I appreciate your experiences and understand where you are coming from Todd. I respect your opinion, I just have different experiences and therefore perspectives.
The CarCraft article was interesting but like most magazine articles was initially undertaken for circulation reasons, filtered for any potential advertising conflict or benefit and then edited to fit the space available without regard for continuity or back story as to why certain phenomena occur. Data collection and environment control are a special discipline all unto them selves. The interpretation of collected data is best performed by yet other individuals with equally specialized data interpretation skills.
Sadly little of that occurred in the article or at least the presentation and interpretation of the data and results in the magazine article. Despite that there are multiple results that while true are not true for the misinterpreted reasons given. It is the misinterpretation that leads the editor and those who read the confused reporting to conclusions that appear correct but are sometimes less than correct. The less than correct understanding of the phenomena leads to later 'experiments' that do not duplicate or build upon the first efforts and result is the often quoted explanation that every engine is different and your mileage may vary.
We know from our consumer experiences that, that is not true! We can buy a car from Detroit that that will perform a complex task like power production or emissions control and do it materially the same and equally well anywhere on the planet under any atmospheric conditions that permit starting the engine. Engine behavior and performance is not a speculative event or endeavor. It is an engineered design of an internal combustion engine (ICE) system calibrated for a particular outcome. What we do is no different, most of us just do not have the same ICE design training or experience that the Detroit teams did when they originally created the engines.
I may have been the first person on this site to have used narrow LSA cams in a race only car. At the time that first 'tight' LSA I experimented with was a 104˚ LSA after having used 110˚, 112˚ and 114˚ LSA cams. I can't claim the vision to have brilliantly come up with the idea (although it should have been obvious). In actual fact it was one of Chrysler's racing engineers from what was then called their Performance Parts Clinics, if I remember correctly. The year was 1966 roughly 50 years ago and the only dyno we had access to was a dragstrip. Chrysler had noodled the cam out in their engine labs and made it available to selected racers through their race program, at that time.
Over the fifty years that have intervened I have used that type of cam phasing in n/a gas, supercharged gas, supercharged alcohol and supercharged nitromethane engines. The original 'tight' LSA approach evolved into an effort to optimize each intake and exhaust event independent of the other. As that effort evolved it produced cam phasing initiatives like I have previously suggested. Each occurred because it produced a better performing race car, we just didn't have dyno time that we could avail ourselves of. Again I respect your opinion and your right to it and to share it. I just have a different opinion.
The tight LSA's substantially picked up the car's trap speed and markedly reduced its et's. I was impressed but didn't, at the time, fully comprehend the dynamics of what brought about the performance improvement. Like most racers who first get introduced to the 'tight' LSA approach I attributed the performance to the LSA. Over time I discovered differently. In fact the performance was due to the advanced intake effect of closing up the LSA. For simplicity lets assume the intake cam and the exhaust cam are identical durations and they are 230˚ If I have a cam with a 114˚ LSA and second cam same profile but with a 104˚ LSA the intake event has actually been advanced 10˚ on the 104˚ LSA camshaft(s).
It is that 10˚ advance of the intake opening point and the improved volumetric efficiency the cylinder now has that produces the increase in torque and power that you see when you race the car. The late opening of the exhaust actually diminishes the engine's potential power production through pumping losses.
One of the reasons you virtually never see early opening exhausts on pushrod engines is exactly the reason Dutweiler and Urban speak to - weak valve train components, primarily pushrods. When we would run the early opening exhaust events we used 7/16" heat treated heavy wall 4130 and 4340 pushrods, it was the best we could get at the time. This was at a time when most other engines uses 5/16 or 3/8 thin wall lesser grade steel pushrods. By the early seventies we had high strength tapered pushrods available from places like D&D and Crower. Later Erson came out with double taper pushrods and today Trend makes H-13 tool steel pushrods available in 7/16", 1/2", and even 9/16" with wall thickness that can be spec'd to as much as some piston pins - 0.200" both to control valvetrain dynamics and because this is what it takes to reliably operate the exhaust side of an engine with an early opening exhaust event. Overhead cam engines that do not have pushrods get a sort of pass on all the pushrod pain and suffering but they still have to open against considerably more pressure when the exhaust opening event occurs just past mid stroke.
You always want to optimize each of the four cycles, Intake, Compression, Ignition/Power, and Exhaust, Cody. To the extent you impair any one of the individual cycles you impair engine performance. That means we want to select an intake cam that is optimized to fill the cylinder. The intake closing point has particular significance in this filling scenario. Most head designs (even our 4V's) have a hard time filling the cylinder at higher engine speeds. To assist in this process the ICE designer closes the intake valve ABDC. This allows additional time for the incoming charge to fill the cylinder. The exact closing point is selected to support a particular volumetric efficiency at a particular engine speed - which is why (if we are careful) we can replace OEM cams with aftermarket cams and improve the engine's performance.
BTW the same things are true of n/a engines and supercharged engines although we may adjust intake phasing (and therefore LSA) slightly to compensate for the lack of boost in the n/a style engines. Whether the engine is boosted or n/a as the piston starts to rise from BDC it begins the process of compressing the charge in the cylinder. As long as the in cylinder pressure is lower than the pressure in the intake port the cylinder will continue to fill. When the pressure in the cylinder from the rising piston equals the pressure in the intake port all flow into the cylinder stops and the intake valve should be closed. When the cylinder pressure exceeds the pressure in the intake port reversion occurs and the just introduced combustible charge is pushed back up the intake - reducing volumetric efficiency, torque and horsepower.
On the exhaust side there is an early opening benefit for both turbo engines, PD engines and n/a engines. If you have a pushrod engine you will have to do a lot of beefing up in the valve actuation machinery, in particular the pushrods. You will also need good oil supply because of the increased loads. As the editor correctly attributes to Dutweiler in the CarCraft article, "
An early-opening exhaust valve can be beneficial for top-end power because even high-efficiency turbos still have to work against some exhaust back pressure."
So lets open our exhaust before the cylinder starts rapidly loosing twisting leverage as the crank pin passes 90˚ ATDC/BBDC. Lets open the exhaust at 80˚ BBDC. this is very tough to do on a pushrod engine and the article's editor correctly attributes to both Dutweiler and Urban, "
attempting to open the exhaust valve too early can cause bent pushrods." On an OHC engine we have no pushrods to consider. When the exhaust is opened early like this it must open against fairly high cylinder pressure. All that high pressure and high velocity and high heat energy is directed directly at the turbo's hot side. It will dramatically accelerate the exhaust turbine and equally dramatically reduce your spool time. As an extra added bonus with the newer higher efficiency turbos you will realize more total boost which you may or may not want - but you will spool much faster. This is blowoff valve country to control the boost to your personal targets.
OK your exhaust is open so where do you shut it? If you shut it late you will have overlap, a lumpy idle, decreased low speed manners and potential exhaust gas dilution in your intake charge which will displace combustible mixture and produce lower apparent volumetric efficiency. If you close your exhaust between 5˚ and 10˚ degrees before the intake opens then you will neither dilute your intake charge or where you boost is higher than your exhaust pressure, supercharge the atmosphere.
So what does all this translate into for your two cam profiles? If you want to use off the shelf cams so they are easily replaced if something hurts them then you want to use the following Comp profiles. Your intake should be Comp's 106260 cam with 222˚ duration and 0.475" lift. Your exhaust should be either the Comp 106460 or 106500 cam both of which have a 240˚ duration and a 0.475 lift. You want to install them like this;
This will violate all the tight LSA religions but it will produce the kind of power and driving experience you are looking for. Now comes the tough part. You got two devils standing on each of your shoulders telling you two diametrically opposed approaches to cam selection and phasing. In the end you are the guy who is writing the check and there is no one more qualified to determine how you want to spend your money than you, sooooo as the quip (with a little theatrical license) goes in the Mission Impossible movies, 'Your mission Mr. Hunt, should you choose to select it, is to pick the correct cams for your engine and install them correctly, based on what you now know...'
Couple of thoughts on the CarCraft Article ...
Dutweiler quote:
"
In the old days it was typical to see 1.5 to 2:1 back pressure ratios," Duttweiler says. "Today the back pressure is actually less than the boost pressure."
What Ken is observing is you can produce more boost at the same exhaust gas pressures you used to produce less boost at. It can easily be misinterpreted that you do not need as much back pressure period. That is only true if you are attempting to produce the same boost the old fashioned turbos used to produce. This begs the question why didn't you get the cheaper old fashioned turbos - to which we all know the answer, we wanted more boost and quicker spooling!
"
All internal combustion engines perform best when tuned with a certain amount of camshaft overlap in which both the intake and exhaust valves are open at the same time. If the exhaust back pressure is greater than the inlet pressure, the exhaust will push back into the cylinder and (given enough time) up into the inlet manifold."
The is not Ken it is the editor. He has taken two different ideas probably from discussion with Ken restated them in his own words and conflated them.
The camshaft overlap is most beneficial to a n/a valve curtain limited engine. It has less applicability in 4V head designs. In fact Ford does not offer any n/a or supercharged 4V designs with overlap. The Cobra engine as delivered by Ford had negative (both valves closed) overlap that was -38˚. The late GT500's run with -18˚, 96-98 Cobra's had -28˚, 2000R Cobras were -16˚. There is a pattern emerging here, high specific power outputs on engines with 4V head designs do not need any significant overlap.
Even in your modified version of the engine it is to your advantage to mimic (to a lesser extent) what Ford already did with the cam phasing for the engine. It will give you excellent performance, good low speed driving manners and rapid spooling of your turbo.
The comment about exhaust back pressure pushing back into the cylinder can only happen if the exhaust valve is left open too long and the exhaust pressure is greater than manifold pressure - which Ken has already informed us can not happen with modern turbos because of their increased efficiency. Don't forget the pressure in the exhaust system is coming from the cylinder not the other way around. This is a prime example of an editor taking good information and restating it in his own words in a manner that hides the original meaning and adds to confusion for readers.
Here is the editor again conflating issues by trying to restate what he thought he heard;
"
According to Dutweiler, today's more efficient, larger turbos reduce that back pressure, which minimizes the power-robbing effect of exhaust dilution. That means the LSA can be tightened, which is contrary to the contention that all turbo cams must have wider 112- to 114-degree LSAs."
The statement about reduced pressure required per pound of boost is correct. The conclusion the editor comes to in terms of LSA is not supported by the data. It is a desirable conclusion for the article but without supporting evidence.
One more editorial embellishment;
"
With newer turbos, the reduced back pressure also means the exhaust valve can be opened sooner and held open longer, which is generally accepted as beneficial to high-rpm power production, just like on a normally aspirated engine."
If the newer turbos produce more boost on less back pressure why would you raise back pressure in the exhaust by an early opening event and a longer duration unless you were after even more boost. Here is the conundrum, if you know early opening of the exhaust is beneficial in terms of increasing boost why haven't you done that with the older turbos? Don't they not need more help than the newer more efficient turbos.
Again what you have is an editor using words to fill empty space that has been allocated to the article he has to write. Sadly he fills it with conundrums and nonsense.
I am not going to shred the rest of the article because it does have some very good information in it. It is however a poster child for why you always have to watch what magazine editors are saying and how they say it. They will start out with high goals and standards, fall back into misstatements and obfuscation because two weeks later they no longer remember what their notes mean and the publication goes to print in six days.
Like the old buyer beware saying, there should be a reader beware saying.
Ed