Exhaust Bypass Valves and Exhaust Controllers

I get asked questions about the use of exhaust controllers and the effect that exhaust bypass valves have on the exhaust system and overall performance. There has been much said about exhaust bypass valves and their effect on performance. I thought I would share here what I have learned.

First, they must improve performance, do they not? Otherwise, why would manufacturers put them on their cars?

Here is a typical exhaust bypass valve used in Ferrari’s and other cars.

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The exhaust bypass valve consists of a flapper valve that is actuated by a lever connected to either a vacuum diaphragm or an electric motor. The illustration above is a valve that uses a vacuum diaphragm actuator. A flapper valve in an exhaust system can serve one of two purposes.

1. As a restrictor within the exhaust pipe to close, or partially close, and restrict the flow of exhaust gases.
2. As a diverter to change the path that the exhaust gases follow.

Why would designers use the first option? Why would you want to restrict the exhaust gases in the exhaust system? At lower engine RPMs, the evacuation of gases from the engine cylinders through the header is improved if there is some back pressure in the exhaust system. By improving the gas evacuation, low-end torque can be enhanced. But, as engine RPM increases, back pressure becomes an impediment to exhaust gas evacuation. Under higher engine RPMs it is desired to decrease back pressure, which increases the exhaust gas flow rate and improves horsepower.

By using a flapper valve to restrict gas flow, designers can develop an exhaust system that is optimized to enhance horsepower at higher engine RPMs. At lower engine RPMs, a flapper valve in the exhaust system can be employed to partially restrict the gas flow ... this increases back pressure. A modest increase in back pressure in the exhaust system will generally improve low-end torque by aiding with evacuation of exhaust gases. The optimal location to induce this restriction is at (or near) the exhaust header collector. The ECU would then be programmed to open or partially close the valve at the appropriate thresholds of RPM and throttle position. Some motorcycle manufacturers employ this concept with single or dual paths for the exhaust gases to follow. However, on motor cars, installing a flapper valve at the header collector does not do anything to alter the sound level. As this is an objective that automotive engineers wish to achieve, automobile manufacturers do not place flapper valves at the header collector location.

The following illustration shows the optimal location for an exhaust bypass valve and the factory location as installed on the Ferrari 458.

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If engineers were to create an exhaust system that optimizes horsepower, it would likely have low restriction to the flow of exhaust gases, especially at higher engine RPMs, and by its nature, would likely by noisy. But by employing a flapper valve to divert the gas flow, the designer can incorporate a path with higher back pressure to enhance low-end torque. This path could also be one that has less sound output to meet noise restrictions. So, now we can have one path for high engine RPM (optimize horsepower) and the other path for lower engine RPMs (improve torque). The high horse power path would be the loud sound path and low-end torque path is the quiet sound path. This is the concept employed in all production cars that utilize bypass valves. The valves are installed downstream of the catalytic converters and provide a quiet sound path and a loud sound path through (or bypassing) the exhaust system silencers. When they are closed, they will increase back pressure a bit, which may enhance the low-end torque. But their location downstream of the exhaust collectors is not optimal, so their effect on low-end torque is minimal – but generally a bit of improvement none the less. When the valves are open, gas is diverted to bypass the silencers, back pressure is minimized in the exhaust system, engine horsepower can reach its design peak and noise output is generally higher.

This next illustration shows the exhaust gas flow with factory installed bypass valves … with a quiet sound path and a loud sound path.

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This cutaway view of the Jaguar F-Type silencer clearly illustrates how that manufacturer employs bypass valves to achieve a quiet sound path and a loud sound path.

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Automotive engineers walk a tightrope when designing exhaust systems to enhance engine performance while at the same time meeting emission standards and noise restrictions. Using exhaust bypass valves, exhaust systems can be designed to optimize power at higher engine RPMs and provide the means to reduce the noise levels in environments where noise levels are restricted … without compromising exhaust emissions standards.

This leads to a question … Would installing an exhaust controller improve engine performance? The answer is no – assuming you are comparing a stock set-up with forcing the valves always open. In a stock set-up, the engine’s ECU is programmed to open the valves under heavy throttle and higher engine RPM … to minimize back pressure and optimize horsepower – not torque. If you were to use an exhaust controller to force the valves always open, you are not changing anything under those conditions. But forcing the valves always open under low engine RPM might actually reduce low-end torque. It is not likely any reduction would be significant, though. But, with the valves always open, you will have that nice aural experience at all times. You might be violating a civic ordinance, but you will sound good doing it.

Another question often asked … Will installing an exhaust controller affect fuel economy? The answer is … probably. Because you might enjoy the sound being emitted, there is a likelihood that you might use the throttle in a manner to promote noise rather than fuel economy.

If you want to increase the sound level of your exhaust, short of installing a new exhaust system or silencer, you have four practical options:

1. Disconnect the electrical connector at the vacuum solenoid valve that controls the flapper valve.
2. Disconnect the vacuum line at the flapper valve or wire the valve to remain open at all times.
3. Install a manual valve in the vacuum line going to the flapper valve.
4. Install an exhaust controller.

Let’s discuss each option in turn.

First, disconnecting the electrical connector at the vacuum solenoid valve is discouraged.

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It is a simple to do, but could lead to problems such as diagnostic trouble codes or other malfunctions. Among Ferrari’s, I only know of one car that this will not cause any issues – the Ferrari 360. There are some other cars such as Camaro and Corvette that this option will also work without problems. You cannot do this with the Ferrari 550, 575M or F355 as this will cause those car’s exhaust bypass valves to revert to closed – not open.

The second option, is to disconnect the vacuum line at the flapper valve or mechanically keep the valve open using wire or a hose clamp. This is an easy mod to perform. If you remove the vacuum line, make sure to plug the end of the tubing to prevent any vacuum leaks.

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Removing the vacuum line will not allow vacuum to reach the flapper valve actuator. On the majority of exhaust bypass valves, the absence of vacuum will result in opening the flapper. The exception is the Ferrari 550, 575M and F355 as this will close the valves. On those cars, you can mechanically force the valves open. Of course, reverting back to stock mode is not very convenient, but the mod is very inexpensive. You can obtain drip tubing end plugs anywhere drip irrigation supplies are sold. If you disconnect the vacuum line, consider placing a short length of vacuum tubing on the nipple of the actuator assembly and plugging the end. This will prevent debris from accumulating inside.

Mechanical restriction of the flapper valve is easily done by using some stainless-steel wire or similar to prevent the flapper valve from closing. You can use this concept on any car so equipped with exhaust bypass valves. If you do this, do not disconnect the vacuum line – or if you do, plug it.

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Installing a manual valve to change the vacuum going to the flapper valves provides greater versatility than merely removing the vacuum line. Obtain a drip tubing coupling valve and place it in-line with the vacuum tubing to the flapper valve. These small valves are available anywhere drip irrigation fittings are sold. Turning the valve off will stop the vacuum from reaching the flapper valve and prevent a vacuum leak. Opening the valve returns operation to normal. Position the drip tubing coupling valve in a convenient location for easy access so you can toggle between open and normal mode. Note: Turn the ignition off before changing the drip tubing coupling valve from open to closed. This will ensure that any vacuum that is present at the flapper valve is released before you close the drip tubing coupling valve.

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The last option is to install an exhaust controller. This provides the greatest flexibility and convenience. This is the only option that will enable easy toggling from normal mode to open and vice versa. The two most common controllers used on Ferrari’s are the Capristo and the Forza.

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Functionally, both controllers are equivalent. They both manage the electrical circuit to the vacuum solenoid valves ... without causing diagnostic trouble codes or other issues. The advantage of using an exhaust controller is to easily toggle between normal mode and always open. The level of complexity to install an exhaust controller is in direct relationship to the difficulty of accessing the car’s vacuum solenoid valves. This varies by car model.

Regards,

Steve

 

I thought to add some words about back-pressure and exhaust bypass valves on cars with turbo-charged engines. In a word, back pressure is bad on cars with turbo charged engines.

In my earlier post, relating to normally aspirated engines, I mentioned that at lower engine RPMs, a bit of back pressure is good thing in the exhaust header to help scavenge spent exhaust gases away from the engine exhaust manifold. Even at higher engine RPMs, a bit of back pressure is desirable, but it is less than at lower engine RPMs.

On turbo charged cars, the turbo charger itself is responsible for the scavenging effect. To enhance the turbo’s capability, all sorts of technologies are employed like waste-gates and blow-off valves. But, first … on turbo charged cars, back pressure in the exhaust system is actually two issues. There is pressure in the post-turbo exhaust system – influenced by the exhaust pipe diameter and length and other in-line restrictions such as the catalytic converter and silencers. These are the same influencers that exist in a normally aspirated engine. The other pressure issue is the pre-turbo pressure. And this is inevitable. Pre-turbo pressure is what spools up the turbo. I am not going to go into the details of pre-turbo pressure – there are many. Suffice it say that the pre-turbo back pressure needs to balance with the size of the turbine, engine RPM and other factors. All really complex stuff.

So, to focus on post-turbo back pressure – pressure bad. The dynamics of turbo charged engines tends to be about the ratio of pre-turbo pressure to post-turbo pressure. From a practical standpoint, the larger the ratio (pre-turbo pressure being higher) the better. One could argue that no exhaust pipes after the turbo charger would be ideal as that would provide the minimal back pressure – effectively none. But that is not practical.

You may have observed that the diameter of exhaust pipes on turbo charged cars is often larger than on normally aspirated cars. The reason … to provide the lowest practical back pressure. This may not be particularly noticeable on the 488 vs 458, but if you had the ability to measure the back pressure in the two exhaust systems you would likely see a lower level in the 488.

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This leads to a question … Will exhaust bypass valves on turbo charged cars in any way enhance low-end torque as they may do in normally aspirated cars? The answer – no. Closure of these valves might increase back pressure, but again as stated before, they are so far downstream in the exhaust system, they are not likely to have much effect. And, if they do increase back pressure on a turbo charged car, it can have only a detrimental effect on the turbo charger’s ability to spool up.

Another characteristic about turbo charged cars is they are inherently quieter that normally aspirated cars. One effect of the turbine wheel in the turbo charged car is to reduce noise output. To gain that noise back in production cars, engineers design the silencers with less noise attenuation construction than silencers on normally aspirated cars. Note in the above illustration ... the Ferrari 488 has only a single silencer vs the 458 has two.

But these silencers have to meet noise restriction where it is applicable (e.g., urban residential areas). By using the exhaust bypass valves as diverters, they can create a quiet sound path and a loud sound path for the exhaust gases to take. These valves serve only one purpose … make more noise. The ECUs will open or close the flapper valves according to thresholds of engine RPM and throttle position. On Ferrari’s they are standard. On some cars (e.g., Porsche) they are an expensive option.

The installation of an exhaust controller on turbo charged cars will have virtually no effect on performance.

 

I agree that that any claim that the bypass valves are there to improve performance are very likely misleading. I so stated in my write up, but not as emphatic.

I do not have any inside knowledge, but I expect that manufacturers design their exhausts to provide optimum horsepower ... designed to optimize power at mid to upper engine RPMs and mid throttle to full throttle positions. Should they so choose, the manufacturers could also design the system to be quiet and still not compromise power. Take, for example, Rolls Royce and high end Mercedes Benz ... powerful yet quiet. But exotic car buyers (e.g., Ferrari) want a more aggressive sound. The silencers on these cars have less noise attenuation and the use of expansion chambers and resonators contribute to the development of an aggressive sound signature. And, to comply with noise regulations, these same manufacturers employ flapper valves to divert the exhaust gases to those components that attenuate the sound to meet those requirements. One only has to follow the routing of exhaust gases through silencers for quiet mode vs bypassing those silencers for loud sound mode to conclude that this is exactly the purpose.

I read somewhere ... don't take this as gospel ... that the EU (and some states in US) are considering outlawing exhaust bypass valves. Or, at least to ensure that testing for noise is performed with the bypass valves set to the "loud" position. The reason? When the current vehicle noise standards were developed, bypass valves were essentially not offered on production cars. The testing that is performed is to have the vehicle proceed through a sound trap at a designated speed. The vehicle must not exceed something like 80 dB(A) or something like that. Manufacturers of cars that employ bypass valves ensure that the car's bypass valves are diverting exhaust gases to the quiet sound path when they have their cars tested for noise. So manufacturers are clearly taking advantage of a regulation that was written before bypass valves were commonly used.

Steve

 

Some interesting tid bits of info. There are many states in the US with poorly written laws about exhaust systems.

For example, in the State of Alabama, as I interpret their code as written ... it might be contrived to be unlawful to operate a Ferrari on their highways ...

Code of Alabama

§ 32-5-216.

(a) Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise and annoying smoke, and no person shall use a muffler cut-out, bypass, a muffler without baffles or similar device upon a motor vehicle on a highway.​

Wording similar to that for Alaska, Arizona, Texas and others. These states have laws prohibiting mufflers with "excessive or unusual noise". No objectivity with respect to measuring the sound output.

California, Nevada, Oregon, Pennsylvania, Maine and Michigan are the only states with an objective method for measuring exhaust noise. For example,

California Vehicle Code

(b) For the purposes of exhaust systems installed on motor vehicles with a manufacturer’s gross vehicle weight rating of less than 6,000 pounds, other than motorcycles, a sound level of 95 dbA or less, when tested in accordance with Society of Automotive Engineers Standard J1169 May 1998, complies with this section. Motor vehicle exhaust systems or parts thereof include, but are not limited to, non-original exhaust equipment.​

I will leave it the curious reader to delve into the details of the SAE Standard J1169. I expect it is loaded with details of how to conduct the test, etc.

Steve

 

I don’t know. Perhaps lawmakers in Alabama have knowledge about these matters that we do not.

 

Check with a salvage yard or used parts dealer to see if you can get a flapper valve assembly.
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One you have the flapper valve, any exhaust shop should be able to cut out the old one and weld the new one.

If you cannot find a used Ferrari flapper valve, there are many such valves available by searching the web.

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For example, here is company that sells these vacuum actuated valves in various sizes ... https://loudvalves.com/9053506/checkouts/42bed795f3f46bccac804e3095869262

I cannot vouch for the quality of the above valves. The valves on Ferrari, BMW, Audi and other cars from Europe are pretty much sourced from the same company. It escapes me at the present that company name. But, there is a firm in the UK called Milltek that manufacturers exhaust system and they have a reputation of providing these replacement valves. You might look them up.

Steve