What is IMRC? What is intake manifold runner control?

A6 All/ S6 All
#1

What is IMRC?

What is intake manifold runner control?

A lot of the info here is general modern mechanics, but some of it, including pictures, points more specifically to the Audi C5 platform S6, because this thread is a side note to the “C5 S6 V8 power build” thread in the A6/S6 section of this forum. Since this is an often misunderstood or unknown topic, I hope this thread will one day show up on Google when searching for “what is imrc?”. I’m gonna break it down for everyone - the DIY guy, the service tech who hasn’t seen it in a long time, and the guy who doesn’t know much about cars at all. I hope there’s lotsa useful info here for you, regardless of whether you drive an Audi or something else.

#2

Before we get into exactly what IMRC is, first let’s talk about engines and incoming air flow. If your understanding of engine basics is at the beginner stage, then check out this bare bones explanation of IMRC: Your engine sucks air, off and on, as each motor cylinder alternates between suddenly sucking in fresh air to burn fuel, and then suddenly blowing out exhaust. When a cylinder is suddenly not sucking air, then incoming fresh air suddenly hits a dead end, bounces, and “echoes” for lack of a better word. If the length of this echo is just right, then the engine sucks air at the moment that the air comes back from that echo - air that now has incoming momentum - and so the engine runs better, and thus produces more torque. This also means more hp, since torque plus rpm equals horsepower (basically). Intake manifold runner control (IMRC) is the result of making this echo “just right”.

And I don’t wanna hear any nonsense feedback about how the proper equation for horsepower does not involve addition of torque and rpm. The above paragraph is for non-technical basic understanding, and with constant torque, horsepower does go up with rpm. We’re gonna go deeper. A lot deeper…

#3

The next two paragraphs are for the readers who know a little more about basic engine anatomy. Let’s go back to the beginning regarding incoming air flow:

Air enters the intake manifold from a main air hole/feed, fed from your engine air filter. Through this main feed, the air enters a part of the intake manifold called the “intake plenum”. The intake manifold then provides nearly equal length air flow paths into separate holes, for each cylinder (in the motor) to breathe. These flow paths are the “intake runners”.

As a cylinder in a motor breathes in air (and fuel), a cylinder head valve opens to suck in fresh air, and then that air-sucking-valve suddenly snaps shut while combustion and exhaust occurs. Closing that valve suddenly as the air is rushing in causes a wave of air to bounce backwards through the intake, kinda like an echo. If the intake runners are just the right length, then this wave of air will bounce back and return at the cylinder head air-sucking-valve JUST as it opens again to suck in fresh air. This uses incoming air momentum to feed air, instead of (a) just leaving the engine sucking at atmospheric pressure, or, even worse, (b) leaving the engine fighting to suck in a fresh air wave that has momentum moving AWAY from the air-sucking-valve.

Intake manifold runner control (IMRC) is the result of making the intake runner length “just the right length”.

#4

Ok now, so what’s involved with intake manifold runner control? How exactly do you vary the length of the intake runners? Here’s how it works on the C5 platform (as well as other platforms):

Looking at the pictures to follow, courtesy of www.GruvenParts.com , you can see the intake runner lengths in blue. The blue paths show the path of a single intake runner length, for the sake of example. At any moment, the air could be going either left or right, depending on which one of the eight cylinders that the intake happens to be feeding at that moment. The main air feed from the engine air filter is the light blue circle in the middle. The entire light blue area is the intake plenum.

C5 Audi cars use three intake runner lengths, or three “stages” of intake runner length. Inside the intake plenum, there are two sets of actuating flaps. Each set of flaps controls the runner length to all cylinders. So if one set of flaps opens, then the intake runner length to all cylinders changes. Stage one involves no flaps. Stage two involves one set of flaps. Stage three involves both sets of flaps. Lemme explain each stage:

#5

Stage 1: In this picture, you can see that with both sets of flaps closed (for Stages 2 and 3), air takes the longest path around. This is the longest intake runner length of all the three stages. At low rpm, this gives you the optimum torque because you get the longest “echo”. I use the term “echo” for lack of a better description of the air bouncing (as cylinder air intake valves suddenly open and close). This is known as “Stage 1”. The number of revolutions per minute (rpm) of the engine is relatively slow compared to redline, so you want the slowest/longest relative echo, with relatively longer intake runner length, in order to maximize torque.

“Lower Speed Range” in picture refers to rpm of the engine, NOT road speed.


http://audirevolution.net/addons/albums/images/515003835.jpg

#6

Stage 2: You can then see that with the BOTTOM flap open, air takes a slightly shorter path than it does in Stage 1. In other words, the intake runner length is slightly shorter. The rpm of the engine is going up at Stage 2, so the time in between the cylinder air-sucking-valves opening and closing is relatively less time than the time taken for the same process at lower rpm in Stage 1. This means that a slightly shorter intake runner length gives a more effective “echo” for the air, in order for the air to bounce back and arrive at the air-sucking-valve just as it opens. This change in intake runner length helps keep engine torque output at a peak/constant value.


http://audirevolution.net/addons/albums/images/225376701.jpg

#7

Stage 3: You can see that with the TOP flap open, air takes the shortest possible path. The rpm is approaching redline, so the air-sucking-valves are opening and closing at a furious pace. In this scenario we want very little echo, that is, very short intake runner length. Again, this means that air arrives at the cylinder air-sucking-valves with incoming momentum, just as the air-sucking-valves open.


http://audirevolution.net/addons/albums/images/378096601.jpg

#8

So aligning

(a) the correct intake runner length with
(b) the particular rpm of the engine

allows you to have a higher and flatter looking torque reading, when looking at a dyno sheet. Power output of the motor, or horsepower, is basically a measure of torque combined with rpm. So keeping the torque near it’s peak value throughout the entire rpm range means better power throughout the rpm range, from a smooth idle right through to high rpm max power output.

#9


http://audirevolution.net/addons/albums/images/658050547.jpg

Without varying the length of the intake runner, your max torque will only exist in a narrow rpm range. Before this rpm range, the torque is only near peak value. After this rpm range, torque kinda falls off again. Why a lack of torque? Because the fresh air does not have momentum moving towards the cylinder trying to suck air. If the echo is going the wrong way, then the cylinder is actually sucking against a wave of air which is moving away from the cylinder. This means less air ends up in the cylinder, which means less gas burned, which means less power.

So IMRC can be thought of as a “torque curve extender”. Looking at the above picture, you can see that if you use only ONE stage instead of three (meaning only ONE intake runner length), then you’ll only have peak torque in one area of your rpm range. Using the varying intake runner lengths, you keep the torque near it’s peak value throughout the rpm rev range. Each of the three sections on the graph show you how the torque would peak at the proper intake runner length for each stage, but then torque could quickly poop out if the other stages did not function correctly.

For example, in the above picture, if only Stage 1 worked (black line in graph), then the torque would build under the green area to peak at around 2800rpm, and taper off around 3300rpm. With only Stage 2 working (grey line), then the peak torque would have to wait for 3300rpm before reaching peak under the grey area, and then would poop out and taper off around 5200rpm. With only Stage 3 working (red line), you can see that the engine would never reach max torque (and would probably show a check engine light), and would only approach max torque briefly in the orange area, around 5200rpm. But if all stages are working correctly, the torque stays near the max possible level throughout the rpm range.

#10

This paragraph is general IMRC. In some cars, under the air intake, between the upper intake plenum and the lower intake manifold, there is a flap for each cylinder (for air flow) that can open and partially close. No “stages”, but rather like a throttle body flap for each cylinder, located within the intake manifold runners. All of these flaps would be linked together for synchronized movement. As the IMRC flaps move from partially open to wide open, this changes the length of the intake runner. At high rpm, the flaps are wide open, and this creates a “short runner intake” (great for high end performance). At low rpm, they are partially closed, and this creates a “long runner intake” when not wide open (good for start ups, smooth cold engine idling, and low rpm torque). So as you may have already gathered, if IMRC works as planned, then you basically get better/smoother torque below 3000rpm, and more horsepower above 3000rpm.

For example, if your intake runner length is “stuck wide open”, aka “too short”, then you might get a rough idle and a CEL (check engine light).

#11

Moving right along, how does it work mechanically? What causes these sets of flaps to move on the C5 V8’s (and other similar platforms)? Well, vacuum lines control diaphragm “pods”. These diaphragm pods attach to “intake manifold runner control arms”, aka “intake actuator arms”. The movement of these arms controls the runner length, because the arms connect in between the pods and the actuators on the front of the intake. These teardrop shaped actuators, also known as “pivot levers”, are connected to the internal IMRC flaps. This will be made more clear in pictures to follow.

Some cars use the vacuum pods to trigger a solenoid, and this solenoid is used to power to the actuators.

#12

What does a pod look like? Ok well here is an IMRC pod from a 2002 (C5 platform) S6. One side of the pod has a little pipe which attaches to a vacuum line, and the other side has the OEM plastic actuator arm attached. In between the pipe and the arm is a rubber diaphragm, or “pod” as most people call them.


http://audirevolution.net/addons/albums/images/484755746.jpg


http://audirevolution.net/addons/albums/images/251429278.jpg

#13

What’s the specific problem with the C5 S6?

Well, in my opinion, it’s kinda like the glove box hinges on some of the Audi models - plastic was used where metal probably woulda been a better idea, since the plastic is prone to cracking with age. Broken plastic actuator arms are often the problem. Or the beginning signs of a bigger problem.

One solution is to replace the plastic arms with metal ones. “Gruvenparts” makes aftermarket intake actuator arms out of aluminum. They come in different sizes, because the A6 is prone to the same problem. The C5 A6 4.2L V8 is a different engine than the C5 S6 4.2L V8. Between the C5 A6, C5 Allroad, C5 S6, and C5 RS6, there are actually five different 4.2L V8 engines. And that doesn’t include other Audi 4.2L V8’s.

#14

Another possible IMRC issue on the C5 platform, to be determined after installing aluminum actuator arms:

If the plastic top clip of the pod/diaphragm busts off while under the hood, from normal engine operation, then it’s probably because the flaps inside the intake manifold seized up, due to build up on the outside of the intake manifold.

#15

Audi will then say that the entire intake manifold needs ($$$!) to be replaced. Luckily, this problem has been addressed without needing to replace the entire intake. You’ll need a Dremel tool to clean the build up around the tear-drop shaped actuators (aka “pivot levers”) which are on the front of the intake manifold.

On another forum, I found someone who made the comment “Replacing the connectors, while necessary, will not solve the problem. The bushings that the shafts for the flaps ride in are corroded and you need to take the front mounts off and clean them out with a dremel. This frees up the shaft so the connectors won’t break.” The “connectors” referred to here are the actuator arms between the pods and the teardrop shaped actuators. The “bushings” referred to in this quote are around the the teardrop shaped actuators.

In addition to making replacement intake actuator arms, Gruvenparts ALSO now makes the pivot levers, so if you have the intake removed to clean it up, then consider replacing the pivot levers.

Gruvenparts has a detailed guide on cleaning up the intake and replacing these parts. I will put info below regarding finding this guide.

#16

I probably shoulda added this earlier, but what does all of this look like under the hood? Ok well here is a shot of under the hood of a 2002 S6. One pod is oriented vertically, and is missing the OEM plastic actuator arm. The other pod has a Gruvenparts aluminum actuator arm installed.

“Actuator” in the picture is also known as the “pivot lever”, or “teardrop shaped actuator” in this thread.

“Aluminum arm” and “OEM arm” shown in picture are also called “actuator arm” in this thread.

“Diaphragm” in the picture is also called the “pod” in this thread.


http://audirevolution.net/addons/albums/images/551855461.jpg


http://audirevolution.net/addons/albums/images/507894430.jpg


http://audirevolution.net/addons/albums/images/260952158.jpg

#17

The next step from there is finding replacement pods/diaphragms, which is an expensive nuisance:

(1) You can try and find an old intake from an online scrap yard, but sometimes they come without the pods. And remember different 4.2L V8 engines have different length actuator arms.

(2) In an A8 discussion on another forum, I’ve read that “Land Rover Cruise Control Actuator” Part# ETC7150 works well as a replacement Audi pod, and is only about $70. The person giving this part number mentioned that he had aftermarket metal intake actuator arms, and that “The pod is the exact same, just a slight angle difference in the ball where the actuator arm connects. My replacement arms are adjustable so this was no problem.”

(3) Lamborghini pods, from 2004-2008 Lamborghini Gallardo V10 Coupe, Spyder, Superleggera, and probably others, since some of the Audi V10’s and some Lambo V10’s are considered by many to be “the same engine” (and they basically are). I have not confirmed a part number for the Lambo pods, so if anyone has a part number, then please post the info here. A Google search for “Lamborghini Gallardo imrc pods” gives a suggested search for 077198327a, but that is an Audi part number. Probably available cheaper than Audi by ordering through a Lambo dealership. I did find a discussion on another forum where the poster said “intake manifold vacuum diaphrams sourced for A8/S8, it’s really odd that Audi doesn’t sell this part”. He confirmed that Lambo pods are only about $60, and the Audi part number that he ordered through the Lambo dealer was 077907327A. Rumor has it that this part is now in the ballpark of $150.

(4) I almost didn’t include this option - going to Audi - because a pod is rumored to be about $500, or a “kit” for about $800. And/or you may have to buy the entire intake manifold in order to get just the pods. Looking at the Google results for 077198327a, it looks like about $350 a piece. On my 2002 S6, I see part number “Hella #004 538” on the body (upcoming picture). The person who posted the Land Rover part number - he noticed this same Hella info on whichever Audi pod that he was looking for. He also noted “Audi part # 077 907 327 A on the tab where the arm connects”.

#18

For part number reference, here is a shot of the 2002 Audi S6 pod that I replaced (with one from a scrap yard intake):


http://audirevolution.net/addons/albums/images/598110899.jpg

#19

You can find replacement billet aluminum intake actuator arms for all Audi/VW 4.2L V8 engines at:

www.GruvenParts.com

They ALSO sell the actuators (aka “pivot levers”) on the front of the intake manifold.

There is a detailed guide on this website regarding changing the arms, changing the pods, changing the actuators, and cleaning up the problematic area on the intake. It can be found under “product description”, and it’s “a DIY on The Billet Intake Linkage Installation”.

#20

Closing trivia: In some BIG power builds, the intake runner control flaps get deleted, to allow just a touch more air flow, in theory. This creates a permanent short runner intake, allowing just a touch more top end power, in theory.

But, if I was to just take a guess at numbers - and this is a guess - we’re talking like maybe a six or seven hundred hp car trying to squeeze out another 10 pounds of torque, and like another ten hp. So in my opinion, deleting the IMRC flaps is a nonsense “upgrade” for most cars, unless the intake manifold becomes a power limiting bottleneck in your power train.

I say “in theory”, because we’re talking about cars that probably would have built engines, with reinforced rods, to extend the redline from 6000rpm to 8000rpm, and have a BIG turbo or supercharger attempting to blow the MAX air through the intake manifold that air flow physics would allow. If this is the case, then it makes sense to delete the flaps, because their mere physical presence forbids a measurable amount of air flow at insane rpm and air pressure.