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

One of the QR25DE's redeeming points is its extremely strong crank support system with the bed plate giving the block maximum structural strength and giving the crank full support.  This is one stout lower end.

Another issue is that the QR has an extremely long 100 mm stroke with a short 143mm rod which means massive piston speeds.  High piston speeds causes the piston rings to have a hard time sealing at high rpm, losing combustion pressure and power.  High piston speeds and the related poor sealing of the rings are a factor 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 RPM and heavy loads.

The lack of journal overlap in the QR25DE crank is easy to see in this picture.

This is because the point of highest acceleration is when the rod to crank throw relationship is at 90 degrees. With a short rod this point is reached early in the stroke so the piston is rapidly accelerated away from TDC putting a lot of stress on all of the reciprocating parts of the engine.  At this point with the piston close to TDC the combustion pressure is still relatively high and the piston wants to cock under load and dig into the cylinder wall.  This also causes a lot of wear, stress and friction.  Since the piston has to accelerate quickly away from TDC there is also little time to fill the cylinder which also has a negative effect on volumetric efficiency.

JWT commissioned Crower to make the longer stronger rod.  The high quality Crower rod is an order of magnitude stronger than the stock part.  It is 5.2 mm longer as well.

To help ease these issues, JWT decided to use the longest connecting rod possible without resorting to having the wrist pin boss intrude into the oil ring groove as this usually has a negative effect on oil consumption.  Crower was commissioned to build the rod with a length of 148.25mm.  At 1.4825:1 the JWT QR's stroke to rod length ratio is still worse than any other current production engine but it is at least close to the venerable and proven Honda F22's 1.49:1.  JWT balanced increasing the rod ratio with doing dodgy things with the oil control ring to come up with a reasonable compromise.  We think Nissan should have designed the engine with a 15mm taller deck height!

You don't need to be an engineer to see that the Crower rod is much stronger than stock!

A problem with QR's that have been turboed is piston failure. Being designed for clean emissions the QR has pistons with the compression rings moved very high on the piston to reduce crevice volume. 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 which increases emissions.  Moving the rings up reduces the size of this trap area for lower hydrocarbon emissions but thins out the top ring land.  This makes the top of the piston weak and susceptible to breaking under the pressure of turbo boost or detonation.

When compared to the SR20DE piston on the right, you can clearly see how the ring lands for the QR25 are thinner, and weaker.  It is clear to see that the QR25DE was designed for emissions and the SR20DE, designed to be turbocharged.