Torque Show Porsche VarioRam

Porsche’s patented VarioRam system is an ingenious way to alter geometry of the induction manifold according to engine needs under different operating conditions. Here’s how it works… Words Shane O’Donoghue Photography Porsche.

TORQUE SHOW

VarioRam is the name Porsche gave to its advanced variable intake system when the technology was first released in 1994. It’s clear, however, from the wealth of Porsche-filed patents in the years leading up to then, that the manufacturer’s engineers had long considered the benefits of varying intake geometry. Indeed, the VarioRam system was preceded by a far simpler variable design that didn’t get much recognition, never mind a catchy marketing name. Before we look at how these Porsche systems work, it’s probably worth us going through an overview of intake tuning. This is a massive field of study, obviously, because it includes the whole inlet system, from engine air intake, filter, throttle and then the plenum, manifold and, finally, the inlet valves. Each and every millimetre of this system affects the tuning and performance of the host engine across the rev range.

SPEAK VOLUMES

When we say tuning, really, what we’re interested in here is volumetric efficiency. This is the measure of how much air — in terms of mass — you can get into an engine’s cylinder, compared with the theoretical amount. More air in the cylinder means it’s possible to burn more fuel, which translates into more energy released in a given combustion cycle. This equates to more torque at the crankshaft. Simply put, higher volumetric efficiency equals more torque and, because output is directly proportional to the torque, power. It’s easy to exceed one hundred percent volumetric efficiency in a turbocharged engine — the compressor lives up to its name and compresses air before it is fed into the cylinder, but in a naturally aspirated engine, it takes a little more effort to achieve high levels of volumetric efficiency.

Inertia charging (or scavenging of the cylinder) is one technique. We covered it in more detail in last month’s issue of 911 & Porsche World. Using valve timing, a pressure wave in the exhaust is created that then scavenges the cylinder and draws the intake air in quicker. Some gases can flow back into the cylinder from the exhaust, too, meaning the potential for more air trapped for combustion and an increase in volumetric efficiency. For the most part, this technique relies on the geometry of the combustion chamber and the valve timing, along with the inlet ports and the rest of the intake system, of course. On the subject of the inlet ports, it’s worth touching on what happens to the air in the ‘runner’, which encompasses the length from the back of the inlet valve to the main plenum chamber in the inlet manifold. When the inlet valve is open during the induction stroke, air is rushing into the cylinder at high speed. When the valve closes, the air compresses itself against the back of the closed valve, causing a pressure pulse to travel back up the runner. When this high-pressure pulse reaches the opening into the plenum, it reflects back down the runner. If the pulse can be timed to return to the back of the inlet valve when it is opened (this may require movement of the wave up and down the runner a few times), then it can greatly increase the air taken into the cylinder and increase the volumetric efficiency. Sadly, this useful effect is limited to a narrow speed range for a fixed runner length. Generally speaking, the longer the runner, the lower the engine speed at which it’s optimised for. This is sometimes referred to as vibrating tube charging.

Resonance charging is another tuning technique, and it’s particularly relevant to engines laid out in a vee or boxer configuration, where, crucially, the firing order alternates between cylinder banks. It’ll become clear why in a moment, but in this configuration, the inlet manifold features two distinct plenum chambers, each feeding air to a bank of cylinders. These plenums have an interconnecting pipe (or pipes, but it’s as easy to think of the setup as a single pipe) and the air flows into this system through a regular throttle valve.

UNDER PRESSURE

Due to the air being sucked into cylinders on alternating sides of the engine, a forward and back pressure wave occurs within the pipe that connects the two plenums. The position of this pressure wave at any given moment depends on the geometry of the inlet manifold and the engine speed. At a certain speed, the pulsing of the air reaches what’s known as its natural frequency and resonance occurs. This means the pulse from one plenum to the other will be in phase with the intake strokes. In turn, this results in higher air pressure in the inlet ports when the inlet valves are open, leading to greater mass of air in the cylinder and, consequently, an increase in volumetric efficiency and torque. The engine speed at which resonance occurs can be tuned by the geometry of the inlet manifold, but it’s always a compromise — there’s a very narrow rev band over which resonance is actually useful. And, of course, there’s also the potential downside of causing an unwanted negative air pressure pulse in an inlet port at another engine speed, which would dramatically reduce the volumetric efficiency (and therefore the torque output) at that point.

With all we’ve described in mind, it’s not difficult to envisage the advantages of an inlet system with variable geometry. Erwin Rutschmann, one of Porsche’s many talented engineers (you’ll find his name on dozens of patents), is credited with creating the first variable resonance inlet manifold for the company, as fitted to the 964’s 3.6-litre M64 engine. As described above, the M64 featured an air plenum on each side, connected by a wide pipe. The main air intake flowed into this interconnecting tube. Underneath this, however, was another pipe — effectively a resonance chamber — linking the two plenums. This pipe housed a butterfly valve that was opened or closed (depending on engine speed and load) by the car’s advanced Bosch Motronic M2.1 electronic engine management control unit. The valve stayed closed until the engine revved above 5,600rpm. This variability would have allowed Porsche to accept a less compromised intake system, resulting in higher torque across the entire rev range.

VarioRam took the idea much further, with more variability for the inlet geometry to optimise volumetric efficiency across a wider range of engine conditions. It varies both the runner length and the resonance in one cleverly packaged intake manifold system.

As expected, intake air is fed to the manifold from the air filter, but airflow can enter via two separate butterfly throttle valves. The upper valve opens by varying amounts at all times, allowing air into the central plenum you can see atop an engine fitted with the VarioRam system. This distributes air to three top runners on each side of the engine, leading into what appear to be further plenum chambers, with lower runners connecting those to the cylinders. What you can’t see from the outside is that the top runners feed movable sections. These are vacuum-operated and computer controlled. At low engine speeds and loads, these moving parts seal the top runners to the lower runners, resulting in a much longer runner length, better suited to the creation of low-speed torque. At this stage, the rest of the manifold is redundant, and the aforementioned vibrating tube charging is key to the engine’s volumetric efficiency.

When engine speed reaches 5,160rpm and the throttle valve is more than fifty percent open, the movable sections retract to allow air from the top runners into the lower plenum chambers. The lower throttle valve also starts to open, increasing overall airflow to the engine. In this scenario, the runner length is much shorter, from the lower plenums to the cylinders, which is better suited to high-speed operation. Additionally, the two lower plenum chambers are interconnected, allowing resonance tuning to suit the given operating condition. As is the case with the 964’s flat-six, the VarioRam manifold features an extra resonance chamber (or interconnecting pipe/tube) operated by a butterfly valve. Providing the throttle is more than fifty percent open, the valve is opened at 5,920rpm, altering the resonance frequency once more to allow higher volumetric efficiency as the engine homes in on its peak power point.

The very first engine to use VarioRam was that of the 993 Carrera RS, though this car’s 3,746cc engine (codenamed M64/20 and marketed as a 3.8-litre unit) was significantly different to the 3.6-litre boxer fitted to the same-generation 911 Carrera and Carrera 4. For a start, the RS lump featured lighter pistons and rocker arms, along with larger intake valves made possible by a two-millimetre increase in bore diameter. Peak outputs of 296bhp at 6,500rpm and 262lb-ft torque at 5,400rpm are, understandably, usefully higher than the 3.6-litre engine’s 268bhp and 243lb-ft (delivered at 6,100rpm and 5,000rpm respectively), the higher revs in the RS befitting its remit in life.

A more important comparison can be made between early 993 engines and those introduced from 1996, the latter identifiable by engine codes starting with M64/21. These later engines received VarioRam as standard, though it didn’t drastically alter maximum output of the 3.6-litre engine: the newer units developed 282bhp at 6,250rpm and 251lb-ft at 5,250rpm. Those are useful gains, undoubtedly, though they don’t fully illustrate the benefits of the VarioRam system. After all, it isn’t designed to introduce a hike in performance, per se. Rather, as explained earlier, VarioRam’s goal is to refine and improve torque output across the whole rev range. In fact, the 3.6-litre 993 engine equipped with VarioRam also received bigger valves, which would have enhanced the engine’s breathing, especially at the top end. Higher peak power is as much attributable to this update as much as anything else, though you might argue the change was encouraged by the flexibility of VarioRam in the first place. As you can see on these pages, our chart highlighting the power and torque curves for the 993 engine with and without VarioRam reveals a great deal. The VarioRam engine produces considerably more torque, notably in the 2,500-4,500rpm range. This is where most people spend their driving time, which is why a 993 with VarioRam feels significantly more responsive in everyday driving environments.

The beauty of VarioRam is that its benefits don’t come at the expense of top-end power: although the updated 993 weighed thirty kilograms more than its predecessor, the Carrera’s sprint from rest to 62mph dropped from 5.6 seconds to 5.4 seconds, with top speed increasing from 167mph to 171mph. Fuel consumption improved, too. What’s not to like?! Well, packaging, weight and complexity, so it would seem. Yep, VarioRam was short-lived — the 996's new water-cooled flat-six reverted to a simpler variable resonance manifold. Of course, the breathing of the newer 911’s engine was improved with a four-valve head, while VarioCam (first introduced to the 968’s three-litre inline-four) allowed variability in valve timing that made VarioRam all but redundant. Nonetheless, there's no doubting lessons learned during the development of VarioRam, knowledge which went on to inform the design of intake systems for all naturally aspirated Porsche engines, right up to the 991.

Above VarioRam was in production from 1995, meaning 993s assembled before this time won't have the system installed as standard kit.

KNOWLEDGE WHICH WENT ON TO INFORM THE DESIGN OF INTAKE SYSTEMS FOR ALL NATURALLY ASPIRATED PORSCHE ENGINES, RIGHT UP TO THE 991

Above VarioRam supplemented the resonance charging system introduced in model year 1989 by allowing the length of the 993's intake manifold to be varied

THE 3.6-LITRE 993 VARIORAM ENGINE ALSO RECEIVED BIGGER VALVES, WHICH ENHANCED THE ENGINE’S BREATHING, ESPECIALLY AT THE TOP END

Facing page Long intake paths at low revs resulted in a gain in torque, while short intake paths resulted in a gain of performance in the upper speed range.

MORE VARIABILITY FOR THE INLET GEOMETRY TO OPTIMISE VOLUMETRIC EFFICIENCY ACROSS A WIDER RANGE OF ENGINE CONDITIONS

Below 3.6-litre 993 air-cooled flat-six complete with VarioRam system