| Author |
Message |
TOM BUCKLEY (Tom_b)
Junior Member
Username: Tom_b
Post Number: 52
Registered: 6-2003
| | Posted on Wednesday, September 03, 2003 - 4:34 pm: | |
So----is a test pipe better than a cat or worse ? Inquiring minds want to know .
Tom
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Harry (Harry)
New member
Username: Harry
Post Number: 41
Registered: 3-2002
| | Posted on Wednesday, September 03, 2003 - 1:54 pm: | |
It's not only about backpressure. The wanted effect is, like James S. says, a resonance of the fluid flow at the "right" engine speed. Due to the intermittent gas flow through the engine, the gas expands (when it is accelerated during the opening of the inlet valves) and gets compressed, when the flow is "stopped" by the closed inlet valve. The same happens at the exhaust: when the exhaust valve opens, the exhaust gas gets accelerated (compressed) and the pressure in the exhaust pipe of this cylinder rises, when the valve closes, the pressure drops.
What does this mean? If you have a multi-cylinder engine, the pressure changes caused by one cylinder in the inlet/exhaust influence the others. Generally, it is good if there is overpressure in the inlet manifold pipe of one cylinder when the respective inlet valve is open and vaccuum in the exhaust manifold when the exhaust valve is open. Of course, in reality it is a bit more complicated, but in the end it is a supercharging effect.
Cams, manifold length and diameter, valve diameter, number and arrangement of cylinders influence the resonance of the inlet/exhaust gas. Generally, the best "resonance supercharging" is at the engine speed with the peak torque (in a 308 at around 5000 revs).
By the way, this is the reason that the Ferrari V8s have a 180 deg crank instead of the usual 90 deg crank. A 90 deg crank results in an uneven sparking sequence, with less charging of the cylinder and less power (but less vibrations as well).
To answer you question: backpressure is generally not good, what you need is a well-desiged inlet and exhaust manifold to have vacuum and backpressure at the right time at the right spot.
Hope this answers your question. |
James Selevan (Jselevan)
Member
Username: Jselevan
Post Number: 735
Registered: 6-2002
| | Posted on Wednesday, September 03, 2003 - 11:01 am: | |
While not an internal combustion engine engineer, my reading and discussion over the past 50 years or so suggests that exhaust manifolds are designed to create a "standing" wave of compression and rarifaction that, when tuned to a particular RPM, aids cylinder intake and exhaust dynamics. The theory is that an area of low pressure will be present at the exhaust port of each cylinder in proper sequence and timing. Exhaust manifold pipes are "tuned" in a manner not unlike a pipe organ, where the length and frequency are related. Inherent in the physics of this design is outflow impedance (pressure). Opening exhaust to the atmosphere through a short pipe MAY not provide ideal flow conditions.
Input from an engine engineer would be interesting.
Jim S. |
Rob Schermerhorn (Rexrcr)
Member
Username: Rexrcr
Post Number: 788
Registered: 11-2002
| | Posted on Tuesday, September 02, 2003 - 9:10 pm: | |
This is up a day and not one response?
There certainly are benefits to exhaust system backpressure, but in general, less is better. None is not necessarily the best for normally aspirated engines, but little or none is the goal.
In reality, for a street-driven car, backpressure is the norm, even for Ferrari. Mainly due to cost of manufacture, packaging, emissions and sound requirements. |
John M.Pisacane (Chazferrari)
New member
Username: Chazferrari
Post Number: 25
Registered: 8-2003
| | Posted on Monday, September 01, 2003 - 6:38 pm: | |
Jp |
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