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.

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. It 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).

Engine Technical Specifications

Engine	RB25DE	RB25DET
Model Years	1991-2002	1993-2004
Displacement	2.5 L (2,498 cc)	2.5 L (2,498 cc)
Aspiration	Natural Aspiration	Turbocharged
Configuration	Straight-Six	Straight-Six
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

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)

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.

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 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.

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.

The Top RB25DET engine upgrades are:

• Boost Controller
• Exhaust
• Front Mount Intercooler
• Tuning
• 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.

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!