THE DRIVE TRAIN
THE BRUTE IN A SUIT
The engine and transmission for this car were given careful consideration. Since we started with a blank slate and cost was not a consideration we could choose anything we wanted. We wanted something with lots of power and yet not something strung out as you would expect to see in a race prepped car. We wanted to have the reliability and durability of a street used car. We also to drive a vehicle that did not need to go to an expensive race garage when service was needed. Race engines often have to be rebuilt every 25 to 50 hours. We wanted to measure useful life in 100's of thousands of miles not dozens of hours. The transmission was different. They can be built to track standards which allows for abuse without having to be rebuilt more often than a street transmission.
The engine of choice is the 1996 dual plug 993 Varioram. We were very lucky to find one with only 9,000 miles on it. Porschephiles will know that this means the engine is barely broken in. Given the low mileage we decided to not crack the case. The bonus feature of this engine is the 115 Amp alternator which we then later removed and replaced with a Classic Retrofit 145 Amp alternator. This allows the install of the cutting edge electric AC unit without modifications to the engine electricals. With this engine we have lots of power in a dual plug engine. The 993 was the last of the air cooled engines. Porsche had by then improved the 911 engine to the point where it was called the most reliable and desirable ever built. Earlier engines, even if rebuilt to stock specs, while good are no match for the more powerful and very reliable twin plug Varioram. Varioram adjusts the length of the air intake ducts in line with the engine RPMs and the speed of the car.
The engine sits on Wevo active motor mounts.
The transmission is a last year built 915 from the factory so it had all the updates from the years prior. In addition we also sourced another low mile piece of the puzzle. We then selected short 3rd and 4th gears so as to make it very fast accelerating 0 - 100 mph. We left a tall 5th for the freeway. 8:31 ring and pinion. 3rd gear ratio is 21 x 30. This ratio was recommended by Mark Mathis who worked for Porsche Motor Sports in the '80's. While building we expected 300 plus HP from the 3.6 Varioram so we thought it best to source a late model ZF limited slip differential unit which was rebuilt with Guard internals set at 80% locking factor. Matt at Guard sets the bar for Porsche LSD. Most Porsche race teams use his Guard parts. The extra strong side covers are also Guard. The beauty of limited slip diffs of ZF, is it allows you to point the steering wheel and step on the gas. The result of the gearing of this 915 is a rocket ride to triple digits.
Porsche’s VarioRam induction system was designed to improve low to mid-range torque by optimizing airflow at different rpm levels. Introduced on the 993 Carrera RS’s M64/20 engine in 1995 model, the technology was then placed into the standard 993 Carrera range with the introduction of the 1995 M64/21 engine.
The previous M64/05 engine made use of a single throttle body, and a resonance chamber tuned to make use of Helmholtz Resonance (where pressure pulses caused by the opening and closing of the intake valves cause air to be bounced off the walls of the resonance chamber and forced into the combustion chamber).
VarioRam adds a second, upper throttle body that flows through to a central plenum. From this central plenum, six vacuum operated induction runners are mounted.
Below 5,000rpm, the ECU electronically triggers these six runners (one for each cylinder) extending them to nearly twice their normal length, using rubber seals to cut off the primary throttle body and resonance chamber.
The resulting increase in runner length makes use of the Venturi effect by reducing the cross-sectional area the speed of the airflow is increased. The increased air velocity aids cylinder charging, while lowering the optimal rpm for resonance induction.
By improving low-range airflow, VarioRam engines enjoy around 20 per cent more torque at 5,000rpm than non-VarioRam units.
Once the engine reaches 5,000rpm, and the throttle is more than 50 per cent open, the induction runners are retracted to their normal length. The primary throttle body is reengaged, and one of the connecting tunnels between the two banks of the resonance chambers is opened via a vacuum operated valve.
Above 5,800rpm the second connecting tunnel is opened, again through the use of a valve. These final two stages operate in the same way as non-VarioRam engines, using the resonance chamber to improve the volumetric efficiency of the engine at high rpm
Engine has had a check valve installed. A one way valve to keep oil on the side of the oil tank to prevent smoke on start up Porsches are known for. This M64/23 engine is engine number 64103865.
This engine has OBD-II style DME which must be flash re-programmed. This has been done on this particular engine by the addition of a special memory socket into the DME. This upgrade may make it tight to pass smog in some areas so the car comes with a second stock DME that can be plugged in for smog testing if needed.
See photos below for :
Twin oil coolers using original Porsche parts where possible and for custom oil lines and hoses with military grade fittings. Rick Hartbrodt collaborated with a builder of Porsche oil lines to accomplish this.
Note twin coil console in engine compartment. Consoles were custom built for car. Contact Rick Hartbrodt for more information.
Car was completely rewired by factory technician at great cost, upgrading wiring harnesses for electric AC, monster aftermarket generator to handle modern electronics.