Joined: Dec 2000 Posts: 5,174teamzr1 Owner - Pays the bills
teamzr1 Owner - Pays the bills Lives in Engine Bay
Joined: Dec 2000
With so many LS3 and L92 engines on the road and new Camaros already starting to be modified, it was high time to look at testing a gaggle of streetable cams perfect for the new Gen IV mill. But of course, then began the debate over what exactly defines "streetability"- one man's idea of wild being another's idea of "almost enough." So the idea of driveability and street manners are wildly subjective. Even power and torque, being quantifiable numbers, can be subjective as to how much is enough (if there is such a thing) or where in the operating range you need it.
So the question becomes: how do you quantify (objectively) how streetable a cam is? Well, we think we have the answer. Follow along as we test out some of the best LS3 street cams on the market for not just horsepower and torque, but also how well the cam matches up to your subjective scale of driveability.
We contacted a large handful of the tuners and manufacturers specializing in LS performance and requested an LS3 cam with street manners for testing. Knowing that it is subjective, we had to put some caps on things to keep it from getting too wild. As such, the cam specs that all suppliers were required to adhere to were:
1. The cam CANNOT require valve reliefs to be cut into the pistons! 2. Minimum 112-degree LSA 3. Maximum 0.650-inch lift 4. Must idle at 950 rpm or preferably less 5. Must have less than 50 kPa manifold absolute pressure at 950 rpm idle point 6. Must keep stock pushrod length and production hydraulic lifters 7. Valvespring replacement is OK 8. All cams will be measured and specs reported in the article Chevy Ls3 Engine Camshaft Comparison Engine Components Close Up
While we tested the power of these cams we also added a vacuum, or manifold absolute pressure (MAP), component to our evaluation. This is a good indicator (amongst many other factors) of how the engine would act on the street.
MAP at Idle & Low Speed
To evaluate the possible street manners of our group of cams we are as concerned about how likely it will operate in the car as to much as power and torque it will produce. For those not wanting to adjust their driving style to gain performance, we are calculating whether the cam will cause poor idle, surging at low speed, and stumbles coming to a stop.
Some can tolerate that-and the power numbers here won't impress you-but hopefully you still gain insight into how to make that wild cam a little more liveable. We decided to test the vacuum at idle [at 950 rpm]. This idle is a little high for a "streetable" cam, but when running on the dyno we determined this would be an idle at which we could easily measure MAP (manifold absolute pressure). In analyzing the data after the testing, we found we inadvertently recorded MAP values for the 1,500-1,600 rpm range, so we used that data point as well.
From experience, cams that fit a large segment of the market's definition of street manners run 35-50 kPa and idle from 750-850 rpm. More aggressive cams-the ones where a smaller segment of the market would classify it as streetable-can be up to 65-70+ kPa at idle and must idle up over 900 rpm. When getting into a 80 kPa range the car has a noticeable rock at idle and likely experiences surge when driving at low speed, low throttle situations (1,400-1,700 rpm, low throttle input). Some people still classify this as streetable, but for the purpose of this article, when you are giving up "manners" or have to adjust your driving style to avoid these situations, it doesn't fit our definition of streetable.
from Calibrated Success, the OEMs actually use statistical covariance analysis to determine and quantify "roughness" of an engine. It is a more linear and directly correlated measure of an engine's stability at idle and low speeds. While MAP is not linear and is not as reliable for predicting driving manners for production cars, we did draw some great correlation to MAP and cam overlap that matches our "expert" in-car experience over the years. Since the covariance analysis on our data did not yield reliable results and it would require extensive explanation on statistics, we opted to stick to data more readily obtained by a reader with an inexpensive scanner.
The Cams As Received
We put the call for cams out and received parts from nine suppliers. They all provided nominal specifications for each cam, but Dart was kind enough to let us roll each one on its Cam Doctor to be sure no "ringers" were substituted with a different spec card. All cams measured to within acceptable grinding tolerances and the measurement system so we accepted them all. Three manufacturers provided two cams, so we selected the one that best fit the intent of our criteria.
Even within our criteria there is room for subjectivity; you can see that there is a large spectrum of cam specifications, and that "streetable" cams are subjective even amongst the experts. It should also be noted that these cams generally fall in the low to middle-of-the-pack [range] for all of these suppliers-much bigger, higher horsepower camshafts are available, though not tested here. Chevy Ls3 Engine Camshaft Comparison Cam Lobe Visual Chart
All the cams were tested on a production 430-horse, 424 lb-ft of torque LS3 engine from GM Performance Parts (PN 19201992), which comes complete with coils, plugs, wires, throttle body, intake manifold, injectors, rails, and much more-ready to bolt right into your hot rod.
With just a little coaxing, this already stout crate motor comes alive using only mild upgrades. West Racing Heads and Uncle Robin were gracious enough to run the GMPP LS3 on its Stutska engine dyno using a stand-alone GMPP E-67 controller using only Hooker Competition Ceramic 1.75-inch headers (PN 2469-1HKR) and an electric water pump to baseline at 471.3 lb-ft and 483.1 hp. One base calibration for pump gas was performed that would work well with all the cams [to keep this a constant] and an Abaco DBX mass airflow meter, that senses standard flow as well as reversion to compensate for airflow differences with the cams, were also used for testing. Since most cams required springs to avoid coil bind, we selected a set of Comp 918 springs, likely the most used and readily available spring out there. Several cams came with different springs that may or may not have affected the results of said cams, but for the sake of timing and consistency we stuck with the 918s for all cams.
For consistent engine dyno testing, we carefully monitored water temp and performed at least three power pulls for each cam. After the first pull, the data was checked for consistency and for motor integrity. If all was good, two more pulls were made back to back. With ten cams, including the stock LS3, we had to ensure quality data and balance time on the dyno. Once the production cam was baselined and a new dyno calibration was created for all the performance cams, no further tune tweaks were made as the following procedure was followed:
1. Warm up engine to 175F water temp 2. All cams were set at the idle point of 950 to check actual intake vacuum 3. Make one power pull and check the data 4. Make two more pulls back-to-back
We recognized that each cam would ideally have its own proper calibration to fully optimize it, but the general tune utilized was checked with each new cam to verify it was still in the ballpark. As many argue on the forums, the calibration or tune is another area that offers a range of options that can balance (or weight to different extremes) durability or performance. A safe dyno calibration was created for the purposes of this testing that targeted 13:1 air/fuel ratio and a maximum of 28 degrees of spark. For street use an even richer A/F ratio may be necessary.
An important question when installing a new cam in an otherwise production engine, is whether the valves will hit the piston with the increased duration and tightened centerlines. We checked the biggest cam with the tightest LSA (that subsequently was not chosen for the test because of too much cam overlap) to verify piston to valve clearance. With a 232/246-duration, 0.614/0.619-inch lift on a 112 LSA, this cam still had 0.100-inch clearance on the intake valve and 0.120-inch on the exhaust valve.
We generally like to see at least 0.080-0.100-inch clearance on the intake and 0.100-0.120-inch on the exhaust, depending on how brave you are and how confident you are the valvetrain is in control.
As one would guess from the range of cams that were submitted, the power results varied a lot as well. Remember that peak power is not necessarily the only factor considered here. If it were, all the suppliers would have provided much different camshafts. When stepping back and looking at the results on a whole-the test cams averaged an improvement over the LS3 of 24.5 horsepower and 23 lb-ft of torque over the entire 3,000-6,700 rpm test range. Peak power gains ranged all the way up to 70 horsepower! While peak power is nice, most driving doesn't occur at 6,200 rpm, so area under the curve is important for seat-of-the-pants enjoyment. So we looked at the area under the curve for various regions of the curve. We would like to evaluate even lower, but often on water brake dynos it can be difficult to properly control WOT runs at 2,000 rpm or lower.
When it comes to identifying how these cams would impact driving manners, we monitored the MAP values with OBD Scanner for each cam. Closely following the MAP values, Robin said he noticed a difference in some cams on the dyno-the lowest MAP value Lingenfelter cam "purrs like a kitten," the Mast Motorsports cam was very smooth, and at the opposite end of the spectrum the Livernois and Lunati cams "had significant lope."
The trend of higher MAP values with larger overlap values are clear from this testing. To try to put all this into perspective and correlate the subjective term of "driveability" into some objective, measurable format we created this cross-reference chart based on experience, the data discovered here and input from other experts in the calibration/cam design field.
Some of the bigger cams with higher MAP at idle oscillated more when coming off wide open throttle, another indicator it would experience some instability in the car, putting it into a lower "driveability" scale level.
Another factor that can't be ignored, is that as changes are made that affect street manners, emissions may be impacted. Our understanding is that none of these cams have gone through the exemption process to be street legal, but all the companies are working toward that with various products or already have cams with EOs.
The entire subject of emissions will have to be addressed by the entire market as we move forward into a new era of regulations. Banish further adds, "bigger cams alone don't destroy emissions, but when combined with good controls-good air-fuel ratio control-they can still meet emissions. The General Motors production cams that have evolved over the life of LS engines are proof of that, they continue to get larger and meet tighter emissions."
Cam Design Strategy
What impacts the driving manners of a camshaft? There are many factors and several, such as calibration, proper functioning of all the sensors in the engine, and other hardware, are outside the control of the cam, but do impact the cars ability to idle and drive in an acceptable manner. Even the closed loop controls algorithms embedded into the ECM can make an impact. Ask any calibrator and they'll tell you that some year Corvettes are easier to tame a big cam than others because of the controller.
The cam itself plays a big part though-let's talk through each factor that plays a role in the performance and driveability of the cam. Number one on the driveability front is the cam overlap. This is the degrees by which the intake and exhaust overlap (or don't). This factor is influenced by the duration and lobe separation angle (LSA). Those along with lift also impact performance of the cam. The profile itself, or the actual shape of the cam, affects accelerations, area under the curve of valve opening and the forces exerted on the valvetrain system (and therefore wear/durability).
Lastly, the timing of the cam as it is installed changes the opening and closing events relative to TDC and usually affects the tuning of the cam-or where it makes peak power.
Each of these cams goes about its business in a different way, trying to meet different objectives. Some utilize tighter LSA centerlines and more overlap to obtain more performance.
Some utilize lift to a greater extent. Higher lift with shorter duration tends to increase the accelerations and forces experienced by the whole valvetrain.
In theory, you want a square waveform for your valve opening and closing events-instantaneous opening and closing of the valve. Since this isn't physically possible, designers choose an acceleration they believe they can live with for the application and open the valve as fast as possible to meet that number.
Acceptable accelerations are limited by the rest of the valvetrain system-the weight, spring load, and natural frequencies of the system. The Mast Motorsports cam, for example, was originally "designed for a 6.0L L76 with solid stem valves. Higher accelerations can be used for the lighter weight LS3 valves, which still maintain control and produce better performance.
This cam offers excellent valve control and durability for people who don't want to risk the health of their engine for the ultimate performance," says Horace Mast. Of course Mast has new cams for the LS3 since this testing was completed, and much larger offerings for those more concerned with greater peak power.
Arguably, the GM Performance Parts Hot Cam has probably been around longer than all the cams submitted, and by today's standard is relatively small, yet still made 17 lb-ft torque and 40 horsepower! This shows the performance potential even with "smaller" camshafts.
"Our philosophy for a cam like this is to use wide lobe centerlines to maintain smooth idle and emissions combined with high accelerations and lift to flow more air," says Jason Haines of Lingenfelter. LPE's cam in this test has some of the smallest durations, the largest LSA (virtually identical to the LS3 production cam), but also the highest lifts for intake and exhaust. Despite short durations and wide LSA, it is above average in performance for the group.
More than half the field submitted cams that were ground by Comp Cams to the supplier's specs. Comp submitted its own spec, and you might say they know a thing or two about cams. Comp's in-house expert, Billy Godbold, recently finished a new family of LS-R camshafts.
"In my 'black book,' they have three peppers on a scale of 1-5. I like spicy food and took that idea from a local Thai restaurant," says Godbold. He feels, "to design a good camshaft you have to flip everything around in your mind and try to think like the air going in and out of the combustion chamber. A good designer considers the valve motion that best suits the application and works backwards to the camshaft." And then test, test, test.
Alan Futral says of Futral Motorsports' strategy, "we design and pick our lobes based upon the application to achieve the quickest lobe and valve acceleration without being hard on the valve and springs, and maintain upper rpm stability.
Our factors for a good street cam mostly is idle quality versus best under-the-curve torque to meet a customer's requests and goals." Futral's cam did offer good performance with mid-pack MAP values.
The highest performing cams were accompanied by higher MAP values and Livernois'
Dan Millen says, "this is the largest cam I would use in a LS3-style engine before needing a stall converter or gear swap." Robin did detect more hunting at idle and off power with the bigger cams, and they likely land on the wilder side of street manners in some people's book.
The summary of all this is that bigger cams tend to make for reduced driving manners, but usually perform better. However, overlap is the actual key to look for. A high-lift, large-duration cam doesn't necessarily have to have poor driving manners if the overlap is kept in check.
Hopefully this gives you a cross-reference to find where your "driveability desires" fall in a very subjective range.
Joined: Dec 2000 Posts: 5,174teamzr1 Owner - Pays the bills
teamzr1 Owner - Pays the bills Lives in Engine Bay
Joined: Dec 2000
In an effort to simplify what actually happens inside an engine, COMP CamsŪ invites you to "take a walk" inside a typical engine, just like the one you might have in your car. We will discuss valve events, piston position, overlap and centerlines.
Although we can not explain cam design in such a small space, we might be able to clear up some of the most misunderstood terms and make clearer what actually happens as the engine goes through its four-stroke cycle.
We will graphically illustrate the relationship between all parts of the engine and try to help you understand how the camshaft affects the power of the engine.
We begin with the piston all the way at the top with both valves closed. Just a few degrees ago the spark plug fired and the explosion and the expansion of the gasses is forcing the piston towards the bottom of the cylinder.
This is the event that actually pushes the crankshaft around to create the power and is referred to as the "power stroke" (figure 1). Each "stroke" lasts one half crankshaft revolution or 180 crankshaft degrees. Since the camshaft turns at half of the speed of the crank, the power stroke only sees one fourth of a turn of the cam, or 90 camshaft degrees.
As we move closer to the bottom of the cylinder, a little before the piston reaches the bottom, the exhaust valve begins to open. By this time most of the charge has been burned and the cylinder pressure will begin to push this burnt mixture out into the exhaust port.
After the piston passes the true bottom or Bottom Dead Center, it begins to rise back to the top. Now we have begun the exhaust stroke, another 180° in the cycle (figure 2). This forces the remainder of the mixture out of the chamber to make room for a fresh, clean charge of air-fuel mixture. While the piston is moving toward the top of the cylinder, the exhaust valve quickly opens, goes through maximum lift and begins to close.
Now something quite unique begins to take place. Just before the piston reaches the top, the intake valve begins to open and the exhaust valve is not yet fully closed. This doesn't sound right, does it? Let's try to figure out what is happening.
The exhaust stroke of the piston has pushed out just about all of the spent charge and as the piston approaches the top and the intake valve begins to open slowly, there begins a siphon or "scavenge" effect in the chamber.
The rush of the gases out into the exhaust port will draw in the start of the intake charge. This is how the engine flushes out all of the used charge. Even some of the new gases escape into the exhaust. Once the piston passes through Top Dead Center and starts back down, the intake charge is being pulled in quickly so the exhaust valve must close at precisely the right point after the top to keep any burnt gas from reentering.
This area around Top Dead Center with both valves open is referred to as "overlap". This is one of the most critical moments in the running cycle, and all points must be positioned correctly with the Top Dead Center of the piston. We'll look at this much more closely later.
We have now passed through overlap. The exhaust valve has closed just after the piston started down and the intake valve is opening very quickly. This is called the intake stroke (figure 3), where the engine "breathes" and fills itself with another charge of fresh air/fuel mixture.
The intake valve reaches its maximum lift at some defined point (usually about 106 degrees) after top dead center. This is called the intake centerline, which refers to where the cam has been installed in the engine in relation to the crankshaft. This is commonly called "degreeing". We will talk about this later also.
The piston again goes all the way to the bottom and as it starts up, the intake valve is rushing towards the seat. The closing point of the intake valve will determine where the cylinder actually begins to build pressure, as we are now into the compression stroke (figure 4). When the mixture has all been taken in and the valves are both closed, the piston begins to compress the mixture. This is where the engine can really build some power. Then, just prior to the top, the spark plug fires and we are ready to start all over again.
The engine cycle we have just observed is typical of all four-stroke engines. There are several things we have not discussed, such as lift, duration, opening and closing points, overlap, intake centerline and lobe separation angle. If you will refer to the valve timing diagram when we discuss these terms it might make things a lot easier to understand.
Most cams are rated by duration at some defined lift point. As slow as the valve opens and closes at the very beginning and end of its cycle, it would be impossible to find exactly where it begins to move. In the case illustrated, the rated duration is at .006" tappet lift. In our plot, we use valve lift so we must multiply by the rocker arm ratio to find this lift. For example, .006" x 1.5 =.009". Instead of the original .006" tappet lift, we now use .009" valve lift. These opening and closing points are circled so that you can see them. If you count the number of degrees between these points you will arrive at the advertised duration, in this case 270 degrees of crankshaft rotation.
In this illustration this is the same for both the intake and the exhaust lobes, thus making this a single pattern cam. Some cam manufacturers rate their cams at .050" lift. If we again multiply this by the rocker arm ratio, we get .075". we can mark the diagram and read the duration at .050" lift.
This cam shows around 224 degrees, standard for this 270H cam. The lift is very simple to determine. You can simply read it from the axis going up. This is the lift at the valve as we said earlier. Sometimes you will hear lift referred to as "lobe lift". This means the lift at the lobe or the valve lift divided by the rocker arm ratio. In this case, it would be .470" divided by 1.5 or .313" lobe lift. The lift is simply a straightforward measurement of the rise of the valve or lifter.
We touched on opening and closing points a little earlier, but now we want to consider them even further. We talked about when these points occur, and how they are measured. As you can see in figure 1, the valve begins to move very slowly then picks up speed as it approaches the top.
It does the same closing, coming down quickly then slowing to a gentle stop. It's kind of like driving your car. If you were to go from 0 to 60 mph in a fraction of a second and stop instantly, you can imagine what that would do to the car, not to mention the driver. It would be much too severe for any valve train to endure. You would bend pushrods, wear out cams, break springs and rockers, and lose all dynamic design. The cam would not run to the desired RPM level as you would have all these parts running into each other.
As the valve approaches the seat, you also have to slow it down to keep the valve train from making any loud noises. If you slam the valve down onto the seat, you can expect some severe noise and a lot of worn and broken parts.
So it is easy to see that you can only accelerate the valve a certain amount before you get into trouble. This is something Competition Cams has learned over the years-how far you can safely push this point.
Looking a bit further at the timing points, the first one we see on the diagram is the exhaust opening point. We have all noticed the different sounds of performance cams, with the distinct lopes or rough idle. This occurs when the exhaust valve opens earlier and lets the sound of combustion go out into the exhaust pipes. It may actually still be burning a little when it passes out of the engine, so this can be a very pronounced sound.
The next point on the graph is the intake opening. This begins the overlap phase, which is very critical to vacuum, throttle response, emissions and especially, gas mileage.
The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine.
If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition.
If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine.
The last point in the cycle is the intake closing. This occurs slightly after Bottom Dead Center, and the quicker it closes, the more cylinder pressure the engine will develop. You have to be very careful, however, to make sure that you hold the valve open long enough to properly fill the chamber, but close it soon enough to yield maximum cylinder pressure. This is a very tricky point in the cycle of the camshaft.
The last thing we will discuss is the difference between intake centerline and lobe separation angle. These two terms are often confused. Even though they have very similar names, they are very different and control different events in the engine. Lobe separation angle is simply what it says. It is the number of degrees separating the peak lift point of the exhaust lobe and the peak point of the intake lobe.
This is sometimes referred to as the "lobe center" of the cam, but we prefer to call it the lobe separation angle. This can only be changed when the cam is ground. It makes no difference how you degree the cam in the engine, the lobe separation angle is ground into the cam.
The intake centerline, on the other hand, is the position of the centerline, or peak lift point, of the intake lobe in relation to top dead center of the piston. This can be changed by "degreeing" the cam into the engine. Figure 1 shows a normal 270 degree cam. It has a lobe separation of 110°.
We show it installed in the engine 4° advanced, or at 106° intake centerline. The light grey curves show the same camshaft installed an additional four degrees advanced, or at 102 degrees intake centerline. You can see how much earlier overlap is taking place and how the intake valve is open a great deal before the piston starts down. This is usually considered as a way to increase bottom end power, but as you can see there is much of the charge pushed out the exhaust, making a less efficient engine.
There is a recommended intake centerline installation point on each cam card, and it is important to install the cam at this point. As far as the mechanics of cam degreeing, Competition Cams has produced a simple, comprehensive video (part #190) that will take you step by step through the process.