ViribusUnits

Anyone got a good explination as to why a little bit of back pressure in the exasute system will equal more low end torque?

Everything I can think of would say that back pressure is bad, always and forever. However, it's commonly accepted that a little bit is good. Which makes no sence to me.

ErnestSw

I think of it as the power you use pushing against a variable spring. The weaker the spring the less power is generated, the stiffer the spring the more power at the bottom end until you reach some equilibrium point where the power needed to push on the spring (move the gasses) starts to represent a net loss at the top end.
Hopefully someone will do a better job than me of explaining it.

PorKen

Backpressure is often a misnomer for gas flow speed. Altering the speed of the gas flow by changing the size of the exhaust pipe or reducing the number/severity of bends will change the breathing characteristics of the engine.

Normy

Backpressure doesn't necessarily equate low end torque; The tuning that gives low end torque has the unfortunate side effect of increasing back pressure.

N-

Thaddeus

Backpressure helps close the exhaust valve a little quicker. Or so I've been told. I've taken that with a grain of salt....

WallyP

In order to achieve max power, you need to move air thru the engine and out the exhaust. One way of doing this is to hold the exhaust valves open longer. The best duration for high-RPM power is too long for max low-RPM power, as the open exhaust valves allow loss of compression at lower speeds as the rising piston pushes fuel/air mix out of the still-open valves.

By increasing the back pressure slightly, you help hold a little more fuel/air mix in the cylinder at lower engine speeds, thus increasing power. Unfortunately, you lose some on top end.

If you want more low-RPM power, it would be much better to change the valve timing - but again, that costs top-end.

Exhaust tuning is a complex art...

2V4V

As has been said, it's not really causality - it's symptomology.

PorKen - New 'Risky Business' avatar huh? Great fun. How 'bout the scene with the tech opening the door full of water and fish next?

Greg

Jim bailey - 928 International

Yea ...what Wally said !! ........ the camshafts are only really efficient for maybe a range of 500 - 1,000 RPM above or below that range they are far less efficient (which is why vario-cam technology exists) . The vario cam changes the relation ship of intake and exhaust events to widen the powerband . Like most things camshaft selection is a compromise low end torque or horsepower but you can not have both (even with vario-cam) for normally aspirated engines .

PorKen

The variable valve opening mechanisms, a la BMW, sound like the way things are going. Instead of a throttle plate, the BMW engine just changes the amount of intake valve opening.
...

2V4V,

It took me 2 hours to downsample that clip to fit in the avatar limit of 20K! (Here's the larger 400K version)

I may try the dock scene or the fish scene sometime.

Jim bailey - 928 International

The new Chrysler Hemi V-8 also has variable timing on a pushrod ( not overhead cam ) engine ...... also has two spark plugs per cylinder