valve overlap?

[rusty duster]

Why do most of the performance cams seem to have more valve overlap than stock ones?Overlap meaning that the exhaust valve and the intake valve are open at the same time.

[amxauto-x]

Cause they have more duration
And they're trying for more scavaging

[hedgehog]

Basically, an increase in overlap(not necessarily alot) over a stock cam will increase power. It helps with scavenging as mentioned above and an engine with a pair of not so good heads can be fooled to think the heads are a little better with more overlap area.

[23T]

It scavenges the exhaust gases better giving the cylinder a better charge of fuel/air for the next combustion stroke. There's more to it but that's a quick shot explaination. If you need more let's know

[jelsr]

Here's a FYI post using the STR from the '60's. The exhaust works on sound waves, when the exhaust opens a sound wave starts down the pipe in a positive value and when it hits the end of the collector it starts back as a negative value. This negative wave hits the combustion chamber and starts back as a positive value. In the process it starts the intake coming in even though the piston is not yet at TDC. The intake continues to flow into the combustion chamber after BDC due to the high velocity attained at peak torque (about 300 FPS). So even though the piston is going up the intake continues to flow in (air has mass, therefore momentum) and when it equalizes the intake valve closes and you have over 100% volumetric efficiency. It only works like this at a given RPM but is still better each side of this optimum RPM. If the overlap is just so, and the intake and exhaust are the correct length it works very well.

[23T]

Originally posted by jelsr
Here's a FYI post using the STR from the '60's. The exhaust works on sound waves, when the exhaust opens a sound wave starts down the pipe in a positive value and when it hits the end of the collector it starts back as a negative value. This negative wave hits the combustion chamber and starts back as a positive value. In the process it starts the intake coming in even though the piston is not yet at TDC. The intake continues to flow into the combustion chamber after BDC due to the high velocity attained at peak torque (about 300 FPS). So even though the piston is going up the intake continues to flow in (air has mass, therefore momentum) and when it equalizes the intake valve closes and you have over 100% volumetric efficiency. It only works like this at a given RPM but is still better each side of this optimum RPM. If the overlap is just so, and the intake and exhaust are the correct length it works very well.

And that my friends is why you should be verrrrry careful porting/ enlarging/ gasket matching the exhaust ports

[rusty duster]

jelsr,could you explain how the exhaust works off the sound waves? Is it a sound wave or a pressure wave?

[amxauto-x]

pressure

[6 packin]

Also in mummy terms, the exhaust velocity going out helps pull the air/fuel in giving it that extra pull. Overlap is simply the amount of rotation (deg) the itake and exhasut valves are open at the same time, That lumpty, lump cam is real clyinder pressure bleeding off in the exhaust. This why bottom end torque is sacrificed for top end all out power.

[jelsr]

A sound wave is a pressure wave, ever watch a bass speaker? Have you ever read (in headers by ed) that a good header will eventually self destruct due to the vibration caused by the sound waves it carries? Not talking streeters here, more like WC or Bush cars where they have to run hours on end at high freq's. You see one come apart ocasionally and they do not run old used equipment, headers included.