Lobe separation

woah, good read! im not an expert on camshafts so this is really helpful information.

back to the side lines!:pop:
 
Soup Nazi said:
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Actually, I use shorter duration exhaust lobes on my power adder stuff than I do on my n/a stuff. If you fill the cylinder better, you will make more power. More power is a by-product of more cylinder pressure and/or holding onto the charge in the cylinder for longer. If your pressure is higher when you open the exhaust valve, it will escape the cylinder faster and all you have left to do is sweep the cylinder clean (the pressure is a large part of what you're trying to get rid of).I have dyno-tested this time and again and always see the same result. The first time I found it, I changed a cam in a supercharged engine. I took over ten degrees of duration off of the exhaust lobe (opening the valve much later between lsa changes and duration changes) and saw three things:
Dramatically more power, even at higher RPM--200+ hp
Dramatically lower fuel consumption
Dramatically lower EGT's--500* lower
All at the same boost level
N/A stuff typically needs more duration depending on desired RPM but also because, I believe, cylinder pressures are lower. All of this follows lobe development that leads one in this direction. A lot has changed in the world of lobe profiles since the 80's. I'm sure that if you looked at the lobes being used even on diesel cams, they would fit nicely into this description.

As for blowdown being slower in diesel engines, I would have to say that it would relate to cylinder pressure vs drive pressure. Cylinder pressure will be higher than drive pressure, so until they equalize, blowdown will still occur. My eperience with this, though, has shown that it tends to limit RPM range of the engine. If your drive pressure is so high that it gets in the way of power potential in a diesel, it seems to me you would want to rethink your turbine size. Just one man's opinion.
 
NickTF said:
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.
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The only way to open the exhaust valve earlier and maintain your overlap # and your lobe separation is to advance the cam or take duration off fo the intake lobe, which would still make you advance the cam, and possibly more, depending on how much you shrunk the intake lobe.
 
vonEric said:
The only way to open the exhaust valve earlier and maintain your overlap # and your lobe separation is to advance the cam or take duration off fo the intake lobe, which would still make you advance the cam, and possibly more, depending on how much you shrunk the intake lobe.
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Why would you want to keep the same LSA when you want to open the exhaust valve sooner. Just use a slightly larger exhaust, re adjust the lsa to whatever it ends up being to get your overlap where you want it and your exh valve opening where you want it and leave the intake where it wants to be. If you readjust the intake centerline to get your LSA where you want it you will affect the engines power band by changing (either more or less) degrees of duration after bdc. This is an instance where desired LSA would take a back seat to valve events.

In the same manner advancing the whole cam could be a step in the wrong direction. Sure you keep the same overlap, LSA, and open the exhaust sooner but you change where overlap occurs in relation to TDC and piston position and move the intake much closer to the piston. The bottom end would be great but the top end would suffer quite a bit.
 
Hamilton Cams said:
Why would you want to keep the same LSA when you want to open the exhaust valve sooner. Just use a slightly larger exhaust, re adjust the lsa to whatever it ends up being to get your overlap where you want it and your exh valve opening where you want it and leave the intake where it wants to be. If you readjust the intake centerline to get your LSA where you want it you will affect the engines power band by changing (either more or less) degrees of duration after bdc. This is an instance where desired LSA would take a back seat to valve events.

In the same manner advancing the whole cam could be a step in the wrong direction. Sure you keep the same overlap, LSA, and open the exhaust sooner but you change where overlap occurs in relation to TDC and piston position and move the intake much closer to the piston. The bottom end would be great but the top end would suffer quite a bit.
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If you will notice the quote in my post, I was responding directly to a proposed situation. I don't necessarily agree with it, but tried to respond to what was presented. Oh, and if you're going to design a camshaft only around available piston to valve you'll always end up with a compromise. I prefer to do what the engine wants and have very good luck running cams advanced, especially ones with agressive lobes. Sometimes its the only way to get valve events where you want them.
 
vonEric said:
The only way to open the exhaust valve earlier and maintain your overlap # and your lobe separation is to advance the cam or take duration off fo the intake lobe, which would still make you advance the cam, and possibly more, depending on how much you shrunk the intake lobe.
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Forgive me if I worded it wrong but in my example the only thing I was throwing out as constant was overlap (or that's what I meant), all the while why varying the exhaust opening point. Sure, as you've stated you could keep lsa the same and advance/retard the cam changing intake opening and closing points but you can also change the lsa which is why i'm making a case that using a certain lobe shape may in fact dictate the optimal lsa. For instance, if your intake opening and closing points are already set/optimal, you need a certain amount of overlap, and want only to change the exhaust opening point you need to change the lsa and that's the only option. Unless you want to get into asymetrical lobes I guess. Understand this is a very crude example and not a specific one. Going back to my original point there could be a case where lobe shape may dictate optimal lsa.
 
Still looking for a befor and after Dyno sheet from a cam install on a 12V.......
 
NickTF said:
Forgive me if I worded it wrong but in my example the only thing I was throwing out as constant was overlap (or that's what I meant), all the while why varying the exhaust opening point. Sure, as you've stated you could keep lsa the same and advance/retard the cam changing intake opening and closing points but you can also change the lsa which is why i'm making a case that using a certain lobe shape may in fact dictate the optimal lsa. For instance, if your intake opening and closing points are already set/optimal, you need a certain amount of overlap, and want only to change the exhaust opening point you need to change the lsa and that's the only option. Unless you want to get into asymetrical lobes I guess. Understand this is a very crude example and not a specific one. Going back to my original point there could be a case where lobe shape may dictate optimal lsa.
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If you change only the lobe separation angle and thats it you will change your overlap duration. To my knowledge there is no way around it in this example. You can figure overlap very easily:

Take your intake duration and divide it in half. Now subtract your lobe sep from that. Do the same for the exhaust. Add the two results together and you have overlap duration. Any change you make to lobe sep WILL change overlap duration if lobe sep is the only change. I hope I'm understanding you correctly. Also, I think if you were to put most cams today on a cam doctor or other such equipment you would find that most lobes are asymmetrical in that the closing ramps are much smoother than the opening ramps, helping to set the valve down easily and without a lot of bounce. I'm not sure if this fits what most people here think an asymmetrical lobe is but its true.
 
vonEric said:
If you change only the lobe separation angle and thats it you will change your overlap duration. To my knowledge there is no way around it in this example. You can figure overlap very easily:

Take your intake duration and divide it in half. Now subtract your lobe sep from that. Do the same for the exhaust. Add the two results together and you have overlap duration. Any change you make to lobe sep WILL change overlap duration if lobe sep is the only change. I hope I'm understanding you correctly. Also, I think if you were to put most cams today on a cam doctor or other such equipment you would find that most lobes are asymmetrical in that the closing ramps are much smoother than the opening ramps, helping to set the valve down easily and without a lot of bounce. I'm not sure if this fits what most people here think an asymmetrical lobe is but its true.
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We are very close in our understandings. I understand what you're saying. To keep overlap the same, vary the exhaust opening point without varying IO or IC, you would have to change the duration of the exhaust lobe and adjust the lsa accordingly.
 
The problem with a before and after dyno on a 12v is that there are too many variables to get clear numbers. On a cr you have your fueling and timing set on X. On a 12v the number one problem I have seen is peoples ability to tune the truck. This will yield varying results that are hard to be duplicated between different setups. In other words one person might see 50hp but another might see 10 because their ability or lack thereof to tune. I have put a lot of cams in 12 valves with positive results. The customers that I have sponsored or gave discounts to give before and after dyno results, typically take the cams and run without dyno numbers.

As far as smoke and mirrors in marketing that is what we are trying to avoid. I will have numbers in the next few weeks.


Fair enough?

Zach Hamilton
 
On a 12v the number one problem I have seen is peoples ability to tune the truck.
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Bingo.......sorry people but there are not many out there that don't know "B/B Chevy with duel points distributor and a carb" tuning

Ryan, good post, that was even before my Navy days!

Jim
 
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