To recap, for those in the back of the room: Unlike a belt-driven blower, a turbocharger relies on exhaust gases to spin its turbine wheel. This turbine wheel is mated to a compressor wheel, which spins as well and forces huge amounts of atmosphere into the engine. In other words, it makes boost. The sooner you can get that turbo to spin, the sooner your motor will experience manifold pressure above atmospheric norm (boost)--and building boost quickly is a good thing for performance.
One of the easier ways to improve turbo response is to eliminate the most restrictive element in the exhaust system, the catalytic converter. California law prohibits driving on public roads with a cat-less exhaust system on any car produced after 1973. The car can be driven on a private road or racetrack. As this car could very well end up on the track, I decided to test a racing exhaust system, supplied by Lindsey Racing, to investigate the difference between a corked and uncorked tailpipe. Testing would be conducted in Precision Motion's Porsche repair shop on its Clayton dynamometer.
The Lindsey Racing exhaust looked nothing short of a full racing system basically a 3-in. pipe from the downpipe back, with a free-flow muffler in the rear. This mild-steel exhaust is manufactured from aluminized exhaust tubing, making it much less expensive to build and much cheaper to buy than stainless steel.
Lindsey Racing offers this exhaust system in four different configurations. First is the basic, straight-through design, with no wastegate pipe tied in and no muffler. It's also the cheapest at just $459. The second system is identical except for wastegate piping to use with your stock or aftermarket wastegate. The third system features a high-flow muffler but no wastegate piping. Both the second and third systems are offered at $559. The fourth system, tested on Project 951, uses both the wastegate piping as well as a rear muffler and costs another $659. For an additional $200, Lindsey also offers a high-flow catalytic converter with any system. If you absolutely have to have stainless steel, Lindsey Racing also offers a 3-in., fully stainless-steel exhaust for a little less than $1,400. My pockets weren't quite that big, but the exhaust system I did order features Lindsey Racing's optional ceramic thermal coating to reduce heat radiation into the cockpit and extend the life of the exhaust system.
Installation is fairly straightforward. The downpipe fit perfectly through the center section, but when Precision Motion owner Don Kravig got to the rear muffler, a little fab work was required. Since the muffler wasn't aligning perfectly with the center section, it would have placed the tip to the right of its normal location. To remedy this, I took the muffler to a local exhaust shop, Redlands Muffler, and had them simply stretch the 3-in. opening of the inlet side about another 1/2 in. This allowed the muffler to be pivoted more towards the exhaust tip opening, after which Don welded it to seal it completely. As a result, the last section--the exhaust tip--came out nicely through the exhaust opening under the factory rear valance.
Compared to many exhaust systems, this Lindsey setup is quite inexpensive. The one aspect of it I didn't care entirely for, however, was the raw look of the tip--it was merely the end of the piping. And although the silver color of the thermal coating helps its appearance, I decided to keep the tip as factory looking as possible. When I was at Redlands Muffler, I found the perfect tip for $35. Don hacked off a piece of the tail end of the Lindsey system and welded the new tip in place.
On the dyno, Don tested the exhaust the same way he tested the B&B system in Part 4 (ec, 03/04), which stayed on the car. Don tested it against a load at certain rpm intervals and recorded the sustained horsepower, measured by the computer. As expected, the release of so much backpressure from removal of the cat did little down low, but we didn't see a loss anywhere else, either. It was after 3000 rpm when the true value of the system became apparent. Thanks to the Lindsey exhaust, the turbo came to life much sooner and showed tremendous gains through redline, peaking at 208 whp at 6400 rpm with just 7.5 psi boost. This compares to 163 whp with the previous 2.5-in stock exhaust system and B&B headers.
Don was impressed. "With the stock exhaust and with the B&B 2.5-in header system, you had nice low-end power and mid-range torque, but there was no reason for going past 6000 rpm with either system. But with a cat-less 3-in. system, you now can take this car to redline and continue making horsepower." It takes about 30 sec. to unload a car from Don's Clayton dyno, and it took about 35 sec. for me to get strapped in and drive off like a kid with a new toy. The difference was amazing: The much quicker spool-up felt as though a smaller turbo had made its way into the engine compartment, yet the top-end pull was reminiscent of a bigger turbo. From 3000 rpm on, there was power all over the place.
The sound from inside the car was now incredible, more than a match for the aural assault. I had ridden in a Ferrari F40 just a couple of days before, and I couldn't stop thinking about it from inside the Porsche's cockpit. On the outside, it was a different story. The car sounded tremendously aggressive, even though the muffler was doing a spectacular job of quieting things down--we tried it uncorked, and dogs went into hiding for miles around. I may be adding a small center muffler sometime in the near future to bring down the decibels (remember, this is my wife's car, too).
Running with a cat-less system also affects throttle response and air/fuel mixtures, so retuning the Kokeln-supplied Link Engine Management system will be a must before turning up the boost.
With so much more power (and much more on the way), I began to fear for some driveline components. Surely the transmission and differential were handling it with ease; my main concern was with the clutch, especially since it had endured lots of hard miles.
Enter Centerforce. With its extensive background in manufacturing clutches for the street and for race cars, I felt I couldn't go wrong with one of Centerforce's units. They're distinguished by a patented weight system that increases performance without increasing pedal effort, important for comfort as well as preventing premature wear on the release mechanism.
Realize, right now, that a 944 Turbo clutch install is a big job. The exhaust, rear subframe, transmission and differential housing all have to come off. It's about a 12-hr job, so make sure that anything which needs replacing gets replaced now. Registering over 80k miles on the odometer, it was no surprise that Project 951 showed a little wear on the clutch release mechanism, so I contacted Performance Products, which carries a wide choice of replacement, maintenance and restoration parts for Porsche and Mercedes-Benz cars. It also sells the Centerforce dual-friction clutch upgrade. Since Performance Products works hand-in-hand with some well-known Porsche dealers, the clutch-release products were sourced from The Auto Gallery, a Porsche dealer in Woodland Hills, Calif. To my delight, the parts arrived the very next day.
Precision Motion's Don Kravig finally got the car all put together and handed me the keys. Centerforce requires 400 miles of stop-and-go driving to fully break in the clutch, which I respected. Immediately, though, I was blown away at the soft feel of the clutch pedal--if she knew how to shift gears, my grandmother could even drive this car.
Centerforce reports its dual-friction clutch system is tested up to a 90% increase in clamping force, plenty for my peak torque expectations in the near future.
(At the time of this writing, the clutch system has been holding up fine, with torque outputs to the ground nearing the 300 lb-ft mark. I'll have more on those tests later.)
•Estimated time:13 hours shop labor @ $90/hour = $1,170
•Lindsey Exhaust: $659
•Thermal coating: $100
•Muffler inlet enlargement: $10
•Exhaust tip: $35
•Clutch release mechanism: $380
•Throw out bearing, pilot bearing: $170
•Flywheel resurfacing and bolts: $80