How and why a VarioRAM works


1995 993 and VarioRAM Intakes

The 1995 manifold is essentially the same as the 92-94 964 manifold with two major differences:

  1. The intake runners are 1mm larger and
  2. It has a mass flow air meter rather than a barn door.

The VarioRAM, however, is completely different.  Note that both have a primary plenum above the primary runners (or stacks).  From that point down they are essentially the same.  Above that, however, they are completely different.

As you may know, all 964/993 motors 2.7 Boxsters and 996 motors have a resonance chamber. This allows the pulses to flow from the left (1-3) bank plenum to the right (4-6) plenum.  It is opened and closed by the computer depending on the throttle position and RPM to maximize volumetric efficiency (how much air gets into the combustion chamber).  The VarioRAM goes one step further.  Whereas the 1995 993 has an upper and lower crossover tube (upper with the throttle body and lower for resonance), the VarioRAM has a total of eight connections, a second throttle body (opened only above 1/2 throttle), and a secondary plenum.  Here’s how it works:

At high RPMs and above half throttle it essentially works just like a 1995 993.  The lower throttle body and crossover look like the upper one on a 95 993.  Now for the upper – the upper throttle body flows to a central plenum which branches out into six runners.  These runners then dump into the primary plenum to join the air coming in through the lower crossover.  In effect, it acts just like the lower throttle body so you get lots of air and the effective runner length is just the lower (primary) runners or the distance from the primary plenum to the intake valve.  T make a long story short the VarioRAM is no better than the 993/964 intake, in fact perhaps a little worse in theory because the air coming through the top runners has further to travel and the whole assembly is pretty heavy (10 lb.?).

Now for the tricky part and why they bothered. At low RPMs (below 5200) the top runners are connected and sealed to the lower runners.  This has several effects:
1) The intake runner length is MUCH longer, about 9″ longer in fact, which means that the optimal charging RPM for tuned resonance induction is much lower.  That means that more air will get into the cylinder at lower RPM.  This effectively boosts low-end torque, below 4000 RPM there is a notable difference. This is also because the air velocity through the single throttle body is increased, which aids in cylinder charging (same old how much air).

2) The lower throttle body is completely closed off from the intake system, as are the primary plenum and resonance chamber.  This is, again, a good thing at lower RPMs.  It is particularly bad, however, at high RPMs (above 5200 and more than 1/2 throttle) since only the upper throttle body is operational, so no matter how hard you mash the pedal you only get about 1/2 throttle!  We recently had a customer who refused to follow directions and hooked up the VarioRAM vacuum canister incorrectly.  He found that the car was absolutely dead above 5200 RPMs, like 180 rear-wheel horsepower.  When the problem was corrected his dyno showed 265 at the rear wheels (I think that’s a bit optimistic but you get the point!)

Here’s how it physically works:

This is a picture of the VarioRAM opened up with the  trumpets retracted.  In the lower portion is the primary stack. In the upper is the primary plenum with trumpets.

This is the VarioRAM in it’s normal position, trumpets extended.  The rubber seals seal against the individual intake runner completely blocking off the primary plenum, secondary throttle body and resonance chamber.

Now here are pictures of the trumpets closed then open.  A pair of vacuum actuators (one per side) retract the trumpets so the primary runners breath from both upper and lower intake tract and throttle bodies.

Note: The DME operated the VarioRAM only at above 1/2 throttle and above 5200 RPM so it is difficult to bench test.

What is the result – the VarioRAM is rated at 10 more HP than the stock 993.  This is because of the valve size alone. VarioRAM engine intake/exhaust valve size is 50mm/43.5mm vs 49mm/41.5mm.  This has nothing to do with the VarioRAM.

Look at the torque figures though:

Model Torque at 4000 RPM (Ft-Lb) Change Max Torque at 5000 RPM (Ft-Lb) Change Max Power at 6200 RPM (HP Change
95 993 (type 06/07) 199 243 272
96-98 (type 21/22/23/24)  238 +20% 251 +3%  282 +3.5%

The following are stylized but based on actual data of the same engine with and without a VarioRAM intake.