David posted some of the best tech I've read regarding resonance tuning in another thread comparing the TFS-R and Box-R intakes. This thread is meant to discuss some of the more technical details of cam design brought up in that thread.
Alright, I'm cracking my knuckles and might be out of my field of expertise here because I don't have a mechanical engineering undergrad or anything, but am inclined to go that direction in grad school. However, I do excel at physics related discussion. First, before going into the specific comments you've made, I'd like your recommended reading material regarding cams so I can up my game a little when it comes to the technical discussion concerning them. So far, you’ve mentioned John Heywood, and now Blair.
More important to me than numbers, at the moment, is an understanding of the physics concepts. Here, you speak of "greater draw" as a good thing. I've definitely heard this before. Explain why, please. I have extremely limited education regarding cams, and even less experience on engine simulations, so I might be way of base in some of my ideas. Allow me to elaborate a bit and see if I'm headed in the right direction:
Negatives
draw
It seems to me that the "draw" itself isn't all positive: First, it causes an increased pressure differential between the inside of the cylinder and the intake runner. The lower pressure in the cylinder is not in itself helpful as it increases the drag on the piston. So, first lets all recognize and agree that the the port velocity gained comes at a cost. Agreed?
The assumption that delaying the intake opening event increases drag on the piston on the intake stroke might only be invalid if there is still more pressure initially in the cylinder than in the intake runner as the piston begins to move downward. This is most likely to be the case in a turbocharged application with significant back-pressure in the exhaust, but could still be the case in n/a engines if the exhaust hasn't had time to completely evacuate, which I think would indicate that the cam wasn't timed correctly to begin with.
overlap
Another possible negative is that delaying the intake opening (IO) reduces overlap (given constant exhaust valve timing), which would most likely be a negative in an n/a engine where delaying the intake valve opening would reduce the benefits of exhaust scavenging.
Since I'm assuming that you're right about this delayed opening being an overall advantage, the positives must outweigh the negatives. Here are the possible positives that I can think of:
Before I get to the possible positive consequences of delaying the intake valve opening, let me define a term in the context that I will use it (though not technically correct, perhaps). 100% VE: air mass in the cylinder = the expected air mass in that cylinder at BDC at rest given atmospheric pressure and the air temp in the cylinder, or in a boosted motor given the applied boost and the in-cylinder air temp). This definition allows for a common reference whether we're talking about a boosted engine or n/a engine regardles of the ambient temperature, or intake air temperature (IAT). If any clarification is needed, please ask.
Positives
Port Velocity
First, delaying the IO, should result in the increase in initial force on the air mass, meaning that acceleration of that air mass will be higher, at least initially. Is it higher, overall? I don’t know. If you’re correct about resulting port velocity being higher, then the answer is yes. And, going with that assumption. It seems to me, that we’re reducing the time that we’re able to apply a force on the air mass. At low RPM, or in a low displacement engine, that’s not a problem because we still have enough time to fill the cylinder, but at high RPM or in a big engine, that may no longer hold true.
Given the earlier definition of VE, my first thought as to the biggest advantage regarding delaying the intake opening event has to do with both increasing intake port velocity, and also timing that velocity with the intake closing event. The only reason that I can see port velocity providing any advantage at all is if the momentum of the air causes an increase in cylinder filling. At the point where the net force acting on the air mass begins to decelerate it, the rate the cylinder is filled is higher because of the momentum of the air mass. One question I have concerning this is: Is that point going to be 100% VE? I think 100% VE should be the point where the pressure in the cylinder equals the pressure in the intake tract. Without accounting for other forces, it would also be the point of maximum port velocity because the net forces acting on the air-mass would be 0. There are other forces involved, like resistance due to friction in the port and when air approaches sonic, though I don’t know how significant an impact they make in normal conditions. Regardless, any air-flow at and beyond which air is no longer being accelerated by the pressure differential would be due solely to momentum of the air column caused by inertia (intake port velocity). The thing that leads me to believe it might be a benefit below 100% VE is that Ed Curtis always talked about the importance of port velocity even in street motors where it is unlikely 100% VE was being reached.
The second part of the port velocity thing is related to the Intake closing (IC) event. I think, theoretically, the optimum IC event without other compounding factors would be the point at which air speed through the port has stalled. Does that actually happen, or are we actually forced to close the intake before that point? In any case, at this point well after the piston has reversed direction but before the intake air reverses direction, you’d want the intake valve to shut. Problem is, the point will vary depending on engine speed. So in terms of crank degrees after BDC, at low RPM I believe the ideal IC would be sooner (in crank degrees) because the cylinder has more time (in seconds)to fill the cylinder, and at high RPM, ideal IC would be later (in crank degrees) because the cylinder has not had as much time (in seconds) to fill. So, choosing the right IC should play a significant part in the power band without even considering resonance tuning.
Now, IC timing should relate back to IO timing for two reasons that I can think of: cylinder filling due to air mass, and resonance tuning. I’ll discuss the first reason in this section. If we slap on bigger heads and do not delay the IO, then the cylinder might fill well more than 100%VE, and then because the intake valve is open too long for the large head, it could actually lose some of the air fuel charge back into the intake port before the valve closes (possibly dropping below 100% VE again. By retarding/delaying (in crank degrees) the IO event, and/or advancing the IC event, we can “tune” the bigger headed combination so that we’re still getting the full cylinder fill with the small engine or at lower RPM.
Here’s another question for you concerning the discussion above: What happens to resonance tuning if the air in the intake tract has stalled? After all, isn’t the pressure wave caused by the impact of the sudden closure in the intake valve on the moving column of air? No pressure wave would be created at the valve shutting because the air there is already not moving. However, I could still see there being a pressure wave that was created earlier by the pressure rise in the cylinder. Is resonance tuning still of any significance at this RPM? If no, then is this our peak torque rpm? Or, is this a non-starter because it doesn’t actually happen the way I’m imagining it?
Overlap:
less overlap = may actually be a good thing for boosted engines. Since you like to cite Bernoulli, you know that air flow causes a local decrease in fluid pressure, which should still help “draw” air from the intake as exhaust flows out. However, it should also be apparent that when the pressure in the exhaust is significantly greater than the pressure in the intake, then minimal overlap is preferred. Thus, delaying the intake valve opening is actually positive anyway. However, due to the increased cylinder pressure in this situation, I don’t think this would create the additional draw that it would in an n/a engine. Perhaps, in a turbocharged application, an even later IO might be called for than would be expected on an n/a motor with big heads.
Resonance tuning
Delaying the IO might result in proper timing to take advantage of the helmholtz effect. I imagine that this would also be of concern regarding the IC event. You seem to have the math figured out… please guide discussion here. I can’t imagine exhaust timing has anything to do with this effect, at least with regard to the intake, so only the IO and IC could. Please enlighten me. How large an effect can playing with this timing have? Is it possible to calculate when the advantages gained from resonance tuning do not outweigh other disadvantages (see port velocity comments regarding IC).
That's pretty much all I can think of at the moment. My brain is out of juice because it's after 4am here, now. What else did I forget to mention?
In any case, that’s enough for tonight. I’ll respond to the rest of your post below tomorrow in a subsequent post in this thread. I definitely want to ask some questions regarding that stuff, too. But the above is already enough for now, wouldn’t you agree?
Alright, I'm cracking my knuckles and might be out of my field of expertise here because I don't have a mechanical engineering undergrad or anything, but am inclined to go that direction in grad school. However, I do excel at physics related discussion. First, before going into the specific comments you've made, I'd like your recommended reading material regarding cams so I can up my game a little when it comes to the technical discussion concerning them. So far, you’ve mentioned John Heywood, and now Blair.
A large cylinder head + intake system may like a delayed intake opening (or at least slow beginning ramp rate) so when the piston speed increases on the intake stroke towards its peak speed, the draw on the intake runner (head + intake) is greater.
More important to me than numbers, at the moment, is an understanding of the physics concepts. Here, you speak of "greater draw" as a good thing. I've definitely heard this before. Explain why, please. I have extremely limited education regarding cams, and even less experience on engine simulations, so I might be way of base in some of my ideas. Allow me to elaborate a bit and see if I'm headed in the right direction:
Negatives
draw
It seems to me that the "draw" itself isn't all positive: First, it causes an increased pressure differential between the inside of the cylinder and the intake runner. The lower pressure in the cylinder is not in itself helpful as it increases the drag on the piston. So, first lets all recognize and agree that the the port velocity gained comes at a cost. Agreed?
The assumption that delaying the intake opening event increases drag on the piston on the intake stroke might only be invalid if there is still more pressure initially in the cylinder than in the intake runner as the piston begins to move downward. This is most likely to be the case in a turbocharged application with significant back-pressure in the exhaust, but could still be the case in n/a engines if the exhaust hasn't had time to completely evacuate, which I think would indicate that the cam wasn't timed correctly to begin with.
overlap
Another possible negative is that delaying the intake opening (IO) reduces overlap (given constant exhaust valve timing), which would most likely be a negative in an n/a engine where delaying the intake valve opening would reduce the benefits of exhaust scavenging.
Since I'm assuming that you're right about this delayed opening being an overall advantage, the positives must outweigh the negatives. Here are the possible positives that I can think of:
Before I get to the possible positive consequences of delaying the intake valve opening, let me define a term in the context that I will use it (though not technically correct, perhaps). 100% VE: air mass in the cylinder = the expected air mass in that cylinder at BDC at rest given atmospheric pressure and the air temp in the cylinder, or in a boosted motor given the applied boost and the in-cylinder air temp). This definition allows for a common reference whether we're talking about a boosted engine or n/a engine regardles of the ambient temperature, or intake air temperature (IAT). If any clarification is needed, please ask.
Positives
Port Velocity
First, delaying the IO, should result in the increase in initial force on the air mass, meaning that acceleration of that air mass will be higher, at least initially. Is it higher, overall? I don’t know. If you’re correct about resulting port velocity being higher, then the answer is yes. And, going with that assumption. It seems to me, that we’re reducing the time that we’re able to apply a force on the air mass. At low RPM, or in a low displacement engine, that’s not a problem because we still have enough time to fill the cylinder, but at high RPM or in a big engine, that may no longer hold true.
Given the earlier definition of VE, my first thought as to the biggest advantage regarding delaying the intake opening event has to do with both increasing intake port velocity, and also timing that velocity with the intake closing event. The only reason that I can see port velocity providing any advantage at all is if the momentum of the air causes an increase in cylinder filling. At the point where the net force acting on the air mass begins to decelerate it, the rate the cylinder is filled is higher because of the momentum of the air mass. One question I have concerning this is: Is that point going to be 100% VE? I think 100% VE should be the point where the pressure in the cylinder equals the pressure in the intake tract. Without accounting for other forces, it would also be the point of maximum port velocity because the net forces acting on the air-mass would be 0. There are other forces involved, like resistance due to friction in the port and when air approaches sonic, though I don’t know how significant an impact they make in normal conditions. Regardless, any air-flow at and beyond which air is no longer being accelerated by the pressure differential would be due solely to momentum of the air column caused by inertia (intake port velocity). The thing that leads me to believe it might be a benefit below 100% VE is that Ed Curtis always talked about the importance of port velocity even in street motors where it is unlikely 100% VE was being reached.
The second part of the port velocity thing is related to the Intake closing (IC) event. I think, theoretically, the optimum IC event without other compounding factors would be the point at which air speed through the port has stalled. Does that actually happen, or are we actually forced to close the intake before that point? In any case, at this point well after the piston has reversed direction but before the intake air reverses direction, you’d want the intake valve to shut. Problem is, the point will vary depending on engine speed. So in terms of crank degrees after BDC, at low RPM I believe the ideal IC would be sooner (in crank degrees) because the cylinder has more time (in seconds)to fill the cylinder, and at high RPM, ideal IC would be later (in crank degrees) because the cylinder has not had as much time (in seconds) to fill. So, choosing the right IC should play a significant part in the power band without even considering resonance tuning.
Now, IC timing should relate back to IO timing for two reasons that I can think of: cylinder filling due to air mass, and resonance tuning. I’ll discuss the first reason in this section. If we slap on bigger heads and do not delay the IO, then the cylinder might fill well more than 100%VE, and then because the intake valve is open too long for the large head, it could actually lose some of the air fuel charge back into the intake port before the valve closes (possibly dropping below 100% VE again. By retarding/delaying (in crank degrees) the IO event, and/or advancing the IC event, we can “tune” the bigger headed combination so that we’re still getting the full cylinder fill with the small engine or at lower RPM.
Here’s another question for you concerning the discussion above: What happens to resonance tuning if the air in the intake tract has stalled? After all, isn’t the pressure wave caused by the impact of the sudden closure in the intake valve on the moving column of air? No pressure wave would be created at the valve shutting because the air there is already not moving. However, I could still see there being a pressure wave that was created earlier by the pressure rise in the cylinder. Is resonance tuning still of any significance at this RPM? If no, then is this our peak torque rpm? Or, is this a non-starter because it doesn’t actually happen the way I’m imagining it?
Overlap:
less overlap = may actually be a good thing for boosted engines. Since you like to cite Bernoulli, you know that air flow causes a local decrease in fluid pressure, which should still help “draw” air from the intake as exhaust flows out. However, it should also be apparent that when the pressure in the exhaust is significantly greater than the pressure in the intake, then minimal overlap is preferred. Thus, delaying the intake valve opening is actually positive anyway. However, due to the increased cylinder pressure in this situation, I don’t think this would create the additional draw that it would in an n/a engine. Perhaps, in a turbocharged application, an even later IO might be called for than would be expected on an n/a motor with big heads.
Resonance tuning
Delaying the IO might result in proper timing to take advantage of the helmholtz effect. I imagine that this would also be of concern regarding the IC event. You seem to have the math figured out… please guide discussion here. I can’t imagine exhaust timing has anything to do with this effect, at least with regard to the intake, so only the IO and IC could. Please enlighten me. How large an effect can playing with this timing have? Is it possible to calculate when the advantages gained from resonance tuning do not outweigh other disadvantages (see port velocity comments regarding IC).
That's pretty much all I can think of at the moment. My brain is out of juice because it's after 4am here, now. What else did I forget to mention?
In any case, that’s enough for tonight. I’ll respond to the rest of your post below tomorrow in a subsequent post in this thread. I definitely want to ask some questions regarding that stuff, too. But the above is already enough for now, wouldn’t you agree?
This is when the curtain area should be maximized in a timely manner. The change in air pressure will increase the air speed (some call it velocity) and will bring inertia with it. You need less duration. Matched parts become crucial when you go with various parts, like small or larger intakes.
With a large port velocity profile (relative to the cylinder volume), you need to "limit" the camshaft valve activity, not try to feed the cylinder more air with longer/larger durations. Thinking about how a basic 4-stroke engine works, you also will benefit with larger exhaust parameters with a large induction system.
There is no perfect set-up (large induction + small cam or small induction + larger camshaft). They just need to be matched, as this dyno session was matched better and it had some strong results to back it up.
Anyways, back on topic. For sake of argument - let's take the aforementioned XE274HR on this 331. Given the specs of that camshaft, the TFS 185 heads, I get a optimized 3rd tuned harmonic of ~12.9". Well, low and behold, the TFS-R BOX (9.0") + TFS 185 (4.75") length equals 13.75". This is less than an inch from ideal. However, add the extra 2.00" on to create the TFS-R runner length + same cylinder head, you get 15.75" - even further from ideal. The TFS-R BOX fits much closer, compared to the previous dyno run using a tuned length an entire two inches off near optimal. I wonder why the power went up? Factor in the assumed increase in taper from the BOX upper to lower, and you get even closer to the needed harmonic for a 12.9" length runner.
It was not a coincidence that at those funny RPM's (3700 and 3200) that we saw variations in HP/FT LB numbers at the wheels - tuned length and other intake parameters played a big part in this.