The Makings of an Autocross National Championship
Nissan Sentra SE-R
This article originally appeared in Grassroots Motorsports March/April 1993 issue. For more info on GRM checkout their website.
Back in the spring of 1991, the editors of GRM got together to sample six cars on the autocross course as part of our annual "Editors' Choice" car test. That test included such heavy hitters as the BMW 3]8is and the Toyota MR2 Turbo, but the car voted the "Best Dual-Duty Car"--as well as the best choice for those on a budget--was the new Nissan Sentra SE-R.
This sporty version of the totally redesigned Sentra offered a terrific combination of civilized manners and quick acceleration, all in a package that could also serve as a carpool workhorse and grocery getter. One of the people present at that car test, Mark Chiles, was impressed enough to go right out and buy a '91 Sentra SE-R. Chiles saw the versatile sedan as a great choice to satisfy his needs for a commuter car that could also zip around an autocross course on the weekends. He also saw something more: the makings of a national champion.
Mark spent that summer preparing his new SE-R for the Solo II National Championships. His hard work was rewarded when his dual duty Nissan-which was one of only three D Stock trophy winners that were actually driven, not trailered, to the event in Kansas--came through with a D Stock championship win. The following details tell just how Mark Chiles developed his Sentra into a national champion. Although his car is now two years old the SE-R is essentially unchanged this year, so the tips in this article will still be useful to owners of '92 and '93 models. These modifications will also make the Nissan NX2000 into a better autocrosser.
The one-piece Revolution Racing Wheels he chose were only slightly lighter than the Nissan Sentra SE-R's OEM aluminum alloy wheels, but the Revolutions were less expensive than the cost of additional OEM alloy wheels. In addition, Mark says he prefers to support a business that has supported the sport of autocrossing. (The owner of RWS, Bill Cobb, has sponsored a number of contingency awards programs at past SCCA Pro Solo and Solo II events.) The SE-R's wheel size is also shared by the first-generation Toyota MR2, the Mazda Miata, the Acura Integra and many other front wheel drive vehicles, so it's a popular and common size of Revolution Wheels. However, the hub center openings of these wheels vary from car to car, and are slightly larger on the Sentra SE-R. This means that RWS had to machine the hub center openings of their Revolutions to create, the larger 59mm diameter hub center openings required for the Nissan Sentra SE-R.
These aftermarket wheels also feature a different wheel offset from stock, which is desirable because a greater offset creates a wider track, aids handling, and allows for better tire clearance of the strut housings. The SCCA's Solo II Stock Category Rules allow wheel offsets to vary by up to 1/4-inch from stock, so the SE-R can run wheels with an offset of up to 39mm. At the time Mark's car was being built, however, RWS already had one-piece Revolution Racing Wheels with a 40mm offset. This was one millimeter less than the maximum permitted offset, but the additional cost from of having specially made wheels shipped from England would have been prohibitive, so Chiles chose to go with the smaller 40 mm offset. Two sets of the Revolution Racing Wheels were purchased, one set in white and the other in black.
Stiffer front and-roll bars reduce body roll on most front wheel drive vehicles, but these bars can also increase the propensity of the inside front wheel to lift and lose traction under hard cornering. So Mark decided to look for aftermarket struts to reduce body roll while at the same time keep the front wheels on the ground as much as possible.
The Sentra SE-R comes equipped from the Nissan factory in Smyrna, Tennessee, with coilover hydraulic MacPherson strut under all four corners. These stock units are manufactured by Tokico of Japan, but when this project was begun, Tokico as well as fellow shock giants GAB and KYB--had no immediate plans to build any performance strut cartridge replacements for the Nissan Sentra SE-R or the Nissan NX2000. So Koni America was consulted about developing some performance struts for the SE-R.
Koni's engineers had developed the externally adjustable strut cartridges used in GRM Toyota FX16 project car (see "Project Toyota FX16 Revisited," GRM January/February 1991, pg.36). Koni accepted the challenge of designing and building a few sets of prototype struts for the Nissan Sentra SE-R, with an eye on future volume production and sales.
After primary investigation, the shock company's engineers determined that the most cost-effective method of creating a performance upgrade for the SE-R's struts was to manufacture adjustable strut cartridge replacements that could be used with a slightly modified OEM Sentra strut housing. These Koni cartridges are not dial-type externally adjustable, so they can only be adjusted when they are off the car. Although the Koni strut cartridges are designed to be complete replacements for the hydraulics and mechanicals of the Nissan Sentra/NX OEM strut, the OEM Nissan strut housings are "sealed-for-life" units and do not contain a strut cartridge. This means that an easy method for aftermarket strut cartridge replacement was not included as part of Nissan's original Sentra/NX strut and strut housing design parameters. So even though the Koni units are designed be installed into the OEM strut housings, the housings require minor surgery before the Koni adjustable strut cartridges will slide into them. Not to worry, though; the procedure is simple.
To prepare the OEM strut housings for Koni cartridge replacement, cut off the welded cap at the top of each strut housing with a large pipe cutter, then remove the hydraulics and mechanicals inside each strut housing (dump the guts). Drill a 14mm hole into the bottom of each strut housing to allow for the placement of a strut cartridge mounting bolt. Before you place the Koni adjustable strut cartridges into the modified OEM strut housings, be sure to twist the strut cartridges to the desired stiffness setting. Now, slide the Koni strut cartridge into the modified OEM strut housing. Bolt the strut cartridge to the strut housing from the bottom of the housing with the Koni-supplied washers and bolt, and you're finished. Wasn't that easy? The final production versions of these Koni strut cartridges for the Nissan Sentra/NX are available. The front strut cartridges are Koni part number 86-2463; the rears are Koni part number 86- 2465. These adjustable strut cartridges have been designed to fit all 1991, ' 92 and'93 models of the U.S.-market Nissan Sentra and Nissan NX. (European- and Japanese-market Sentra/NX models have different suspension and strut configurations.)
The Chiles SE-R was fitted with new Aimco brake pads while it was up in the air at the Koni America facility. The front of the car received Aimco Super Premium Semi-Metallic brake pads (part number SPM-509), which have a very high metallic content. Aimco Premium Asbestos Free pads (p/n PF-511) were mounted at the rear. The asbestos free brake pads and shoes have less metallic content than the semi-metallic brake pads used at the front of the Sentra, which helps to maintain the proper front-to-rear brake bias. Aimco's semi-metallic pads and shoes feature an unlimited mileage lifetime warranty, while the asbestos-free pads and shoes carry a 12 month/12,000 mile warranty. These Aimco friction products are marketed as replacement products and are not considered racing applications, but Mark says they did eliminate the brake fade that he had been experiencing under repeated hard braking. They also helped to reduce wheel lockup under hard braking and shortened the braking distances normally required for maneuvers.
Now, halfway through the 1991 model year, the part number changed for the replacement lower mounting bolts from Nissan Genuine Parts. And low and behold, these new replacement lower mounting bolts, which are available through local Nissan dealer parts departments, are of a smaller diameter than the earlier bolts. This gives the performance tuner an opportunity to provide for additional negative camber.
The new part number for the replacement lower mounting bolts is 40056-71L00; these parts retail for about $2.50 each. Order several of these lower mounting bolts from your Nissan dealer and micrometer each of them. Due to manufacturing tolerances, some lower mounting bolts are of a smaller diameter than others. Use these variations to your advantage by placing the smaller of the mounting bolts in the upper mounting location of the struts, and the larger of the bolts in the lower mounting location of the struts.
There is a common practice among autocrossers of assuring that a vehicle has the maximum negative camber available by having large, burly friends simultaneously pull on the bottom of the disc brake rotor and push on the top of the disc brake rotor while a person tightens the lower mounting bolts of the struts. Unfortunately, this method of assuring maximum negative camber only works on vehicles that have one lower mounting bolt for the strut located above the centerline of the disc brake hub AND one lower mounting bolt for the strut below the centerline of the disc brake hub. On the Sentra SE-, both lower mounting bolts for the struts are located above the centerline of the disc brake hub, so this practice will actually increase unwanted positive camber on the Nissan Sentra SE-R. So Sentra owners have to take another approach to assure that they have dialed in maximum negative camber available.
To do this, take a large C-type clamp and hook it around the back of the lower strut housing and the upper part of the knuckle, perpendicular to the upper bolt of the lower mounting bolts of the strut (see photo). Turn the screw on the clamp to make sure that the holes for the lower mounting bolts of the strut are properly aligned for maximum negative camber, then tighten both of the strut's lower mounting bolts to the proper torque specifications. This method will assure maximum negative camber is obtained and can be performed by one person, reducing the number of large burly friends you have to maintain.
Using these methods, you should be able to obtain at least one degree of negative camber at all four wheels of the Nissan Sentra SE-R. But be forewarned, too much negative camber on the Sentra SE-R can cause the inside sidewall of the tires to run against the outside of the strut housings under hard cornering, and may also prevent the SE-R from keeping a full and useful tread patch on the pavement while the car is under straight-line acceleration. As far as toe settings are concern, Chiles set his Nissan at 3/8-inch toe out in the front and 1/64-inch toe in at the rear. He found that a large amount of toe out in the front gave the SE-R a very quick steering response, while the small amount of toe in at the rear helped to maintain a well-balanced and quite neutral autocross handling. These toe settings will make the car difficult to push in those long staging lines, however, and will create abnormal wear patterns on street tires. Therefore, we cannot recommend these settings for the street!
PICKING THE BEST TIRES
The lower profile 205/55ZR14 A008RS has the same diameter as the same tire in 185/60HR14 size, yet allows for almost an additional inch of tire tread width. These 205/55ZR14 tires are a tight fit on the Sentra's 5-1/2" wide wheels and due to stiff sidewall construction, require more air to seat the bead. In competition, the 205/55ZR14 tires also require more air then the 185/60HR14 tires in order to help prevent the tires from rolling over onto the sidewalls. And finally, the combination of the wider tires and the increased negative camber meant that Mark's earlier desire for aftermarket wheels with a 40 mm offset became a requirement.
There had been some concern that the Sentra would experience a loss of steering response due to the combination of wide tires and skinny wheels, but Chiles reports that there did not seem to be any steering response lag. In addition, the Sentra SE-R's steering, suspension, struts, brakes and limited slip differential all worked infinitely better with the lower profile Yokohama A008RS 205/55ZR14 tires. In fact, with the new tires, the Sentra SE-R's cornering limits were greatly increased, and braking distances were also reduced on Chiles' non-ABS-equipped Sentra.
Not satisfied with just these improvements, Chiles also conducted back-to-back tire testing of the two Yokohama A008RS tire sizes to prove that the 205/55ZR14 tire combination was the fastest. The tests were conducted with the Nissan Sentra SE-R on the sealed asphalt Parking lot of the Baseball City Sports Complex near Orlando and on the concrete runway of Sebring International Raceway. A number of drivers participated in the tests, and all were at least a second quicker on the nearly 60-second-long autocross course with the 205/55ZR14 tires. Some of the drivers said they preferred to drive the Sentra SE-R on the skinny 185/60HR14 tires, however, because they felt the car was more "tossable."
The remedy for this Sentra's clogged arteries was to continue with 2-1/2" i.d. stainless steel exhaust pipe out of the catalytic converter outlet flange. Then, using only two mandrel-style bends, the exhaust was routed under the right rear lower control arm. After that, a slight mandrel-style bend upwards led to a small Walker DynoMax turbo-style muffler placed in the same location as the OEM muffler. The exhaust exits the muffler, via a chrome tip, under the rear bumper near the OEM location. As many stock exhaust system hanging points were used as possible.
This new exhaust system changed the SE-R's exhaust note to a slightly rasping and louder sound. However, it doesn't wake the neighbors, and is still able to meet the CFR/SCCA's rigid Solo II sound control requirements.
The aftermarket exhaust also brought quite a noticeable difference in the Sentra's acceleration and throttle response. Self-timed acceleration runs made with the Sentra SE-R both before and after the exhaust mods showed that low-end torque was improved, as 0-60 MPH times were shortened by an average of .51 seconds. Second gear, 20-50 MPH sprints that were .22 seconds shorter displayed that throttle response had also improved.
Mark Chiles cautions that building your Sentra SE-R to the above specifications will not fully guarantee you a SCCA Solo D Stock National Championship-for that, you'll also need certain driving skills and that all-important bit of luck. But the modifications he shares on these pages are sure to give these Nissan products a little extra zing, which can translate into a lot of extra fun for their owners.