Why do engines lose torque when changing the exhaust system?

Question

i've heard that if you change the stock exhaust system of a car you lose torque when you incremente too much the diameter of the pipe or if the length of the pipe is too short. Is this true? if the answer is yes, why does it happen.
Any links?

Answer 1

Hello,

exhausts introduce a restriction to the engine to push out burnt mixture from the cylinders. This resistance depends from the RPM. A small-diameter, long exhaust is very restrictive, therefore is helps at lower engine speeds to maintain torque of the engine for it helps the gases to stay in the cylinder during the valve-overlap phase rather than letting it to the exhaust, as the piston tries to compress. This also prevents some exhasut gas to exit the cylinders, but that results an EGR (exhaust gas recirculation), which improves emission. However, this long restrictive exhaust emits even larger resistance as the RPM goes up, therefore reducing power as the RPM increases. That's why tuned engines have large-diameter pipes to permit free gas flow, but which also lets mixture to exit prematurely (without combustion), because tuned engines have camshafts with long valve-overlap periods and opening times.
The length of the pipes affect the 'resonant' range of the exhaust systems, the RPM range at which the exhaust exhibits it's most positve effect on using a certain cylinders' pressure wave to help another cylinder.

You don't necessarily loose torque with the change, maybe you gain more at another RPM than you loose elsewhere in the RPM range.

Regards

Answer 2

It really depends on the engine - I've seen a lot of dyno charts where an engine picked up torque throughout the entire RPM range from a performance exhaust.

The problem happens on some engines with a large amount of camshaft overlap. This means that the intake valves and exhaust valves are open at the same time for a significant period. If the exhaust backpressure is too low, the air and fuel coming in the intake valve can get sucked right out the exhaust valve. Since backpressure climbs with RPM, this effect only happens at low RPM.

Like I said, though, it doesn't happen on every engine.

Answer 3

Well, yes, in certain cases. If your not doing anything else to the engine, it might lose a little. Thats because of this term called back pressure. Like some 4cyl cars, with the 5" exhaust, there basically is no back pressure to work with. Length of pipe doesn't really matter, is more diameter.

Answer 4

The cars engine is tuned to overcome the back pressure in the exhaust, basically the amount of force needed to make the gases flow through the system, change that and your engine will be out of tune and lose power, the same is not true of a turbo car where reducing back pressure increases power. Any change to the cars exhaust must match changes to the engines setup, hence the existence of tuning companies who will re-map your engine using a rolling road, if however you just change the back box this will only have a marginal effect.

Answer 5

Some Engines seem to lose torque , others gain. A lot depends on the engine's compression as well as operating temperature and the size of intake and exhaust valves. Talk to a performance shop's machinist about a specific motor and application.