Valve Overlap???

[nudge1201]

What is valve overlap?? If someone could explain this to me and make it fairly easy to understand, Because if goten a few vering answers from people i work with that don't make to much sence to me. I would think it is were both the intake and exhaust valve are open at the same time during there cycle. and why is it expained in degrees??(in the celica something in between 4 to 94 degrees or something like that. and how is is it benifficail or not??? any explaination would be helpful thank.

[Acilec]

I'm not to sure about this, but I would also ask this in the performance section of the website. Let me know if you want me to move the thread there.

Fin

[Acilec]

Here's what I found on it laymen terms:

Exhaust flow characteristics are important as well. Every time the exhaust valve opens, a pressure wave is released. Again, at 5000 rpm, this occurs about 42 times per second. At this speed, more than one wave will be present in the exhaust stack at any given time. Towards the end of the stack, the wave experiences a reverse in direction and tries to travel back towards the exhaust port. This is partially due to valve overlap. At some point in most engines, the intake and exhaust valves will both be off their seats at the same time. One is closing and the other opening. Low pressure created by piston travel from top dead center to bottom dead center will attempt to draw the pressure wave back towards the combustion chamber. The length of the stack will to some degree determine what affect this will have on engine performance. If not properly timed, waves collide at the wrong time and restrict the flow out of the stack which in turn effects the flow into the combustion chamber from the intake port. If the exhaust can't get out of the way in time, the intake charge has to battle to get into the combustion chamber. If you can't fill the combustion chamber with fuel and air, you're not at your full power potential. Next time you're at the library, look up Volumetric Efficiency in a good automotive racing book. Changes in the exhaust stack length can tune an engine to help produce the maximum power at the rpm range that it will do the most good.

[Acilec]

Here's the best one so far:

Another spec you need to look at when selecting a cam is the relative timing of the intake and exhaust valves. This can be expressed either as "valve overlap" (the time during which both the intake and exhaust valves are both open) or "lobe separation" (the number of degrees or angle between the centerlines of the intake and exhaust lobes). Decreasing the lobe separation increases overlap, while increasing the separation decreases overlap.

Most stock replacement cams with durations of less than 200 degrees will have lobe separations of 112 to 114 degrees. Higher duration cams for mid-range performance typically have 110 to 112 degrees of lobe separation. With racing cams, you

[brolly33]

http://www.speeddemonmotorsports.com/Tech/articles/camtiming.html

and the graph I was actually looking for:

http://ctfba.tripod.com/main/technical/cams/cambasics/GraphAttack.htm

With VVTi, you can actually move the whold curve to maximise the power at any given RMP for a given cam profile.
<<< WRONG: see following post, only intake is variable<<<

With VVTi-L and the second cam, you have a second cam profile to add to the mix, allowing you the huge 94 degree overlap, taking the greatest advantage of the Ehaust gas scavenging effect when you really want it with no sacrifice of low end torque.

.

Really quite brilliant.

[nudge1201]

thank for the informative and understandable explanations they are very helpful. Its a good thing I was a "little" sceptical of some of my friends explinations.

brolly33 is there any overlap on the low RPM cam??? just curious.

[brolly33]

Well, I looked for about an hour on the web and was not able to find cam specs for the GT-s anywhere. Perhaps one of the members that have access to more detailed specs could post them?

I seem to remember that 4 - 90 degrees overlap mentioned in some article I was reading on this engine but I could not find a link to it.

Oh wait, I found it!

http://www.sae.org/automag/globalview_01-00/08.htm

now... the critical info for this question is here:

Low speed cam:
Intake valve starts opening at: 10 after to 33 before top dead center (depending on the variable timing)
Exhaust closes at: 14 after Top Dead Center (TDC) (not variable???)

As the piston travels up toward TDC, expelling the exhaust gasses, the intake valve can open as early as 33 before TDC and the exhaust at 14 after for a total of 47 degrees of crank travel in overlap. That would be where the computer would set the cam toward the top end of the power band. At idle I imagine that the computer would reduce that intake spec towards the other setting of 10 after TDC to make it idle really smooth with a very small overlap of 4 degrees.

(Interesting that they give the intake timing specs as variable and the exhaust as static. That seems to mean that only the intake is variable timing AND that the lobe separation is variable as well. You essentially can change only the intake half of the overall cam timing with this system. This actually makes VVT-i more dynamic and useful for getting maximum performance out of this engine. Can anyone who really knows verify this please?)

The aggressive cam:
You experience the most extreme intake setting of 58 before TDC and the exhaust of 36 after TDC for a total overlap of a whopping 94 degrees of crank travel that both the exhaust and intake valves are open at the same time.

The computer probably adjusts intake timing this by having a "suggested" value for a given RPM and adjusting off of that number by sniffing the exhaust gasses for too much unburned mixture.

The other joyous part of the aggressive cam is that the lobes are higher, making the valves open further (as well as leaving them open longer) SO the engine can pump more air, more efficiently and put out more total power. (engines are after all, only big air pumps)

A good bit of this post is interpretation, as I do not have easy access to the engineering papers from the engine designers. If anyone has different theories, I would love to hear them.