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Porsche 911 SC Carrera

Engine Build Part 2: Building a better bottom

Ralph B Hollack
Jan 4, 2006 SHARE
0507_01z+porsche_911_sc_carrera_coupe+left Photo 1/10   |   . Beautiful, isn't it? Better yet, this car is designed to be a durable, daily-driven reality.

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.

0505ec_proj964_02_z Photo 2/10   |   . Mahle Motorsport 100mm forged piston and cylinder. The pistons are machined with valve reliefs to clear high lift camshafts.

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.

0505ec_proj964_03_z Photo 3/10   |   Engine case spigot bore opened to 105mm and main bearing webs boat-tailed for increased airflow inside the case.

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.

0505ec_proj964_04_z Photo 4/10   |   Inside the engine case. Note the dowel pin mating holes on the inner main bearing webs to minimize case movement at higher rpm.

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.

0505ec_proj964_05_z Photo 5/10   |   Inside the engine case. Note the dowel pin mating holes on the inner main bearing webs to minimize case movement at higher rpm.

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.

0505ec_proj964_06_z Photo 6/10   |   100mm piston with cylinder that has been machined with a small moon-shaped cut on each side to improve airflow.

Cylinder Head Studs

Broken head studs in 911 motors is an issue that could be viewed as a chink in Porsche's vaunted design armor. From the 2.7 all the way up to the 964 3.6-liter motors, Porsche has implemented (some might say experimented with) different head stud materials in an attempt to rectify the breakage problem. A variety of materials have been used, some with more success than others. For the SC and Carrera motors, steel was the material used for the upper intake row of head studs, and a special "dilivar" material in the lower exhaust row. The steel units generally work well and create few problems, but the dilivar units are infamous to Porsche owners and mechanics alike. The dilivar alloy was designed to have a similar thermal expansion rate to that of the aluminum engine case, cylinders, and heads. In theory, this would help balance the stresses involved during expansion and contraction of the aluminum parts. In reality, larger displacement motors with their additional stress and corrosion issues have seen a significant number of failures, regardless of mileage accrued, thanks to faulty head studs.

0505ec_proj964_07_z Photo 7/10   |   AASCO Performance valve springs help eliminate valve float and extend the rev capabilities to 8500 rpm.

While Porsche finally seemed to lick the problem with the fully threaded steel units installed top and bottom on the 3.6-liter 993, I wasn't going to roll the dice with my 3.5. Chris Brown from ARP recommended his company's high-grade head stud kit that comes beautifully packaged complete with the appropriate (24) studs, washers, 12-point nuts and special moly assembly lube. ARP is well respected in motorsports circles for its engine fasteners and hardware. The head studs are manufactured from premium grade 8740 alloy steel and heat-treated to 200,000 psi. After heat-treating, the studs are centerless ground to make them as concentric as possible. The ARP head studs are also thread rolled after heat-treating, providing for ten times better fatigue strength than studs threaded prior to heat-treating. ARP head stud products are not the cheapest on the market, but their quality and attention to detail does provide for a premium part that should be considered a necessary replacement for any 911 motor using dilivar head studs.

Popular Piston/Cylinder Sets for 3.0L SC & 3.2L Carrera Motors

3.0L SC Displacement Options
BoreDisplacementPiston TypeCRStrokeWrist PinCase BoreMachine Case?Notes
95mm3.0LCIS/Motronic8.5:170.4mm22mm103mm 1978-1979 U.S. spec SC
95mm3.0LCIS/Motronic9.3:170.4mm22mm103mm 1980-1983 U.S. spec SC
95mm3.0LCIS/Motronic9.8:170.4mm22mm103mm 1981-1983 Euro spec SC
98mm3.2LCIS/Motronic9.8:170.4mm22mm103mmNo3.2L "Short Stroke", Twin Ignition optional
98mm3.2LCarb/MFI/EFI10.3:170.4mm22mm103mmNosame as above, Twin Ignition required
100mm3.3LCarb/MFI/EFI9.5:170.4mm22mm105mmYesTwin Ignition required, rarely built
98mm3.4LMotronic/CIS9.8:174.4mm23mm 103mmNo3.2L Carrera crank & rods. Twin Ignition optional
98mm3.4LCarb/MFI/EFI10.3:174.4mm23mm103mmNosame as above, Twin Ignition required
100mm3.5LMotronic/CIS9.8:174.4mm23mm105mmYessame as above, Twin Ignition required100mm3.5LCarb/MFI/EFI10.3:174.4mm23mm105mmYessame as above
100mm3.6LMotronic/CIS9.8:176.4mm23mm105mmYes964 3.6L crank w/ 3.2L rods.Piston mods & Twin Ignition required
102mm3.7LMotronic/CIS9.8:174.4mm23mm107mmYes3.2L crank, aftermarket rods
& Twin Ignition required
3.2L Carrera Displacement Options
BoreDisplacementPiston TypeCRStrokeWrist PinCase BoreMachine Case?Notes
95mm3.2LMotronic/CIS9.5:174.4mm23mm 103mm 1984-1989 U.S. spec Carrera
95mm3.2LMotronic/CIS10.3:174.4mm23mm103mm 1984-1989 Euro spec Carrera
98mm3.4LMotronic/CIS9.8:174.4mm23mm 103mmNoTwin Ignition optional
98mm3.4LCarb/MFI/EFI10.3:174.4mm23mm103mmNoTwin Ignition required
100mm3.5LMotronic/CIS9.8:174.4mm23mm105mmYesTwin Ignition required
100mm3.5LCarb/MFI/EFI10.3:174.4mm23mm105mmYessame as above
100mm3.6LMotronic/CIS9.8:176.4mm23mm105mmYes964 3.6L crank, Piston mods
& Twin Ignition required
102mm3.7LMotronic/CIS9.8:174.4mm23mm107mm Yes3.2L crank, aftermarket rods
& Twin Ignition required

Mahle original cast and Mahle Motorsport forged piston & cylinder sets distributed exclusively by SSF Auto Parts and Andial. JE Pistons also offers custom forged pistons in a variety of sizes and compression ratios for the engine builder that may have a special requirement.

Rod Bolts

The connecting rod bolts are another source of concern for 3.2-liter Carrera motors. Prior to the introduction of the 3.3-liter Turbo motor in 1978, Porsche rod bolts were 10mm in diameter and provided the extra safety margin Porsche usually engineers into its motors. Unfortunately, when Porsche borrowed the 74.4mm crank and rods from the 3.3-liter Turbo to make a 3.2-liter engine for the Carrera in 1984, the smaller 9mm diameter rod bolts first seen on the earlier 3.3-liter Turbo were used as well. The stock 9mm rod bolts are considered a weak point and the motor should not be revved past 6800 rpm with any regularity. Many professional engine builders recommend replacing the rod bolts with higher quality units whether they're doing a stock rebuild or a race prepping a motor.

Although I don't anticipate my 3.5, with the original injection system and relatively mild camshafts, spinning past 6800 rpm, this was another area where the factory offering could be improved upon. The most important fasteners in any engine are the rod bolts, as they mate the rod halves and must support the tension loads caused by each cycle of the crankshaft. Wanting to ensure a virtually bulletproof bottom end, I once again turned to the ARP catalog and selected the Pro Wave ARP2000 rod bolt kit. According to the company, these ultra heavy-duty rod bolts are made from an exclusive hybrid alloy developed to deliver superior strength (about 220,000 psi) and better fatigue properties. The patented Wave-Loc design contacts the rod and cap for optimum alignment and provides a snug fit for all OEM connecting rods despite wide ranges of factory rod bolt hole tolerances. ARP rod bolts are requisite for any 3.2L-liter rebuild that will see more than 6800 rpm, whether it's intentional or not.

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Sources

AASCO Performance, Inc.
Anaheim, CA 92805
ANDIAL Road & Racing
Santa Ana, CA 92704
By Ralph B Hollack
5 Articles

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