The last installment of Project 911 SC/Carrera left off with a very sexy bottom end, a butt built for speed if you will. Ollie's Porsche Machining in Santa Ana, Calif., hot-tanked the case and removed the original fasteners to make way for the ultra-tough ARP units. The case spigot bores were enlarged from 103mm to 105mm to accept larger 100mm cylinders. Ultimately, displacement would be increased from 3.2 to 3.5 liters and filled with custom pistons, reconditioned rods and AASCO Performance springs. This is basic old school engine building stuff, virtually unchanged for decades. And given the quality of hardware, craftsmanship and realistic performance expectations, that's how long we expect this engine to last.
Machine work-top end
Once the bottom end machining and balancing was completed, Ollie's focused on the top end. The rocker arms were reconditioned and the shafts polished in preparation for running with the new camshaft profile. The cylinder heads were disassembled and cleaned for inspection. The 3.2-liter heads feature large 40mm intake and 38mm exhaust ports, sufficient in stock form to flow well over 300 hp. The ports were polished but not increased in size to maintain low- and mid-range drivability. The heads were also drilled and tapped for the second spark plug hole in preparation for twin ignition. Each head was also surfaced and beveled for use with Mahle's 100mm cylinders. All 12 valve guides were replaced with special phosphorous bronze units that Ollie's has made to its own specifications. These guides should last for upwards of 150,000 miles and resolve a known weak spot in the 3.2-liter Carrera engine. The intake and exhaust valves were measured for wear and all six intakes replaced, while the six exhaust valves were deemed serviceable and simply reground. A three-angle performance valve job suitable for street use was performed. Then the heads were carefully reassembled and the valve spring height set in preparation for installation.
I preferred to use the RSR-style pistons with valve reliefs machined into the piston crown to provide the increased piston-to-valve clearance necessary for high-lift camshafts. However, the common plenum and single throttle body used with Motronic injection, and more importantly the emissions restrictions in California, necessitated a compromise in the camshaft profile I wanted to use. From my days at Andial, I knew the factory 964 grind was a common upgrade over the SC and Carrera profiles and would still meet emissions requirements. Web-Cam in Riverside, Calif., has been hard welding and grinding cams since 1945 and offers a wide range of off-the-shelf and custom profiles for the earliest 2.0-liter motors all the way to the fire-breathing 993 GT2. I called co-owner Laurie Dunlap to ask if there was another grind available that yielded slightly more lift with similar duration compared to the 964 profile, but would still work with Motronic injection and pass smog. She suggested using a 20/21 grind to meet my criteria. The 0.485-inch intake/0.452-inch exhaust lift and 258-degree intake/246-degree exhaust duration cam pair is developed specifically for SC/Carrera/964 engines retaining CIS or Motronic injection and will provide more mid-range and top-end power than the more commonly installed 964 grind. Web-Cam offers hard-welded and reground cams on an exchange basis, or the profile of your choice can be ground on new O.E. quality billets.
A big problem with increasing the bore and compression ratio on an SC or Carrera motor is the propensity for harmful detonation to occur depending on the octane of the gas available. Bear in mind the 3.0 and 3.2-liter motors were just entering the electronics age in the 1980s and did not have a knock sensor provision to retard the ignition timing in the event that detonation occurred. Porsche alleviated this problem with the introduction of the 3.6-liter motor in 1989, incorporating twin-ignition and a knock sensor for each bank of cylinders. With these modern attributes, Porsche's new 3.6 safely operated at a lofty compression ratio of 11.3:1.
The larger cylinder bores, combined with high-dome pistons used to increase compression and allow the use of more radical camshaft profiles, also effectively cut the combustion chamber in half. The 49mm intake and 41.5mm exhaust valves used in SC and Carrera motors are so large there is not enough room in the combustion chamber for placement of a center-mounted spark plug, which is the ideal location for efficient and complete combustion. To combat this problem, a second hole is drilled and tapped for a second spark plug opposite the stock location. Two plugs per cylinder allow the combustion process to proceed on both sides of the combustion chamber, especially important on larger bore motors where the flame front has a bigger area to cover. It gives a slight increase in horsepower along with better fuel economy and a smoother running engine. Generally speaking, the addition of twin-ignition allows for an extra point of compression while still using the same gasoline octane level.Twin-ignition implementation for 3.2-liter Carrera motors is straightforward. The 3.2L Carrera uses Motronic injection: The fuel and spark are both controlled by the DME computer located under the driver's seat. The major parts required to upgrade to a twin-plug ignition system are a distributor from a 964 or 993, a special signal splitter developed by Andial that mounts next to the DME computer, a second ignition coil and the lower spark plug wires. The triggering is done at the flywheel by the OEM sensor and the distributor simply does what its name implies: It distributes the sparks.
Since the SC/Carrera motor is not originally equipped with twin-ignition, the lower spark plug wire harness must be either custom made using original connectors, boots, terminals, etc., purchased in bulk from a local auto parts store or sourced from an aftermarket parts vendor. This can be expensive and time consuming and the end result is usually less than aesthetically pleasing, unless you also redo the upper set of wires to match the new lower set. One call to Steve Brown at Magnecor in Farmington Hills, Mich., resolved this dilemma. Magnecor manufactures race-proven 8.5mm or 10mm spark plug wires that are ideal for stock or modified engines relying on electronics to control ignition and fuel systems. I chose the KV85 Competition wire set primarily because its high-quality connectors eliminate misfires caused by faulty resistors commonly found in the stock plug connectors. Electromagnetic interference (or EMI) caused by poor quality wire sets can cause engine computers to receive incorrect information, leading to drivability problems. Magnecor's KV85 Competition wire sets provide proper suppression for EMI and are frequently touted as the last set of spark plug wires you'll ever need to purchase.
Intake plenums and throttle body
The stock 3.2 Carrera intake plenums are known to flow uneven amounts of air through the individual runners. While the horsepower gains are typically small on a stock or mildly modified 3.2-liter motor, making the airflow consistent across all cylinders will increase smoothness and drivability due to each cylinder receiving equal amounts of air and fuel. In the old days, the plenums were removed from the motor, cut open, the internals polished by hand, and then welded up and reinstalled on the motor. The problem with this method was the welds on the plenums were clearly visible.
|Cylinder||Stock cfm||Extrude Hone cfm||cfm Gain||cfm Increase %|
this stock and modified comparison table illustrates the greater and more uniform airflow across all cylinders provided by the Extrude Hone AFM process
Extrude Hone originally developed its AFM (Abrasive Flow Machining) process in the late 1970s for motorcycle engines. What can be best described as semi-solid putty with silicon carbide acting as an abrasive is forced through heads and manifolds under strictly regulated conditions. This process is so consistent that Ford approved AFM for the heads and manifolds used on the production Contour SVT and Cobra SVT models. The AFM process is also successfully used in aerospace and industrial applications. An added benefit of extrude honing vs. cutting, polishing and welding is that externally the plenums maintain their stock appearance.
The table shows the stock and modified airflow (in cfm) after extrude honing for a typical 3.2-liter intake plenum. Note the airflow variation between the best and worst performing stock runners is 37%. After the extrude honing process, the variation decreased to only 7%, leading to a much more uniform intake charge to each of the six cylinders.
The last restriction on the intake side is the throttle body. While a stock 3.0 or 3.2 probably will not see any significant horsepower gains simply by enlarging the throttle body, a larger displacement with the rest of its induction system opened up can indeed benefit from this modification. Steve Becker from B. Precise Machining measured the stock throttle body diameter at 62.99mm and increased the diameter to 66.77mm to try and wring out every last little bit of power.
The standard exhaust system routing on all 3.0-liter SC and 3.2-liter Carrera motors is restrictive due to Porsche having to meet stringent emission standards starting in 1974. SC and Carrera owners can realize significant power gains simply by replacing the standard heat exchangers and single-inlet muffler with stainless-steel headers wrapped inside heater boxes combined with a dual-inlet sport muffler. These exhaust modifications require removing the catalytic convertor and are legal for off-road use only, so check your local laws before proceeding.
If the car is raced or resides in a warm climate where the heater is not required, the equal-length mild steel headers crafted by George Narbel at European Racing Headers make excellent power and torque. An added benefit is these headers not only lower the overall weight of the exhaust system, they also make the motor run significantly cooler. With heat no longer being viable, the heater blower assembly is removed from the left side of the engine bay. This allows the fan to turn additional air back over the cylinders for improved cooling.
The exhaust gases will flow from Narbel's headers into a T304 stainless-steel Dansk sport muffler. Dansk is a Danish exhaust manufacturer that provides OEM parts to Porsche. Dansk's dual-inlet sport muffler with dual 84mm exhaust tips fits perfectly, just like the original muffler, but that's where the similarity ends. Whereas the stock muffler is demure in sound and appearance, the Dansk unit elicits a growl under acceleration which notifies curious onlookers that this 911 means business, without overshadowing the wonderful fan shriek that makes the engine note so distinctive. The excellent build quality ensures maximum power is attained without the harsh resonance and drone under load that describes many inferior muffler manufacturers' products.
Ready to build: final assembly
Now that the all of the machine work has been performed, the performance parts chosen, and the standard replacement gaskets, seals and bearings purchased, it's time to assemble the motor. The final installment will find Steve Becker and myself building the motor, Dwain and the gang at Vision Motorsports prepping the car and installing the motor, and finally breaking it in and putting it on the dyno. Will the goal be attained?