EmirOfDenmark
Emira Fan
As many of you have been considering and trying an aftermarket upgrade of the exhaust system, I would like to share my learnings, along with the dealer advice.
The background (as explained to me by the dealer)
Here is what is going on. If you are considering about upgrading the exhaust, the last stage of the exhaust is what you are replacing. That connects to the Valve Controller (VC). The VC effectively controls a valve from a range to positions from fully open to fully closed. It is controlled by the ECU, connected to how you drive and which drive mode you are in. In Tour mode, it is mostly closed, but is still opens up slightly at higher revs (1). In Sport mode it starts somewhat opened and opens up fully in higher revs. In Track mode is behaves exactly the same as in Sport mode, but the idle RPMs are higher and the rev limiter is higher by about 200 RPM (this is why people think it is different, but what you are hearing is the higher idle RPMs). So, the way it is designed is that ECU sends a message to the valve (e.g. open up by 20%), and the valve needs to send back a response "valve opened by 20%". If the valve gets physically stuck, the valve controller will not respond, and the ECU will go "oh, the valve is not opening, that is unsafe, there is not enough respiration for the engine, I will limit it to 4000rpm".
My Initial Experience - the problems
Last year, I bought a Miltek exhaust system for my V6. I drove it to a mechanic (not Lotus), that is recommended by Miltek, and they installed it relatively quickly, no fuss.
As soon as I drove off from the shop, I got Limp mode, i.e. the engine was limited to 4000rpm. What happened was, that the mechanic has connected the Miltek exhaust with the VC in a position that was not the last recorded position in the ECU. E.g. the ECU would have thought is is in 20% Open, but the connected position would have been 30% open, by pure mechanical positioning of the valve during installation. So, the first time the ECU tried to open it up fully, it couldn't, it interpreted the valve as being stuck, and triggered limp mode.
The solution - recalibration
To solve this problem, you need to take the car to the Lotus dealer and they will recalibrate the VC to the mechanical position of the valve. Only Lotus dealer can do this, you cannot do this with a custom tool. Clearly the position of the valve is so important that Lotus doesn't want anyone to mess with it, and the moment the ECU thinks then valve is not doing what it needs to be doing, it limps the engine. So the valve is really important for the engine respiration. (2)
The workaround - the valve controller
Then I ran into the Miltek Valve Controller (MVC) that can control the valve manually (via a fob). It has three positions: Open (full open), Close (fully closed), and Stock (let the ECU decide). In any of the modes, the MVC returns an affirmative response to the ECU, and thereby the car will never go into limp mode. You are also removing an obviously important feedback loop that Lotus has built in, that when the valve misbehaves, the engine needs to be limited. (3) I installed the MVC, and the limp mode went away.
More problems - O2 sensor slow response
While I drive it fully open most of the time, I do sometimes like to close the valves, e.g. if I am on a long highway journey and want a more quiet experience. When I do that, I could get the CEL, which my OBD would read as "O2 sensor slow response". Hmmm, is it safe this? Turns out that because the air mix and air pressure is different from what is expected at certain RPMs, the error could occur. In other words, the closed valve is triggering an invalid reading from the O2 sensor.
The conclusion - as verified by the Lotus mechanic
The Miltek exhaust lets out more air flow than the stock exhaust, and generally they trust Miltek on the exhaust itself. However, the controller was a big concern for them.
Open mode - ok, you may get some O2 sensor errors there as well, but the engine can breathe properly, all good.
Stock mode - depending how the valve is calibrated. If there is a big discrepancy between the perceived position and real position you can still have the same problems as in Closed (but you would hear that).
Closed mode - this is a major risk. Do not use it. Note the points 1, 2, and 3 above. Even in Tour mode, the engine needs to open the valve to breathe (1). There is a serious feedback loop being built into the engine respiration pathway, to prevent engine from revving if there is a risk of that pathway being closed (2). Removing that feedback loop is a risk to the cats and the engine itself, it goes against the programmed behaviour (3).
Unless, you drive it in Open mode.
In other words - when you buy the Miltek Valve Controller, you are not buying a valve controller, you are buying an Open only mode, if you want your engine to be safe.
The result and the experience
I now have confidence that the Miltek exhaust itself is ok, and that the controller is safe as long it is in Open mode. That is why I basically left the fob safe at home, and the MVC in Open mode in the car. The result is a great sounding car for me. The stock car sounds good too, but with this setup, it sounds much better (for me). What is better?
At low revs - sounds deeper
At mid revs - more howl
At high revs - louder and more race-like sound
Overruns - more rumbles and pops, more drama
Final note, without the MVC, I was underwhelmed when I just had the Miltek Exhaust. It sounded a bit louder, but I was questioning whether it was worth it. If you want to do this, get the MVC as well, as that makes huge difference, but be aware you are buying an Open mode only.
TLDR; if you want a exhaust upgrade, get the Miltek exhaust AND get the Miltek valve controller, but never use it in any other mode than Open, if you want to keep your car safe.
Hope this clarifies many questions I've been seeing here.
NOTE: This is just me sharing my experience to the best of my knowledge. You take responsibility of your own car modifications.
The background (as explained to me by the dealer)
Here is what is going on. If you are considering about upgrading the exhaust, the last stage of the exhaust is what you are replacing. That connects to the Valve Controller (VC). The VC effectively controls a valve from a range to positions from fully open to fully closed. It is controlled by the ECU, connected to how you drive and which drive mode you are in. In Tour mode, it is mostly closed, but is still opens up slightly at higher revs (1). In Sport mode it starts somewhat opened and opens up fully in higher revs. In Track mode is behaves exactly the same as in Sport mode, but the idle RPMs are higher and the rev limiter is higher by about 200 RPM (this is why people think it is different, but what you are hearing is the higher idle RPMs). So, the way it is designed is that ECU sends a message to the valve (e.g. open up by 20%), and the valve needs to send back a response "valve opened by 20%". If the valve gets physically stuck, the valve controller will not respond, and the ECU will go "oh, the valve is not opening, that is unsafe, there is not enough respiration for the engine, I will limit it to 4000rpm".
My Initial Experience - the problems
Last year, I bought a Miltek exhaust system for my V6. I drove it to a mechanic (not Lotus), that is recommended by Miltek, and they installed it relatively quickly, no fuss.
As soon as I drove off from the shop, I got Limp mode, i.e. the engine was limited to 4000rpm. What happened was, that the mechanic has connected the Miltek exhaust with the VC in a position that was not the last recorded position in the ECU. E.g. the ECU would have thought is is in 20% Open, but the connected position would have been 30% open, by pure mechanical positioning of the valve during installation. So, the first time the ECU tried to open it up fully, it couldn't, it interpreted the valve as being stuck, and triggered limp mode.
The solution - recalibration
To solve this problem, you need to take the car to the Lotus dealer and they will recalibrate the VC to the mechanical position of the valve. Only Lotus dealer can do this, you cannot do this with a custom tool. Clearly the position of the valve is so important that Lotus doesn't want anyone to mess with it, and the moment the ECU thinks then valve is not doing what it needs to be doing, it limps the engine. So the valve is really important for the engine respiration. (2)
The workaround - the valve controller
Then I ran into the Miltek Valve Controller (MVC) that can control the valve manually (via a fob). It has three positions: Open (full open), Close (fully closed), and Stock (let the ECU decide). In any of the modes, the MVC returns an affirmative response to the ECU, and thereby the car will never go into limp mode. You are also removing an obviously important feedback loop that Lotus has built in, that when the valve misbehaves, the engine needs to be limited. (3) I installed the MVC, and the limp mode went away.
More problems - O2 sensor slow response
While I drive it fully open most of the time, I do sometimes like to close the valves, e.g. if I am on a long highway journey and want a more quiet experience. When I do that, I could get the CEL, which my OBD would read as "O2 sensor slow response". Hmmm, is it safe this? Turns out that because the air mix and air pressure is different from what is expected at certain RPMs, the error could occur. In other words, the closed valve is triggering an invalid reading from the O2 sensor.
The conclusion - as verified by the Lotus mechanic
The Miltek exhaust lets out more air flow than the stock exhaust, and generally they trust Miltek on the exhaust itself. However, the controller was a big concern for them.
Open mode - ok, you may get some O2 sensor errors there as well, but the engine can breathe properly, all good.
Stock mode - depending how the valve is calibrated. If there is a big discrepancy between the perceived position and real position you can still have the same problems as in Closed (but you would hear that).
Closed mode - this is a major risk. Do not use it. Note the points 1, 2, and 3 above. Even in Tour mode, the engine needs to open the valve to breathe (1). There is a serious feedback loop being built into the engine respiration pathway, to prevent engine from revving if there is a risk of that pathway being closed (2). Removing that feedback loop is a risk to the cats and the engine itself, it goes against the programmed behaviour (3).
Unless, you drive it in Open mode.
In other words - when you buy the Miltek Valve Controller, you are not buying a valve controller, you are buying an Open only mode, if you want your engine to be safe.
The result and the experience
I now have confidence that the Miltek exhaust itself is ok, and that the controller is safe as long it is in Open mode. That is why I basically left the fob safe at home, and the MVC in Open mode in the car. The result is a great sounding car for me. The stock car sounds good too, but with this setup, it sounds much better (for me). What is better?
At low revs - sounds deeper
At mid revs - more howl
At high revs - louder and more race-like sound
Overruns - more rumbles and pops, more drama
Final note, without the MVC, I was underwhelmed when I just had the Miltek Exhaust. It sounded a bit louder, but I was questioning whether it was worth it. If you want to do this, get the MVC as well, as that makes huge difference, but be aware you are buying an Open mode only.
TLDR; if you want a exhaust upgrade, get the Miltek exhaust AND get the Miltek valve controller, but never use it in any other mode than Open, if you want to keep your car safe.
Hope this clarifies many questions I've been seeing here.
NOTE: This is just me sharing my experience to the best of my knowledge. You take responsibility of your own car modifications.
. Is the point about rapid vs progressive exhaust noise drama. However, still worried about the OEM mode apparently always returning "all ok" to the ECU, even if the valve is misaligned. 
