Can too much intake lobe area decrease performance?

[groberts101]

On your typical stock Chevy?.. NOPE! Been there and done that many times over on many generations of SBC.

Assuming a stock casting build scenario, once you really understand that you're moderately undervalved AND sunstantially port limited?.. it’s not hard to understand why increasing the area under the lift curve with more lobe and/or rocker ratio only allows for positive mass airflow gains. Basically, if you can't get more air mass in(or out) with a larger valve and killer valvejob?(including idealized throat cuts).. then you must rely on the cam/rocker/spring to compensate for those others shortcomings. Basic facts of engine building.

And guys.. this is a 5,000 rpm max we’re talking about here. Not anywhere near the ragged edge of design by any stretch.

I also think it's fair to say that given the constrains of most piston pins/rings and chambers.. pretty tough to overdo it unless you reach the point that your shit starts wearing out too fast and/or breaking. On the extreme end.. enigma does make have a valid point when you think about the added airflow abilities associated with a 4 valver. Expose those ports way too fast(think.. 2 stroke) and way too suddenly?.. you knock the ass out of a slow moving pistons low speed torque production capability. Timing the highs and lows of varying pressure differentials surely matters when designing these things. Otherwise.. how hard would it be to just make a really fast on/off solenoid based valve that just suddenly pops the valve open and closed when a computer tells it to? Well.. actually probably pretty hard to do for mass production.. but you get the point. AFAIK..currently that technology is being used for the variation of cam timing/phasing only.

When do the engines with pneumatic valvetrains open their valves? Anyone know those timing/lift spec’s compared to an aggressive lobed poppet cam?

[user-9274568]

As the valve is closing and the piston is moving toward the head you have to increase the intake port pressure as much as possible to keep the valve open OR close the valve earlier.

Or something like that.....

[MrBo]

The actual question was “is there a point where the intake valve can be opened and closed too quickly for a given duration?”
The answer has to be yes. You need curtain area restriction to create air velocity.

The very word "restriction" would indicate that you are opposing flow. Is velocity not created by air moving through the port? The more air you move through the port for a given size the higher the velocity through the port is until you max the port out and it won't flow any more. "curtain area" in and of itself does not increase or decrease velocity through the port or affect intake ram ability.

Think of it this way........ you open the valve .050". You have already started air moving in the port. At .050" though, the head will only move 25cfm (at 28 inches). 25 cfm in a port that moves 300 cfm @ .600" would indicate that velocity through the port is very, very slow and you very little ram effect.

Now, if you had the same pressure differential as the above scenario but the valve was open .300" at the same point and you could move 180 cfm of air, the velocity of the port would be much higher and ram effect would be much higher as well.

The problem we run into is doing all of this at the wrong time.

Not sure I agree.
The valve curtain area is the control that meters the build up the kinetic energy (velocity) in the airstream at rate to combat pumping losses when the piston goes down but still get the desired airspeed.
The kinetic energy in the air (velocity) is then carefully metered out buy the curtain area to extract the energy to cylinder pressure at IVC.
Optimum valve motion for a given displacement will vary with piston motion (stroke or rod length).
It needs to be metered in my opinion. That is the job of the lift curve.

[Roadknee]

Thanks for the informative discussion. In this particular small-cam example, the intake valve is only open about 0.040" at TDC. It makes sense to me that flow into the cylinder between TDC and BDC will increase the quicker we open the valve. The first page of this thread was focused on what happens between IVO and BDC, and then Chad posts this:

As the valve is closing and the piston is moving toward the head you have to increase the intake port pressure as much as possible to keep the valve open OR close the valve earlier.

Or something like that.....

I had to think about this for a while. At low engine speeds, let's say 1/2 peak torque rpm, wouldn't the more aggressive lobe bring the pressure in the cylinder and port closer to one another resulting in more reversion between BDC and IVC? The lobe is much larger than what is needed to fill the cylinder at this low speed, so the loss due to reversion could be greater than any gain the aggressive lobe may have provided between IVO and BDC. Put another way, an engine with the more aggressive lobe produces less torque than the mild lobe at low engine speeds, but greater torque at higher engine speeds. The aggressive lobe also extends the usable rpm range of the engine. Thoughts?

[MrBo]

A late edit & something that made my first post confusing as well.

Not sure I agree.
The valve curtain area is the control that meters the build up the kinetic energy (velocity) in the airstream at rate to combat pumping losses when the piston goes down but still get the desired airspeed at the curtain area.
The kinetic energy in the air (velocity) is then carefully metered out buy the curtain area to extract the energy to cylinder pressure at IVC.
Optimum valve motion for a given displacement will vary with piston motion (stroke or rod length).
It needs to be metered in my opinion. That is the job of the lift curve.

[900HP]

Thanks for the informative discussion. In this particular small-cam example, the intake valve is only open about 0.040" at TDC. It makes sense to me that flow into the cylinder between TDC and BDC will increase the quicker we open the valve. The first page of this thread was focused on what happens between IVO and BDC, and then Chad posts this:

As the valve is closing and the piston is moving toward the head you have to increase the intake port pressure as much as possible to keep the valve open OR close the valve earlier.

Or something like that.....

I had to think about this for a while. At low engine speeds, let's say 1/2 peak torque rpm, wouldn't the more aggressive lobe bring the pressure in the cylinder and port closer to one another resulting in more reversion between BDC and IVC? The lobe is much larger than what is needed to fill the cylinder at this low speed, so the loss due to reversion could be greater than any gain the aggressive lobe may have provided between IVO and BDC. Put another way, an engine with the more aggressive lobe produces less torque than the mild lobe at low engine speeds, but greater torque at higher engine speeds. The aggressive lobe also extends the usable rpm range of the engine. Thoughts?

This is where we get into timing. Because the area of the more aggressive lobe is larger for a given duration, you can shorten the overall duration and maintain the same area. This makes MORE torque.

[user-9274568]

Thanks for the informative discussion. In this particular small-cam example, the intake valve is only open about 0.040" at TDC. It makes sense to me that flow into the cylinder between TDC and BDC will increase the quicker we open the valve. The first page of this thread was focused on what happens between IVO and BDC, and then Chad posts this:

As the valve is closing and the piston is moving toward the head you have to increase the intake port pressure as much as possible to keep the valve open OR close the valve earlier.

Or something like that.....

I had to think about this for a while. At low engine speeds, let's say 1/2 peak torque rpm, wouldn't the more aggressive lobe bring the pressure in the cylinder and port closer to one another resulting in more reversion between BDC and IVC? The lobe is much larger than what is needed to fill the cylinder at this low speed, so the loss due to reversion could be greater than any gain the aggressive lobe may have provided between IVO and BDC. Put another way, an engine with the more aggressive lobe produces less torque than the mild lobe at low engine speeds, but greater torque at higher engine speeds. The aggressive lobe also extends the usable rpm range of the engine. Thoughts?

We have cylinder pressure climbing because our column of mass is decreasing in size, causing a increase in cylinder pressure. But we are trying to put more and more air in it, so we need to increase our port pressure to do so. One way is closing the valve faster.

Or something like that....

[groberts101]

Put another way, an engine with the more aggressive lobe produces less torque than the mild lobe at low engine speeds, but greater torque at higher engine speeds. The aggressive lobe also extends the usable rpm range of the engine. Thoughts?

Mostly true in the general sense where the port/induction is already sized for high rpm airflow. But in this particular case where a stock style port is severely lagging behind that scenario?.. the added lift area under the curve will increase mass flow through the engine. Think about all the ten's of thousands out there who simply swapped to higher ratio rockers and found gains. However small they may be.. a net gain is often worth fooling with bolt on parts.

And aggressive lobes typically cause increased overlap at any given valve lift.. so it's actually just the opposite of your analasys above. The aggressive lobe usually makes an engine feel peakier.. not broader.

In another way.. the title of this thread could also be named.. "Can too much rocker ratio decrease performance?"

With the wrong combo of parts it can almost be a near certainty at times.

[panic]

Re: "No pressure differential=no port flow"

Only true if the valve throat is the same size as the bore...

[MrBo]

In another way.. the title of this thread could also be named.. "Can too much rocker ratio decrease performance?"

With the wrong combo of parts it can almost be a near certainty at times.

Yeah.
When Chev NASCAR engines were still using 18* heads there was a dyno test with intake rockers in the Chevy Power book. They used higher and lower ratios on the intakes of different cylinders.
They didn’t just use the higher ratio on all the intake valves.

[groberts101]

Re: "No pressure differential=no port flow"

Only true if the valve throat is the same size as the bore...

I can't say that I really agree with some of the things being said above either.

Basically, we're being FORCED to work within this constantly changing environment that relies on gravity, pressure, and vacuum to control mass airflow in, through, and out of an ICE. This is obviously only accomplished through precisely timed pressure balancing which allows improved directional movement and redistribution of the mass and its available energy. Basically, it doesn't even matter how we achieve said improvement as long as those physics can be controlled and the component/s used can actually survive the task. Timing and controlling all these wildly fluctuating pressure differentials forces engine builders to become some sort of "pressure analyst" at this juncture of a buildups valve-train theorem. Typically, most guys would just pick a cam already.

All that I currently know about engines is based on factual evidence that you simply cannot.. MUST NOT.. suddenly expose an already overly large valve/port which was originally sized and developed for high-rpm airflow.. out in front of a slow moving piston with a smaller swept mass volume. Seems rather silly to think that we should rightfully expect that any and all sudden availability of this additional mass will definitively increase mass throughput to give greater BMEP. Not even remotely possible on every parts combo and we must still stick to designing in that sweet spot. Now, those sweet spots are surely getting damned big and MUCH wider nowadays.. but you'll never EVER squeeze ultra-high efficiency out of an ICE due to inherent design and metallurgical restrictions. Physics is our friend and foe. Just think.. 200 years from now it'll all be extinct dinosaur tech and future generations will wonder what the hell all the time, money, and hubbub was all about anyway.

And think about it for 5 seconds longer. If this "give er' all you got" methodology worked on every engine ever made?.. we'd surely ALL be running really short .700 lift cams in our stock Toyota or Dodge twin-cam 4 bangers by now. Hell.. even your cheapest lawnmowers would probably have .600 lift lobes. lol The plain and simple fact is that today's newest lightweight roller/hydraulic valve-train technologies are fully capable of controlling such a "short time/high lift" regime.. albeit at some obvious loss of lifespan. Hell.. I'd still buy it if it made more power!

Unless I missed something really big here.. why don't we utilize this max airflow methodology more often if it's really just as simple as grinding the cam and rocker to.. let it eat?

I'll answer my own question by copying off the above posters test paper. Kinetic energy.

[groberts101]

In another way.. the title of this thread could also be named.. "Can too much rocker ratio decrease performance?"

With the wrong combo of parts it can almost be a near certainty at times.

Yeah.
When Chev NASCAR engines were still using 18* heads there was a dyno test with intake rockers in the Chevy Power book. They used higher and lower ratios on the intakes of different cylinders.
They didn’t just use the higher ratio on all the intake valves.

No doubt in an attempt to balance things out due to the inherent mass imbalances associated with our beloved common plenum shared crank platform. They were simply trying to control kinetic energy through the use of velocity.

[900HP]

The factory GM LS-7 engine comes with 1.8:1 rocker arms 211º/230º duration @ .050 and almost .600" lift. I wonder what GM is attempting to do here and still meet emissions requirements and durability/warranty needs as well.

[groberts101]

The factory GM LS-7 engine comes with 1.8:1 rocker arms 211º/230º duration @ .050 and almost .600" lift. I wonder what GM is attempting to do here and still meet emissions requirements and durability/warranty needs as well.

We get the main point.

So, with that well developed and optimized combination of parts .. why not just up the lift even more and shorten up the seat timing to compensate for that faster net filling? Or why not simply use a 1.9 rocker ratio? Would we also then be guaranteed that an aftermarket set of 2.0 ratio rockers would add power?

On a well developed platform.. you simply cannot affect one thing in a positive way without negatively affecting another. There limits to everything being discussed here is the main point. Not just the physical parts limitations.

[900HP]

The factory GM LS-7 engine comes with 1.8:1 rocker arms 211º/230º duration @ .050 and almost .600" lift. I wonder what GM is attempting to do here and still meet emissions requirements and durability/warranty needs as well.

We get the main point.

So, with that well developed and optimized combination of parts .. why not just up the lift even more and shorten up the seat timing to compensate for that faster net filling? Or why not simply use a 1.9 rocker ratio? Would we also then be guaranteed that an aftermarket set of 2.0 ratio rockers would add power?

On a well developed platform.. you simply cannot affect one thing in a positive way without negatively affecting another. There limits to everything being discussed here is the main point. Not just the physical parts limitations.

Reliability has a lot to do with it. Using a hydraulic lifter and controlling the valvetrain is the other part of it. Look at our EMC engine (or others as well) with 2.1:1 intake rockers and .850" lift with only 242º duration at .050" and it makes stupid amounts of torque. We are pushing the limits of reality with that and it certainly isn't a 100,000 mile package and it's horrifically expensive to get the parts that will hold up for even a moderate amount of usage.

We are up against valve spring limitations, lobe design limitations, valve seat limitations, lifter limitations, etc, etc, etc.

edit: and we had a 2.35" intake valve on a 428 cubic inch engine, is that enough curtain area for you for the displacement? Yet we still use violently aggressive valvetrain to make the steam. This is relatively low rpm stuff too, 6500 max pulling from 2500 up.