cheekychimp
Well-known member
I read a thread a few days ago about exhausts and ended up looking at a number of manufacturers who produce exhausts with valves that can be opened and closed (primarily to allow the user to make the exhaust quieter when the vehicle is at idle or cruising).
All interesting stuff but Barry then put me onto a site about Active Exhaust Systems. Basically the same stuff but they had an option to install a black box that measures back pressure in the system and automatically adjusts the degree by which the valve is opened in accordance with the amount of back pressure that is detected. I can't really see that being of any benefit in a forced induction engine but it was supposed to help low end and mid-range power in normally aspirated vehicles.
So that got me thinking. I realized that I really don't understand torque. I know long intake runners generate it, but I have absolutely no idea why. I know NA engines need back pressure to generate it, but again I have no idea why. I also remember being told that a lot of rally cars had engines capable of producing over 500 hp but most only made about 300 hp because the rules stated they had to run 34 mm restrictor plates. The throw off from this of course was that they made barrel loads of torque. Again I realized I didn't understand why.
So what creates torque exactly? And does a restriction in the exhaust or at the turbo inherently create torque or is the engine simply tuned to produce torque (rather than power) because airflow has been restricted?
If a valve in the exhaust can affect the torque characteristics of a normally aspirated vehicle, then couldn't a 'variable restrictor' in a forced induction engine create torque at low rpms and allow power to develop at higher rpms?
Or does the engine have to be retuned everytime the size of the restrictor is changed?
All interesting stuff but Barry then put me onto a site about Active Exhaust Systems. Basically the same stuff but they had an option to install a black box that measures back pressure in the system and automatically adjusts the degree by which the valve is opened in accordance with the amount of back pressure that is detected. I can't really see that being of any benefit in a forced induction engine but it was supposed to help low end and mid-range power in normally aspirated vehicles.
So that got me thinking. I realized that I really don't understand torque. I know long intake runners generate it, but I have absolutely no idea why. I know NA engines need back pressure to generate it, but again I have no idea why. I also remember being told that a lot of rally cars had engines capable of producing over 500 hp but most only made about 300 hp because the rules stated they had to run 34 mm restrictor plates. The throw off from this of course was that they made barrel loads of torque. Again I realized I didn't understand why.
So what creates torque exactly? And does a restriction in the exhaust or at the turbo inherently create torque or is the engine simply tuned to produce torque (rather than power) because airflow has been restricted?
If a valve in the exhaust can affect the torque characteristics of a normally aspirated vehicle, then couldn't a 'variable restrictor' in a forced induction engine create torque at low rpms and allow power to develop at higher rpms?
Or does the engine have to be retuned everytime the size of the restrictor is changed?
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