Nissan RB25 Engine Guide
Though it’s not as talked about as some of its cousins, Nissan’s RB25DET was still one of the best tuner engines of the 1990s. Nissan first released the naturally aspirated version, the RB25DE, in 1991, and in 1993 they brought out its turbocharged variant. Ever since its introduction, the RB25DET has been highly regarded for its ability to make lots of power reliably. Many enthusiasts often compare the RB25 to its older brother, the RB26 twin-turbo (RB26DETT).
This guide is here to cover everything you need to know about Nissan’s RB25 engine. We’ll look briefly at the engine’s history, before digging into the technical specs, engine specs, common problems, and most importantly, performance and mods.
Nissan RB25 Engine History
Nissan initially released the RB25DE in 1991 in the Skyline 2.5 GTS-25 Type X and Laurel Medalist. The new RB25 was a replacement for the outgoing and widely used RB20. The RB25 also soon found its way into the Cefiro in 1992 and Stagea 20s in 1996. Nissan released the turbocharged version in 1993, putting it first in the Skyline 2.5 GTS-25t. Eventually, Nissan also put the RB25DET into the Cedric/Gloria 2.5 Gran Turismo, Laurel 2.5 Club S, and the Leopard 2.5 XJ Four.
Nissan rated the RB25DE between 180-197 horsepower and 170-188 lb-ft of torque. The RB25DET bumps up power a bit, making between 235-276 horsepower and 200-246 lb-ft of torque. Though Nissan initially stopped making the engine in 2004, in 2019, they started making parts for older engines, like the RB25, available again through the NISMO Heritage Program.
In addition to the standard RB25s, Nissan also released Nissan Ecology Oriented (NEO) versions. These RB25s had NEO heads for improved emissions, which allowed them to be regulated as Low Emissions Vehicles (LEV).
While it was out of production for many years, the RB25 became an incredibly popular engine for swaps. Many people have swapped RB25s into their 200sx (S12) and 240sx (S13/S14) to upgrade the stock SR20DET and KA24DE.
It’s certainly not as popular or heralded as the RB26, but the RB25 still has many loyal fans and supporters. It also has a wide aftermarket community, both JDM and in the US, which is pretty incredible considering it was never offered stateside.
Nissan only sold the Skylines in Japan, and briefly in Europe and Australia, never offering them in North America. RB25s have been eligible for import into the US since 2014 (RB25DE) and 2018 (RB25DET), though, so they have finally started to legally make their way over.
Eventually, the RB25 was superseded by Nissan’s VR and VQ engine series in the early-2000s.
RB25DE and RB25DET Engine Technical Specifications
|Displacement||2.5 L (2,498 cc)||2.5 L (2,498 cc)|
|Compression Ratio||10.0:1||8.5:1; 9.0:1 (NEO)|
|Bore and Stroke||86mm × 71.7mm||86mm × 71.7mm|
|Valve Train||DOHC, 24 Valve||DOHC, 24 Valve|
|Fuel System||Electronic Fuel Injection (EFI)||Electronic Fuel Injection (EFI)|
|Head/Block Material||Aluminum/Cast Iron||Aluminum/Cast Iron|
|Horsepower Output||180-197 Horsepower||235-276 Horsepower|
|Torque Output||170-188 Torque||200-246 Torque|
Nissan RB25 Car Applications
RB25DE Car Applications
- 1991–2002 Nissan Laurel (C33, C34, C35)
- 1991–2001 Nissan Skyline GTS (R32, R33, R34)
- 1992–1994 Nissan Cefiro (A31)
- 1996–2001 Nissan Stagea 20 (WC34)
RB25DET Car Applications
- 1993–2001 Nissan Skyline GTS (R32, R33, R34)
- 1994–2002 Nissan Laurel (C34, C35)
- 1996–2001 Nissan Stagea 25 (WC34)
- 1997–2004 Nissan Cedric/Gloria (Y33, Y34)
- 1997–1999 Nissan Leopard (Y33)
Nissan RB25DE and RB25DET Engine Design Basics
The RB25DE and RB25DET are relatively similar motors with the exception of the turbocharger. The engine code can be broken down as follows: RB – “RB” engine series; 25 – 2.5 liter displacement; D – DOHC; E – Electronic Fuel Injection; T – Turbocharged. Compression on the RB25 turbo is 8.5:1, while the naturally aspirated version is 10.0:1 due to the lack of boost.
They are both straight-six engines with cast iron blocks and aluminum heads. They also both have DOHC twin-cam setups with 4 valves/cylinder, making 24 valves total. The bore and stroke of the RB25 block is 86mm × 71.7mm, and it was based on the outgoing RB20 block. The pistons and rods were also derived from the RB20 but were stronger and bigger.
Originally, the RB25DE did not have the Nissan Valve Timing Control System (NVTCS), but beginning in 1993 Nissan started using it on the intake cams. The RB25DET had NVTCS on the intake cams from its initial introduction in 1993. The RB25DET turbo is a Hitachi 45V1 with hybrid T3/T28 housing/internals and has a ceramic wheel.
In 1995, Nissan revamped the RB25 series and made several changes. The turbo was made slightly bigger (45V2), though stated horsepower stayed the same – a likely indication it made more than the 276 horsepower advertised. Other improvements included new electrical systems, new ignition coils with built in ignitors, a new airflow meter, mass airflow sensor, throttle position sensor, camshaft position sensor, and ECU.
The RB25 NEO Variants
In 1998, Nissan introduced the NEO version of the RB25DE and RB25DET. These versions used Nissan Ecology Oriented (NEO) technology on the cylinder heads to improve emissions and increase gas mileage. The NEO heads had solid lifters, instead of the hydraulic ones in the non-NEOs, and also had revised cams. The NVTCS was still included but had an on/off solenoid now. Nissan also revised the intake manifold for improved flow.
The RB25DET turbo was also again made larger and the ceramic wheel was replaced with a stronger steel version. Compression was increased on the turbo version from 8.5:1 to 9.0:1. Nissan gave the RB25DET NEO forged connecting rods from the RB26DETT and the oil pump from the N1 spec RB26.
While it wasn’t a gigantic revision, it did make for more improved flow and better performance.
Power actually increased on the NEO engines, despite the increased emissions technology. The RB25DET jumped from 250 horsepower up to 276 horsepower – though it’s really considered closer to 300 horsepower.
Nissan RB25DET vs RB26DETT
Ever since Nissan introduced the RB25DET in 1993 enthusiasts have compared it with the slightly larger RB26DETT. The two have some similarities, but overall are vastly different engines. The RB26 block is essentially a stroked version of the RB25, with a stroke of 73.7mm vs 71.7mm. Its total displacement is 2.6 l, just 0.1 l larger than the RB25.
The RB26 has twin-turbos, instead of the RB25 single-turbo setup, though they are slightly smaller (T25). The RB26 also does not have NVTCS, though it can be added as an aftermarket mod with HKS’ V-Cam system. The internals are different and stronger in the RB26, except the RB25 NEO has RB26 forged steel connecting rods.
In general, the RB26DETT is known for making horsepower easier than the RB25DET. It lags less and has stronger internals – hence the NEO version getting the RB26 rods. Still, the two are pretty comparable, and the RB25 is really no slouch. You can think of the RB25 vs the RB26 as similar to the 1JZ vs 2JZ debate. The RB25/1JZ are a little smaller and not as well known as the RB26/2JZ, but they are still incredible engines.
The RB25 was used much more extensively by Nissan, as they only put the RB26 inside the Skyline GT-R and briefly in the Stagea 260RS. They put the RB25DET inside not just the Skyline GTS-4/25, but also the Stagea, Laurel, Cedric/Gloria, and Leopard. As a result, RB25 blocks and parts have long had better availability compared with the RB26.
However, now that the NISMO heritage program has started rebuilding older RB blocks and parts again, the RB25 doesn’t have the same advantage. While previously, RB25 swaps might have outnumbered RB26 swaps, that might change going forward.
Nissan RB25 Common Problems & Reliability
Like the rest of the RB series, the RB25DE and RB25DET are known for their reliability and longevity. The naturally aspirated version has been known to last a little longer, but both of them can take some serious mileage. They can easily break the 100,000 mile (160,000 km) barrier with proper maintenance and care, and it’s not entirely uncommon for 200,000 mile (320,000 km) examples to appear.
The main thing to keep in mind on the RB25 series is going to be timely and proper maintenance. As we’ll get into, the RB series all have documented oiling issues. Making sure proper oil changes are done every 3-5,000 miles is incredibly important.
The timing belt is also known to be a weakness on the RB engines, and should be changed every 60,000 miles at the most. The RB25 IS an interference motor, so a broken timing belt can have serious – and expensive – repercussions.
RB25 Crank Collar and Oiling Issues
The two big problems on the RB25 relate to the crank collar and the oiling system. This isn’t specific to just the RB25 series, as the RB26 has noted issues with the oil pump, and the RB20 also has its own oiling system issues.
RB25 Oil Control System Issue and Fix
The issue with oil control on the RB25 is that the oil pump pushes too much oil into the cylinder head in the upper RPM range. This leads to oil pooling on top and not draining back down into the oil pan because the oil return passages are too small.
While it’s not super common on street driven cars, on the track this can lead to serious oil starvation issues. Oil ends up coming out of the cam breathers and into the inlet system, taking it away from the rotating assembly which needs it.
To fix the RB25 oil control problems it takes five steps. First, you want to block off the rear oil feed gallery to prevent excess oil flowing into the head. Second, you want to add a restrictor to the open oil gallery to control the oil flow to the head better. Third, you’ll want to run an external oil line from the head to the sump for better drainage from the head. This Head Oil Drain Kit from Franklin Performance is made for the RB series.
Fourth, you’ll want to machine out the oil return galleries by 1mm for better drainage. Finally, you’ll want to machine around the galleries so they can take in the oil better (and thus drain it quicker). Additionally, after you have taken these steps, you can also run a larger sump and oil pump to increase oiling. For very high horsepower builds, a dry sump is the ultimate solution.
Here is an excellent guide with a more detailed step-by-step process to solve the problem. The guide also covers the below issue with the crank collar.
RB25 Crank Collar Issue and Fix
The RB25 crank collar issue is actually very common on several RB motors, including the RB20DET and pre-1993 RB26DETT. The problem with the crank collar is because the part of the crank’s snout that touches the oil pump drive is too small and wears easily. Both the crank and the pump can become damaged, resulting in the oil pump gear cracking and breaking and a complete loss of oil pressure.
The issue is exacerbated by repeated high RPM engagement, and as you can imagine, no oil pressure at high RPM means big problems. This is probably the most common issue on the RB25.
To fix the problem the stock crank collar needs to be machined off and a replacement put on that allows for more engagement of the inner oil pump gear. Here is an RB25 crank collar from Franklin Performance.
Apart from these two main issues, as long as adequate maintenance is kept up on the RB25 series will run for lots of miles.
Nissan RB25DET Performance and Top Mods
The Nissan RB25 has a solid reputation for performance, both stock and modded. While Nissan listed the rated horsepower of the RB25-equipped GTS at 276 horsepower, that’s always been considered a low estimate.
Due to the “Gentleman’s Agreement” that the major Japanese auto manufacturers engaged in, major companies like Nissan, Toyota, and Honda all vowed to keep their engines below 280 horsepower. They did so as to discourage horsepower wars between manufacturers – like is commonplace in America.
However, most companies were known to be ignoring this, and several models are thought to greatly exceed the stated 280 hp limit. One of them was the RB25DET powered GTS, which most estimate to make north of 300 horsepower.
RB25DET Power Limits
The RB25 is considered a relatively stout engine and handles upgrades well, it can take some serious power with minor internal upgrades, similar to the RB26DETT. The block itself is capable of well over 700 horsepower without issue – not too bad from a 2.5 liter. The pistons and connecting rods need to be upgraded at 450 horsepower, even the NEO with RB26 rods. Head studs are also a good idea, as are both the crank collar and oil control fixes listed above.
The Top RB25DET engine upgrades are:
- Boost Controller
- Front Mount Intercooler
- Turbo Upgrade
These are the top upgrades for the RB25DET. Since the majority of RB25s are going to be swaps and rare spec Skyline GTS models, finding aftermarket parts in the US can be a hassle. Keep that in mind before starting on your RB25 build, as if you aren’t capable of the fab yourself you’ll want to start talking with a shop as soon as you can to start planning.
RB25 Boost Controller
One of the first mods that should be made on any RB25DET build is an aftermarket electronic boost controller. The stock boost controller has two modes, a high and low boost mode. The low boost mode is active until 4,500 RPM, after which the high boost mode kicks in. Low boost is 4-5 PSI and high boost is 7-8 PSI.
If you have the stock boost controller still, you can follow this guide to modify it to always run in high mode. Basically, the system works by going into high mode whenever the ground is activated and the circuit is complete. The mod makes the circuit always complete so you always run in high mode.
The other option is to get an electronic boost controller. With these, you can set the PSI to whatever you want and it’s very easy to use. We don’t recommend running more than 11-12 PSI on the stock turbo, with 10 PSI being a good sweet spot. Upping boost is the quickest and easiest way to add horsepower to the RB25DET.
For a long time, the Greddy Profec B Spec II was the preferred RB25 boost controller. However, Greddy has upgraded to the Profec OLED Boost Controller, which is the best option now.
RB25 Exhaust Upgrade
The next step is to upgrade the RB25 exhaust, both the dump pipe and catback. The dump pipe attaches to the exhaust manifold, and is sometimes known as a downpipe. The cat-back connects to the dump pipe and completes the exhaust through the muffler.
Getting a 3” high-flow dump pipe and full 3” cat-back exhaust will net you 10-20 wheel-horsepower alone. The lack of back pressure will also increase turbo spool and make the car much more responsive on the low-end. Here is a Tomei Dump Pipe for the RB25 we highly recommend.
RB25DET Front Mount Intercooler Upgrade
Now that you have upgraded the exhaust and started to push more boost, your next step is to upgrade the intercooler. The Skyline GTS has the intercooler located behind the radiator, which really cuts down on its effectiveness. While it does help the radiator, you really want that increased airflow to be cooling your boost instead.
Routing the intercooler to the very front of the engine bay, just behind the bumper, allows for max airflow. This means the intercooler will be able to cool more efficiently and recover quicker. While it won’t add horsepower by itself, it will stop you from losing power due to heat soak on warmer days and during extended pulls.
RB25DET Turbo Upgrade
After you have done the above mods, your next step will probably be to start thinking about upping the boost with a bigger turbo. The stock turbo maxes out at about 350 horsepower (13.5 PSI), at the most, and trying to consistently run it at max boost will quickly wear it out and lead to premature failure.
Adding a bigger turbo will allow you to run more boost reliably. If you are considering reliably pushing 350 horsepower or more, getting a bigger turbo is a must. Some of the most popular turbos for mid-ranged RB25DET builds are the HKS 2835/3040/3037, HKS GT-RS, and GCG GTS-T Highflow turbos. These turbos will give a good combination of spool and mid-ranged power without running out of steam up top.
RB25DET ECU Tuning
Tuning your RB25 ECU is one of the best bang for your buck mods available. Getting your car tuned will make sure all of your mods are running together harmoniously as well as delivering their max reliable output.
One of the benefits of adding things like bigger turbos, larger exhausts, and better cooling, means you can run more boost, more ignition timing, and more optimal air-to-fuel ratios. Tuners take care of all of this to make your RB25 produce peak power while staying safe.
The standard tuner for the RB25 in the Skyline GTS is going to the tried and true A’PEXI Power FC standalone ECU system. The Power FC comes with several pre-loaded base maps and allows for customization of all kinds of parameters. Either you can make these adjustments yourself, if you have experience tuning, or you can have a reputable local tuner make the adjustments.
Tuning will easily add 10-20% in power over what you’re already making, and as stated it’s good for the health and safety of your engine. You don’t want to find out after you have a melted piston that your fueling has been running too lean.
RB25DET Supporting Mods
In addition to the above mods, you’ll also want to keep these supporting mods in mind too. Increasing the airflow and boost pressure will also necessitate improvements to the oiling and fueling systems, too. For starters, you will want to make sure you have taken care of the oil control fix mentioned earlier, and if you are building the engine from the start definitely get the upgraded crank collar.
Besides that, you’ll want to get an upgraded water pump and larger N1 oil pump – keeping in mind the larger oil pump only helps if you have done the oil control mod. An upgraded intake is also a useful mod but not totally necessary until you get a larger turbo. You’ll also want larger injectors, a bigger fuel pump (like a Walbro 255 lph), a fuel pressure regulator, and upgraded head gasket.
The stock internals, as we mentioned, are good until about 450 horsepower, after which you’ll want forged pistons and rods and head studs. Larger duration and higher lift cams are also a good upgrade once you start to reach the 400+ horsepower range.
Nissan RB25 Legacy
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 known to last for some serious mileage, and only require pretty basic 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 have or will sound better than the Nissan RB series, and the RB25DET is no exception. Just listening to the RB25 with an open exhaust is truly glorious.
Let us know about your RB25 experience in the comments below!