Sport Compact Car - July '97
Project 200SX SE-R
by Mike Kojima
[Put into HTML format by Ken Pratte]
Part II: The process of making more power with an OBD-II equipped engine.
PHOTOGRAPHY: Les Bidrawn, Dave Coleman, Scott, Dahlquist, Mike Stute
As you will recall, in the June 97' issue we tackled the suspension and braking system on Project 200SX SE-R. So far, these mods are working well, but there is still some fine tuning to be done with brake bias and anti-roll bar pre-loading. These issues will be covered as soon as the car is tuned to our satification. Meanwhile, the vast leaps forward in handling and braking prowess have left us yearning for more power.
Building an engine that has some power while passing undected through California's roadside smog machines is a challenge. Add to this making power without triggering OBD-II. Doing both these things takes thorough planning, not to mention a healthy amount of trial and error. Some horsepower has to be sacrificed to keep the electronic eyes happy but luckly not much. If you want an 11 second machine that you might be able to drive on the street for a while before you get caught (and you will get caught), this article wont be for you. If you are a regular guy that cant afford both a street car and a race car, then read on.
First off, extreme modifications is out. Super high-compression, hugely ported heads, large duration/overlap cams and other extremes will all trigger the OBD-II system. Turbos and other forced-induction devices can be done but tons of money and an expert in fuel and ignition mapping must be called in. Reasonable camshafts, mild porting, a properly done header and exhaust, air intakes and NOS are well within the realm of OBD-II passable modifcations, if the work is handled correctly.
After baseline-testing the engine we added some relatively inexpensive bolt-on mods that are condsidered to be compatible with the 200SXs OBD-II system.
Exhaust Tech provided a custom header designed to work with the Nissan OBD-II system. Nissans OBD-II system uses two oxygen sensors, one controls closed loop operation (when O2 sensor readings are used to automatically adjust the air- fuel ratio under light cruise conditions) and the other is used to gauge catalytic converter effectiveness and light off time. With this electronic watchdog, it was deemed important to try to conserve the heat in the header to help the cat warm up quickly so the computer would continue thinking all is well. Thick-walled tubing was used in the base followed with a coating of plasma sprayed aluminum. This prevented heat from radiating out of the headers pipes and kept the cat at normal operating temperature.
The size and locations of the O2 sensor port and the EGR port were also critical. The location of the O2 sensor fitting was picked to be the same distance from the exhaust port as it was in the original manifold in an effort to maintain proper O2 sensor temperature. Care was taken to make sure that the EGR valves orifice was the same size (8mm) as on the stock manifold and in the same location to prevent the EGR valve from prematurely opening due to back pressure.
The headers primary tube length was tuned to resonate at about 6000rpm, and a larger-than-typical, 1 5/8" diameter to provide enough flow for the 2000cc engine. The relatively low resonance point was placed between the horsepower and torque peaks. The reasonance point, combined with the tri-y design, was chosen to provide the broadest possible power band and to be less sensitive to cam and induction changes that we plan to experiment with later. A tri-y header may be down a few peak ponies from a full race 4-into-1 design, but on a street motor the broader powerband of the tri-y is usually better. The primary tube dimensions were determined through computer modeling. The final dimensions were compromised somewhat to ease installation, make the part easier to produce and to compensate for future cam changes. Since street cars must most always compromise their design to be able to fit in a production chassis, (often totally equal length primary's are not practical), tri-ys usually come out on top again due to their relative insensitivity to minor tuning variables.
For this improvement, we turned to Jim Wolf Technologies. The Wolf ECU works best with the timing set at the factory maximum of 17 degrees BTDC. Advancing the timing more than this will cause a loss of power, especially on the top end. The Wolf ECU modifies the fuel and spark maps for optimal performance while maintaining OBD-II compatibility. Wolf removes the top-speed-limit fuel cut function from the program. Wolf also raises the stock rev limit of 7000 rpm to 7700 rpm, which was a big help because we were continually bouncing off the factory rev limiter with the stock ECU.
The Wolf ECU also limits the amount of high temperature and or knock sensor response retard. This is an area where Wolf's years of experience in racing shows. Wolf discovered that the SR20DE's stock tuning is overly sensitive to knock. Normally, when the knock sensor recognizes the onset of detonation, the ECU will retard ignition timing to prevent further detonation. Unfortunately, retarted ignition timing also tends to increase coolant temperature, which, in turn, increases the tendency knock, which causes the ECU to further retard timing, etc. In the end, coolant temperature soars and power output suffers. Though the conditions needed to start this thermal overrun are rare for the average Joe, aggressive driving can bring coolant temperature into the range where this will happen frequently. The reduced ignition retard response of the Wolf ECU prevents this vicious cycle from starting in the first place.
These extensive program modifications are no small task considering that the vehicles engine control processor talks to the OBD-II monitoring processor. Any disagreement between the two results in error codes and headaches.
The Wolf ECU made a very noticeable difference with improved throttle response and greater top end power. Best of all no check engine lights appeared indicating zero error codes. Because of the advanced ignition timing and reduced knock sensor retard, the Wolf ECU requires the use of 92 octane gas, but if you want the most power out of the stock engine, 92 octane is a must anyway. Wolf also has ECUs available for almost all Nissans and Infinities, some of which have CARB OE numbers.
Jim Wolf Technologies once again stepped in to help design a camshaft. Duration and overlap were limited to keep the OBD-II system happy. Lobe center angle was also carefully considered in an effort to make these cams consistent performers. One of the positive features of this is the cams are installed straight up with no need to dial them in using adjustable timing gears as Wolf has already done this for you. Besides simplicity and consisten performance, this also ensures the OBD-II system is not triggered by someone dialing in the cam with too much overlap. The highest lift the stock vavletrain can tolertate reliably was used. The Wolf cams are ground on brand new genuine Nissan billets, not reground on old cams. Regrinds reduce the base circle diameter which can cause problems with the hydraulic lash adjusters and valve train geometry. The end result is often a customer with a noisy, fast wearing valve train that does not deliver the proper lift duration and lobe center. Mild regrinds that do not cause valvetrain problems typically can't achieve lift and duration numbers that can make much of a difference.
A low-budget cam alternative is to use the intake cam from a 90 - '92 Sentra SE-R. This cam is significantly bigger than the stock 200SX SE-R cam and has been tested to provide six more HP at the front wheels. This cam does not upset the OBD-II system and is available for a reasonable price from your friendly Nissan dealer or even scrap yard.
Of course, the exact specs of the Wolf cam are propriertary, but the performance figures are not. The cam produces a slight lope at idle but low speed performance and driveability is not sacrificed as is typical with most high-performance camshalfs. A trip back to DPRs Dyno indicated 166 hp at 7200 rpm, an honest gain of 29 hp. The engine lost no low end power below 3000 rpm. All of the horsepower gains came in at 3000 rpm and above. Gas mileage remained unchanged averaging 27-30 mph in mixed cycle driving which consisted of mostly bumper-to-bumper freeway traffic and around-town hops. For strictly highway, driving the mileage soared to over 30 mpg if the speed was kept below 75 mph. The car feel like it received more than 29 hp as the bottom end torque is still as good as stock with the improved response that the Wolf computer gives actually makes it feel like there is greater than stock low end because of the transient improvements. Typically high-performance cams cause a loss of bottom end power because increasing duration sacrifices good scavenging at low rpms to improve it at high rpms. The raised rev limiter give the driver more headroom for when you might have to hold out a gear for the next corner. Overall the engine feels much more flexible, willing and freer breathing than the stock motor.
The engine runs cleaner than clean with the inside of the exhaust tip remaining shiny instead of the typical sooty black even after thousands of miles of use. The car should enable you to run past those roadside smog machines without having to don a rubber Richard Nixon mask.
With good bottom end power, excellent midrange, way more top end, clean emissions, no error codes and good mileage to boot we have ended up redefining the no-compromises engine. Not bad for pure bolt ons.
In the future installments we'll examine the benefits of cyclinder head work and a hi-tech nitrious system for Project 200SX SE-R.
Reprinted with Permission