The 964,
to RSR Specification.

Contents

The Builders

Foremost Speed Shop and the sourcing decisions

Foremost Speed Shop is an independent Porsche shop in Buckhead, Atlanta. It works on both air-cooled and water-cooled flat sixes: full rebuilds and in-house custom engine builds. It is the shop regularly recommended in the Rennlist and Pelican Parts threads on independent Porsche shops in Atlanta, and warrants all work for 24 months or 24,000 miles.

Rothsport supplied the top end as a developed system: the cylinder heads, machined in its own shop to its RS-valve specification, 993 RS cams, the big-bore ITB kit with plenum, fuel rails, injection and coil packs, the MoTeC ECU and harness, and the requirement that the M&M exhaust be used so the package ran on the exhaust it was modeled against. Wrightwood Racing supplied the bottom-end hard parts to fit that package and the factory rods: the Mahle pistons and cylinders in the wide-pin-boss slip-in configuration, the gasket set, and the case materials. The calibration was likewise sublet to EMS, an expert in air-cooled dynamometer tuning.

The engine made 298 whp on the Dynojet, nearly 350 hp at the crank, with a street idle. The specifications a future shop will need are held by Rothsport, Wrightwood and EMS.

Rothsport Racing

Rothsport Racing is Jeff Gamroth's shop in Sherwood, Oregon, founded in 1998 after a decade of road racing Porsches in SCCA, IMSA and PCA competition, including the Daytona 24 Hours and Sebring 12 Hours. Gamroth has developed individual-throttle intake systems for the flat six for more than twenty years, and the shop's ITB-and-MoTeC packages are widely used on air-cooled engines that need to start and idle as road cars.

Rothsport's intake development began with stable idle, clean cold start and a usable manifold-pressure signal on a street 911; the shop's Baja-raced 964s ran the package under heat, dust and vibration. The big-bore kit on this engine is the current version of that work: Jenvey-derived throttle bodies under a GT3-style plenum with a cone filter, Rothsport fuel rails and injection, coil-pack ignition and a MoTeC ECU, developed together. Rothsport supplies the kit only after a consultation, as part of a full flow package in which the heads, cams, throttle bores, injectors and exhaust are specified together for the engine it will go on.

Wrightwood Racing

Charlie Spira of Wrightwood Racing polished the crankshaft and supplied the engine's hard parts as one sourcing effort: the case materials and gasket set, the Mahle pistons and cylinders, and the Rothsport big-bore top end. Wrightwood is a small California firm best known among 911 engine builders for custom-assembled Porsche gasket sets and as the sole source for certain OE-spec seals, and it works by phone and COD. Given the plan (factory rods, slip-in 107 mm barrels, a Rothsport-specified head and induction package), the parts were selected to fit together.

The Motor

The engine follows the factory's 3.8 RSR specification, adapted for street use. In that specification the power gain comes from removing each restriction in the airflow path in turn. A stock 964 is 247 hp at the crank and roughly 205–210 at the wheels. This one made 298 at the wheels, which at a typical 13–15% driveline loss is 340–350 crank hp, between a 3.8 Cup car (325 PS) and a 993 RSR 3.8 (350 PS). The factory got there with 102 mm bores, larger valves and ports, RSR cams, individual throttles and a free exhaust; Rothsport's package reproduces each of those steps in a form that idles, starts cold and runs a 24-month warranty.

Changing one element alone yields little; larger valves without an exhaust, or an exhaust without cams, move the restriction to the next item in the path. What the package does, in order of airflow:

1. The M&M header-and-muffler system, which meets Rothsport's required exhaust specification, replaces the heat exchangers with long-tube, equal-length primaries into a 2.5" motorsport collector, removes the catalyst and the second muffler. On an air-cooled flat six this is the single largest restriction; M&M claims 25 hp and 23–35 lb-ft from the system alone on an otherwise stock 964.
2. The RSR-style valve job raises the head's flow ceiling. Larger Euro/RS-pattern valves on 8 mm stems, new guides and seats, and blended ports lift peak flow and shift it higher in the lift curve, so the port continues to flow at the lift the cam provides.
3. The 993 RS cams add lift and duration. With the exhaust's scavenging and the head's flow in place, extra overlap pulls charge through the cylinder.
4. The ITB kit gives each cylinder its own throttle under a GT3-type plenum. That deletes the AFM and the long plenum runners, gives sharp throttle response and strong intake pulse tuning, and because the plenum sits upstream of the throttles it still provides a filtered air source and a usable manifold-pressure signal.
5. The 3.8 kit (102 mm Mahle RSR pistons and cylinders) raises displacement 5.5% and compression to roughly 11.3–11.5:1.
6. MoTeC with a full sensor set (crank trigger, CHT, oil pressure and temperature, lambda) provides per-cylinder fuel and ignition trim, a cold-start and idle strategy for six independent throttles, and knock-safe timing on an 11:1+ air-cooled engine.

The 964 3.8 RS/RSR and the 993 3.8 RSR were homologation and customer-race engines whose cylinder heads carried 51.5/43.5 mm valves on 46/42 mm ports, fully threaded head studs, and individual slide or butterfly throttles. An RSR flow package is the head, cam, induction and exhaust specification designed around the 102 mm pistons. This engine carries each element of that specification: fully threaded 993 RS-type head studs (993 101 170 54), cylinder heads machined for Euro valves, 993 RS camshafts, the Rothsport ITB kit, and the M&M exhaust.

The street-use choices are butterfly ITBs under a plenum (filtration, MAP signal, cabin-friendly induction noise), a 993 fuel-pressure regulator with Bosch EV-type injectors, and a Dynojet calibration targeting a flat torque curve to a street redline.

Bottom end

The case was split for the bottom-end reseal and bearing replacement; the cylinder spigots were left at factory size. The piston and cylinder set is a Mahle 3.6-to-3.8 "RSR" slip-in kit — 102 mm Nikasil cylinders in 107 mm sleeves on the stock 76.4 mm stroke, 3,746 cc — which fits the factory case bore without boring. Charlie at Wrightwood Racing supplied it in the wide-rod-eye configuration, with the wrist-pin size selected for the factory 964 connecting rods, so the crank and rods were retained. The installing shop considered the 109 mm machine-fit kit, which requires boring the case spigots, and chose the slip-in version on a risk and scope basis. The crank went to Wrightwood for polishing. Bearings are new throughout.

Cylinder heads — the 993 RSR-style valve job

The heads were sent to Rothsport, which machined them for Euro valves; they were assembled with a new head gasket set. This is the 993 RS/RSR head specification applied to the 964 castings, which never changed and accept it directly.

Every 993-pattern valve has a thinner stem than the 964's, so all 24 guides come out and 8 mm guides go in, honed to fit. A larger intake valve needs a larger seat, so the intake seats are replaced (heads heated to around 200 °C for the install), then cut to the factory three-angle spec with a narrow seat width. The step the bigger seat leaves below it is blended into the port, the guide boss is knife-edged, and the castings' flashing and porosity are smoothed. Done well this adds roughly 10 hp on its own and raises the lift at which the port continues to flow, which the higher-lift cam requires.

Camshafts

The cams are the 993 RS grind, installed with new flange gaskets, O-rings and Woodruff keys. On a 964 the M64 runs solid lifters and adjustable rockers, so a cam change is a direct swap with a valve-lash set. The RS profile was Porsche's road-legal performance cam for the 993 Carrera RS (3.8 L, 300 PS), one step below the Cup and RSR race grinds; it is ground on 964-compatible blanks for this application.

The RS grind suits the rest of the package: enough lift and duration to use the larger valves and the 2.5" exhaust, with a street idle and torque below 4,000 rpm.

Induction & Management

The induction and management are the Rothsport big-bore ITB kit: six Jenvey-derived individual throttle bodies, a GT3-style plenum with a cone filter, Rothsport fuel rails and injection, Rothsport coil packs for ignition, the MoTeC ECU, chassis and engine wiring harness, sensors, distribution plug, TPS and pressure transducers. A Bosch high-flow fuel pump was added separately. The induction half of the package addresses the 964's largest intake restriction, the single throttle and air-flow meter.

How the kit is laid out on this car: two sand-cast aluminum GT3-pattern plenums, one per bank, each with three short runners down to the heads, joined across the top by a center resonance tube with a corrugated coupling at each end. The inlet feeds the center section from a single filtered source. Beneath the plenums sit six Jenvey big-bore individual throttle bodies — the throttle linkage rods are visible crossing under the castings — with fuel rails and Bosch EV-type injectors in the runner flanges and an OEM 993-pattern fuel-pressure regulator on the linkage pivot. The factory AFM, throttle body, plenum and long runners are gone.

The calibration was sublet to EMS, producing the Dynojet sheet in the records at 298 whp. On a Dynojet with a typical 13–15% loss for a G50 car that is about 340–350 hp at the crank, and the oil was changed immediately after the tune, which is the normal sequence for a fresh engine's break-in and full-load mapping.

The Exhaust

The exhaust is M&M part 64C012-3, the 964C Power Kit in dual-exit form, imported from Germany and fitted with an O2 bung. Rothsport specified this system as a condition of the flow package: the head, cam and induction work is modeled against a known exhaust, and the M&M header length, primary diameter and collector are the ones the package was developed on. M&M Exhaust GmbH, of Annaberg-Buchholz in Saxony, is the fabricator behind the race exhausts run by Porsche customer teams — Manthey, Black Falcon, Farnbacher, Konrad, Frikadelli, Kremer among them — and its 993 RSR and GT3 Cup systems are what the FVD Brombacher catalog sells under its own name for the 964, 993 and 997 Cup cars. Street systems are handmade to order, built from German 1.4301 stainless.

M&M's own claim for the system on a stock 964 is about 25 hp and 23–35 lb-ft. On this engine the system sets the scavenging that the cam overlap and the exhaust-port work were modeled against: a long equal-length primary into a 2.5" collector sets the pulse timing that the 993 RS cams were ground against. It is a cat-bypass system and not emissions-legal; the M&M part number and the customs line indicate it was sourced direct from M&M.

Driveline & Chassis

The driveline is a rebuilt G50 with a torque-biasing diff and an RS flywheel and clutch.

The RS single-mass flywheel sharpens throttle response, and the Wavetrac keeps the torque usable out of corners with no clutch packs to wear or chatter. The gearbox keeps the stock G50/03 ratios with the 3.818 first, so the 1–2 gap the 964 is known for remains, with more torque to fill it. A 3.154 RS/RSR first-gear set is the factory-pattern change if that gap is ever addressed. Some of the dealer listings said "6-speed"; the car has the G50 5-speed.

Body, Lighting & HVAC

The body work is carbon where the factory used steel or plastic, RS aerodynamic pieces and period-correct lighting, plus an electric A/C conversion. The ducktail is Stuttgart Classica carbon fibre, in the same prepreg carbon as the frunk insert and the interior pieces.

Interior

The interior is built on a Stuttgart Classica smooth interior tub in carbon/Kevlar composite, with Stuttgart Classica CNC-routed floor boards beneath the carpet, a Stuttgart Classica 964/993 short centre console with the shift cup integrated, and a carbon centre console tray kit; the front trunk carries the Stuttgart Classica carbon frunk insert over the space-saver. The door cards are Stuttgart Classica blank RS panels (0.125" ABS, 1969–97 pattern), carrying Stuttgart Classica GRP door pockets with integrated armrests, built from fibreglass, stainless steel, wood and aluminium and supplied uncovered for trimming. Every surface was then retrimmed in Relicate materials: hand-woven Nappa and Alcantara in Bourbon (5 mm strips of Nappa Italia Bourbon 6465 leather and Alcantara 2930 Hazelnut, woven in Italy, 39" wide) on the door cards, the lower dash and glovebox face, the tub and the seat centres; and smooth Nappa Italia Bourbon 6465 (1.0–1.2 mm, matte automotive topcoat) on the seat bolsters, the transmission tunnel, the armrests, the door pockets and the remainder of the tub. The carpet and mats were cut from Relicate German Square Weave in Natural 326, a 100% wool, 71"-wide, 3/16" carpet of the type Porsche fitted in period, with Silverlake Projects aluminium floorboards in both footwells: 5000-series aluminium, hand-dimpled and anodised black, made in Los Angeles. The seats are custom Recaro LXCs trimmed in the same two materials; the dash top, the door tops, the rear shelf and the adjoining trim were retrimmed in Relicate Nappa Italia Ferrari Black 6300 (1.0–1.2 mm, matte automotive topcoat, made in Italy); the headliner is new; the shift knob is laminated wood on a leather gaiter; the steering wheel is a leather-wrapped MOMO Prototipo, the three-spoke wheel Porsche fitted to the 911 R and the early RS cars; the cluster was rebuilt by Hamps with red gauge faces. A factory 964 third brake light was added at the rear window: the 964's tail lights sit low, and a taller following car can lose sight of them under braking.

Before the trim went on, sound deadening was added in key locations: the chassis floor and tunnel, the inner door skins, the centre console and the tub, for a "rock solid" feel all around. A 964 shell with its factory insulation removed and a composite tub over the steel floor resonates: the door skins, floor pans and tunnel are large, thin panels that ring at the engine's firing frequency (a flat six at 3,000 rpm fires 150 times a second) and over road texture, and a lightweight composite tub adds its own drumming. Constrained-layer damping mat applied to the centre of each panel raises its mass and converts that vibration to heat, so the panel stops ringing; the door then closes with a single thud, the console stops buzzing against the tunnel at idle, and road noise drops, leaving the ITBs and the exhaust as the only sounds penetrating the cabin.

The audio system is a Porsche Classic head unit driving a JL Audio amplified system: a four-channel RD400/4 amplifier, C3-650 and C3-570 component speakers, rear speakers, and marine-grade wiring, with a Blaupunkt adapter at the head unit.

From Here: Refining the Package

The car is sound as delivered, and the next phase is service, calibration and finishing. The work falls in four stages, in sequence, with one engine-bay item alongside. The parts going on in stages 1 through 3 are shown together at the end of the chapter.

Replication Cost Estimate →