Don Terrill’s Speed-Talk

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?

Roadknee wrote: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?

As long as you are able to control the valvetrain there is no such thing on the intake side as opening or closing to quickly. Ideally we wouldn't have an intake valve in the way at all. That being said, the faster you get the intake valve out of the way the better until it becomes a durability or control issue. This is where the spin-tron is an invaluable tool.

I agree that the spintron would be the only way to know what the valvetrain was actually doing with the more intense lobes. But, I do feel that even with the valvetrain under control that their are situations and combinations where the intake valve could be opened to quickly.

CGT wrote:I agree that the spintron would be the only way to know what the valvetrain was actually doing with the more intense lobes. But, I do feel that even with the valvetrain under control that their are situations and combinations where the intake valve could be opened to quickly.

Only if it was opened at the wrong time

Lift it to quickly and the pressure differentials will implode into one another. No pressure differential=no port flow. You absolutely can lift it too quickly. The ideal is to chase the piston, lagging a little behind it to keep the differential, until max piston velocity. Then start gaining.

Lift it to quickly and the pressure differentials will implode into one another. No pressure differential=no port flow. You absolutely can lift it too quickly. The ideal is to chase the piston, lagging a little behind it to keep the differential, until max piston velocity. Then start gaining.

Sorry about the double post.

Enigma wrote:Lift it to quickly and the pressure differentials will implode into one another. No pressure differential=no port flow. You absolutely can lift it too quickly. The ideal is to chase the piston, lagging a little behind it to keep the differential, until max piston velocity. Then start gaining.

So how does a rotary valve 2-stroke even run?

You are speaking in theory here. I do not believe that mechanically we are able to open a valve fast enough. Even in your scenario, let's say the valve opened instantly, you would wait on the valve opening until there was enough pressure differential to do what you needed.

You do realize that the intake valve opens before the piston reaches TDC and yet we still have flow due to pressure differential from the exhaust leaving the chamber? The main reason why we follow the piston with the valve is so we don't damage the valve. There are mechanical limitations as to how fast you can realistically open the valves. With aggressive flat tappets (more aggressive off the seat than a roller even) and 2.2-2.4:1 rocker arms the NASCAR engines seem to run just fine and I'm betting the valve events are quite insane

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.

Enigma wrote: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.

Please explain to me how port activity will stall? If the port is designed correctly you could remove the valve completely and it will still work. Pressure differential between the chamber and the port is controlled by the valve (that's why we need them) but once the valve is open pressures try and equalize. All you are doing by keeping the valve in the way is delaying how fast you can fill the cylinder.

I understand that you are saying once the volume is filled there is no more differential and if you fill it too fast the differential will end prematurely but that seems like a timing thing to me, not a valve open issue.

And to the OP, sorry this got so side tracked......... none of the 3 cams you list are in any danger of being "too fast" for a mild street engine.

This discussion is beyond my pay grade, but it did bring up questions:

Back in the day of the carburetor and vacuum advance point-type distributors, we thought high intake manifold vacuum was critical to highway fuel mileage. Aftermarket vacuum gauges were widely sold and used. Shift down when the vacuum dropped.

Today, with EFI/DI and ECU ignition, taller gears, lower RPM, wider throttle openings and low manifold vacuum are current best MPG science.

It would seem slow intake valve opening and thus less lift at any degree point would theoretically produce higher manifold vacuum and quicker opening would reduce intake manifold vacuum.

Since the OP didn't specify carb or EFI, distributor or ECU, it is to be assumed his choice of fuel supply and ignition would affect the cam choice? Since an IR intake can still idle with much more overlap than a single plane 4-bbl intake, the intake manifold might be in the equation?

Enigma wrote: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.

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.

Don't some current BMWs have constant WOT and modulate power delivery request from the "throttle pedal" via valve lift and duration modifications?

This avoids any problem with too much lift too soon at too low RPM.

Still, with our available technology in an unoptimized low-buck- low-RPM 2-valve pushrod V8, is the OP going to see any real drive-ablilty problems or horsepower loss with the most aggressive of the Comp Cams? I'd say probably not.

Lifter/lobe wear problems with extended street miles? - quite probably.

MrBo wrote: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.