Lobe separation

As my Moderator previously requested, you guys can cut this out or I'll close the damn thread...
 
Soup Nazi said:
There you go again. Another snipe. These are the same lobes the Helix used since 2004.

Anyone can understand that a lobe with less duration VS a lobe with more will almost always have a faster rate of lift as long as the lobe lift is similar in height. In other words a longer duration cam is more round, less peaky.

The CR cam has a short duration and has nearly .300 lift. The Helix is much longer duration and roughly the same lift. This is an easy mental picture to understand when compared to a mountain range. A peaky mountain with a fast rate of climb will have less real estate ( so to speak ) between its sides. A fatter mountain will have more real estate and will take more time to climb to the other side. The duration of time the Helix lobe spends lifting the tappet is simply longer. Clearly the duration figures I gave are longer.
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I think we're getting rate of lift and duration confused here. Rate of lift is just that, how fast the valve is reaching lift points as dictated by the lobe. A cam (call it cam A) that takes 30 degrees to get from .050" to .100" (just an in theory example) vs. a cam (call it cam B) that takes just 20 degrees to get from .050" to .100" has a slower RATE of lift from .050" to .100" regardless if Cam A is 230 degrees at .050" and Cam B is 280 degrees at .050". Sure its very likely cam B will be opening the valve much sooner and farther for the given degree of crank position unless the intake/exhaust center lines are incredibly different but that doesn't mean Cam B has a greater rate of lift.

But wait, I suppose the case could be made that rate of lift is nothing more than the position of the valve relative to the crank position; and as was stated by Soup Nazi a cam with more duration would have a higher rate of lift if that's the terminology being used. Boy, there's alot to this isn't there;)

And just to add to what's been said here without un needed negativity, there's simply too many parameters in a camshaft to single out just one of them. It pretty much comes down to four valve events, intake opening and closing and exhaust opening and closing and the lobe profile. There's a millions ways to keep some of these parameters similar while changing others to get very different results. Your best bet is to find someone who's invested the time and effort finding out what events produce the best results and what is durable and what is not.

If anyone is truly interested to learn more I suggest going past comp diesel and checking out some of the gas motor sites. They've been discussing valve events for far longer given the rpms associated with gas motor and the fact that n/a is a very popular setup for them.

There's much that can be learned and shared if people just try to learn and don't tell others they are flat out wrong or right. Many ways to skin a cat. :Cheer:
 
From a gas engine builder's point of view,most of the people commenting here are all headed in the right direction, just not collectively.
First, most racing engine builders will tell you that the most important valve event is exhaust valve opening. Why? When the exhaust valve opens, the first thing that happens is you stop making power. The longer you can keep the exhaust valve closed, the longer you can burn the mixture in the cylinder. This means you are still building cylinder pressure. This helps in two ways. First, you make more power (assuming you did a good job of filling the cylinder). Second, the higher pressure in the cylinder helps clear the cylinder during blowdown. Blowdown occurs immediately after the exhaust valve leaves the seat. The bulk of the pressure (and spent gasses) leave the cylider very quickly at this time, leaving the piston to sweep the cylinder clean during the rest of the exhaust stroke. In power adder engines, (boost or nitrous) I typically try to use lobes that are fairly agressive, moving the valve quickly and spending more time (duration) open at higher lifts (.200+). Lobes such as this do a very good job of evacuating the cylinder with a side benefit of shortening up the overlap event. This works very well on most internal combustion engines regardless of how they are ignited or fed. Obviously absolute numbers will vary depending on RPM, whether or not it is force-fed, etc.
On the intake side, lobes that make good power should obviously help feed the cylinder well.To this end, they should move the valve as quickly as possible and duration in the higher lifts (.200+) should be as long as possible without sacrificing overall system reliability. To that end, the Cummins has some features that help this. The cam core diameter is one as is the diameter of the followers. Some of them have fairly high-ratio rockers, which, when combined with proper springs will give good results.
Finally, lobe separation helps get overlap duration where it ought to be as well as placing valve events where you want them. Making statements like
""wide lobe separation cams are doggy" means that whoever came up with the grind didn't pay much attention to valve events.
The reality of most of the cams I see guys talking about on diesel sites is that they will probably have very peaky, somewhat disappointing torque curves and fairly high EGT's. They also will sacrifice engine efficiency relative to how much fuel it takes to make a given amount of power. And unless I missed something, that basically undoes what a diesel engine is all about.
As far as horsepower, it is a function of torque and RPM. If you make the torque and spin the engine high enough, you will make the power. Cam the engine for an unrealistic RPM range and you will be disappointed by the results.
 
Wow nice analogy! Lobe centerlines are a product of the ramp rate and size of the lobe factoring for the correct valve events. LSA's are a by-product not the means to an end.


Zach
 
Lobe centerlines can be oriented in any position of your choice, thereby establishing lobe separation. To say that lobe size and shape dictates lobe separation angle doesn't make sense.
 
vonEric said:
Lobe centerlines can be oriented in any position of your choice, thereby establishing lobe separation. To say that lobe size and shape dictates lobe separation angle doesn't make sense.
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I think it could make sense, if a setup requires a certain amount of overlap but an earlier opening of the exhaust valve as an example having a set requirement of overlap would dictate the amount of lsa you can work with. Add in the differences in the profile of the thousands of lobes in existence and it gets complex fast. There's a bunch of ways to solve my very simple example but point is I could see a case for what Zach said.

Zachs way of thinking is just another way to skin a cat.

Further, in my experience people aren't that concerned with exhaust opening point. Many would say it's not that important especially in a race gas motor where combustion happens extremely fast at high rpm, high compression, and top shelf combustion chamber designs. Most just open the SOB as early as they can because of the very small size of the exhaust valve in comparison to the intake valve in race (pro stock) apps and the fast burn not suffering from any loss associated with shortening the power stroke. Intake closing point is where most I know would give the number one importance.
 
"Lobe centerlines can be oriented in any position of your choice, thereby establishing lobe separation. To say that lobe size and shape dictates lobe separation angle doesn't make sense."

... unless you consider the larger view and factor in the valves' allowed range-of-motion in relation to physical constraints (piston clearance, coil bind, etc.), and desired flow characteristics as dictated by engine type (compression-ignition turbocharged) - not to mention the effect on LSAs from asymmetric profiles.
 
Asymetric lobe profiles typically are different after the point at which they open--in other words, you don't see a change in duration only on the opening side of the lobe. If you did it would change the lobe centerline. If you wanted to open the exhaust valve earlier but maintain your lobe separation you would add duration which would be split between both the opening and closing ramps. By definition if you added only to the opening side you would change where the center of the lobe is. If you moved the centerline of the lobe to retain your lobe separation number you would again change your opening point. As for Pro Stock engines exhaust opening point,if you tried to duplicate that in an engine that spins to 4500-5000 RPM you wouldn't make much power. In the stuff I've done, force-fed engines usually get less exhaust lobe. They don't need the overlap duration for scavenging. Boost will fill the cylinder.
 
vonEric, can you give us some background information on yourself? You seem to have a lot of knowledge on this subject and with only four posts it may be helpful for people to accept what you have to say.
 
mech2161 said:
vonEric, can you give us some background information on yourself? You seem to have a lot of knowledge on this subject and with only four posts it may be helpful for people to accept what you have to say.
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Apparently you don't understand what he spent so much time explaining.
 
mech2161 said:
vonEric, can you give us some background information on yourself? You seem to have a lot of knowledge on this subject and with only four posts it may be helpful for people to accept what you have to say.
Click to expand...

I have worked as a racing engine builder for almost twenty years. I've been involved in everything from street car drag racing (lots of nitrous and boost) to NHRA pro stock. I've worked as a professional head porter, been involved in developing parts for new-to-racing engine families and focused on how to make engines more powerful as well as reliable. Some of my stuff has set records but my main focus is to learn from everything I do. My interest in diesels stems from owning one and helping a few of my friends try to meet their perfrmance goals.
 
thank you vonEric for the back ground and a different view on things. If I am understanding all of the info that is provided a wider LSA is for a truck turning high rpm and the narrow LSA is for a truck turning lower rpm but that is only half the story becasue you still have to take into account the ramp rates and lift of the lobe. correct me if I am wrong.
 
vonEric said:
Asymetric lobe profiles typically are different after the point at which they open--in other words, you don't see a change in duration only on the opening side of the lobe. If you did it would change the lobe centerline. If you wanted to open the exhaust valve earlier but maintain your lobe separation you would add duration which would be split between both the opening and closing ramps.
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Yes, but by adding duration you would increase the overlap which in my example I through out, is not benefecial. My point is I can see how lobe size would dictate the "optimal" lsa. That being said lobe size by itself does not dictate lsa, they can be moved independantly of each other. Maybe that's where you're disagreeing with what Zach (Hamilton Cams) posted.


vonEric said:
As for Pro Stock engines exhaust opening point,if you tried to duplicate that in an engine that spins to 4500-5000 RPM you wouldn't make much power.
Click to expand...

I would agree with this, but when you sited race engines pro stock came to mind. Ofcourse a low rpm motor, especially a diesel with an extremely long power stroke and slow burn, is a very different creature.
 
vonEric said:
First, most racing engine builders will tell you that the most important valve event is exhaust valve opening. Why? When the exhaust valve opens, the first thing that happens is you stop making power. The longer you can keep the exhaust valve closed, the longer you can burn the mixture in the cylinder. This means you are still building cylinder pressure. This helps in two ways. First, you make more power (assuming you did a good job of filling the cylinder). Second, the higher pressure in the cylinder helps clear the cylinder during blowdown. Blowdown occurs immediately after the exhaust valve leaves the seat. The bulk of the pressure (and spent gasses) leave the cylider very quickly at this time, leaving the piston to sweep the cylinder clean during the rest of the exhaust stroke.

On a turbo engine, blowdown is slower. The exhaust gas runs into a restriction ( the turbine ) on its way out. Also, the volume of exhaust gas we need out is very high. The N/A gasser can dump its volume faster and may get away with a late exhaust event, but I think we should define what we mean by late and early first. The helix 2 opens the exhaust valve at 39 BBDC. In the gasser world that may seem late, but RPM ranges are usually higher.


On the intake side, lobes that make good power should obviously help feed the cylinder well.To this end, they should move the valve as quickly as possible and duration in the higher lifts (.200+) should be as long as possible without sacrificing overall system reliability. To that end, the Cummins has some features that help this. The cam core diameter is one as is the diameter of the followers. Some of them have fairly high-ratio rockers, which, when combined with proper springs will give good results.

Even at idle, the Cummins has some pressure on the intake manifold over atmo. Ever remove the boost pipe to the manifold and feel the volume air rushing out when the engine is idling? A turbo engine fills the cylinders fast. Just a point I think can get lost on anyone with a gasser background. Dont get me wrong I am a old gasser guy myself. I tuned a alcohol funny car for years before moving on. Heck my old ass worked at Harry Eberle's Super Shops in the early 80's if anyone remembers them??

Finally, lobe separation helps get overlap duration where it ought to be as well as placing valve events where you want them. Making statements like
""wide lobe separation cams are doggy" means that whoever came up with the grind didn't pay much attention to valve events.
The reality of most of the cams I see guys talking about on diesel sites is that they will probably have very peaky, somewhat disappointing torque curves and fairly high EGT's. They also will sacrifice engine efficiency relative to how much fuel it takes to make a given amount of power. And unless I missed something, that basically undoes what a diesel engine is all about.
As far as horsepower, it is a function of torque and RPM. If you make the torque and spin the engine high enough, you will make the power. Cam the engine for an unrealistic RPM range and you will be disappointed by the results.
Click to expand...

reply shows above in blue
 
vonEric said:
I have worked as a racing engine builder for almost twenty years. I've been involved in everything from street car drag racing (lots of nitrous and boost) to NHRA pro stock. I've worked as a professional head porter, been involved in developing parts for new-to-racing engine families and focused on how to make engines more powerful as well as reliable. Some of my stuff has set records but my main focus is to learn from everything I do. My interest in diesels stems from owning one and helping a few of my friends try to meet their perfrmance goals.
Click to expand...

VonEric is the man behind Goalie990 and my new engine program, He has designed our camshafts, valvetrain, and cylinder head port design, and helped us develop any and every other need we have had building these cummins engines.
 
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