Nissan Spec-V: Part 16: It’s a Turbo
10/11/2006
Since the long stroke QR25DE does not like high rpm, we have chosen to limit its maximum revs to 7000 rpm and make power by adding turbo boost. In our experience, it is difficult to get front wheel drive cars with more than 350 wheel hp to be able to use the power without excessive wheel spin under road racing conditions. In front wheel drive cars, excessive wheel spin quickly overheats the front tires and leads to severe understeer, a condition to be avoided.
In our planning we decided that 350 whp is the most power that we should aim to produce. Jim Wolf Technology designed our turbo system to produce no more than this, over the widest possible range of RPM. The JWT system was also designed for simplicity, lightness and reliability over maximum power.
Garrett’s GT28RS turbo is designed for reliability, high-quality power for its size and quick spooling.The heart of the system is the turbocharger; JWT selected the Garrett GT28RS. This turbo has the potential for up to 350 whp and is designed for rapid response and minimal turbo lag. It features the ultra efficient BCI16 compressor wheel in 62 trim in a TO4B housing. On the exhaust side a 76 trim NS111 turbine wheel resides in a .86 A/R turbine housing. This turbo is also known as the Disco Potato as it was born to power another well known B15 Sentra, the famed Nissan concept Disco Potato Sentra. The GT28RS also features a low drag ball bearing center section for improved reliability, faster spool and better throttle response. The QR25DE’s over 2500cc of long stroke power pulses should spool this turbo so fast that turbo lag should be nearly undetectable.
JWT’s long manifold is shown here under construction; it is designed for crack free reliability.A log type manifold was fabricated from schedule 40 stainless steel weld els. A log manifold is not the best for power, but logs generally spool faster than tubular turbo headers. A long tube equal length turbo header is also more likely to fail under the extended full throttle action that road racing cars see. JWT decided to use an internal waste gate. To reduce complications a big external waste gate plumbing was not needed.
Our Spearco intercooler is kept to a moderate size so as not to block airflow to the radiator. The coating is Swain Technology’s black body emitting coating, which helps speed heat transfer.To reduce the possibility of heat damage and to prevent setting our carbon fiber hood on fire, JWT constructed stainless steel heat shields over the exhaust manifold and turbine housing to reduce the radiant heat created to less than stock levels.
The exhaust system is unique. The turbine housing was modified by JWT to separate the wastegate and turbine flows. This has been found to boost power and to improve turbo spool time. The flows are kept separate by a hand fabricated stainless O2 sensor housing and are introduced into a venturi created with a Burns Stainless long transition megaphone, leading to a 3” downpipe. This is designed to improve exhaust velocity for improved bottom end torque but to provide adequate flow at high rpm. The downpipe is connected to the 3” exhaust via a sprung slip flange and a V-Band clamp.
Cleanly laid out and simply routed plumbing connect the turbo to a moderately sized front mount Spearco intercooler. A too large front mount will block airflow to the radiator, airflow that is needed to keep a road racing motor cool. A BOV from a Skyline GT-R was re-circulated into the intake; good design practice for MAF equipped cars like this one. The air intake was equipped with a JWT POP charger filter drawing cool air from the front of the car.
To keep things from getting heat damaged, heat shields were built to go around the turbine housings, downpipe and exhaust manifolds.
The exhaust housing was modified to keep the wastegate and turbine flows apart to reduce flow restricting turbulence. This increases power and speeds spool time.
This fabricated stainless O2 sensor housing keeps the flows separate then smoothly merges them together far away from the turbo.
As you can see, there is a physical divider between the turbine and wastegate ports.The entire exhaust system was fabricated from lightweight 0.035” thinwall 321 stainless, provided by Burns Stainless. Instead of a turbulence inducing flex pipe, a muffler bearing was used. That’s right, a Burns Stainless muffler bearing! First the exhaust was terminated by Burns Stainless’s ultra light race muffler. This muffler is quite good at keeping the noise down while weighing only 4 lbs. The muffler bearing is a tubular cradle for the muffler that is mounted to the chassis, which allows the muffler to slide back and forth inside it so it can move with the engine. Our entire system only weighs about 16 lbs total, about one-third the weight of a normal exhaust.
The O2 sensor housing turns into a megaphone fairing up to 3” in diameter. This keeps exhaust velocity high for maximum torque but still flows well enough for top end power. An extra O2 sensor bung is provided for tuning on the dyno with a wideband.
Ritchie Watanabe tig welds our exhaust system on the bench after tack welding it on the car. Thinwall 321 mist be tiged with a fine 0.035 wire.
The super light Burns Stainless muffler only weighs 4 lbs and really cuts the noise.
Our downpipe uses spring clamps and a slip joint so it can move with engine torque without cracking.
The muffler bearing allows the whole exhaust to move around in response to engine torque.Our turbo has plastic bearing separators, which require that the center section be water-cooled for maximum life. We plumbed the water and oil to the turbo with a combination of fabricated hard lines and Earl’s Teflon lined braided steel hose as well as fittings for maximum reliability.
With our built motor in place and the turbo system fabricated, we will now turn our attention to the peripherals to make our system functional.
It is starting to look like a car again!