All this talk of Exhaust pressures making a difference in performance got me
thinking:
5 psi is not alot of pressure, how can it rob an engine of so much power?
If the pressure in the exhaust system is 5 psi, then that means that when
the exhaust valve closed, the exhaust gases in the cylinder would also be at
5 psi. When the intake valve opens and the piston begins to move downward,
NO air or fuel will be sucked into the cylinder until that 5 psi has been
relieved. WORSE YET, as soon as the intake valve opens, that 5 psi of
pressure pushes OUT the intake valve into the intake manifold, then the
downward stroke of the piston must create enough vacuum to stop and reverse
the flow of the air in the intake manifold.
So, it’s not the power required to push the exhaust through a muffler/cat
etc that consumes the power, it’s the residual pressure in the cylinder
disrupting the intake of clean air and fuel.
By the same token, a study some years ago by, dare I say BMW, documented the
effects of throttling the intake, therefore, increasing the energy used just
to charge the cylinders. The conclusion was pretty obvious to anyone who
tries to breath through tightly-pursed lips (as we do when a BMW mechanic says
how much the latest trouble will cost) – the throttle should always be as
open as possible. This means that the engine should be running in the highest
gear (lowest rpm) possible, if efficiency is the goal.
This, of course, also matches the philosophy that I share with a few others
and was admonished for in Driver’s Ed – having a heavy foot on the gas.
Alex
79xj6
“Wasson, Tim” wrote:>
All this talk of Exhaust pressures making a difference in performance got me
thinking:
5 psi is not alot of pressure, how can it rob an engine of so much power?
If the pressure in the exhaust system is 5 psi, then that means that when
the exhaust valve closed, the exhaust gases in the cylinder would also be at
5 psi. When the intake valve opens and the piston begins to move downward,
NO air or fuel will be sucked into the cylinder until that 5 psi has been
relieved. WORSE YET, as soon as the intake valve opens, that 5 psi of
pressure pushes OUT the intake valve into the intake manifold, then the
downward stroke of the piston must create enough vacuum to stop and reverse
the flow of the air in the intake manifold.
So, it’s not the power required to push the exhaust through a muffler/cat
etc that consumes the power, it’s the residual pressure in the cylinder
disrupting the intake of clean air and fuel.