Multi-Ram vs. Mantzel Power Resonator

12-18-2010

Intakes, Airboxes, and Plenums



Let's get one thing straight. The plastic box the air filter lives in isn't the Catera's true airbox. The plenum sitting atop the engine is. More on this later. I know from experience that if you remove the airbox on a motorcycle and replace it with individual air-filter pods you may gain power at the top of the power curve but you lose torque all along that curve. The airbox creates pressure waves that force more air/fuel mixture into the cylinders when their intake valves open. This is the purpose of the plenum sitting on the intake runners of the Catera's L81 V6 engine.

The main benefit of using an airbox and intake runners is a slight pressurization of intake air. That's because the airbox is a Helmholtz resonator. That is, a resonance effect occurs when you connect an enclosed volume of a suitable size and shape to an engine’s intake stacks. This causes the air inside to resonate at a frequency that results in high pressure waves which coincide with the opening of the intake valves. This can theoretically increase power by 10–15% within a particular RPM range by boosting airflow into the engine's combustion chambers.

Airboxes need to be well-sealed and stiff in construction to maintain these resonance characteristics. Ours is made of cast aluminum alloy. A simple way to illustrate this phenomenom is to blow across the mouth of an empty bottle. The sound you hear is the natural frequency of the bottle acting as a Hemholtz resonator. If you change the shape and free air volume of the airbox, you change its resonance frequency, and the engine RPM at which it enhances filling the engine with air. Add a little water to the bottle and you change its shape and volume thus changing its resonant frequency. Add too much water and the frequency will have shifted to a point where we can't blow hard enough to make any sound. Same with the engine. Change the shape or volume of its airbox and an engine can't use the new resonance frequency.

The L81 V6's plenum functions as an airbox, a space where the airflow is smoothed so as to eliminate turbulence prior to rushing down the velocity stacks and into the combustion chambers. In fact, if you  could place your finger anywhere near the edge of the top of a velocity stack while the engine is running you'd see dyno power dropping off due to the introduced turbulence. The air in the plenum is considered "free air" meaning it's unhindered. That is, it’s already passed through the filter and air tract so it's supplied to the engine without any further flow restriction. Our engine has an integrated plenum so what we call the "airbox" is merely a filter housing. The Catera's true airbox sits right over the velocity stacks.

Dual & Multi Ram Assemblies



The Catera's Dual ('97-'98) and Multi ('99-'01) Ram assemblies use the Bernoulli Effect to regulate the velocity of intake air through the plenum over the engine's entire RPM range. Bernoulli explains how the velocity and pressure of confined moving air depends on the dimensions of the path through which it travels. It's the same principle that creates downforce under Formula 1 and Indy Cars. A long narrow intake path accelerates the air at low engine RPM. That's good for bottom end torque. But that set-up chokes the intake at high RPM and limits top end HP. If the intake path is short and wide you get fast air at high RPM. That's good for top end HP, but bad for bottom end torque. The Ram Assembly allows a good compromise across the entire RPM range. The system uses butterfly valves in the plastic intake tubes and plenum to effectively change length and width of the intake path, at preset RPM stages, to optimize the airflow into the cylinders. Failure of the valves will have a debilitating affect on engine performance and efficiency.

Despite the "Ram" in its name this isn't an air compression system. It allows the air drawn in, by the vacuum created by its pistons, to enter the cylinders at the most efficient velocity for a specific RPM. Using the auto industry's standard calculation of air volume required for efficient flow within the air intake system, you need at least 130% of engine capacity in available air volume between the throttle butterflies and the air filter element. This is called the Intake Tract. This means for the best results from our 3.0L V6 engine the air volume of its intake tract should be at least 3.9L which I'm sure the Catera's Multi-Ram system has. Replacing it with a Mantzel Resonator, or clone, reduces the Intake Tract volume.

The Mantzel Resonator



The Mantzel Resonator and its clones are aftermarket devices meant to replace the Dual/Multi Ram assemblies. The manufacturers claim an increase of HP and Torque across the entire RPM range. They consist of a box inserted in the intake tract and have no internal moving parts. Its volume and shape, thus its resonance frequency, is locked in when it's manufactured. The airflow's velocity through it can't be altered to match engine intake valve frequencies. It's probably optimized for wide open throttle. The Catera's ECU sets the engine redline at 6,500 RPM (by cutting off the fuel injectors) so the Mantzel Resonator probably works best just short of that. Since the 4L30E transmission's Sport setting moves its shift points up to redline an L81 V6 equipped with a Resonator would be at home on a dragstrip or freeway on-ramp. However, anywhere else on the power curve and performance will drop off affecting low and mid range driveability. This will hinder performance on country backroads where throttle changes are many and full throttle is rarely used.

Filter Cones (aka Cold Air Intake Kits)



These are the usually cone shaped oiled filters that replace the air filter housing with its paper filter element. Using one eliminates the filter housing and its intake tubing whose opening is located front of the radiator. The cone filter still uses the MAF Sensor and the Dual/Ram assembly. Though often called "Cold Air Intake Kits" they really aren't since they have no direct access to cooler outside air and ingest hot under-hood air. They do eliminate the restrictions inherent to the intake tube and air filter box by eliminating them entirely. That should allow the engine respond better to throttle changes. The hot under-hood air it does "inhale" supposedly helps improve fuel mileage according to the information I've read on high gas mileage sites like EcoModder.com. I've got one in my Catera and haven't noticed any great leap in top end power, but she does seem to pull harder from a stop.
(Besides, the intake howl is sweet.)

The Bottom Line



The OEM Ram Assemblies are tuned to optimize the airflow into the cylinders across the engine's rev range, where the Resonators aren't. Though I use one on my Catera, the Cone Filter may or may not improve engine breathing and performance. In my opinion the Dual/Multi Ram is the best option. The Cone Filter is a personal choice though.