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Has anyone tried the electronic cutouts. If I do go with them I would have them installed close to the mufflers. Installing them right after the headers would, I think, be too loud.
MBH headers with stock exhaust. Considering valved exhaust also. Probably less things to go wrong with full exhaust system but don't know if I can get the same sound as the cutouts.
W204 C63 Coupe, W166 ML350 BlueTEC, 928GT, C5 Z06 & IS300 race cars, EQE 4Matic+ on order
Aside from any noise considerations, installing them right after the headers would burn up your exhaust valves in short order and you'd probably lose some power. The M156 needs some back pressure on the exhaust side.
I would do them somewhere near the back, after an x-pipe. a lot of the CTS-V guys dump them behind the diff I think, at least the one I saw did it there. it made the ground shake
'04 E55 (Gone but not forgotten), '13 C63 P31 (RIP), another '13 C63 PP
Originally Posted by Diabolis
Aside from any noise considerations, installing them right after the headers would burn up your exhaust valves in short order and you'd probably lose some power. The M156 needs some back pressure on the exhaust side.
I've never understood this concept... An engine is an air pump. Everything we do is to make that air pump more efficient and effective. Back pressure is contradictory to that efficiency.
You talk about burning the valves up? What? More back pressure would put more pressure, wear, and strain on those valves.
yep back pressure is the resistance to flow, having your engine strain at getting the exhaust out wouldn’t benefit it... what most people mean by “back pressure” is proper flow which can be negatively affected by improper exhaust design...
As for cutouts most people put them right behind the collector on the exhaust or right after your crossover if you are using one
W204 C63 Coupe, W166 ML350 BlueTEC, 928GT, C5 Z06 & IS300 race cars, EQE 4Matic+ on order
Collected from several posts on PistonHeads...
For a turbo, you want none; that's why turbo cars have such huge exhausts. Back pressure will [slow down] the turbine which is undesirable.
For N/A and supercharged cars the question is a bit more nuanced. Low back pressure is generally good, but a bit of back pressure allows the scavenging effect to work, which can be worth a few ponies if it's managed properly.
You always want the lowest pressure possible in the relevant branch of the manifold as the exhaust valve opens as this will allow as much of the contents of the cylinder to be ejected as possible before the valve closes, thus allowing more oxygen to be drawn into the cylinder on the next intake stroke.
As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
W204 C63 Coupe, W166 ML350 BlueTEC, 928GT, C5 Z06 & IS300 race cars, EQE 4Matic+ on order
Another one from PistonHeads:
It's a fairly common conception that back pressure is bad, with the idea being that it stops exhaust flowing effectively out of the cylinder and robs you of cylinder space for the fresh charge. However, it's not as simple as that, and while having no back pressure is better than having loads right by your cylinders' exhaust valves, what you actually want to do is shape the manifold (and engine timing) to cause some pressure elsewhere, and a vacuum near the valves. As there's only going to be a finite amount of gas in the exhaust, deliberately raising the pressure in one place reduces it in others, which is beneficial when done correctly.
If you make the exhaust flow more freely, yes you get less obstruction to the exhaust gas escaping, but you also lose the lower-pressure vacuum near the cylinders, and don't scavenge all the exhaust gas out as effectively. Resonance waves exist in the manifold (this applies to the intake one too), and are very finely tuned so that each cylinder's exhaust gas assists the next one in scavenging more thoroughly. Buggering around with it makes you unlikely to achieve the same benefit.
On a turbocharged car, as has been said, you cannot scavenge effectively anyway so needn't worry. But for naturally aspirated engines, while "too much" or "incorrectly placed" back pressure is bad, do not think removing it completely is the best way forward!
02-11-2018, 12:53 PM
