We left off with my 3.2-liter Porsche 911 motor torn down and in numerous pieces. To reach the desired goal of 80 hp/liter while retaining street manners and utilizing 91 octane pump gas, I chose to increase the displacement from 3.2 to 3.5 liters using larger 100mm forged pistons and cylinders with an advertised 10.3:1 compression ratio. With the larger cylinder bore and increased compression, twin-plug ignition is an absolute must for maximum performance and reliability. The breathing capabilities for the big motor will be improved on the induction side by modifying the heads, intake plenums and throttle body. An appropriate camshaft grind will be used that not only improves the mechanical characteristics of the motor but also remains California emissions compliant. Exhaust gases will be evacuated more efficiently using larger diameter equal length headers and a dual inlet and outlet T304 stainless steel sport muffler. Most importantly, known reliability problems associated with 3.0- and 3.2-liter motors will be addressed with higher quality head studs, rod bolts, valve springs and valve guides. Once the motor is assembled and running in the car, it will go through the usual 1,000-mile break-in period. After break-in, a specially mapped aftermarket computer chip will be installed. This modification is designed to optimize the fuel and timing curves, and will be the final piece of the puzzle before heading back to Vision Motorsport's Dynojet 248C chassis dyno to measure the results.

Displacement Increase Options
For a normally aspirated engine, the most widely accepted method for increasing horsepower and torque is simply making the motor larger. "There's no replacement for displacement" is the classic American hot-rodder's rule of thumb. Unfortunately for a 3.0 SC or 3.2 Carrera, the larger pistons and cylinders (and sometimes crank and rods in addition) needed to achieve such a displacement increase are very expensive and can easily represent more than half the cost of the parts and machine work needed for a standard rebuild. The SC and Carrera share the same 95mm bore (although the piston wrist pin diameter is different), but the Carrera borrows the crankshaft and rods from the 3.3-liter 930 to gain extra displacement over the SC. The cases and many internal parts are interchangeable so either one can be a candidate for an engine pushing up to 3.7 liters. The table on page 66 lists various displacement options available to SC and Carrera owners. The least expensive, and thus most popular, conversions are 98mm pistons and cylinders for both motor types, yielding 3.2 liters from a 3.0 and 3.4 liters from a 3.2, respectively. The slip-in 98mm units literally replace the stock 95mm pieces and do not require splitting or machining the engine case. The only modification necessary is a small chamfer on the cylinder heads. The piston dome, compression ratio and octane availability will determine if twin-plug ignition is recommended or required.

Knowing my engine's case would be split and twin-plug ignition would be used regardless of the chosen bore diameter, I took the next step up and decided on 100mm forged pistons and cylinders from Mahle Motorsport. The 100mm units increase the displacement on a base 3.2-liter motor from 3164cc to 3506cc, a 10.8% increase that makes a true 3.5-liter motor. Mahle is a respected German firm well known for making high quality pistons. In addition to manufacturing pistons and cylinders for the Porsche aftermarket, Mahle is also an OEM supplier to Porsche, supplying pistons and cylinders for both production and competition engines. Mahle's forged pistons and special Nikasil-coated cylinders are widely considered the best available for a 911 engine rebuild, but their high cost and limited applications breeds alternatives. JE Pistons also manufactures forged piston sets for 911 motors and they have two major advantages over the Mahle components: They can be made to the bore (provided you can find the appropriate cylinders) and compression ratio specifications of your choice. Time will tell if JE's pistons will share the same legendary reliability that Mahle's enjoy, but for a race motor whose endurance is measured in hours rather than miles they pose an attractive option for an engine build.

Machine Work - Bottom End
Since the machining and reconditioning of the vital internals generally has the longest lead time, these tasks were first on the agenda. Given the high cost of replacement parts and the special tools and knowledge necessary to undertake a high quality Porsche 911 engine rebuild, it is extremely important to source a machine shop that has extensive experience with 911 engines. One must be careful to minimize the risk of tolerances not measuring up to Porsche's original high standards.

George Boley and Ike Arriola at Ollie's Porsche Machining in Santa Ana, Calif., both have more than 35 years of experience with 911 internals and quite possibly have performed more 911 machine work over the years than anyone else outside the Porsche factory. Their work makes the grade for noted tuners Andial, Porsche Motorsport, Vision Motorsports and many others from across the country. It is not uncommon to see between 25 and 40 engine cases at any one time waiting for modification. The guys at Ollie's know their stuff.

Before starting on the machine work, my case was hot tanked and the oil galleys and piston squirters were given a thorough cleaning. The old cylinder head studs were removed and discarded. The case spigot bores were then enlarged from 103mm to 105mm to accept the larger 100mm cylinders. The case was also boat-tailed to streamline the main bearing support webs for increased airflow, as well as shuffle-pinned (dowel pins installed to minimize case movement) for additional strength at high rpm. The crank and rods were Magnafluxed to check for any imperfections, then the crank was micro-polished and the rods reconditioned. The 100mm cylinders also received a small angular cut on each side commonly referred to as "mooning," which is generally performed as a complementary modification with boat-tailing. After the flywheel was resurfaced, the reciprocating internals were fully balanced to keep operation smooth.

Valve Springs
911 valve springs have a history of being robust and generally are only replaced when the engine is being rebuilt or prepped for track use. One of the best sources for 911 racing valve springs is AASCO Performance in Anaheim, Calif. Vince Howard at AASCO states that although stock valve springs removed from a high mileage motor may seem visually perfect, they usually show fatigue when measured on a spring compressor. Vince recommends replacing the stock valve springs with stiffer AASCO racing valve springs if the motor will turn more than 6500 rpm. AASCO racing valve springs should be on the list of parts for any 911 engine rebuild, especially when sport camshafts with increased lift will be used. AASCO racing valve springs are rated for up to 0.512 inches (13mm) of cam lift and 8500 rpm, much more than my street-driven 3.5-liter will ever see. AASCO valve springs are cheap insurance that can help a potential missed shift become an "oops" rather than an "ouch." The stock steel retainers were reused. Vince said high dollar titanium versions are only necessary for race motors that rev over 7500 rpm.

By Ralph B Hollack
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