The Question for the night

[speedy400]

Since 2-strokes can have powervalves, why can't 4-strokes have a powervalve or electonical(or mechanical) device in the exhaust that restricts the exhaust more at low r's and less at high r's? Seems like a stupid question but it's boggled my mind for the past couple years. thanks-darren

[dawnchris]

I'm no expert, but I believe that many cars use such a thing- Variable valve timing. Less lift and duration at low RPM, and more at higher RPM. Probably not cost efficient on an off road bike.

[wilkin250r]

This thread just SCREAMS my name. :rolleyes:

To understand this, you need to know and understand the difference between 2-stroke and 4-stroke engine operation. If you haven't already, check out the tech section Two-Stroke Motor Explanation (http://www.exriders.com/techarticles/twostroke.html)

When the piston is near the bottom of the stroke, both the intake and exhaust ports are open at the same time. The incoming fuel/air mixture is pressurized and comes rushing into the cylinder through the intake port, and much of it can escape out the exhaust port. If you are losing all your fuel/air, you are losing power with it.

At higher RPMs, there is a pressure wave coming from the exhaust pipe that stuffs all this fresh fuel/air back inside the cylinder, helping to build cylinder pressure and increasing power output.

At low RPMs, this pressure wave is not there at the right time, and thus you lose all your power out the exhaust port.

This is where the powervalve comes in. By lowing the top of the exhaust port, it closes sooner. This not only limits the amount of fuel/air that escapes, but it helps to build compression. In a 2-stroke motor, you don't start building compression until the exhaust port is closed (the intake port always closes first, so it's already closed). The lower the exhaust port, the longer the engine can build compression, and higher compression equals more power.

A lower exhaust port isn't always desirable, because a very low exhaust port gives terrible top-end performance. You need to raise the exhaust port to allow the engine to breath at high RPMs.

In a 4-stroke, the compression isn't based on the exhaust port. The exhaust valves close, so you start building compression from the very bottom of the compression stroke. Not only that, you don't lose any fuel/air out the exhaust port, because the intake and exhaust isn't open at the same time.

[dawnchris]

Yea, what he said.:D

[toby400ex]

:huh

[AtvMxRider]

Originally posted by wilkin250r
This thread just SCREAMS my name. :rolleyes:

To understand this, you need to know and understand the difference between 2-stroke and 4-stroke engine operation. If you haven't already, check out the tech section Two-Stroke Motor Explanation (http://www.exriders.com/techarticles/twostroke.html)

When the piston is near the bottom of the stroke, both the intake and exhaust ports are open at the same time. The incoming fuel/air mixture is pressurized and comes rushing into the cylinder through the intake port, and much of it can escape out the exhaust port. If you are losing all your fuel/air, you are losing power with it.

At higher RPMs, there is a pressure wave coming from the exhaust pipe that stuffs all this fresh fuel/air back inside the cylinder, helping to build cylinder pressure and increasing power output.

At low RPMs, this pressure wave is not there at the right time, and thus you lose all your power out the exhaust port.

This is where the powervalve comes in. By lowing the top of the exhaust port, it closes sooner. This not only limits the amount of fuel/air that escapes, but it helps to build compression. In a 2-stroke motor, you don't start building compression until the exhaust port is closed (the intake port always closes first, so it's already closed). The lower the exhaust port, the longer the engine can build compression, and higher compression equals more power.

A lower exhaust port isn't always desirable, because a very low exhaust port gives terrible top-end performance. You need to raise the exhaust port to allow the engine to breath at high RPMs.

In a 4-stroke, the compression isn't based on the exhaust port. The exhaust valves close, so you start building compression from the very bottom of the compression stroke. Not only that, you don't lose any fuel/air out the exhaust port, because the intake and exhaust isn't open at the same time.


Couldn't have said it better myself;)

[cheatin mother]

You do have one! Its called a camshaft!:eek2:

[speedy400]

Alright, THANK you for explaining that! One more thing, so why do 4-stroke exhausts with more backpressure have more low and ones with less backpressure has more top end. And if this is all true, why dont they make some kind of mechanical backpressure depending device to have the correct backpressure?thanks again-darren

[UglyMotha™]

Originally posted by AtvMxRider
Couldn't have said it better myself;)




jerry that's probably cause you couldn't have said it at all :o :p

[wilkin250r]

Originally posted by speedy400
Alright, THANK you for explaining that! One more thing, so why do 4-stroke exhausts with more backpressure have more low and ones with less backpressure has more top end. And if this is all true, why dont they make some kind of mechanical backpressure depending device to have the correct backpressure?thanks again-darren

Quite simple, it's not cost-effective.

Very high-strung top-end 2-stroke motors are difficult to ride, because the powerband is so sharp. It's like a light-switch and it wants to rip your arms off. A powervalve will smooth out the powerband and give more low-end power, so you can port for more top-end power without it being difficult to ride. With the powervalve taking care of the low-end, you can get port for 5-10 more HP on the top end without that "light switch" feel. The powervalve will actually allow you to get more peak horsepower, so people are willing to pay lots of money for them.

A 4-stroke motor already has lots of low-end, and the difference between a high-backpressure motor and a low-backpressure motor is only about 1-2HP, and it's all on the low-end. Most racers don't care much about the low-end on a 4-stroke, for two reasons. It already has lots of low-end power to begin with, and they are usually in the upper rpms anyways.