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Turbo Cobra cams

38K views 91 replies 19 participants last post by  CalBoy101 
#1 ·
I've been doing a ton of reading and trying to educate myself on cams. Up till this point I always left cam selection up to "professionals" however I feel that if I continue to work on people's cars I should be able to select and explain why certain cams should be used. Now, I understand many of the points Ed's articles have made for supercharged cars. My confusion stems heavily on the turbo side, there are so many conflicting ideas. I've called three people and gotten three different grinds and defense of each grind. Many people still stick to the old "wide lsa" design for turbo cams. However, I feel if the correct duration is selected to put the power at the correct RPM a narrow LSA will provide much better performance on a full out turbo car. I struggle with people arguing about how to help spool and others arguing about scavenging and blowing out boost.

I'd like to encourage some technical discussion on something that seems to be almost voodoo and left for people to pick them as "turbo cam stages" and/or at the mercy of custom grinders that vary widely.

My own personal build is a teksid ~10.5:1 motor with ported GT heads, shortened FR-500c intake, on e85 with twin 62/68 forced induction etr borg warners. I plan to shift north of 7500 and therefore have been leaning towards a narrower lsa (110-112) with a duration in the upper 230's. I have no real evidence this is the best option and is just hobbled together from light reading and discussion with people who know more than me. I've also leaned heavily on Todd (N/Asvt). His grind suggestion is similar to what I've looked at.
 
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#2 ·
my 11.0:1 teksid, ported navi heads, short-runner hogan intake, e85, w/ 88mm shifting at 7000rpm(will be going up to 8000). 24-30psi.

My turbo cams are custom from comp cams
lift: .475/.475
duration@.050 : 238/234
LSA:116(intake set at 116)
(thats were comp cams said to put them)
They made great top end power but the mid range spool time wasnt the greatest

Todd (NASVT) suggested
intake set to 108
exhaust set to 115

results the turbo spool time and midrange felt a lot stronger without suffering to much top end. Currently this is were mines at.


John Mahovitz suggested
intake: 104
exhaust: 106

Ive yet to do this
 
#3 ·
my 11.0:1 teksid, ported navi heads, short-runner hogan intake, e85, w/ 88mm shifting at 7000rpm(will be going up to 8000). 24-30psi.

My turbo cams are custom from comp cams
lift: .475/.475
duration@.050 : 238/234
LSA:116(intake set at 116)
(thats were comp cams said to put them)
They made great top end power but the mid range spool time wasnt the greatest

Todd (NASVT) suggested
intake set to 108
exhaust set to 115

results the turbo spool time and midrange felt a lot stronger without suffering to much top end. Currently this is were mines at.

John Mahovitz suggested
intake: 104
exhaust: 106

Ive yet to do this
See, three different recommendations lol. I do notice that manufacturers tend to stick to the old school thought of wide LSA's. That seems to hurt spool on bigger turbo's badly on these little engines. I'm all for a tighter LSA to get the turbo lit and trying to make up for some of the up top with duration for the correct RPM but I'm a noob.
 
#4 ·
I am jammed for time right now Cody, but I promise to stop back and talk about it and I'll try to say it differently and in a more understandable fashion.

Great thread you started here!


Ed
 
#6 ·
Cody,

Our engines are what is called Otto Cycle or Otto engines. They are named after Nickolaus Otto who along with partner Eugen Langen actually designed (invented) three different engine types. Wikipedia has a very good summary of them here => Otto Engines. The one they designed (actually invented) in 1867 that was known as the Otto Cycle engine was a four stroke, single cylinder, low rpm engine that fired every other stroke because of the Otto cycle that was the foundation for the design. Today we call that engine a gasoline engine and it has four cycles; Intake, Compression, Ignition (power) and Exhaust.

When we want to optimize the performance of one of these engines the best way is to optimize each of the cycles. With the exception of the intake and exhaust cycle the other cycles have minimal impact on any of their brethren. The intake and exhaust however can and do. The primary interaction occurs at TDC overlap where the potential for both the intake and exhaust valves to be open occurs.

Ideally you want these events to be distinct and separate from one another. Practically there are those situations where the two events can complement each other and at other times and installations counteract each other. Lets talk briefly about the complementing phenomena first.

In a 2V cylinder head design, even a very efficient layout like a hemispherical chamber or a modern day polyspherical chamber (current day ProStock and BBC) the intake charge need both a little encouragement to get moving and also a little bit of an opportunity to scavenge the chamber of the exhaust gases from the last combustion event. To achieve this we open the intake valve before TDC and close the Exhaust valve after TDC. This provides a period of time with both valves open that allows the incoming intake charge to get moving but also allows it to "sweep" the remnants of the last combustion cycle out the exhaust providing the least number of dilutive components of the last combustion cycle the opportunity to displace combustible air and fuel in the next combustion cycle.

With emissions playing an increasingly larger role in engine design over the last 40 years things like EGR where exhaust gas is recycled for emissions and operating considerations have become increasingly common on Detroit production engines. For a race application they have much less attractiveness if any at all.

Back to the performance perspective. When using a 4V (or more) head design the 4V geometry provides the absolute best flow performance compared to any 2V or 3V alternative. We get an indication of just how impressive this performance is when we check the OEM intake profile for our engines. The intake cam uses 185˚ intake event that Ford installs at a 114˚ centerline with the intake valve closed until the piston is already 22˚ down the cylinder on the intake stroke! Despite this day late and dollar short approach to valve timing the engine makes well over 1 HP per inch of displacement n/a. When we look at the supercharged versions of the engine with aftermarket KB and Whipple blowers 700 hp is a walk in the park even with this wimpy intake event!

For space considerations we will only look at intake and exhaust events. In order to optimize the power output we want to optimize each of the four cycles independent of the other three. For the intake event, only because we have such good low lift air flow, we can begin to open the intake at exactly TDC. Internal combustion engines upper rpm performance is limited by how well the engine can fill the cylinder with combustible fuel and air. A measure of this volumetric efficiency (V[SUB]e[/SUB]). If we were to close our intake valve at exactly BDC and the engine was operating at 2000 rpm we would have fairly good (V[SUB]e[/SUB]). Raise the engine speed 1000 rpm and the (V[SUB]e[/SUB]) would decrease incrementally. These incremental decreases would continue as engine speed continued. To maintain the (V[SUB]e[/SUB]) at higher engine speeds we would have to hold open the intake valve longer to provide adequate time for the cylinder to fill.

When we put a blower on the engine the (V[SUB]e[/SUB]) or Load as Ford likes to talk about tends to be table flat from low speed to somewhere past the torque peak of the n/a equivalent. How far above becomes a mix of intake valve closing point, blower inlet flow capacity, and manifold pressure. Essentially what you are trying to do is coordinate the pressure of compressing volume in the chamber and the intake valve closing point so that when they are equal you close the intake valve. The obvious upshot here is that higher intake manifold pressures and engine speeds will like later closing events and lower intake pressures and engine speeds will like earlier closing points. Things get mor complicated when you have lower intake manifold pressures and higher engine speeds. The obvious answer is they will like later closing events but the pregnant question is how much later.

For readability and brevity (such as it is) I am going to close this portion of the story here and return later for the exhaust side and turbine story.

Ed
 
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#7 · (Edited)
+1 Bump on this thread..

Before / After data would be awesome but is almost never available as most people install cams along with their build.


I have a set of comp 106500s I'd like to use but am not confident in doing so. (.475/.45 lift, 242/240 duration, 114° LS. )

Most input suggests that duration is too aggressive, yet I have to believe there would be some top end power gains. I suppose defining how much mid range is lost and how spool is effected is key..
 
#8 ·
Back to back testing is so difficult for the "shadetree" Cobra guy. There are only a handful of people that have done back to back tests between camsets and more importantly where to degree a particular set of cams and how that changes the power characteristics of a given application.

That said, I've always HEAVILY advanced my intake and had the exhaust 3 or 4 degees behind.
 
#10 ·
I can tell you this, every turbo combo that I've tightened the LSA on has picked up a ****load of power and MPH. Turbos work great with overlap as the higher exh pressures will not allow the fresh intake charge to exit through the exh valve. With that, if the flange is small and the AR low, too much overlap can cause exhaust gasses to backup into cylinder. I've run 109 LSAs with 235/233 @.050" in a t3 flange 76mm in the .98AR range successfully. Almost all of the turbo cams I spec these days with durations longer than 220 are 108-109 lsa. Those with less duration have wider LSAs to keep the RPM at which peak occurs up high. The old school turbos required less overlap due to their design but since those are no longer used the old school cam thinking needs to go out the window.
 
#17 ·
There are so many other options to help spool. What trans? from a dig? I can make a pro mod 94 with 370 cubes and no fancy cam work spool 12#s from a dig in seconds. Are you trying to make it zippy from a roll on the street? What are your goals?
 
#24 ·
Apologies for the long delay Cody. The alligator level in my personal swamp needed maintenance.

Just like we know that more fuel and more air (in a combustible form) produces more power we also know that turbos rely on a volume of hot, rapidly moving exhaust to spool. If we allow heat energy to diminish by loss through the headers, or if we allow the volume of exhaust gas reaching the exhaust turbine to decline or slow down then, the exhaust turbine has greater difficulty spooling the turbo. So lets assume we want to do things that will cause the exhaust turbine to spool more quickly.

I suspect that you have already read the article on cams that I posted a while ago. If you haven't here is the link => Cam Selection & Phasing I will build upon what I said over there.

We already know a turbo thrives on hot, high volume, high pressure exhaust gases. We also know from my Cam Selection article that there is little work done between 90˚ ATDC firing and BDC. The volume of the cylinder however does double. The doubling has two effects, the first is a cooling of the burnt charge attributable to the expansion it experiences and the second is the reduction in cylinder pressure as the volume doubles.

Both of these results contribute to diminished exhaust turbine performance and a corresponding reduction in compressor performance. Suppose we could hit the exhaust turbine with a decidedly hotter, higher pressure and higher velocity stream of exhaust gas. Yup, you guessed it the turbine would respond by spooling more quickly and producing a higher intake boost level sooner (better throttle response).

When we provide a higher intake manifold boost level anywhere but especially at low engine speeds the PCM responds with additional fuel and the engine responds with a mountain of torque that translates into vehicle acceleration if we don't spin the tires. BTW the additional fuel and air in the engine produce higher volume, higher velocity and higher temperature exhaust which in turn adds yet again to the turbo's spooling performance.

I just took delivery of a new twin turbo 4.4L BMW. In it's low tune state it hits 18 psi intake manifold pressure and 480 ft/lbs of torque at 1700 rpm. Yup, that's right 1700 rpm. The Dinan guys are about 5 miles from me and they tell me they can easily add 100 ft/lbs of torque at 1700 rpm and a boat load of hp up top with their entry level retune (remember PCM controllable variable cam timing). The trick is exhaust gas volume, exhaust gas temperature and exhaust gas velocity. The way you get those attributes is with an early opening exhaust profile. Look at my article on cam selection and you will see what I believe will put an extraordinary smile on your face.

BTW the intake cam duration and phasing is directly related to the boost in the intake. For a moment consider what happens in a supercharged engine format when the intake valve is closing. If the pressure in the cylinder is less than the pressure in the intake the volumetric efficiency and therefore torque and horsepower is low. If the pressure in the cylinder is the same or similar to the intake manifold we have optimized our volumetric efficiency and for the manifold pressure, camshaft and head design we are as good as it gets. If the cylinder pressure is higher than the manifold pressure and the intake valve is open we push combustible mixture back up the intake port and do not burn it - bad volumetric efficiency again.

How can you get a higher pressure in the cylinder than in the intake manifold? Suppose your cylinder has achieved a complete fill 25˚ ABDC but your intake valve does not close until 55˚ ABDC. With an open intake valve and a rising piston, as soon as the in-cylinder pressure reaches manifold boost levels any further rotation of the crankshaft pushes intake charge up the intake port just as surely as the same piston pushes exhaust gases out the exhaust port.

Bottom line don't get crazy with increasing the intake event (duration) and run the intake cam as far advanced as intake closing point and TDC overlap PTV clearance will allow. Don't over reach. This is one of those situations where less is easily more.

Ed
 
#26 ·
Even with a large flange and a high A/R housing higher velocity, higher volume and higher temperature exhaust will improve performance. I don't recall ever seeing an exhaust turbine improve spooling performance by reducing exhaust volume, reducing exhaust velocity and/or reducing exhaust temperature. The reduced volume, velocity and temperature have always had a negative impact on turbine performance.


Ed
 
#27 ·
1/4 mile performance after changing the LSA from 115-109 says it all Ed. You can dispute with MPH gains in the 7-10 range. The spool time also decreases significantly and these are real world improvements (not dyno) as seen on many cars.

I will never run a 115 or greater LSA on a turbo combo ever again as a result if what I've experienced.

Here is a very good article from someone who has significantly more experience than all if us: http://www.hotrod.com/how-to/engine/ctrp-1106-turbo-camshaft-guide/
 
#30 · (Edited)


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;

Font Parallel Circle Screenshot Number


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
 
#31 ·
Again, I've done wide, narrow and moderate LSAs in 2v, 3v and 4v engines and all have made more power and spooled quicker with tight LSAs. This is especially true on 4v engines with Sullivan, 2Vs with the Edelbrock and 3V with the short runner JPC or similar intake manifolds.

Those combinations with a 7000rpm shift point and a stock intake (one that has equal length, long runners) can work with an LSA in the 115 range and ILCs of 110-112. However, as the duration increases the ILC must be lowered to prevent an IV closing that wants to make power where the stock intakes don't work (upper RPMs).

I respect your opinion Ed but like I said earlier, what performs on paper doesn't always do so at the track.
 
#32 ·
Ed, I don't know enough to argue your points on a technical level. What you say makes mechanical sense to me but I'm not an engineer (I'm an economist lol). However, my car is 90% race and 10% street. With 7500+rpm shift points and twin 62/68's hanging on my ported GT500 heads and shortend FR500c intake with a ~4.5k stall in a th400 3.31 gears with goals in the 1200-1300rwhp the intake event on the cams you selected seems fairly mild. Mihovetz and Todd have both spec'd me much larger intake duration and a tighter LSA (their intake centerline is advanced as well). Comp suggested something closer to what you spec'd out but the cams they "custom" spec'd for me were almost identical to their stage 3 blower cams.

I need to buy a set of cams in the next few days so I can button up my long block and get it in the car to start fabbing the turbo kit. I am still confused on why there is such a broad spectrum of opinions on this lol.
 
#34 ·
I'm not here to get told what to do, I want to understand WHY. I work on other peoples cars, I have always avoided selecting cams for people and have always called up an "expert" to grind them with no idea if what I just told someone to spend 1300 bucks on is worth anything or not. I don't have a full on speed shop but I'd like for people to remember when they came to me that I actually had a working understanding of what parts work with what rather than just asking them to open a checkbook and me select the expensive things.

To add, there is a common (and I felt this way for a long time as well) idea that the stock cams are just fine. In my opinion this resulted from people not understanding charging 1300 bucks plus the cost of springs and retainers to grind something they didn't fully understand and ended up giving very little if any improvement. I've seen a car pick up 60rwhp across most of the curve with the correct cam, I've also seen a three letter companies "stage 3.5" cams hurt performance up to about 6000rpm from a set of degreed 98's even though they swore it would give him 90rwhp increase with no sound reason on why it would.

I want to remove the "magic" that surrounds cams and understand how our OHC cars actually work with cams.
 
#36 ·
I understand where you are coming from Cody. Like yourself I like to understand the answer to the question why. PM Joe Genovese (BlownBlu97) in NY. Joe has a freshly built turbo version of the engine he is waiting to be tuned. The only tune he has is his old turbo tune until his tuner becomes available. Joe had to boost limit his car to I think 5 psi while he waits for the tuner. His cams are different but degreed as I have suggested.

If it helps I can provide some of the math to explain what is happening. Try talking to Joe. He might possibly give you the extra data point or so to help you to make your decision.

I know it is stunningly difficult to chose. In many ways you are like the fictional Ethan Hunt in the Mission Impossible series, you have incomplete information available to you and you have to decide what to do. It is not easy ...

Ed
 
#39 ·
Font Parallel Circle Symmetry Screenshot


Here is my cam card, were actually 124 on the exhaust but the car is so drivable on the street out of boost in blown away. See how it reacts on the dyno soon but I'll tell ya, I'm petaling it to keep it from getting into boost. She wants to spoil the turbo much quicker than with my old engine.
Here are my cam specs
Font Number Rectangle Parallel Screenshot
 
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