overlap

[GARY C]

Here is the original, looks to be a 4 valve head, it is an intake stroke, pressure, fire and exhaust, not sure why they didn't show overlap...it could be totally bogus.
http://www.youtube.com/watch?v=ieuT2GNscbE

[lorax]

Here is the original, looks to be a 4 valve head, it is an intake stroke, pressure, fire and exhaust, not sure why they didn't show overlap...it could be totally bogus.
http://www.youtube.com/watch?v=ieuT2GNscbE

Probably because its not mentioned in the original Otto patent. Its never shown 4 cycle event charts. Its not a REAL timing event.

I think its all just BS and the cam grinders just stick it in there to give the engine a cool sound and make their work seem more confusing and mysterious.

[Kevin Johnson]

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.

http://www.youtube.com/watch?v=cAlw9RpTZ4I from Comp Cams

[CamKing]

many of you are making this harder then it is.
Air flows from high pressure to low pressure.
When you open a valve, the air won't flow unless the pressure on one side of the valve is less then the pressure on the other side.
Overlap allows the exhaust pulse to lower the pressure below the intake valve(combustion chamber), so when the intake valve opens, it will start flowing.

The only tricky part is figuring out how long the exhaust needs to lower the pressure in the combustion chamber, before the piston takes over, and what the exhaust curve needs to be to accomplish it.

[BigBro74]

Mike has stated this perfectly- I think he has just made the tricky part (his job in most cases) seem less tricky by the short answer
I feel like it gets trickier when the intake becomes more restricted, mostly because there are not many sources for free info for poor shmucks like me....
probably easily quantifiable by someone's experience in this area (like his). Jason

[lorax]

Mike has stated this perfectly- I think he has just made the tricky part (his job in most cases) seem less tricky by the short answer
I feel like it gets trickier when the intake becomes more restricted, mostly because there are not many sources for free info for poor shmucks like me....
probably easily quantifiable by someone's experience in this area (like his). Jason

I think it an get can get pretty tricky when the exhaust becomes restricted as well, like log type manifolds and single muffler.

[BigBro74]

Mike has stated this perfectly- I think he has just made the tricky part (his job in most cases) seem less tricky by the short answer
I feel like it gets trickier when the intake becomes more restricted, mostly because there are not many sources for free info for poor shmucks like me....
probably easily quantifiable by someone's experience in this area (like his). Jason

I think it an get can get pretty tricky when the exhaust becomes restricted as well, like log type manifolds and single muffler.

Very True!

[CamKing]

I think it an get can get pretty tricky when the exhaust becomes restricted as well, like log type manifolds and single muffler.

Yes, that's a lot trickier then a restricted inlet.

With a restricted inlet, you have less pressure in the intake, but because you're pulling less mass into the engine, you have less pressure in the combustion chamber, and exhaust port too.

With a restricted exhaust, if you can't drop the pressure on the exhaust port below the pressure in the chamber(as the piston moves toward TDC), you're skrewed.

[nitro2]

Hmmm...... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

[lorax]

Hmmm... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

Do you consider a dirt track car with a 2 bbl on a iron dual plane a "restricted" engine? They are as common as the dirt they race on. Are you suggesting there is some pressure wave in the intake runner when the intake opens?

[CamKing]

Hmmm...... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

I never think about atmospheric pressure.
I llok at all the pressures as absolute pressures, and the differences between them.
Air is constantly trying to find it's own level(pressure).

[GARY C]

Hmmm...... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

What do your test show the intake air doing throughout the lift range?

You would probably be one of the best sources for this type of discussion as you have the equipment to measure a running engine.

[nitro2]

Hmmm... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

Do you consider a dirt track car with a 2 bbl on a iron dual plane a "restricted" engine? They are as common as the dirt they race on. Are you suggesting there is some pressure wave in the intake runner when the intake opens?

I'm only forgetting about restricted engines because they are a further twist on the topic, and the topic in its simplest form (WOT unrestricted) isn't even sorted out yet.

Every simulation software program with pressure traces, and every port pressure measuring equipment such as we sell, will show pressure waves (big ones) in the intake. The better the engine the bigger the waves, with magnitudes up to +/-6 psi. When an intake wave of -6 psi is timed with overlap there won't be much airflow into the cylinder at overlap, if any.

[nitro2]

Hmmm...... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

Air is constantly trying to find it's own level(pressure).

That is true but the pressure generated by the intake changes drastically from one rpm to the next. Over the range of +6 psi to -6psi for a well built engine and roughly +3 to -3 psi for a modest engine. Air will go from high pressure to low pressure but unless the pressures are known the direction and intensity of flow is unknown.

[CamKing]

Hmmm...... Just to be sure we're all on the same page here, (and forgetting about restricted engines and all that bad stuff), we do all realize that the pressure in the intake doesn't just sit there happily at near atmospheric pressure waiting patiently for the intake valve to open and the exhaust suction to pull it in, right ?

Air is constantly trying to find it's own level(pressure).

That is true but the pressure generated by the intake changes drastically from one rpm to the next. Over the range of +6 psi to -6psi for a well built engine and roughly +3 to -3 psi for a modest engine. Air will go from high pressure to low pressure but unless the pressures are known the direction and intensity of flow is unknown.

The pressure below the valve changes with RPM also. As long as it's below the pressure above the valve, you'll have flow.
I've never seen the pressure wave lower the pressure above the intake valve during overlap.
I've seen it do that as the intake valve begins to close, but not when it opens.

Of course what is the correct overlap at 8,000rpm, isn't correct at 4,000rpm.
That's why in most cases I design the valve events to be most efficient at max HP RPM, because that's where the advantages of the exhuast pulling on the intake are going to have the biggest benifit.