Can too much intake lobe area decrease performance?

[CamKing]

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?

It can actually be either, in most cases it's from opening it too soon, but not in all. I was working on a Pro Stock project a few years ago, and we had the cam working very well. It was well developed, and one of the fastest engines out there. On the spin-tron, the cam was good, well past where we were turning, so we decided to get more aggressive. I designed a cam with the same duration and lift, but wil more area. It was fine on the spin-tron, but lost 100hp on the dyno, and the CFM's were way down. I opened and closed the valve at the same spot, but I opened it too fast.

[MadBill]

Per Mike's comments, seems like on the opening side it might just be a matter of matching lift rate and IVO to take best advantage of the exhaust suction pulse and a very fast opening could be paired with a later IVO, (possibly with matching changes to exhaust dimensions) resulting in at least equal top end power but more mid-range. Approaching IVC however, a less aggressive closing rate would result in a smaller L/D 'jet' in say the 40-80° ABDC range, to enhance mixture velocity for greater inertia ramming. FWIW, optimum power IVC in all the Dynomation sims I've run occurs a number of degrees after the cylinder pressure rises past the port pressure and a small amount of reverse flow has developed.

[Enigma]

"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!

"It can actually be either, in most cases it's from opening it too soon, but not in all. I was working on a Pro Stock project a few years ago, and we had the cam working very well. It was well developed, and one of the fastest engines out there. On the spin-tron, the cam was good, well past where we were turning, so we decided to get more aggressive. I designed a cam with the same duration and lift, but wil more area. It was fine on the spin-tron, but lost 100hp on the dyno, and the CFM's were way down. I opened and closed the valve at the same spot, but I opened it too fast."

Point made. Stick a 26* intensity lobe (I know, Mike, I know) in the current corvette and see what happens. Too much curtain area too early to allow the air columnto build momentum is too much curtain are too early to allow the air column to build momentum.......whether it be in the form of intensity, or duration.

[Enigma]

Per Mike's comments, seems like on the opening side it might just be a matter of matching lift rate and IVO to take best advantage of the exhaust suction pulse and a very fast opening could be paired with a later IVO, (possibly with matching changes to exhaust dimensions) resulting in at least equal top end power but more mid-range. Approaching IVC however, a less aggressive closing rate would result in a smaller L/D 'jet' in say the 40-80° ABDC range, to enhance mixture velocity for greater inertia ramming. FWIW, optimum power IVC in all the Dynomation sims I've run occurs a number of degrees after the cylinder pressure rises past the port pressure and a small amount of reverse flow has developed.

This is exactly Harolds ideas pertaining to lobe design. "open it later, open it quicker, set it down easy with an unsymetrical lobe". And that has everything to do with allowing the piston to create enough of a void to maintain a higher pressure differential and build momentum in the column.

[nitro2]

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.

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.

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.

Sorry, disagree on all counts, and many years worth of pressure data on everything from soup to nuts to back it up.

[900HP]

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?

It can actually be either, in most cases it's from opening it too soon, but not in all. I was working on a Pro Stock project a few years ago, and we had the cam working very well. It was well developed, and one of the fastest engines out there. On the spin-tron, the cam was good, well past where we were turning, so we decided to get more aggressive. I designed a cam with the same duration and lift, but wil more area. It was fine on the spin-tron, but lost 100hp on the dyno, and the CFM's were way down. I opened and closed the valve at the same spot, but I opened it too fast.

So you ended up with more area under the lift curve and lost power. I'm just thinking out loud here but if you had opened the intake valve say 1 or 2 degrees later and maybe closed it one degree earlier (not sure I like this idea) so that the overall area was the same as the previous cam what do you think the results would have been?

[novadude]

Doesn't rate of volume change come into play? In Camking's post, he mentions "overfilling" before BDC and pushing back out of the intake valve after BDC because the cylinder and port pressure have equalized (paraphrasing). In the hypothetical scenario someone posted about removing the valve altogether, if the piston speed is low, is the SBC port really a restriction? At a hypothetical 100 rpm is the cylinder REALLY sucking through a straw? Seems to me that if the rate of volume change in the cylinder is slow enough, a SBC port would have no problem filling the cylinder before BDC, especially if the exhaust gave it a head start.

However, back the the OPs question: I personally believe that with the short duration cams he has referenced, there is no such thing as too fast (unless you care about valvetrain stability and long life). How much is the piston actually moving after BDC before the valve is back on the seat with such short durations? Do you even have enough overlap area to allow the exhaust to "help" much?

I am just a home garage hack, so I could be way off base. I personally wouldn't choose the most aggressive lobe anyway, as something with such a small cam is probably a daily driver / tow vehicle. 100k mile longevity would be more important to me than getting the last lb-ft of torque from an aggressive lobe.

[Enigma]

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.

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.

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.

Sorry, disagree on all counts, and many years worth of pressure data on everything from soup to nuts to back it up.

Take a typical sbc, remove all 8 intake valves, put pressure sensors in all 8 combustion chambers, and spin that engine up as high as you want. Get back to me with those pressure readings. Maybe the laws of physics work differently with soup.

[Enigma]

Doesn't rate of volume change come into play? In Camking's post, he mentions "overfilling" before BDC and pushing back out of the intake valve after BDC because the cylinder and port pressure have equalized (paraphrasing). In the hypothetical scenario someone posted about removing the valve altogether, if the piston speed is low, is the SBC port really a restriction? At a hypothetical 100 rpm is the cylinder REALLY sucking through a straw? Seems to me that if the rate of volume change in the cylinder is slow enough, a SBC port would have no problem filling the cylinder before BDC, especially if the exhaust gave it a head start.

However, back the the OPs question: I personally believe that with the short duration cams he has referenced, there is no such thing as too fast (unless you care about valvetrain stability and long life). How much is the piston actually moving after BDC before the valve is back on the seat with such short durations? Do you even have enough overlap area to allow the exhaust to "help" much?

I am just a home garage hack, so I could be way off base. I personally wouldn't choose the most aggressive lobe anyway, as something with such a small cam is probably a daily driver / tow vehicle. 100k mile longevity would be more important to me than getting the last lb-ft of torque from an aggressive lobe.

Rate of volume change (rpm) is the key. The slower the rate of change, the less likely you are to build any momentum in the air column with too much curtain area early in the intake stroke. Less inertia=less charge fill. You manipulate that with a slower lift curve.

[Warp Speed]

The exhaust scavenging, alone, creates an exponentially higher pressure drop than the piston ever will.

That is simply not true!

[groberts101]

The exhaust scavenging, alone, creates an exponentially higher pressure drop than the piston ever will.

That is simply not true!

I think that, very much like what's happened with this question, he may have pulled that Vizard quote a bit out of context and extrapolated it across a much broader range of crank rotation, mass flow, and curtain area, is all.

Do you guys sensor the entire motor during some of your pressure tests.. exhaust pressure fluctuations too? Are they fairly high rpm motors?.. and what's the highest pressure drop you've witnessed on an exhaust port?

Thanks ahead if you're willing and able to share more detail.

[CamKing]

[ I'm just thinking out loud here but if you had opened the intake valve say 1 or 2 degrees later and maybe closed it one degree earlier (not sure I like this idea) so that the overall area was the same as the previous cam what do you think the results would have been?

That's what we do when the ports are too big for the application, but that wasn't the OP's question.

[RevTheory]

That's what we do when the ports are too big for the application, but that wasn't the OP's question.

That's interesting. Does the opposite hold true if you've sized your ports a little on the smaller side when torque, throttle response, shift recovery, etc. is the goal instead of an rpm target and the ability to drive over the peak hp?

[CamKing]

Does the opposite hold true if you've sized your ports a little on the smaller side when torque, throttle response, shift recovery, etc. is the goal instead of an rpm target and the ability to drive over the peak hp?

If you've sized your port on the small size, because "torque, throttle response, shift recovery, etc. is the goal instead of an rpm target and the ability to drive over the peak hp", then you're port is not too small for the application. Adding duration would just help the things you don't care about, and hurt the things you do.

[nitro2]

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.

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.

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.

Sorry, disagree on all counts, and many years worth of pressure data on everything from soup to nuts to back it up.

Take a typical sbc, remove all 8 intake valves, put pressure sensors in all 8 combustion chambers, and spin that engine up as high as you want. Get back to me with those pressure readings. Maybe the laws of physics work differently with soup.

Perhaps you have forgotten that there is a port underneath the valve, even if you take the valve out. We do bench tests equivalent to that (valveless) all the time -6 psi no sweat. I'm not saying it's the way to go, just that it's not -1 psi even valveless, unless the rpm is low. The laws of physics work the same everywhere, it's the understanding that can vary.