Porsche 964 Doppelturbo Cabrio

The Porsche Carrera C4 is one of the hottest Autobahn bombers on the road. With its all-wheel drive system, the Porsche is ready to duke it out at a moment’s notice-damn the road conditions. As potentially devastating as the C4 is, the crew at BEGI (Bell Engineering Group, lnc.) couldn’t wait to get their hands on one. 

BEGI, once the U.S. distributor for the Haltech pro­grammable engine management computer under the name Cartech, definitely knows how to set up custom fuel injec­tion systems. But the company’s newest line of turbo system products, sold under the BEGI labe! are designed in line with a different philosophy-maintaining the stock fuel injection computer on turbo conversions for easy emissions compliance in all 50 states. BEG I has achieved excellent results in turbocharging the late-model Acura NSX, the Ford Mustang, the Mazda Miata and the Porsche Carrera 4 featured here. This was accomplished by maintaining stock engine management under all conditions while manipulat­ing fuel pressure under turbo boost conditions via a special, variable rate of gain Cartech fuel pressure regulator to produce proper air/fuel ratios. 

BEGI owner Corky Bell is something of a fanatic about the Aerocharger Variable Area Turbine Nozzle tur­bocharger. which needs no wastegate. no external oiling. and can even function with the shaft axis turned vertically­as is the case with the twin VA TN turbos on this Porsche C4. Of course, the main advantage of the Aerocharger turbos is that the nozzles through which exhaust gases pass to exert force on the turbine are made effectively smaller at lower engine speeds to increase the velocity of exhaust gases and get the turbine/compressor shaft spinning fast right now, in order to eliminate turbo lag. At higher rpm/loading, with heavier and faster exhaust gas flow. the VA TN nozzles open wider to reduce backpressure and raise peak horsepower. 

General Approach 

Overall, the BEGI objective was to produce a 35 percent torque increase from 2000 rpm to redline. while maintaining stock emissions. There was to be no alteration to the Porsche’s driving quality, and the car was to be com­patible with 93-octane fuel. 

BEGI set about to build a low boost pressure, high response turbo system designed to keep charge air tempera­tures near ambient. This high response mandates excellent low end torque. which implies maintaining the stock C4 compression ratio of 11:1, thereby eliminating the expense and complexity of internal engine modifications. But high compression ratios-and even higher effective compression ratios produced under turbo boost-require good combus­tion chamber design, lowest possible combustion tempera­tures, good gasoline, and ideal air-fuel ratios. in order to combat detonation. Low combus, ion temperature implies cool inlet air, which implies intercooling. Porsche already took care of the combustion chamber design, but owners of the BEGI twin-turbo C4 will be wanting the best street gasoline available. 

Due to the C4 motor’s high compression ratio. heat in the intake charge was a major hurdle in the BEGI system design. One of the design goals was selecting a compres­sor that would reach peak efficiency at the point of maximum boost at redline. According to Aerocharger compressor flow maps, peak efficiency is 78 percent. Testing showed that the actual com­pressor output temperature was 76 degrees above ambient, seven degrees off on the high side. Working the equation backwards. seven extra degrees indicates the actual efficiency was 71 percent. 

Achieving very high compressor efficiency in turbo system design generally requires a compressor so large that it is sluggish at low speeds. However. the V ATN design t”eature ended up powering the compressors suffi­ciently to achieve full five psi boost at 1400 rpm! Raad testing of the finished design would reveal. in fact. an unforeseen problem: At maximum response adjustment, the turbos were so sensitive to small throttle increases ( .. jumpy”) that unwanted spurts of boost had to be elimi­nated by adjusting the minimum discharge area of the VA TN turbos to a larger opening. 

In case you’re thinking about installing a BEGI C4 system and pumping up the boost with race gas, know that boost pressure on a system like this is limited not only by detonation considerations. but also fuel pressure. The Cartech regulator increases fuel pressure under boost at a ratio of at least four to seven psi of fuel pressure increase per pound of boost in order to prevent leaning out. Given injector and fuel pump limitations, fuel pressure cannot normally rise much above 90 psi, which leaves about 40 to 50 pounds of surplus pressure to work with. Therefore, total boost limitations of five to 10 psi are essential (perhaps even a little less to provide a margin of error). 

Layout 

BEGI mounted two Aerochargers with the shafts on a vertical plane, both on the driver’s side of the car. Each turbo compressor feeds into a single intercooler. The inter­cooler’s single outlet feeds into the Bosch air flow meter. Both turbos exit directly into the catalytic convertor. and then to the muffler. The above plumbing is short enough to maintain good heat for the turbos, the catalyst, and the oxygen sensor. 

Turbos 

The variable area turbine nozzle (VA TN)-as used by BEGI on the C4 solves the problem of turbo lag with an ingenious approach to turbo design. Aerocharger’s manu­facturer Aerodyne first utilized the concept of the variable­vane turbocharger in the late ’70s, producing one of the first commercially available units in the early ’80s. The unit uses a relatively conventional-although extremely efficient­compressor section. But the turbine section is revolution­ary. Exhaust gases traveling at high speed around the cir­cumference of the turbine housing are brought to bear and the turbine wheel-not through a single fixed-size orifice as in a conventional turbocharger. but through a continuous slot around the entire circumference of the turbine housing. A series of hinged f! apper vanes are arranged radially around the turbine wheel in the slot. If the vanes are point­ing directly nearly so-toward the center of the turbine wheel, there is little resistance to exhaust gas flow. and exhaust gases impact the turbine wheel at relatively low speeds with little resistance or backpressure. But when the vanes rotate such that they are almost lying down against eachothere scales an afish nearlyy tangential to the circumference of the turbine wheel. exhaust gases must accelerate to very high speeds as they flow through the narrow path between the vanes. they are hitting the turbine blades at extremely high speed. Even at low engineairfloww, exhaust gases can be accelerated quickly to make a lot of low-rpm boost. 

Interestingly, the Aerocharger is, in effect. its own “wastegate.” An actuatorandn the Aerocharger rotate a wheel which simultaneously changes the angle of all vanes at once. The closed vanes act like a very small AIR orifice at low engine speed. which progressively open at higher speeds airfloww) to control turbine speed. The actuator is adjustable similarly to some wastegate actuators. and is referred to manifold pressure like a wastegate. 

The Aerodyne variable \’ane turbo has yet another unique feature for a turbocharger: The oiling system is self­contained. Every 20,000 miles you add 100cc of synthetic oil to the unit, which shares no oil with the vehicle’s engine. In other words. the Aerocharger unit has no oil lines! The oil wick can be set up at manufacturing time to allow vertical placement. 

Aerocharger compressor sections are sized in seven increments from 70 to 290 horsepower. BEGI uses two 220-horsepower units on the C4. 

The exhaust from the two Aerochargers is routed through the Borla stainless steel alloy two-path muffler with eight square inches of flow area.  

Charge Cooling 

With the Porsche C4. space con­siderations suggested sufficient charge cooling was possible at 80 per cent effi­ciency with 0.5 psi pressure loss. The intercooler is located behind the pas­senger side rear wheeL However. inter­cooler efficiency of a hoped-for 90 per cent turned out to be elusive. The charge air temperature exiting the intercooler was 21 degrees above ambient. indicating actual efficiency was more like 71 per cent. BEGI never determined the reason for this. although suspicion centres around the large thickness of the core material. In retrospect. BEG I decided thick cores were unavoidable. due to the need to avoid significant pressure losses.

Fueling 

The Cartech Variable Rate of Gain (VRG) fuel regulator is located downstream of the stock Bosch fuel pressure regulator. it has no effect during naturally-aspirated conditions. allowingfree floww of fuel into the fuel return line. Under boost conditions. the manifold-referenced VRG regula­tor progressively limits fuel return to provide a rising rate of fuel pressure gain up to a maximum of 85 psi fuel at t1vs psi boost pressure. BEGI left the Bosch Air Flow Meter in its original position. blowing through it to avoid –air-fuel ratio lag” based on the system layout which put the turbos some distance from the meter. Bell felt that five psi would not overly stress the body of the flow meter. Fears that crankcase breather fumes would produce flow meter problems. thereby requiring a condensing filter. turned out to be groundless. 

Performance

On the road. the thrill is exactly 

like a C4. only more so. If you’ve read the C2 and C4 reviews. you know the C2 is alittlee q uicker-except where the all-wheel drive comes into play-like on dead-stop launches and such. The BEGI C4 is noticeably quicker than either. BEGI claims the C4 is the hottest five-psi turbo system the ·’vee ever driven. feeling more like a 10 psi car. Output is estimated by BEGI to be in the 320-to 330-horse range. BEGI’s approach to increasing the power-to-weight ratio on the C4 was to keep the 3.6-liter motor with 240 horsepower and 240 lbs-ft of torque, twin-plug configuration and 11: 1 compression ratio as stock as possible. With the VATN turbos in place, the engine makes great power to 8000 rpms resulting in 0-60 times of roughly 4.6 seconds. according to BEGL with 5.5 seconds being stock. The Aerocharger V ATN concept eliminates any sense of hesitation followed by the power surge of some conventional turbo including many factory Porsche turbo cars! 

The expertise on the Car describes it as condition 2-

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