backpressure

[KiwiBacon]

Originally Posted by UncleBob

This goes back to the basics of an engine. An engine is an air pump. The more efficient the air pump, the more power its capable of. Back pressure reduces air flow, therefore its bad.

Not only does backpressure reduce airlflow, it directly sucks up an engines power in pumping loss.
Pumping loss = pressure * flowrate.

[curtis73]

True, but lets not confuse power (measured in HP) with the original spirit of the question which concerns torque.

Its quite true that more flow equals more hp, but that is not necessarily true with torque. As a general guideline, exhaust velocities of about .35 mach are where things start getting restrictive. So (again, very general ideas here) if a person were to choose an exhaust that sustatained just under .35 mach at WOT at redline, then it would probably be the best torque production you could expect without a significant loss of HP. If you're going for all out HP, go bigger. If you want a quieter more torque-producing exhaust, choose something a little smaller that coincides with the torque peak.

[KiwiBacon]

Originally Posted by curtis73

True, but lets not confuse power (measured in HP) with the original spirit of the question which concerns torque.

Its quite true that more flow equals more hp, but that is not necessarily true with torque. As a general guideline, exhaust velocities of about .35 mach are where things start getting restrictive. So (again, very general ideas here) if a person were to choose an exhaust that sustatained just under .35 mach at WOT at redline, then it would probably be the best torque production you could expect without a significant loss of HP. If you're going for all out HP, go bigger. If you want a quieter more torque-producing exhaust, choose something a little smaller that coincides with the torque peak.

Torque and power are different ways to measure the same thing (engine output).
Anything which requires power (like pumping against exhaust backpressure) also requires torque.

So where the pumping loss is: Power = pressure * flowrate.
The torque loss is: Torque = (pressure*flowrate)/rotating speed.

Where the crossover lies between torque lost (to creating high speed exhaust pulses) and torque gained (through scavenging) is obviously the subject of much R&D engineering.

[UncleBob]

its a definition thing.

When most people say "that engine is torquie" what they really mean is, that engine produces good HP at low RPM's.

Low back pressure does not necessitate a "non-torquie" engine. Quite the contrary. A low back pressure system with properly sized exhaust/ports will greatly enhance the "torquie"ness of an engine....it very well could cause suffering in higher RPM's of course

[cody_e]

Somebody told me if you didn't have any backpressure or little backpressure you'd burn the valves.

[UncleBob]

that is only true if you have extremely short exhaust. As in, exhaust that is a total length of 6 inches.

Back pressure isn't what holds the heat, its the metal that the exhaust is made of. If its of sufficient length (and it doesn't have to be very long, really) then it will prevent the exhaust valves from cooling off too quickly.

[curtis73]

Originally Posted by KiwiBacon

Torque and power are different ways to measure the same thing

Well... yes and no. Mostly the "no" part.

Engines produce torque. HP is a derivative of that torque. HP=TQxRPM/5250, so HP is completely dependent on torque production.

Consider this formula....

MKA=AGE x SEX

Where, MKA = Mary Kate and Ashley
AGE = 18
and SEX = when we can bang them legally.

MKA is producing torque, but they aren't making HP until age 18, so we can't make sex with them until a certain RPM... wait...... no, I mean sex with torque...

crap, I messed it up again.

[curtis73]

Originally Posted by cody_e

Somebody told me if you didn't have any backpressure or little backpressure you'd burn the valves.

Partially an old wive's tale. If you run an engine with very little exhaust (like open headers or no manifolds at all) you run the risk of cracking valves. After running an engine and getting the valves screaming hot, the lack of exhaust means ambient air is exposed to the valves. Screaming hot valves plus ambient air can mean cracked valves.

[psychopathicdude]

Originally Posted by curtis73

Mostly the "no" part....

Nice use of (MK+A)(O). It's going into my Calculus notes.

[cody_e]

Oh man this is funnny: http://www.noiseoff.org/exhaust.shtml

[2.2 Straight six]

Originally Posted by UncleBob

that is only true if you have extremely short exhaust. As in, exhaust that is a total length of 6 inches.

Back pressure isn't what holds the heat, its the metal that the exhaust is made of. If its of sufficient length (and it doesn't have to be very long, really) then it will prevent the exhaust valves from cooling off too quickly.

since bikes are you area, maybe you can help answer this.

i read that bikers that fit open race pipes to their bikes often find that they desroy valves due to such low back pressure, so the exhaust isn't any shorter, but it is far more free-flowing. so what is it about the more free-flowing exhaust that destroys the valves?

[Steel]

Originally Posted by KiwiBacon

Torque and power are different ways to measure the same thing (engine output).
Anything which requires power (like pumping against exhaust backpressure) also requires torque.

Ah...no. Torque is a measure of work, Power is a measure of work over time. Engines can produce massive torque while not making much power, and vice versa, they can produce massive power without much torque. Long stroke diesel vs formula 1 engine as examples.

Remember though, that power is just a mathematical toy, derived from torque. Torque is the only thing an engine actually produces.

[UncleBob]

Originally Posted by 2.2 Straight six

since bikes are you area, maybe you can help answer this.

i read that bikers that fit open race pipes to their bikes often find that they desroy valves due to such low back pressure, so the exhaust isn't any shorter, but it is far more free-flowing. so what is it about the more free-flowing exhaust that destroys the valves?

I've yet to see such a situation, so I say BS

[UncleBob]

Originally Posted by Steel

Ah...no. Torque is a measure of work, Power is a measure of work over time. Engines can produce massive torque while not making much power, and vice versa, they can produce massive power without much torque. Long stroke diesel vs formula 1 engine as examples.

Remember though, that power is just a mathematical toy, derived from torque. Torque is the only thing an engine actually produces.

torque is a really handy measurement in a plane of existance without time.

Since we don't exist in such a plane.....

[Moppie]

Originally Posted by UncleBob

Since we don't exist in such a plane.....

Is that a royal "we"?
Because I have blue telephone box in the garage that can get around a few time related issues.