smaller exaust pipe for mpg

[stonebreaker]

Originally Posted by Frank Lee

One thing that has stopped me from pursuing this further is not wanting to have the system "neck down" as IMHO that would be bad. Does that mean the ports in the heads are too big? Exhaust valves too? Probably.

The bigger the exhaust valves, the faster the cylinder can blow down, decreasing pumping losses. In other words, the faster the exhaust blows out of the cylinder, the less work the engine has to do pumping it out of the cylinder into the exhaust system.

In a performance situation, you worry about filling the cylinder as much as you do about emptying it, so you make both valves as big as possible, with the exhaust valve ideally flowing about 75% of the intake flow.

In a hypermiling situation, the intake valve is always going to be plenty big. So if I were building an engine from scratch to increase FE, my focus would be on making the exhaust flow as much as possible at low lift numbers. That means as big an exhaust valve as I can fit in the head, with regard to other parameters such as shrouding.

[stonebreaker]

Originally Posted by Christ

And as it expands, it cools, and as it cools, it slows, and as it slows, it creates a stacking effect, increasing pumping losses in the engine.

Go to the hardware store and blow through a 5 foot section of 1/4 inch copper tube and a 5 foot section of 1/2 inch copper tube and let me know which one is harder to blow through. By your reasoning it should be the 1/2 inch tube.

[Christ]

Originally Posted by stonebreaker

Go to the hardware store and blow through a 5 foot section of 1/4 inch copper tube and a 5 foot section of 1/2 inch copper tube and let me know which one is harder to blow through. By your reasoning it should be the 1/2 inch tube.

Blowing doesn't involve pulse harmonics and scavenging, nor does it involve a gas which is cooling and expanding as it flows. Bad comparison, to say the least.

[stonebreaker]

Originally Posted by Christ

Blowing doesn't involve pulse harmonics and scavenging, nor does it involve a gas which is cooling and expanding as it flows. Bad comparison, to say the least.

There is no pulse scavenging via harmonics or anything else at hypermiling speeds. See for yourself.

Thus it's a perfect example.

[Christ]

You're trying to tell me that at 2,000 RPM there is no pulses in the exhaust? That's funny, I seem to remember watching smoke puffing from the exhaust at idle. Maybe I was seeing things?

There is no pulse scavenging or harmonic tuning because it's not done from the OE's. That doesn't mean it can't be done.

BTW - That calculator suggests that at 2,000 RPM for a 1.0 with a 12* EVO, the ideal pipe diameter is 0.00". I'm pretty sure it wasn't designed for the type of information we need, here.

[stonebreaker]

Originally Posted by Christ

You're trying to tell me that at 2,000 RPM there is no pulses in the exhaust? That's funny, I seem to remember watching smoke puffing from the exhaust at idle. Maybe I was seeing things?

No, it means that at 2,000 rpm a set of headers needs to be twice as long as the car to create pulse scavenging'

Originally Posted by Christ

There is no pulse scavenging or harmonic tuning because it's not done from the OE's. That doesn't mean it can't be done.

Yeah, actually, it does, when the header has to be 18 feet long to work at 2000 rpm.

Originally Posted by Christ

BTW - That calculator suggests that at 2,000 RPM for a 1.0 with a 12* EVO, the ideal pipe diameter is 0.00". I'm pretty sure it wasn't designed for the type of information we need, here.

You told it your engine was 1 cubic inch in displacement.

[Christ]

Originally Posted by stonebreaker

No, it means that at 2,000 rpm a set of headers needs to be twice as long as the car to create pulse scavenging'

Yeah, actually, it does, when the header has to be 18 feet long to work at 2000 rpm.


You told it your engine was 1 cubic inch in displacement.

Missed that part. Thanks!

The pipe doesn't have to be that long for harmonics to take effect, and would only need to be that long for ideal pulse scavenging, and then that would also depend on the amount of flow and the speed/density of that flow.

[stonebreaker]

Originally Posted by Christ

Missed that part. Thanks!

The pipe doesn't have to be that long for harmonics to take effect, and would only need to be that long for ideal pulse scavenging, and then that would also depend on the amount of flow and the speed/density of that flow.

It means that if there is a muffler within that length, 18 feet in this case, it will damp any pulses. That's what mufflers do.

[Christ]

Once again, the pipe doesn't need to be 18 feet to benefit from scavenging. That's the "ideal" length for a given engine size at WOT flow, assuming (I believe) 100% VE.

In reality, that wouldn't be happening most of the time, and considerations could be made to take advantage of pulse scavenging, although not in the most ideal sense.

Harmonics are a different beast altogether, and can be taken advantage of on increasingly smaller amounts after primary harmonics, but can still be usable by length/cross section tuning.

[Christ]

The myth of exhaust backpressure [Archive] - My Pro Street

This pretty much has it.