Extreme Engine Tech: Jim Wolf Technology's Turbo Nissan QR25DE Powerhouse Part 1

Jim Wolf Technology's Turbo Nissan QR25DE Powerhouse - Part 1

By Mike Kojima

Due to some rather unfortunate decisions within Nissan's corporate structure, the QR25DE engine has found its way, probably by default to the top of Nissan's product line up as its flagship performance 4 cylinder engine.  The QR25DE is found in the B15 Sentra SE-R and Spec-V as well as the ultra hideous Sentra B16 SE-R and Spec-V.  In North America, the QR is also found in the Altima, Rogue and as a base Frontier truck engine.

It is easy to see how the SR20 rod shown above on the left is much stronger than the QR part.  Look at the general lack of robustness of the QR25DE part on the right and note the thinner rod bolts. The stock SR rod can easily withstand 20 psi of boost and over 400 hp stock.  That cannot be said of the stock QR parts.

We have a love/hate relationship with the QR25DE.  We felt that Nissan completely missed the boat with the engines first iteration which was released in 2002.  The engine had a lot of torque and a wide powerband, but the power output trailed it's competitors such as the Honda K20A, K24, F20 and F22. The QR did not have the delightful high revving fistfuls of power that these Honda engines made, nor the boat anchor robustness of the Toyota 2AZ-FE. The engine's low, class trailing 6100 rev limit simply hamstrung it from a power perspective and made the engine very unrewarding to drive.  The engine was surrounded by controversy from its first release, starting with Sport Compact Car Magazine's discovery and publication that Nissan had seriously stretched it's power claims.  This was confirmed by owners who independently dyno tested their own cars.

The QR25 has an open deck.  Although this offers improved cooling, open decks often suffer from cracked cylinder walls, poor head gasket sealing and cylinder flexing when subjected to high boost and increased rev limits.  We are not planning for high boost levels or really high rpm so we did not bother to sleeve and close up the block deck like many Honda tuners do.  Sleeving blocks with integrated decks also creates issues with block structural integrity and flex which often hurt bearing life and ring seal so we are going to first try to see if we can make the engine live without sleeves.

The QR25DE seemed to have been released before development was complete as the QR initially was plagued with horrendous design related quality and reliability problems that were so bad, they completely tainted the halo image that the Sentra SE-R had made with enthusiasts over the years.  The QR's problems were unheard of as Nissan has historically always had a reputation of releasing solid, industry leading engines with exceptional performance and durability.

Here is a sectioned SR20 block compared to a QR25 block.  As you can see the QR block is taller to accommodate its much longer stroke at 100mm vs 86mm.  Note the thinner sections of the block.  Even though the QR has 500cc's more displacement it is actually a lighter engine.  Modern casting techniques and computer modeling has allowed Nissan to pare away a lot of excess metal.

Compared to the late great SR engine family, the early Nissan QR25DE has a spotted history as a performance engine.  In the performance world it has earned a reputation for being fragile and the many recalls that Nissan has had for serious problems from blowing head gaskets to blowing rods out the side of the block has furthered this reputation.  The engine's redeeming features are a cylinder head that flows extremely well, a light overall weight and a strongly supported crank and lower end. A fragile engine is not going to work for our time attack Spec-V so we are going to address the engine’s weak points and see if we can turn the QR into a solid performer capable of racing levels of performance and reliability.

When comparing both side by side, its easy to see that the SR20DE rod and piston on the bottom are much stronger than the QR25DE parts on top.  Compare the thickness of the rod beams.  This is not entirely a result of poor engineering.  The SR20DE was designed to be a robust turbocharged engine while the QR25DE was designed to be lightweight for its displacement.

 In 2006 Nissan revamped the QR with many improvements, mainly those which strengthened its reciprocating parts allowing a higher 7000 rpm redline.  This much improved engine also made considerably more power.  This is how the engine should have been in its initial release and if it had been so, the B15 Sentra SE-R Spec-V might have become a lovingly adored vehicle like its predecessors.

The QR block makes extensive use of external ribbing to strengthen it without excessive weight.  The QR25DE at 2500cc is 500 cc larger in displacement than the 2000cc SR20DE while weighing less, a significant feat.

Jim Wolf Technology, perhaps the top Nissan tuner is North America was drafted for our engine development program.  JWT brings over 25 years of Nissan experience to our program. We immediately decided that the engine had to be turbocharged for reliability.  The QR25DE has a long 100mm stroke and a short 143.05mm rod with an 89mm bore. Its 1.43:1 stroke to rod length ratio is the worst of any production engine by far.  Obviously this engine was designed for low end torque with no regard for controlling piston speed. 

The QR block features this bed plate which firmly holds the crank in place.  This is much stronger than the traditional main bearing caps that most engines use and is a feature usually found only in racing engines.  The bedplate supported bottom end is one of the QR's best design features.

To make power naturally aspirated would mean significantly increasing its low 6100 rpm redline.  Long stroke/short rod engines have difficulty staying together at high rpm.  The RTR World Challenge SE-R would go through 3 engines per race weekend.  As we don't have a factory works team budget; to ensure reliability, we decided not to exceed 7000 rpm.  With such a low redline we would have to turn to forced induction to make power.

The QR crank on the bottom, is only half counterweighted compared to the fully counterweighted SR crank pictured on top. The SR crank's counterweights are easier to see here.  A fully counterweighted crank is less stressed by torsional vibrations at high rpm.  A bad feature of the QR crank is the lack of overlap between the rod and main journals.  This means that the crank is stroked to the limit and is weak in the area between the journals.