SE-Rious Sentra Part II
Tuning: The Crossroads Of Performance
[Put into HTML format by Dan Thompson]
This article originally appeared in the Turbo & Hi-Tech Performance July 1995 issue and was reprinted with permission. For more info on Turbo Magazine check out their website.
After tracking down hard-to-find parts and working through numerous logistical mazes to ensure everything comes together, we were still not home free with project Sentra. With the turbo system plumbed and at the ready, we come to the performance crossroads - tuning. Proper fuel enrichment and ignition timing are paramount to power and reliability. Making the correct choice at the performance crossroads will be the difference between having a nightmare car that resembles the Deathmobile of "Animal House" fame, or a dream machine that runs like an IndyCar. Our first and last inquiry was to Jim Wolf Technology, Inc. (JWT). It was the company's experience with Nissan's IMSA race cars of the '80s and its expertise with Nissan 300ZX street cars that made them the obvious choice.
Before turning our attention to whipping the 2.0-liter into shape, we decided to hedge our bets and add a couple of insurance policies, namely a GReddy blow-off valve and a GReddy Turbo Timer. The GReddy blow-off valve is designed to improve turbo response between shifts and extend the life of the turbo. It relieves boost pressure when the throttle is suddenly closed. Without a blow-off valve the engine would experience compressor surge wherein boost pressure hits the closed throttle and backs up until it impacts the compressor wheel, bringing it to a standstill in extreme cases. This condition can be damaging to the turbo's compressor wheel and bearings. From a performance standpoint, the GReddy blow-off valve reduces turbo lag as the boost charge is redirected into the atmosphere resulting in quicker spool up. The Turbo Timer is an electronic device that keeps the engine idling for a preprogrammed period of time after the key is removed from the ignition. This gives the turbo a chance to cool down after hard use which prevents oil from baking onto bearing surfaces. Both of these items, and an Autometer boost gauge, were installed at the Sentra's home away from home - Dynamic Autosports in Irvine, California.
The expertise of Jim Wolf Technology was the only reason this project, with its high static compression, high boost expectations and stock bottom-end componentry, was feasible. Jim and Senior Programming Engineer Clark Steppler worked their magic in the Nissan's computer to make the 2.0-liter purr like a kitten at idle and roar like a lion at WOT.
Since 230 to 250 horsepower was the objective, JWT knew from the get go that the factory-spec 259cc injectors would have to be scrapped. They were replaced with 370cc squirters from an Infinity Q45. JWT tested the Mass Air Flow meter (MAF) and found it to be in good shape but 250hp seemed to be its limit.
When delivered to Jim Wolf Technology headquarters in El Cajon, California, the turbo system utilized a blow-through MAF setup. While from a packaging standpoint, and to a certain extent a performance standpoint, having the MAF near the throttle was a good way to go, Jim and Clark were leery of the MAF becoming susceptible to shock waves and reversion pulses as the throttle is closed during in-boost operation. Flutter in the sample chamber of the MAF at this time can transmit erroneously high airflow values to the ECU. So JWT swapped the system back to a draw-through MAF
With the MAF where they wanted it, Jim and Clark began the tedious job of equipping the SE-R with a complete data acquisition system. The JWT system records rpm, boost pressure, MAF voltage, injector pulsewidth, engine inlet temperature, compressor outlet temperature, throttle position and detonation sensor output. That was just the tip of the iceberg as the subcompact Sentra was also stuffed with a Nissan Consult analyzer, Horiba precision air/fuel meter, ECU breakout box, ROM emulator, ROM tracker, A/D data breakouts and a laptop PC.
An initial "Base" program, prepared with larger fuel and ignition map dimensions, was downloaded to the ROM emulator connected to the SE-R's ECU. The larger maps are necessary to control fuel and ignition timing during high-horsepower operation. Without these larger maps, the computer program's "access pointer" would "slam against the side of the maps" shortly after appreciable boost is generated, leaving no means of precisely calculating the new fuel and ignition information. The JWT "Base" program kind of picks up where the stock ECU leaves off and provides maps for injector pulsewidth and ignition timing that are conservative but close enough to start the process. The base program also contains information about the flow rate and time delay between applying current and fully opening the injector pintle, called invalid time, of the new 370cc injectors.
Although Jim Wolf Technology has a Superflow engine dyno and a 500-horsepower chassis dyno at its new facility, Jim prefers to refine performance in the real world on the road. "For this type of project," he says, "you can dial everything in on the dyno and still end up spending two weeks on the road to get the crispness and driveability right."
The SE-R, which resembled a mad scientist's rolling laboratory with all the electronic gadgets inside it, was ready for the first run on the "Base" program. This run was primarily intended to check the air/fuel ratio. Wolf reports that the SE-R has an excellent combustion chamber design that can safely handle 12:1 air/fuel ratio while under boost. (He reports a 300ZX needs a richer 11:1 ratio to achieve a similar threshold of detonation.) Once the "Base" program was deemed acceptable, the boost level was slowly increased on successive runs by using the T-BAC III. The fuel maps were continuously edited and updated so a 12:1 ratio was maintained.
With the air/fuel ratio stabilized, the tuning process concentrated on the ignition timing map. Jim says, "advancing the timing curve too much results in an increase of detonation sensor activity, too little timing advancement raises the exhaust gas temperatures. Mapping ignition timing is a process of finding a window between these two indicators for every cell in the map. In some areas of the map these windows are extremely small. In these areas, the air/fuel ratio is enriched by up to one-half ratio in corresponding cells of the map. The ignition map is again tuned until the value is safely free of detonation and high exhaust gas temperatures." When adjustments can no longer create an acceptably large window, the limits of tuning for that cell have been achieved. Extending tuning beyond this point requires a change in octane, compression, air temperatures of some other external variable. Jim notes that dumping extra fuel to try and push the limit makes for a very unstable fix and a sloppy tuning job. He says this will result in a car that is slower than one that is properly tuned.
"Crisp driveability and throttle response, as well as good warm-up and idle characteristics, take 70 percent of the time spent tuning a good program," says Jim. "This must be done on the street to get acceptable results." Each morning a cold-start and warm-up test was logged on the SE-R, using programming adjustments gleaned from the previous morning. This procedure was a sidebar to the main programming since it could only be done once a day.
For the main driveability tuning, Jim paid particularly close attention to the "attack and decay" algorithms that enrich fuel and retard timing relative to throttle movement. If too much timing retard occurs relative to throttle opening, the car feels dull under acceleration. This is also true of fuel enrichment, not only if too much or too little fuel is added, but also if the enrichment is not properly tapered back to normal after the throttle stops moving. There are more than 100 variable attributes that must be tuned correctly in a typical program. Each of these has a unique tuning procedure and all must be tuned in a specific order. "By the end of the second week the Nissan SE-R Turbo development program yielded a tremendously driveable car," says Clark, "with great power not only at wide open throttle but in all ranges of rpm, load and throttle position."
Jim Wolf Technology had dialed in a max boost setting of 13.5 pounds because of spiking at high-boost. We found that by using the T-BAC III's boost curve control we could safely up the boost to 15 psi and even out the spiking. An interesting aspect of the Sentra's turbo system is that the fuel system is basically stock and everything is maxed out. The mass air meter is at its capacity, as are the 370cc injectors, and the turbo itself is moving about as much air as it can.
Finally we got to drive the car for a while, and the g-forces it created were quite intoxicating. We like the responsiveness of the small T25 turbo and are happy with the power output. In fact, with the max of 15 psi dialed in the prospects of hooking up quickly became a fantasy as the tires would spin continuously through the gears. The clutch, which had a good deal of hard abuse under its belt already, seemed to fade away at higher rpms as the boost level peaked.
These two maladies (especially poor traction) made 0-60 performance totally non-representational, although it improved from 6.48 seconds to 5.89 seconds. For sure-handed gear engagement, the Sentra now relies on a custom-built Action Clutch. We opted for a full-face setup to keep the SE-R as streetable as possible. This particular unit features high static pressure to provide extreme power handling capabilities at all rpms. Action designs its turbo and nitrous clutches in this manner since these types of motors can produce tremendous amounts of power at relatively low rpms. While having the clutch installed at Dynamic Autosport, the crew tracked down and fixed a small oil leak in the turbo drain line and went over the entire turbo assembly with a fine-tooth comb to make sure the system was oil tight. The car was still very "traction challenged" but the high-rpm slippage problem was now a thing of the past.
Being staunch supporters of improving braking performance in conjunction with upping the ante under the hood, we had a GRIP cross-drilled brake package installed on the car. Our stock rotors were wafer thin which made us realize that GRIP brakes are perfect as a replacement/upgrade to any serious performance car. The GRIP package includes application-specific vented rotors and high-performance brake pads. Installation is billed as a remove-and-replace affair. After the recommended break-in period the GRIPs do what their name implies. Their added bite is especially evident on hard stops from higher speeds. There was no noticeable fade after four back-to-back high-speed stops.
As of press time, we haven't really had the opportunity to drive the car for more than a week straight. In upcoming issues we tell you what its like to live with the car, finish the intercooler ducting that was outlined in Part I, dress up the engine bay, spruce up the interior, get some accurate horsepower ratings using K&N's trick DynoJet single-roller chassis dynamometer and, even though it's not meant for the strip, run some quarter-miles.