Nissan Spec-V: Part 14: Making a Motor
10/5/2006
The Nissan QR25DE has had a spotted history as a performance engine. In the performance world, it has a reputation as being fragile and the many recalls have furthered this reputation. 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 level performance and reliability.
While we have been working on the chassis at Technosquare, Jim Wolf Technology (JWT) has been working on our engine development program. Jim Wolf is perhaps the industry’s leading specialists in Nissan tuning and they bring over 25 years of Nissan experience to our program.
Early on it was decided that the engine had to be turbocharged. The QR25DE is a long stroke, short rod engine designed for low end torque. To give power to the naturally aspirated engine we would have to significantly increase its low 6100 rpm redline. Long stroke engines have difficulty staying together at high rpm so 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.
One for the first issues that JWT addressed in our motor is the problems of seizing balance shafts. If you raise the revolution limit past the low stock limit of 6100 rpm, the balance shafts seize in their bores creating a chain of events that destroys the engine. Balance shafts works to cancel out 4-cylinder engines inherit up and down shaking moments and spin, at twice the crank rpm. This means that at 6100 rpm the shafts are zinging at 12200 rpm.
The stock balance shaft is in the bottom of the oil pan submerged in oil. It is the weakest link in the chain when the QR25DE is modified.
JWT’s balance shaft eliminator kit removes the problem and features a pan baffle and windage tray to help control oiling better as well.The balance shafts are submerged in the oil pan and when spinning at 12,000+ rpm they churn the oil to a frothy mixture of oil and air. If ingested by the oil pump, this aerated oil can cause the bearing failures that this motor has.
JWT removes the balance shaft assemblies to address these problems. On the QR it’s a separate assembly bolted to the main caps. JWT used their balance shaft removal kit, which has spacers for the main bolts, plugs for the oil passages and instructions on how to remove the drive chain for the shafts. The kit also comes with a windage tray to help speed the oil return off the crank and free up some power. The kit also includes a pan baffle to help ensure that the oil pump can still be submerged in oil even under hard cornering. The total oil capacity is increased by a quart as the balance shaft assembly is submerged in oil and its removal makes room for more oil. Eliminating the balance shafts also increases oil pressure and makes more oil available for the main and rod bearings. The balance shaft removal kit eliminates several sources of potential failure and hardly increases the level of detectable vibration produced by the motor. As a very positive side effect, the kit frees up an additional 8-10 wheel hp over stock.
Another issue is that the QR has an extremely long 100 mm stroke, which means substantial piston speeds. High piston speeds cause, the piston rings to have difficulty sealing and loose compression pressure and power. High piston speeds and the related poor sealing of the rings are other factors contributing to excessive oil burning. With a long stroke and relatively short rod, there is a lot of side loading going on the cylinder walls as the piston moves up and down in the bore. This creates friction and wear inside the engine at high piston speeds and heavy loads.
Since it was designed for clean emissions, the QR has pistons with the compression rings moved very high on the piston. The crevice created in the area from the top of the piston to the top of the first compression ring is a trap that hydrocarbons can hide in increasing emissions. Moving the rings up reduces the size of this trap area but makes the top of the piston weak and susceptible to breaking under the pressure of turbo boost.
To help ease issues with side loading and piston speed JWT decided to use the longest connecting rod possible. A longer rod slows piston acceleration away from top dead center and reduces angularity of the rod and crank reducing side loading. Strong forged pistons were designed to lower the compression from nearly 10:1 to 8.5:1 and move the piston pin as high as possible to make room for a longer rod.
When compared to the SR20DE piston on the right, you can clearly see how the ring lands for the QR25 are thinner, and weaker.
A close up of the forged JWT piston is shown here in place, taken during the mock up fit up in the sectioned block.
The longer rod and piston were fit up in a sectioned scrap block to assure that there would be no contact issues internally.
JWT commissioned Crower to make the longer stronger rod.
Here is a sectioned SR20 block compared to a QR25 block. As you can see the QR block is taller in order to accommodate its much longer stroke.
The forged JWT piston has a dish to lower the compression ratio to a more turbo friendly 8.5:1
The JWT pistons feature full skirts because a strut type piston although lighter, may not have enough skirt contact area to avoid scuffing when you consider the side load of the long stroke short rod engine. A pressure equalization groove and lateral gas ports were used between the compression rings to help them seal better under high piston speeds. Dual pin oiler holes were also used to ensure good lubrication to the piston pin.
Undercrown milling was used to lighten the piston as much as possible to reduce the stress on the rod and crank.Longer JWT rods were used, replacing the QR25 rods. We have included photos of the QR parts in comparison with the popular SR20DE engine to show the internal weakness of the QR compared to the SR, the engine that was the power plant of the former SE-R.
A damaged QR25 motor was cut apart to study the internal clearance with the parts and the fits ruled as acceptable. Follow us in the next installment of our project to see other tricks that Jim Wolf Technology used in the build up of our motor.
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 achievement.
The QR block features a bedplate, 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 QR block and bedplate are shown here with the crankshaft in place. Note the full support of the crank in the block.A negative 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.
The QR25 has an open deck. Although this offers improved cooling, when subjected to high boost and extreme rpm this sort of deck often suffers from cracked cylinder walls, poor head gasket sealing and cylinder flexing. We are not planning for high boost levels or high rpm so we did not bother to sleeve and close up the block deck.
When compared to the beefy SR20 crank, it is easy to see that the QR lacks any overlap of the rod and main bearing journals. The QR crank is only half counterweighted compared to the fully counterweighted SR crank. 
The SR crank’s counterweights are easier to see here. A fully counterweighted crank is less stressed by torsional vibrations at high rpm. 
It is easy to see how the SR20 rod and crank shown above is much stronger than the QR part. The stock SR rod and crank can easily withstand 20 psi of boost and over 400 hp stock. 
When looking at the rods big end, you can see that the SR rod on the left is clearly much stronger than the QR rod.