The SE-R's QR25DE powerplant is poised to be Nissan's four-cylinder powerhouse for many years to come. The SR20 loyalists, though, have received the new powerplant with no small amount of skepticism. The huge, 100mm stroke, lightweight construction and torque-biased tuning make it a world away from the crank-the-boost robustness and 7800-rpm redlines that the SR20 crowd is used to.
Despite the differences, the QR25 is a powerhouse in its own right. We've spent more than a year helping develop the first wave of QR25 bolt-ons with our SE-R Spec-V rally car, and have found 35-hp gains with literally no sacrifices in driveability. Peak power is about what a fully built, high-compression street SR20 would make. Torque output is more like a mildly turbocharged SR20. Because our testing occurred between rallies and while parts were being developed, our tests are not necessarily in the most logical order, and there is a hole or two in the data, but the final package we found is quite effective.
The QR25DE is a lightweight engine designed primarily for torque, responsiveness, and low noise and vibration. It's the workhorse of all base-model Altimas, and plays a performance role only when placed in the SE-R Spec-V. In both cars, however, it is in exactly the same trim. The priorities of fuel economy, torque and low noise are evident everywhere in its construction, with lightweight valve springs, spindly connecting rods and a huge stroke.
These design elements conspire to keep the rev limit low, with a brutally abrupt fuel cut coming in at 6200 rpm. Engine management is handled with a new-generation ECU that has yet to be cracked, so the rev limit cannot be moved. Even if it could, the piston speeds are already critically high. Sunbelt Performance's Jim Thompson, who built the engine for Joe Hermes' SPEED World Challenge Spec-V, says he would hesitate to push the stock internals even to 6800 rpm.
The cylinder head, on the other hand, flows exceptionally well--better than any SR20. In many ways, the QR25 is reminiscent of a Honda engine, but without the high redline.
There are a few characteristics of the QR25 that any Spec-V owner should be aware of. First, the knock sensor appears to be overly sensitive. Knock sensors are simply microphones listening for the distinct sound of detonation. Unfortunately, other normal engine noises sometimes get interpreted as knock, causing the engine to retard timing unnecessarily.
It's this knock sensor activity that led us to comment in an earlier review that the Spec-V was one of the least consistent cars we had ever dyno tested. Simply unplugging the knock sensor will scare the ECU into retarding timing across the board. Instead, we unbolted it from the engine, wrapped it in a towel, and zip-tied it to a power steering hose behind the engine. It's still plugged in, and we had to run a ground wire from the sensor to the engine blcok, but the towel prevents it from hearing anything that might be considered knock. The clean alternative is to unplug it, put a 500,000 Ohm resistor across the terminals, and then run a ground wire from the left pin (looking into the connector with the tab on top) to the engine block. This is now one of the most consistent engines we've ever tested.
In all our testing and rallying, we've used piss-poor 91-octane California gas, even in triple-digit temperatures, and have never heard a single knock, so we're quite confident in running without a knock sensor. The sensor's purpose is probably to protect the engine when some nimrod uses 87-octane gas. Don't do that.
The QR25 seems to be very sensitive to break-in treatment. The piston rings are very hard and take up to 6,000 miles to break in properly. Oil consumption during break-in can be quite high, so the oil level should be checked often. Continued high oil consumption can be something more insidious. The close-coupled cat is prone to overheating during hard driving, and the ceramic core can start to disintegrate.
The QR25, like many modern engines equipped with variable valve timing, has no exhaust gas recirculation (EGR) valve. Instead, it uses large valve overlap at cruise and under engine braking to suck exhaust gas back in from the exhaust manifold. If the cat has started to disintegrate, chunks of ceramic can be sucked back into the cylinders where they will score the cylinder walls. It's not clear how common this problem is, but when we removed our cat after 1,000 miles of relatively mild driving, a fine ceramic dust poured out the runners.
Step 2: Jim Wolf Technology
Given the dangers of a disintegrating cat, a header, which would eliminate the close-coupled cat, should be step one, but as we said earlier, we did our tests in an odd order.
So, the POP-Charger is a simple affair, nothing more than a free-flowing oiled cotton gauze air filter (with blue oil, not the red oil in our photo) and an airflow-smoothing air horn to minimize turbulence in the mass airflow meter. The induction noise, especially after the secondary intake manifold runners open, adds at least 10 hp of sound. The dyno has no ears, though, so the peak power gains are a more modest 4.4 hp at 5000 rpm.
Step 1: Hot Shot header
Do it for reliability. Do it for power. Whatever your reason, just do it. Needless to say, this is not a CARB-legal modification, since it eliminates the first catalytic converter. There is, however, another cat under the car, and although it will take longer to reach operating temperature, it will still be cleaning the exhaust. Whatever with the cat already; look at the dyno chart on page 198! More than 15 hp at redline and 11 at the power peak. And this with Hot Shot's relatively simple shorty header. To keep costs low, the header bolts to the stock B-pipe (the one with the second cat in it) with the stock doughnut-style flex joint. The header's bottom flange is a complex, CNC-machined piece with threaded holes for the stock spring bolts and a precise fit for the flex doughnut. The threaded holes can get filled with the ceramic coating, though, so chasing the threads with a tap is a good idea.
Over a season of rally abuse, we did manage to crack our header. The culprit may be vibration, or it might have something to do with the fact that we tore all the exhaust hangers off on rocks and the entire exhaust system is hanging on the header flange. Either way, Hot Shot is adding a brace to secure the header to the engine block just in case.
Installation is relatively straightforward, but the alternator needs to be removed during installation. There is plenty of clearance between the primary tube and the alternator once it's installed, but the header has to slide back about an inch on the manifold studs during installation.
Steps 4 and 5:
Balance shaft eliminator and early prototype camshafts Now, it seems like an exhaust would be a natural next step, but there was a rally coming up and eliminating the balance shafts offers some critical durability enhancements that we wanted to take advantage of. The purpose of the balance shafts is to cancel the natural imbalance of an inline four. This imbalance is minimal on a small-displacement engine, but as the pistons get bigger and heavier and the stroke increases, the imbalance gets obnoxious. This is purely a noise and vibration issue and has no effect on performance. In fact, it takes a significant amount of power to spin most balance shafts.
Most balance shafts are integrated into the block, but the QR25's are in a self-contained assembly in the oil pan. Where they're mounted, the counterweights on the shafts froth the oil like a pair of eggbeaters spinning at up to 12,400 rpm. It's unclear how detrimental this oil frothing is, but we didn't want to find out during a rally.
Jim Wolf Technology makes a kit that replaces the balance shafts with a pair of windage trays and some spacers carefully designed to cap off the oil galley that once fed the balance shaft bearings. The bolts holding the balance shafts in place also passed all the way through the lower half of the split crankcase into the engine block. In other words, they're major structural bolts, and the JWT spacers are critical to getting them back to their structural duty.
Unfortunately, there was a problem with the third-party dyno we tried to use between installation of the balance shaft kit and the first set of cams we tested, so we weren't able to produce a dyno chart of just the balance shaft gains. Jim Wolf tested the kit later on another Spec-V and found a constant 6 lb-ft gain in torque across the powerband, which equates to about 7 hp at redline.
Surprisingly, there's virtually no noticeable difference in engine vibration without the shafts. Apparently, the QR25's reciprocating bits are so light that even with the massive stroke, there isn't that much vibration. The QR25's engine mounts are also so soft, they absorb any additional vibration the shafts aren't canceling. Removing the balance shafts from the pan frees up an extra quart of space in the oil pan, bringing total capacity to 6 quarts.
We did have some unexpected problems in the two rallies following the balance shaft removal. Fuel injector wires started having fatigue failures about a quarter inch from the connectors, and one of the electronic throttle wires had a similar failure. Our best guess is that the unsupported sections of the harness were resonating, causing fatigue failures in the wires. It's not clear if this is from extra vibration from the missing balance shafts, or simply from the abuse of rallying. After replacing the harness, we zip-tied any free sections of wire either back on the connector, or to the nearest solid piece of engine. The problem has not come back.
The first in a long series of camshaft tests was next. The QR25 has no hydraulic lash adjusters, so changing cams involves an extra step of adjusting valve lash. Like any direct cam-on-bucket valvetrain, in which the cam lobe pushes directly on the valve bucket instead of using a rocker arm, this adjustment means swapping shims (or, in this case, buckets of different thickness) rather than something simple like turning an adjuster screw. Because there's only a limited range of bucket thicknesses, re-ground cams are impossible. Regrinds start with a stock cam and then grind down the base circle, which increases the bucket thickness needed to achieve the proper valve lash. Jim Wolf's cams are ground from a fresh cam billet, so the base circle is the same as stock.
All the prototype cams we tested, and whatever final cam gets produced, are designed to work with stock valve springs, at least up to the stock rev limit. Regardless, we used Jim Wolf's valve springs as insurance against valve float in the event of a red-misted over-rev during a bad downshift. Not that we ever do such things.
The QR25's variable intake cam timing eliminates valve overlap at idle, so even with cams, the car idles perfectly and is as civilized as the stock car. Most of the cams lost significant amounts of torque below 3000 rpm and didn't start making serious gains until 4000 rpm. The QR25 makes so much torque at that low engine speed, though, that the torque lost honestly wasn't missed. The engine is so torquey that in street driving (which rally cars do still have to do) we still frequently pull away from stops in second gear, even with the cams.
Step 3: Exhaust
Halfway through our cam testing, we finally tried an exhaust. BRM makes a simple, relatively inexpensive system that picks up from the stock B-pipe with xx-inch piping and flows through either one or two straight-through Magnaflow mufflers. The standard exhaust comes in aluminized steel with only the rear muffler, but a pre-silencer under the car is optional, as is stainless-steel construction. We tested a stainless-steel system without the pre-silencer. Stainless steel is harder than mild steel, so it withstands impacts better. This is very important on a rally car.
With the stock exhaust manifold and restrictive cat, the sound level is, according to BRM, quite reasonable, but with the Hot Shot header, the pre-silencer is dearly missed. While quite reasonable for a rally car, we wouldn't want it to be this loud on a street car. Luckily, the pre-silencer, being a straight-through perforated core design, should have no effect on power output. We highly recommend the pre-silencer.
Surprisingly, the QR25's exhaust note isn't as deep as you would expect from a 2.5-liter. Instead, there's a tinny edge to the sound. The pre-silencer may cure that as well.
The BRM exhaust is broken into three pieces, which makes shipping costs more reasonable. One of the two resulting joints is a slip joint, while the other is a simple ball-and-socket flex joint. These joints give some installation flexibility, allowing you to make it fit perfectly, or completely screw it up, depending on how much you pay attention. The trick is to attach all the hangers, tighten the flange where the exhaust meets the B-pipe, and then manually push the exhaust up into place before tightening all the joints. We were able to achieve a nice, tight fit with a centered tip and without any rattles on the first try.
Step 2 again: AEM intake
AEM's intake came out only after we had done several of the other modifications, so it was tested last. Of course, starting from scratch, we would have installed it second. The AEM intake puts the filter in the driver's front wheel well. On a street car, this is protected by the inner fender liner, but in a rally car it would be pelted with rocks. We only used the AEM intake on the dyno and stuck with the POP-Charger for competition. If you drive in areas where deep puddles are an issue, you should think twice about the AEM intake, as there is no room for AEM's bypass valve.
Anywhere dry, though, the pulse tuning and inertial effects of the AEM's long intake tube mean another 5 hp above the POP-Charger's gains.
Finally, the wheel hop
The Spec-V's massive torque, short gearing and soft engine mounts don't always get along. There is so much engine movement, even at stock power levels, that the exhaust flex joint can actually pull apart under hard acceleration in first gear, making a "pffft, pffft, pffft" noise as exhaust leaks pass the sealing doughnut. When the engine gyrations get together with the soft suspension bushings, massive wheel hop can result.
We solved this problem by filling the voids in two of the four engine mounts with urethane windshield adhesive, a sticky black goo that hardens into a very similar material to the rest of the rubber engine mount. Only the front and rear mounts, which resist engine torque, need reinforcing. This does noticably increase vibration, but it still isn't as bad as many of the polyurethane-mounted hybrids we've driven. The payoff is reduced wheel hop and a car that handles throttle transitions with much better composure.
Holy huge power gains! It was a long and sometimes circuitous route to find the parts comb
Jim Wolf Technology's POP-Charger is a simple open filter adaptor and bracket that mounts
The simple POP-Charger intake was good for 4 hp just about everywhere. The induction noise
Hot Shot's simple header produces amazing gains, thanks to the horribly restrictive stock
The bottom flange on the header is CNC-machined to accept the factory flex doughnut and sp
A header is not only a simple way to get ridiculous power gains from the QR25, it's also g
The QR25DE's balance shafts sit below the crank in the oil pan. In addition to frothing th
Unfortunately, rally schedules and the ineptitude of a third-party dyno shop conspired to
The balance shaft assembly is self-contained and, amazingly, can come out by simply removi
The only ugly part of the process is cutting the chain that drives the balance shafts. Aft
The upper tray is serrated to help collect the oil streaming off the crank. Make sure you
The lower windage tray holds the oil pickup.
The added vibration from the missing balance shafts is barely noticeable with the stock en
We tested several prototype Jim Wolf camshafts before settling on the best combination aft
The front bearing cover seals against the top hoop of the front cover and is tricky to get
Cam installation on the QR25 is not overly difficult, but there are a few surprises. After
The BRM exhaust uses a simple, but effective, flared flex joint. The flange that holds ten
This slip joint in the middle of the exhaust will make or break the installation. Slide it
The first thing you'll notice about our exhaust test is that the baseline is lower than th
In stainless steel, the BRM exhaust is tough. It took a BIG rock to do this. Luckily, perf
The AEM filter's placement in the front fenderwell means we can't actually use it on the r
We can't use it in competition, since the filter is too close to the dirt-flinging tire, b
Only the front and rear mounts, both of which are on the longitudinal engine cradle, need
The Spec-V's engine mounts are horribly squishy. This tube of 3M Windo-Weld fast-curing ur
Hot Shot Performance
Jim Wolf Technology
Advanced Engine Management
2205 126th St., Unit A