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- Apr 15, 2006
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- Sth Auck, NZ
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- HSV VS Senator, VX s2 Calais S/C
If you're running a supercharger, turbocharger, or nitrous, you need to tune your engine accordingly. We help you figure out which camshaft is best for you.
If ever you've had to choose the right camshaft for your engine you understand how tricky it can get. It boils down to knowing what you want the engine to do and how the vehicle will be driven most of the time. Where cam selection gets dicey is when you add power boosters like supercharging, turbocharging, or nitrous. What then?
How much power can you get from forced induction? Under optimum conditions you can experience as much as a 50 percent increase. However, this is the real world and your numbers are likely to be less. You have to go the extra mile to even get close to that 50 percent increase.
When you opt for power adders like superchargers, turbochargers, or nitrous you are changing your engine's dynamic. More importantly, your engine has to be ready for it. You are going from naturally aspirated to forced induction where incoming air is forced into the cylinders under pressure known as boost, which changes how the engine breathes and makes power. You don't want to run forced induction with the dynamics of a naturally-aspirated engine. It has everything to do with valve timing events. And then there's bore and stroke along with rod ratio that determines whether or not boost is even worth the investment. It also has to do with your engine's bones and whether it can handle the boost safely. Too much boost with an unprepared engine can mean disaster.
Compression ratio is a huge factor with forced induction. Too much compression with forced induction can destroy an engine in nanoseconds at wide-open throttle because detonation happens quickly. Compression ratio and forced induction are a series of tradeoffs. With increased compression you get more horsepower and torque. With increased boost comes more horsepower and torque. When you combine the two, engine life hangs in the balance. With supercharging and turbocharging compression must be conservative in the 8.5:1 to 10.0:1 range if you're going to run 5 to 8 pounds of boost and 92-octane pump gas. Ignition timing and air/fuel ratio should also be conservative and programmed by a qualified tuner.
An engine has to be fully prepared for boosted induction or you're just wasting time and money. Cast or hypereutectic pistons will not survive excessive amounts of boost because they will not withstand the excessive combustion temperatures you get with boost.
When you go from naturally aspirated to supercharging, turbocharging, or nitrous you also have to go to a camshaft designed to work well with forced induction. Once you make that decision, you have to fine-tune valve timing events to work well with the type of forced induction you intend. Supercharging performs differently (more quickly) than turbocharging, hence the need for a different cam profile. Turbo lag is why you need a different cam profile. Nitrous is yet a completely different element because it comes into play at wide-open throttle and full power. Nitrous and supercharger cams generally employ very similar profiles because both do their best work at high rpm.
Before we go any further, let's talk about Lobe Separation Angle (LSA) or what's also known as Lobe Centers. Narrow lobe separation angles range from 106 to 109 degrees. Wide lobe separation angles span 110 to 118 degrees. Narrow lobe separation angle results in lumpy idle quality due to valve overlap.
Cam selection really depends on who you talk to about blower cams versus stock cams. Some seasoned builders and racers swear by stock camshaft grinds for blower applications because there's not as much valve overlap and wider lobe separation with a stock grind. The concern with valve overlap and narrow lobe separation is losing precious boost pressure via the exhaust valve during overlap.
One expert we know in this field is Richard Holdener, who has a tremendous amount of dyno testing experience, who tells us cam duration beyond stock is used in a wide variety of blower applications to determine effective engine speed. He adds that with a normally aspirated engine, cam specifications, primarily duration, can be used along with intake manifold runner length to determine where the engine will produce peak horsepower and torque. You can't just look at it as cam and blower, but the entire engine package, including displacement, rod ratio, cylinder heads, induction, and exhaust.
We learn via experience it takes more than just cam profile to make power. There are many other engine elements that work along with cam profile. Cylinder heads, port configuration, valve size, chamber shape and size, compression, header tube sizing and configuration, and even axle ratio affect how your boosted engine will perform where the rubber meets the road.
Holdener adds that moving the torque curve higher in the rpm range will normally produce a higher peak power level, but low-end torque will suffer in the balance. You might suspect that this tradeoff has a negative effect on power once you add boost, but Holdener's dyno-testing experience has been known to prove otherwise. He has experienced extraordinary results with forced induction and a variety of cam profiles.
Cam selection has to wait until you've chosen a specific type of forced induction. Positive-displacement superchargers work best at lower-rpm ranges where they deliver an abundance of torque, however, they tend to taper off at high rpm. There are always exceptions to this rule depending on the supercharger and how the engine is tuned.
Centrifugal superchargers do their best work at high rpm due to the very nature of their design. The faster we spin them the more boost they make. Nitrous requires a cam profile similar to that of a centrifugal supercharger—which is high-rpm friendly.
These factors play into the type of cam profile you're going to select. Because we're forcing a bunch of air into the cylinders under pressure, how we manage exhaust scavenging is everything to power. You don't want the air/fuel mixture to be contaminated with spent exhaust gases from the previous power cycle that haven't left the chamber. You also want more lift and duration on the exhaust side to aid scavenging. Add to that the need for wider lobe separation to reduce valve overlap and lost power. It is a fine line between valve overlap and exhaust lift and duration.
Shane Pulido of Crower Cams tells us "There are many different avenues you can go down when it comes to choosing a cam designed for boosted engines, but the basic premise is a wider "Lobe Separation Angle" (LSA), which keeps boost pressure from going out the exhaust during valve overlap." He adds at the same time you're going to want more duration and lift on the exhaust side when the exhaust valve finally opens.
Shane goes on to say, "Supercharging and nitrous are the closest in nature with more numbers between intake/exhaust duration. For example 240/255-degrees duration (valves off their seats) at 0.050 inch with a wide lobe separation angle somewhere between 112-120 degrees." However, he adds, "This is based on many variables, such as cylinder head flow numbers, chamber size and shape, valve size and shrouding, compression ratio, and the rest of it." We cite these factors because the overwhelming amount of cylinder pressure needs to get out."
When we asked Shane about turbocharging, he had this to say: "Turbocharging is a whole different approach when it comes to planning cam profile. However, here at Crower we have learned over the past 50 years to increase in intake duration while shortening up exhaust duration, which contradicts what we've said about superchargers." He adds, "For example, with turbocharging, you going to want 240/235-degrees duration at 0.050 inch with a wide lobe separation angle of 112-118 degrees. Our logic behind this is to close the exhaust valve and keep the intake valve open longer to get a good healthy cylinder charge before compression and power strokes." Shane further adds this will also help spin the turbo aggressively for a faster spool up. And with that faster spool up you get more turbo boost quickly along with the resulting power.
A good article and it even mentions someone's hero but funny what they say about wider LSA's.