JZ, but it is still a fantastic engine.
Nissan used the RB25 significantly more extensively, as the RB26 was only used in the Skyline GT-R and briefly in the Stagea 260RS, whilst the RB25DET was used in not just the Skyline GTS-4/25, but also the Stagea, Laurel, Cedric/Gloria, and Leopard. As a result, RB25 blocks and components have long been more readily available than RB26 blocks and parts.
However, with the NISMO heritage programme now rebuilding older RB blocks and parts, the RB25 no longer has the same advantage. While RB25 swaps may have previously outnumbered RB26 swaps, this may change in the future.
Nissan RB25 Common Issues and Reliability
The RB25DE and RB25DET, like the rest of the RB series, are noted for their dependability and lifespan. The normally aspirated variant has been known to last a little longer, but both can handle a lot of miles. With proper maintenance and care, they can easily break the 100,000 mile (160,000 km) barrier, and 200,000 mile (320,000 km) instances are not unusual.
The most important consideration for the RB25 series will be timely and adequate maintenance. As we’ll see, the RB series has a history of oiling problems. It is critical to ensure that proper oil changes are performed every 3-5,000 miles.
The timing belt is also a recognised vulnerability in RB engines, and it should be replaced every 60,000 miles at the most. Because the RB25 is an interference motor, a broken timing belt can have serious – and costly – consequences.
Crank Collar and Oiling Problems on the RB25
The crank collar and the oiling system are the two major issues with the RB25. This isn’t limited to the RB25 series; the RB26 has also reported oil pump troubles, and the RB20 has its own oiling system issues.
Problem and Solution for the RB25 Oil Control System
The RB25’s oil control problem is that the oil pump pours too much oil into the cylinder head at high RPMs. Because the oil return passages are too small, oil pools on top and does not drain back down into the oil pan.
While this isn’t very common in street cars, it can cause serious oil starvation issues on the track. Oil escapes from the cam breathers and into the inlet system, diverting it away from the rotating assembly that requires it.
There are five ways to resolving the RB25 oil control issues. To begin, close the rear oil feed gallery to prevent extra oil from entering the head. Second, you should add a restrictor to the open oil gallery to better control the oil flow to the head. Third, for improved head drainage, run an external oil line from the head to the sump. Franklin Performance’s Head Oil Drain Kit is designed for the RB series.
Fourth, machine out the oil return galleries by 1mm to improve drainage. Finally, machine around the galleries to allow them to absorb more oil (and thus drain it faster). After you’ve completed these steps, you can increase oiling by running a larger sump and oil pump. A dry sump is the ideal choice for really high horsepower setups.
Here’s an amazing guide with a more extensive step-by-step procedure for resolving the issue. The following issue with the crank collar is also addressed in the book.
RB25 Crank Collar Problem and Solution
The RB25 crank collar problem is common on various RB engines, including the RB20DET and pre-1993 RB26DETT. The crank collar has an issue since the part of the crank’s snout that meets the oil pump drive is too small and easily wears. Both the crank and the pump can be damaged, resulting in cracking and breakage of the oil pump gear and a complete loss of oil pressure.
The problem is compounded by frequent high RPM engagement, and as you may understand, no oil pressure at high RPM spells disaster. This is most likely the most common problem with the RB25.
To correct the issue, the stock crank collar must be taken off and replaced with one that allows for greater engagement of the inner oil pump gear. Franklin Performance offers an RB25 crank collar.
Aside from these two major issues, the RB25 series will run for many miles as long as proper maintenance is maintained.
Nissan RB25DET Performance and Modifications
Both stock and modded, the Nissan RB25 has a solid reputation for performance. While Nissan listed the rated horsepower of the RB25-equipped GTS as 276 horsepower, that has always been regarded as a low estimate.
Because to the “Gentleman’s Agreement” that the main Japanese automakers signed, Nissan, Toyota, and Honda all pledged to keep their engines under 280 horsepower. They did this to discourage manufacturer horsepower battles, which are typical in America.
However, most manufacturers were known to disregard this, and several models are thought to far exceed the stated 280 hp limit. One of these was the RB25DET-powered GTS, which was expected to produce more over 300 horsepower.
Power Limits of the RB25DET
The RB25 is regarded a relatively strong engine that handles modifications well; with simple internal upgrades, it can take some considerable power, akin to the RB26DETT. The block itself is capable of producing well over 700 horsepower with ease – pretty bad for a 2.5 litre. At 450 horsepower, the pistons and connecting rods must be upgraded, even on the NEO with RB26 rods. Head studs are also a good idea, as are the above-mentioned crank collar and oil control modifications.
The following are the top RB25DET engine upgrades:
- Boost Control Unit
- Intercooler with Front Mount
- Turbo Enhancement
These are the best RB25DET upgrades. Because the majority of RB25s will be swaps or rare spec Skyline GTS models, finding aftermarket parts in the US can be difficult. Keep that in mind before beginning your RB25 build, because if you can’t do the fabrication yourself, you’ll want to start chatting with a business as soon as possible to start planning.
Boost Controller RB25
An aftermarket electronic boost controller should be one of the first modifications made to any RB25DET setup. The stock boost controller has two boost modes: high and low. The low boost mode operates until 4,500 RPM, at which point the high boost mode takes over. The low boost pressure is 4-5 PSI, and the high boost pressure is 7-8 PSI.
If you still have the stock boost controller, you may tweak it to always run in high mode by following this instructions. When the ground is activated and the circuit is complete, the system switches to high mode. The hack ensures that the circuit is always complete, allowing you to run in high mode at all times.
An electronic boost controller is another possibility. You can tune the PSI to anything you want with them, and they’re really simple to use. We don’t recommend pushing the stock turbo over 11-12 PSI, with 10 PSI being a decent sweet spot. Increasing boost is the simplest and quickest way to add horsepower to the RB25DET.
The Greddy Profec B Spec II was the favoured RB25 boost controller for a long period. Greddy, on the other hand, has upgraded to the Profec OLED Boost Controller, which is currently the best alternative.
Exhaust Upgrade for RB25
The RB25 exhaust, including the dump pipe and the catback, will be upgraded next. The dump pipe connects to the exhaust manifold and is also referred to as a downpipe. The cat-back connects to the dump pipe and completes the exhaust system by passing it via the silencer.
Adding a 3″ high-flow dump pipe and a full 3″ cat-back exhaust will increase your wheel-horsepower by 10-20%. The lack of back pressure increases turbo spool and makes the car considerably more responsive in the low end. We highly recommend the Tomei Dump Pipe for the RB25.
Front Mount Intercooler Upgrade for the RB25DET
After upgrading the exhaust and starting to push more boost, the next step is to upgrade the intercooler. The intercooler in the Skyline GTS is located behind the radiator, which significantly reduces its efficacy. While it is beneficial to the radiator, you really want that increased airflow to be cooling your boost.
The intercooler is routed to the very front of the engine bay, directly under the bumper, for maximum airflow. This means the intercooler can cool more efficiently and recover faster. While it will not add horsepower on its own, it will prevent power loss due to heat soak on hotter days and during long pulls.
Turbo Upgrade for the RB25DET
After you’ve completed the above modifications, your next step will most likely be to consider increasing the boost with a larger turbo. The factory turbo has a maximum output of 350 horsepower (13.5 PSI), and running it at full boost will quickly wear it out and lead to premature failure.
Adding a larger turbo allows you to run more boost consistently. A larger turbo is required if you want to consistently push 350 horsepower or more. The HKS 2835/3040/3037, HKS GT-RS, and GCG GTS-T Highflow turbos are among the most common turbos for mid-ranged RB25DET builds. These turbos will provide an excellent balance of spool and mid-range power without running out of steam on the high end.
Tuning the RB25DET ECU
Tuning your RB25 ECU is one of the most cost-effective upgrades accessible. Having your car tuned will ensure that all of your changes work together nicely and deliver their maximum reliable output.
One advantage of installing larger turbos, larger exhausts, and greater cooling is that you can run more boost, more ignition timing, and more ideal air-to-fuel ratios. Tuners handle all of this to ensure that your RB25 produces maximum power while remaining safe.
The A’PEXI Power FC standalone ECU system will be used as the standard tuner for the RB25 in the Skyline GTS. The Power FC comes with many pre-loaded basic maps and allows full parameter adjustment across the board. If you have prior tuning experience, you can make these adjustments yourself, or you can have them made by a competent local tuner.
Tuning will easily add 10-20% more power to what you’re already creating, and as previously said, it’s beneficial to the health and safety of your engine. You don’t want to discover, after a melted piston, that your fueling was too lean.
Related : The Nissan RB26DETT Engine Manual
Modifications for the RB25DET
In addition to the mods listed above, keep these supporting mods in mind. Increasing airflow and boost pressure will demand changes to the oiling and fueling systems. To begin, make sure you’ve addressed the oil control issue noted previously, and if you’re constructing the engine from the ground up, certainly obtain the updated crank collar.
Aside from that, you’ll want to get an upgraded water pump and a larger N1 oil pump, though the larger oil pump will only help if you’ve done the oil control mod. An enhanced intake is also a handy improvement, but it isn’t completely necessary until you obtain a bigger turbo. Larger injectors, a larger fuel pump (such as a Walbro 255 lph), a fuel pressure regulator, and an improved head gasket are also recommended.
As previously stated, stock internals are adequate up to 450 horsepower, after which forged pistons, rods, and head studs are required. When you reach 400+ horsepower, larger duration and higher lift cams are also a good upgrade.
Legacy Nissan RB25
Overall, the Nissan RB25DE and RB25DET are two incredibly stout and reliable motors. The turbo version is capable of some serious performance, while the naturally aspirated version is no slouch for mid-90s straight-six. They are reputed to have a long service life and just require minor maintenance.
The RB25DET is one of the most popular Nissan engine swaps among modern enthusiasts, and it’s clear why. With just a few upgrades you can easily start pushing north of 350 horsepower while revving out to a screaming 7,500 RPM. Few engines sound better than Nissan’s RB series, and the RB25DET is no exception. Listening to the RB25 with an open exhaust is simply magnificent.