overlap

[302ford]

Been thinking a bit about this lately.
What does overlap do?
What is the function and why is it needed?
Lets discuss why and what is actually happening.

[John Wallace]

Better to talk about the intake closing and exhaust opening events.

A few pieces of cam events will make a huge difference in power output.
They all need to work together.

[wildbill]

id be curious to see why overlap aids in cylinder fill. from my understanding its the lower barometric pressure in the cylinder that fills it, not the vacuum from the downstroke.

[302ford]

Does overlap vary with the cfm demand of the intake runner

[twl]

The generic answer would be that it gives some ramp time to get the intake valve open to a decent amount by TDC, and it allows the low pressure zone following the exhaust stream to help get the intake mixture flow started early, and the exhaust valve stays partly open for enough time for the chamber to be evacuated of exhaust and replaced by fresh mixture prior to closing to allow the rest of the intake stroke to handle the rest of the intake work.
The length of this overlap period is related to the rpm range, because the faster the rpms, the less time is available for this stuff to be accomplished, so longer overlap periods come along with high rpms.

That's the generic short answer.
A lot more details could be included.

[CamKing]

It's simple.
Overlap allows for the intake flow to start, before the piston starts to pull on the intake.
Air want to move to the lowest pressure.
On the exhaust stroke, as the piston slows down toward TDC, the velocity of the exiting exhuast lowers the pressure in the cylinder below the pressure in the intake port. opening the intake valve as soon as the pressure in the cylinder gets below the pressure in the intake port, causes the air to flow from the intake port into the cylinder, even before the piston has reached TDC. This get the air velocity in the port up, until the piston pull can take over.

[Mr.Mater8]

302Ford...

I've had my head swimming in camshaft physics to get the "perfect" cam for my endurance race engine. You may want to buy David Vizard's book on "Building Horsepower"- he is extremely knowledgeable and experienced- it's well worth the cost. He has a comprehensive section on cam science, and I think he is probably one of the most experienced engine design/build and test persons out there, but not without controversy from many others.

Anyway, all modern gas/diesel engines must use some degree of overlap to draw the new charge of air/fuel mixture into the cylinder using the vacuum that immediately follows the escape of the high pressure exhaust gas thru the header tube. Some estimates place the percentage of induced intake volume attributed from the vacuum pulse to be 80% or more, and only 20% (or less) from the piston downward stroke (obviuosly we are talking about an efficient high speed race engine). Actually- this is exactly how pulse jet engines work- they can even sit on the ground with zero ram air pressure at the inlet, and run only on the effect of vacuum pulse exiting the tuned exhaust tube to draw in the new air/fuel charge.

Controversial, but Vizard's methodology on selecting a cam starts with the most effective LSA (lobe separation angle) that a typical valve diameter-to-cylinder volume ratio may be (say for a SBC350, about 22) and relating that to the most effective LSA for max HP, about 108 degrees. His next step is choosing overlap by the engines application- towing, street performance, competition and then outright full race, somewhat modified by the head's ability to efficiently flow. His approach is systematic: the engine's expected rpm range, and, it's ability to fill the cylinder. Under breathing engines need more overlap as also higher rpm engines. Amazingly, when LSA and overlap are known, average intake and exhaust duration can then be uniquely solved as the last variable of the overlap (algebraic) equation. I my case, the the resulting cam duration was correct based on the cam manufacturers description.

Vizard's message is that overlap is pivotal to optimizing the engines HP & torque curves, efficiency and useable speed range. As an example, a typical SBC 350 race engine reaching 6500 rpm needs between 75-85 degrees overlap, assuming a reasonable flowing head and 2.02/1.6 valve sizes. A typical SBC 400 with great (but underflowing) stock Vortec heads with 1.94/1.5 valves needs about 10 dgrees more overlap, about 85-95 degrees to help fill the cylinder for the same 6500 rpm.

As an important side note (that all the experts will tell you), dont even think about using high overlap without considering at least a moderate increase in compression ratio (like 10:1). With longer overlaps, late intake valve closing causes lower compression and poor lower speed power, even though the calculated (static) compression based on swept and head volume haven't changed.

Hope this helps!

[randy331]

. Some estimates place the percentage of induced intake volume attributed from the vacuum pulse to be 80% or more, and only 20% (or less) from the piston downward stroke (obviuosly we are talking about an efficient high speed race engine).

So 80% of trapped VE can happen before the ex valve closes?

Randy

[lorax]

302Ford...

Some estimates place the percentage of induced intake volume attributed from the vacuum pulse to be 80% or more, and only 20% (or less) from the piston downward stroke (obviuosly we are talking about an efficient high speed race engine)

That's not a pressure pulse, that's a SUPERCHARGER!!

[MadBill]

...As an important side note (that all the experts will tell you), dont even think about using high overlap without considering at least a moderate increase in compression ratio (like 10:1). With longer overlaps, late intake valve closing causes lower compression and poor lower speed power, even though the calculated (static) compression based on swept and head volume haven't changed.

Hope this helps!

No reflection on the rest of your post Mr. Mater, but the intake valve closing point, not overlap, determines cranking compression and dynamic compression ratio. In fact, all else (i.e. lobe duration) being equal, more overlap results in earlier IVC and higher cranking compression.

[lorax]

...As an important side note (that all the experts will tell you), dont even think about using high overlap without considering at least a moderate increase in compression ratio (like 10:1). With longer overlaps, late intake valve closing causes lower compression and poor lower speed power, even though the calculated (static) compression based on swept and head volume haven't changed.

Hope this helps!

No reflection on the rest of your post Mr. Mater, but the intake valve closing point, not overlap, determines cranking compression and dynamic compression ratio. In fact, all else (i.e. lobe duration) being equal, more overlap results in earlier IVC and higher cranking compression.

The mistake Mater is making is that overlap is directly tied to duration, which is not necessarily true. That somehow extending the duration automatically means increased overlap. Its a common mistake to attribute longer duration, and longer overlap with killing the low end, when in fact the longer duration's later IVC was the primary cause for the loss of low end, not a later EVC and earlier IVO

[CamKing]

Someone drank way too much KoolAid.

[nhrastocker]

Someone drank way too much KoolAid.

[nitro2]

. Some estimates place the percentage of induced intake volume attributed from the vacuum pulse to be 80% or more, and only 20% (or less) from the piston downward stroke (obviuosly we are talking about an efficient high speed race engine).

So 80% of trapped VE can happen before the ex valve closes?

Randy

Must be an interpretation error. I'd like to see the actual quote on that one.

[lorax]

. Some estimates place the percentage of induced intake volume attributed from the vacuum pulse to be 80% or more, and only 20% (or less) from the piston downward stroke (obviuosly we are talking about an efficient high speed race engine).

So 80% of trapped VE can happen before the ex valve closes?

Randy

Must be an interpretation error. I'd like to see the actual quote on that one.

I have said this before on these boards, as well in conversations with others,....... mis-quotes, misunderstanding, and general mis-information past on by DV loyalists have done more harm to his reputation than anything he ever actually wrote.