SE-Rious Sentra Part IV

The Second Turbo

Text & Photos By Evan Griffey

[Put into HTML format by Dan Thompson]

This article originally appeared in the Turbo & Hi-Tech Performance January 1996 issue and was reprinted with permission. For more info on Turbo Magazine check out their website.

Three heat-stress cracks in the turbine side of the original T25 robbed the turbo of exhaust gas energy. These fissures were caused by a restriction in the catalytic converter.

Here we see the first T25 turbo (left) and our current T28 hybrid. The top photo is of the compressor housings. The T3 housing (right) is about as big as you can get with the cast iron manifold. The bottom picture illustrates the difference in the turbine wheel.

As the SE-R saga continues we are in the midst of bolting up a T28 turbo from Turbonetics. While examining the previous unit, a T25, we discovered three cracks around the flapper valve of the wastegate. These cracks may have bled off significant exhaust gas energy. The trio of fissures were caused by extreme heat which was generated by intense backpressure. The Sentra was at Dynamic Autosports where technician Simon Kim was in charge of swapping turbos.

The headliner in this latest power play is a slick hybrid turbo created by Chris Weisburg of Turbonetics. The turbo features a T25 turbine housing with a .86 A/R ratio and a trimmed T28 turbine wheel. This wheel is standard fare for Nissan's hardest hitting performer - the Skyline GT-R. The compressor side of our hybrid turbo is made up of a T3 housing with a .42 A/R and a Super 60 small-shaft compressor wheel. This combination should provide stratospheric power thanks to its improved efficiency, especially on the top end, compared to the T25.

Once bolted to the exhaust manifold, Simon and our esteemed tech editor Michael Ferrara worked on positioning an external wastegate on the exhaust manifold. After trying a number of different wastegates, space limitations dictated he use of a Turbonetics Deltagate. Once properly positioned, the exhaust manifold was delivered to Scott Murphy at Belanger's Muffler for welding. Since there was very little clearance between the wastegate and the edge of the engine's valve cover, precision was paramount. Scott did an excellent job and from his shop the header went back to Dynamic Autosports for coating. To brighten up the dull looks of the manifold, Dynamic Autosports applied its Oxy-Aluma 2000 coating. This metallic-oxide coating resists corrosion and will not discolor like some other ceramic coatings.

With the front of the car torn down, we took this opportunity to up the 2.0-liter's cooling capacity by installing a Nissan Motorsports radiator. The Nissan Motorsport unit is 22-percent more efficient than the stock unit it replaces. We also had Simon install an adjustable short-throw shifter from SMC Products. The Sentra shifter comes with a weighted shift knob. The unit's adjustability comes from the leverage ball which is threaded to the stick. By loosening an Allen bolt, the ball can be spun upward or downward on the stick depending on the desired effect. The shifter is a vast improvement over the previous short-throw shifter and has quickened our shifting noticeably.

Here's the SMC Products adjustable short-throw shifter. Notice how the leverage ball is threaded into the shifter. By moving the ball up or down, the user can alter the shifter/linkage geometry and change the length of the throw.

Here's our damaged converter, dubbed the "Lava Flow" cat. The fore portion no longer resembles the honeycombed aft portion. A Random Technology cat will replace the Lava Flow unit.

After putting the turbo assembly back on the engine with a borrowed Deltagate, we made an intoxicating two-psi revving in the parking lot. Unfortunately, the engine only made four-psi on the road - the Sentra was a boostless wonder. Believing the wastegate was at fault, we awaited delivery of our own unit with a seven-pound spring. Once bolted into place we still saw no improvement. Deciding to go over the obvious first, we disconnected every pipe in the intake tract looking for obstructions and replaced it - no tangible improvement. Since most of the exhaust system was already removed we decided to take off the catalytic converter. What a relief when we discovered the catalytic converter was our problem. Our "Lava Flow" cat from the acclaimed "Chernobyl Series" was created when the engine experienced lean and rich conditions under a good deal of load during the tuning process. As the photos will show a meltdown of the Nth degree resulted in a great restriction that caused cracks in the T25's turbine housing. Once compromised, the cat continued to lose flow until it smothered the turbo's ability to make boost. At this point, we figured that the cat or the breakage of a jury-rigged spring on the T25's wastegate were responsible for the poor showing on K&N's DynoJet dyno. With the Lava Flow cat removed we could rev the engine in neutral and see five psi from the T28. Figuring we were on the right track, we took the car for a test run. In Second gear we jumped on it and the boost needle leaped like it had been shocked. Before we could get off the gas the needle was well past 15 psi - we were live once again. With the mean, aggressive exhaust note of a MerCruiser big-block at full song, our Serious Sentra was really tugging at the bit. We set our boost controller at nine psi because we knew the fuel system could provide sufficient fuel at that level. The setting felt as strong as 13 to 14 psi from the T25, a testament to the efficiency of the new hybrid turbo.

Before sending the SE-R to Jim Wolf Technology for fine tuning, we installed a custom-fabricated exhaust system that features a GReddy RX-7 can and a monster 130mm GReddy polished tip. The three inch system includes a Random Technology replacement cat which should live quite happily as the tuning process will be much kinder this time around. The whole enchilada was expertly welded up by Scott at Belanger's Muffler.

In our next update, we will get some performance and possibly dyno numbers with the current fuel system, install the new system, tune the car and possibly get some new numbers on it as well.