Can too much intake lobe area decrease performance?

[nitro2]

Sometimes things look clearer by taking an extreme case and working backwards. Imagine you had a normal size NA V-8 and for some crazy reason you needed it to have say 120+ VE at a mere 1500 rpm. Ignore combustion/mechanical/durability issues.

I know how I'd do it and it would not use small valves, nor low lift, nor small rocker ratios. Cam would be a bit unique though.

[MrBo]

Some F1 engines a few years back had very short duration cams (in my opinion), but they still had to have the idle speed up around 5000 rpm.
Huge valve area opening like a light switch would be my guess as to why they had to idle so fast.

[Enigma]

The original question posed didn't indicate budget, intended usage, or rpm. It didn't relate to who has done or is doing what. It simply asked if there's a point at which lifting a valve too quickly can hurt performance. Again I say.......yes. There is a point at which lifting the valve too quickly can hurt performance. Roberts101 has answered much more eloquently than myself, but the valve is what continues to create the needed pressure differential up to the point that inertia takes over. That point is generally a little shy of max piston velocity. 900, you keep mentioning timing.......what timing are you referring to? Valve timing. The lift curve. Yet.......you contend that a valve is all but unnecessary. The valve timing/lift curve is necessary to develop enough momentum, early on. Then, a little before max piston velocity, you speed the valve up to get it out of the way. But the early lift needs to be exact for any given application. Otherwise all the custom cam grinders would only need one question on their applications: "do you have a valve, check yes or no". Every design needs a specific lift curve to create max cylinder pressure at x rpm. Again......it's simple physics and fluid mechanics......make the valve disappear instantly at TDC, and your engine will be a slug. You'll be depending on the piston itself to initiate port activity.......not a good situation.

[900HP]

Are they opening valves before TDC these days? That's pretty high tech.

Look at the R&D research capital available in NASCAR. With that kind of budget.....they could easily lift the valves much quicker than they do. So, based on your premise that it's just some plug in the way fouling things up, why don't they? Because the rate that they lift them is perfect for an engine that spends most of it's life above 8000-9000 rpm. Try that rate of lift on the average street/strip engine that spends most of it's life below 6500 rpm and see what happens. Scavenging initiates port flow, but once that exhaust valve close......lift the intake too quickly and the differential will diminish and port activity will stall. It's simple physics.

We lift them as fast as current spring technology will allow. As 900 said, unless you do it at the wrong time, it is pretty difficult to open them too fast.

This is the wall we ran into with our 6500 rpm and below EMC engine as well. We are not able to open the intake valve too fast. It's not physically possible at this time.

[900HP]

[quote="Enigma"]The original question posed didn't indicate budget, intended usage, or rpm. It didn't relate to who has done or is doing what. It simply asked if there's a point at which lifting a valve too quickly can hurt performance. quote]

Here is the original question:

"Assuming parts are selected to adequately control the valvetrain, is there a point where the intake valve can be opened and closed too quickly for a given duration? Take for example a low hp/ci 350 sbc truck engine with a desired peak rpm around 5,000. Comp offers the following lobes:

Lobe Family 0.006" 0.050" 0.200" Lobe Lift
HE 262° 206° 116° 0.300"
XE 258° 206° 128° 0.320"
XFI 258° 208° 134° 0.348"

Would the XE increase output over the HE and the XFI over the XE? Would part throttle response and efficiency be adversely impacted? Is there any downside to the most aggressive XFI lobe in this application?"

None of those cams even come close to opening the valve "too fast". They are super-mild street cams especially with standard rocker ratios. Now, if you put a 2.2:1 rocker arm on them you may have some control issues but the "too fast" wouldn't be because of power it would be due to loss of control.

[CamKing]

Yes, you can open the valve too fast.
If you normalize the pressure between the cylinder and port after the piston reaches BDC and before the valve closes, you're not taking advantage ofthe duration beyond BDC, and you'll lose mass in the cylinder.

[groberts101]

Yes, you can open the valve too fast.
If you normalize the pressure between the cylinder and port after the piston reaches BDC and before the valve closes, you're not taking advantage ofthe duration beyond BDC, and you'll lose mass in the cylinder.

Thank you!

[900HP]

Yes, you can open the valve too fast.
If you normalize the pressure between the cylinder and port after the piston reaches BDC and before the valve closes, you're not taking advantage ofthe duration beyond BDC, and you'll lose mass in the cylinder.

Please explain to me what I'm missing. If you are experiencing the above scenario isn't it because you opened the valve too soon, not to fast?

[groberts101]

Yes, you can open the valve too fast.
If you normalize the pressure between the cylinder and port after the piston reaches BDC and before the valve closes, you're not taking advantage ofthe duration beyond BDC, and you'll lose mass in the cylinder.

Please explain to me what I'm missing. If you are experiencing the above scenario isn't it because you opened the valve too soon, not to fast?

You lose the major advantage of inertial filling that would have been available had you correctly controlled the rate of pressure drop.

As you well know.. it's all about the combo of parts and size does matter.

[Enigma]

Yes, you can open the valve too fast.
If you normalize the pressure between the cylinder and port after the piston reaches BDC and before the valve closes, you're not taking advantage ofthe duration beyond BDC, and you'll lose mass in the cylinder.

Thank you!

X2. Thanks Mike. Harold Brookshire calls it "overshooting the port". David Vizards terminology for it is "what you lose in the first half of the induction stroke, you cannot make up for in the second half". Simply put: the rate of lift, especially in the early stages, is what creates the pressure differential needed to instigate port activity. It must be slow enough to maintain the differential until momentum takes over. Then you get it out of the way.

[Roadknee]

Simply put: the rate of lift, especially in the early stages, is what creates the pressure differential needed to instigate port activity. It must be slow enough to maintain the differential until momentum takes over. Then you get it out of the way.

I've spent a fair amount of time reading these informative posts. I just can not understand how holding a valve partially closed creates more port flow in the first half of the induction cycle than a fully open valve or no valve altogether. In a typical SBC the intake throat area is about 22% of the bore area. As the piston moves down the bore it is effectively sucking through a straw even in the hypothetical scenario where the valve is removed. The pressure in the cylinder is always going to be less than the pressure in the port until BDC, and air will flow from the port into the cylinder.

Let's look at an extreme unrealistic example for illustration purposes. Assume the entire top of the cylinder is an intake valve. That is, the intake valve area and cylinder area are equal. Further assume we can open and close this intake valve instantaneously. If we instantaneously open the intake valve at TDC and close it at BDC flow into the cylinder would be unimpeded and the pressure in the cylinder would be equal to or very near atmospheric pressure. Now, take the same scenario, but reduce the area of the hypothetical intake valve to something significantly less than the cylinder area. Now instantaneously open the valve at TDC and close it at BDC. The air pressure inside the cylinder would be something less than atmospheric because of the pressure loss across the smaller restrictive intake valve. Now, apply this principle to a running engine. If we only look at cylinder filling between TDC and BDC, the camshaft that opens the valve to maximum port flow the fastest without striking the piston will provide the greatest cylinder filling.

Unfortunately, it's not this simple. We also have to consider cylinder filling between BDC and IVC. Both Mike and Chad point out that the intake port pressure must be higher than the cylinder pressure until IVC. Otherwise, the piston will push air back into the port and torque will be reduced. Somewhere around the ICL maximum airflow and velocity through the port is achieved. Between ICL and IVC the valve continues to close which restricts port flow. As the velocity through the port is reduced, the velocity energy is converted to pressure energy. Another way to think of this is the intake port flow stacks up against the closing valve and the port pressure increases. This is a good thing because during this same time the piston comes to a stop at BDC and continues up the cylinder which increases the pressure in the cylinder. Ideally, the pressure rise in the port exceeds the pressure rise in the cylinder right up until the valve closes, achieving maximum VE and torque.

I could see where the cam lobe that provides the greatest filling during the first half of the induction cycle might not produce the highest VE if this same cam causes port pressure and cylinder pressure to equalize prior to IVC resulting in reversion and reduced VE.

Thoughts?

[Enigma]

Simply put: the rate of lift, especially in the early stages, is what creates the pressure differential needed to instigate port activity. It must be slow enough to maintain the differential until momentum takes over. Then you get it out of the way.

I've spent a fair amount of time reading these informative posts. I just can not understand how holding a valve partially closed creates more port flow in the first half of the induction cycle than a fully open valve or no valve altogether. In a typical SBC the intake throat area is about 22% of the bore area. As the piston moves down the bore it is effectively sucking through a straw even in the hypothetical scenario where the valve is removed. The pressure in the cylinder is always going to be less than the pressure in the port until BDC, and air will flow from the port into the cylinder.

Let's look at an extreme unrealistic example for illustration purposes. Assume the entire top of the cylinder is an intake valve. That is, the intake valve area and cylinder area are equal. Further assume we can open and close this intake valve instantaneously. If we instantaneously open the intake valve at TDC and close it at BDC flow into the cylinder would be unimpeded and the pressure in the cylinder would be equal to or very near atmospheric pressure. Now, take the same scenario, but reduce the area of the hypothetical intake valve to something significantly less than the cylinder area. Now instantaneously open the valve at TDC and close it at BDC. The air pressure inside the cylinder would be something less than atmospheric because of the pressure loss across the smaller restrictive intake valve. Now, apply this principle to a running engine. If we only look at cylinder filling between TDC and BDC, the camshaft that opens the valve to maximum port flow the fastest without striking the piston will provide the greatest cylinder filling.

Unfortunately, it's not this simple. We also have to consider cylinder filling between BDC and IVC. Both Mike and Chad point out that the intake port pressure must be higher than the cylinder pressure until IVC. Otherwise, the piston will push air back into the port and torque will be reduced. Somewhere around the ICL maximum airflow and velocity through the port is achieved. Between ICL and IVC the valve continues to close which restricts port flow. As the velocity through the port is reduced, the velocity energy is converted to pressure energy. Another way to think of this is the intake port flow stacks up against the closing valve and the port pressure increases. This is a good thing because during this same time the piston comes to a stop at BDC and continues up the cylinder which increases the pressure in the cylinder. Ideally, the pressure rise in the port exceeds the pressure rise in the cylinder right up until the valve closes, achieving maximum VE and torque.

I could see where the cam lobe that provides the greatest filling during the first half of the induction cycle might not produce the highest VE if this same cam causes port pressure and cylinder pressure to equalize prior to IVC resulting in reversion and reduced VE.

Thoughts?

You have to start by realizing that the piston isn't "sucking" anything. Well, to be fair, the piston is "barely" sucking anything. The exhaust scavenging, alone, creates an exponentially higher pressure drop than the piston ever will. All the piston is doing is opening a void. Think about this: you have two cylinders of equal size, stacked atop each other and separated by a thin plate. In one cylinder you have positive pressure, in the other cylinder you have negative pressure. What happens if you instantaneously make that thin plate disappear? Boom, the pressures instantly equalize and the event is over with. Instantly. Air, being fluid, has mass. As such, it builds momentum. Correctly timing the rate of lift of the valve allows the air/fuel emulsion to build momentum as it try's to equalize the pressure differential between the port and the cylinder. The piston itself only creates less than 1 PSI of pressure drop. If you instantly removed the valve, you'd be dependent on this 1 psi pressure drop to fill your cylinder. And you'd never even reach 100% VE, much less 105% or so. But by creating momentum in the air column, with the correct lift curve, we can then get the valve out of the way and let inertia do the job you you were hoping (fruitlessly) that the piston would do. But we have to create that inertia first. And we do that by correctly timing the lift curve.

Two things to keep in mind: the piston itself is barely sucking in anything......and, at the moment the valve leaves the seat, there is positive pressure in the port due to the reflected pulse waves in the intake tract.

[groberts101]

Basically boils down to this. There is simply not enough time and pressure differential available within this particular atmosphere to pack it all into the cylinder before BDC. Therefore we MUST use any physical aid possible to add more mass into the cylinder. This is called the 5th cycle or inertial ramming. If air and fuel wasn't so heavy?.. it would be a whole different ballgame.

We give some potential efficiency away here(IVO/BDC).. so we can gain even more over there(BDC/IVC). Same thing we haft'a do on the exhaust side too. We give away/use some exhaust energy by adding overlap to better pull in more fresh charge. Perfect solution?.. no. Better for power?.. YES!

I do enjoy the conversation but we are talking about things from a pure efficiency standpoint vs max power production. Two entirely different aspects altogether.

[Enigma]

Basically boils down to this. There is simply not enough time and pressure differential available within this particular atmosphere to pack it all into the cylinder before BDC. Therefore we MUST use any physical aid possible to add more mass into the cylinder. This is called the 5th cycle or inertial ramming. If air and fuel wasn't so heavy?.. it would be a whole different ballgame.

We give some potential efficiency away here(IVO/BDC).. so we can gain even more over there(BDC/IVC).

Exactly. And, to be fair, I'll say this: I understand what you're saying, 900. I've read your thoughts, ideas, and accomplishment for years. You like big heads and big rpm. In that world, you'll generally run into mechanical limitations before you run into a situation of too much curtain are too quickly. But it simply is possible to hurt performance by lifting a valve too quickly. And the more mild the engine is, the easier it is to do. "too much duration" is exactly the same as lifting the valve too quickly. Many an engine has lost performance by being over-cammed. But it all boils down to too much curtain area too quickly to build momentum in the air column. That's where correctly timing the lift curve comes into play.

[PackardV8]

"too much duration" is exactly the same as lifting the valve too quickly

How many agree completely with this? Back in the bad old days of valve springs not up to the task, we worked with long, slow ramps, long duration and low lifts and resultant poor idle and fuel economy. Today's better spring technology has enabled higher intensity/quicker-higher lift/descent with much less duration. More power and economy with good idle, emissions and warranty. Think current Corvette.

Many an engine has lost performance by being over-cammed.

Agree completely with this second half, but only as far as duration is concerned.

Back to the OPs question; does anyone really think his low-RPM street engine is going to fall on it's face because he chooses a catalog CompCams of higher intensity but the same duration and lift? Will the highest intensity lobe make more valve train noise and wear cam/lifters/springs/rockers/valves? Fer sure!