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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
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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 won’t be for you. If you are a regular guy that can’t 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.

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Project 200SX SE-R was first baseline dynotested on the Bosch chassis dynamometer of Dan Paramore Racing. The Bosch dyno measures driveline drag and an attempt to convert measured drive-wheel horsepower to flywheel horsepower. The result was 137 hp at 6700 rpm with a redline of 7000 rpm. Nissan rates this engine at 140 hp at 6400 rpm, so it seems as if the Bosch technique for finding corrected flywheel horsepower is accurate. Our stock 200SX SE-R felt reasonably peppy but the rev limiter seemed to kick in before the right shift point was reached and the car seem more anemic than the earlier Sentra SE-R, despite the fact they have the same rated horsepower.

After baseline-testing the engine we added some relatively inexpensive bolt-on mods that are condsidered to be compatible with the 200SX’s OBD-II system.


The 200SX SE-R's stock exhaust manifold is made of thick cast iron or covered with insulated heat shields for a reason. Catalytic converters need to be hot to work. Very hot. The thin walls of conventional tubular headers can conduct enough heat away from the exhaust stream to delay catalytic converter light-off. This can greatly increase the amount of time it takes for the cat to start working from a cold start, the point where modern cars make the most pollution. If a lot of heat is lost, even the oxygen sensor’s operation can be affected. Oxygen sensors also need to be hot to operate correctly. A cold, sluggish O2 sensor causes all kinds of drivabilty problems. Some cars arrive stock from the factory with tubular headers but many of them use close-coupled cats because of the heat loss. The close-coupled cats can offset the amount of gain that the headers provide by disrupting the acoustic tuning. As you can see, headers on a modern OBD-II equipped car are not so simple. Any offsets in cat outputs, activation time or O2 sensor operation will trigger error codes.

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Just to prove that our engine modifcations are running clean, the inside of our new GReddy exhaust has stayed clean and soot-free even after more than 1500 miles of hard street driving.

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.

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Primary tube diameter and length were important considerations in designing a header for the SE-R, just as with any car, but we also had to deal with O2 sensor location and EGR port placement.  The O2 sensor is clearly visible on the number 3 primary tube. The EGR port is located on the number 4 primary under the black box.

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-y’s usually come out on top again due to their relative insensitivity to minor tuning variables.


Catalytic Converter
The catalytic converter and the rear O2 sensor were retained for obvious reasons. Most modern monolithic matrix catalytic converters are pretty free flowing. Cats got their reputation as power eaters when they first appeared. Those early cats forced the exhaust to flow through a bed off pellets, creating a lot of back pressure. Modern cats use a ceramic honeycomb with straight through holes which add very little backpressure to the exhaust system. Before deciding to stick with the stock cat, we tested it for backpressure. During testing the cat only added a maximum of .5 PSI of backpressure, and under most conditions the cats effect was too low to measure. The difference between different brands of performance mufflers was greater than that! The lesson here is to leave that cat alone. Unless you are trying to double your horsepower with a turbo or your cat is greatly undersized from the factory, there is little to be gained by tinkering with it.


Exhaust system
In our testing, the GReddy cat-back exhaust system was selected because it has the least amount of backpressure (2.5 PSI with cat, others had as much as six PSI) and was also among the quietest. The GReddy unit has a pleasant, deep, medium-loud-tone with little irritating high frequency resonance. The GReddy system is also very high-quality, constructed of mandrel-bent 60mm stainless steel with a polished muffler and a 130mm tip. The large shiny tip doesn’t exactly fit into our sleeper theme but it sure looks good. Big shiny tip or not, GReddy also has the advantage of actually certifying their exhaust systems to the stringent standards of the California Highway Patrol. There are no federal or state standards for noise on aftermarkit exhaust systems so the CHP standard was the toughest GReddy could find. GReddy can send you a certificate of compliance to the standard if you ask. Just the thing to give to the officer if you get pulled over for an apparent violation. If the officer still insists on giving you a ticket, you can always go to court with your certificate in hand.


Air intake
A Jim Wolf Technologies POP charger was used to replace the stock airbox and air filter. The POP charger uses a cone-type open filter element with an additional cone of air filter material mounted in the front cap of the air filter element. This cone adds filter area, reducing the pressure drop needed to pull air throught the filter. The filter clamps to a machined aluminum velocity stack that bolts to the airflow meter. The Nissan airflow meter is designed to be accurate under laminar flow conditions.  A sharp entry into the meter can cause the meter to read inaccurately, leading to inapproiate fueling and lost power.  The fact that the stock air filter box incorporates a small velocity stack is a hint at what a good idea a velocity is on this particular engine.  The POP charger is a high quality unit overall, and it carries a CARB OE number making it legal in all fifty states.


For engine modifications, Nissans have a big advantage over many other sports compact cars. Since Nissan’s ECU fuel and spark tables are driven by the airflow meter output, the ECU can automatically compensate for changes in volumetric efficiency unlike the speed density systems on Hondas. In otherwords, mild camshafts, headers and head work can be used without automatically causing the engine to lean out.  The greater the airflow the more fuel the computer will inject up to the limit of the airflow meter voltage curve or injector capacity.  Even with this inherit advantage, there are still comprises in the stock ECU programming. The stock ECU is tuned to be able to run on bad gas, and to maintain drivability and clean emissions even with poor maintaince.   Since this car is well cared for and feed a diet of high octane-fuel, there is room for improvement.

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With a very careful selection of components, we were able to find an honest 29-hp gain without sacrificing horsepower anywhere, and without triggering any OBDII trouble codes!

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 ECU’s available for almost all Nissans and Infinities, some of which have CARB OE numbers.


Perhaps the most difficult part of Project 200SX SE-R‘s build-up was the design of the camshafts. Aftermarket cams and OBD-II don't get along for a variety of reasons.   High overlap cams can cause pulsation in the intake manifold at low rpm.  The pulsation in turn cause wildly erratic airflow meter and MAP sensor readings that make it impossible for the ECU to properly deliver fuel.  Not surprisingly, such erratic behavior will be noticed by the OBD-II system, which will assume a system malfunction and start logging error codes.  A lumpy idle caused by a big cam can also make the computer think a misfire condition is occurring.  OBD-II systems are required to look for misfires and log their frequency.  The only way they can currently do that is by closely monitoring crankshaft speed.  Erratic changes in rpm (i.e., rough idle) will be interpreted as frequent misfires, and again, error codes will be triggered.  Cams tuned to work well at high rpm often sacrifice low-rpm pumping efficiency.  This, in turn, causes low manifold vacuum that can interfere with EGR operation.  Very high overlap cams will exhibit an "eight stroking" phenomena at idle where each cylinder will completely misfire on every other compression stroke.  Thus there is only one power stroke for every eight strokes instead of over four when everything is operating properly.  This phenonmenon is responsible for the loping idle of the older hot-rod V-8s.  Unfortunately, with every other exhaust stroke pumping out a cylinder full of unburned gas, hydrocarbon emissions soar.  The over-rich exhaust will not contain enough oxygen for the catalytic converter to complete its breakdown of unburned hydrocarbons, so even with the cat in place it will become ineffective.  Clearly, our cam's profile had to be slightly compromised in order to get a smoother, more stable idle with steady airflow meter voltage and consistent manifold vacuum.
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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.

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The filter on the JWT POP Charger conceals this beautifully finished cast aluminum air horn.  nissan air mass sensors can become inaccurate if the air entering the sensor is too turbulent.  The fact that even the stock air filter box has a small air horn in it tells us that this air horn is no gimmick.  The tiny plastic snorkel shown above the air horn is what the stock air filter pulls all of it's air through.

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 DPR’s 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