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Discussion Starter · #1 ·
This is a compilation of things to keep in mind when selecting a camshaft. I did this for a member a while back and inspired by another thread I've decided to go ahead and post it on the boards. This was compiled using personal experience, talks with other engine builders, as well as multiple published written materials.



Before we get to it, know that engines perform best when the system is optimized as a whole! EVERY component must be considered when selecting other upgrades. Don't expect a miracle cam to make you the ultimate engine. The system has to work together. A cam that works well for engine A may not for engine B.



(This is just a list of main considerations. I know this is redundant but it needs said again... Cam choice needs to be made on many engine characteristics and components so the things below may not always hold true for your application.)



OK, you've waited long enough...



The cam is the brain of an engine. It helps determine the engines characteristics at every rpm and can be optimized for the power delivery you wish to attain.



If you open the intake valve sooner then you give the incoming charge a head start in filling which can help high rpm power. This also increases the valve overlap period and the exhaust gases can be used to help pull in new charge. The low rpm power can suffer from a greater overlap duration though. Make sure you check valve-to-valve and valve-to-piston clearance when opening the intake sooner.



The most important part of a four stroke is the intake valve closing because it defines when the power producing period begins. To improve high rpm volumetric efficiency hold the valve open longer. It does this by using the inertia of the intake charge to help ramming before closing. The downfall is the low rpm torque will drop off because of low rpm port velocities being low and allowing the flow to reverse.



The exhaust valve opening point is also important because it determines when the pressure from the combustion begins to release. Opening the valve to late will increase pumping losses but to quick will kill cylinder pressure to soon. Early opening of the exhaust valve is generally used for high rpm and compression. High compression lets the charge burn quicker and it builds pressure sooner in the stroke. Early opening can be hard on the exhaust valve(s) and guide(s). An efficient exhaust port and system can allow for a later valve opening.



The exhaust valve closing point determines the end of duration and exhaust stroke. At high rpms a late closing lets the engine scavenge the exhaust better. At low rpms though, gas velocity is low and reversion occurs.



The optimum overlap is defined by various engine characteristics, including, combustion chamber design, intake and exhaust flow characteristics, and even piston dome. A tall dome may require more duration to allow for better scavenging. A lot of overlap works well with an open free flowing exhaust. If you are restricted to a poor exhaust system reduce overlap to improve volumetric efficiency. Also use less overlap when low lift flow is high and ports are not lazy.



The most common mistake with choosing a cam is to much duration.



Increase valve event duration or lift aggressiveness when rpm and/ or displacement are bumped up.



Shorter duration increases torque but more duration helps build horsepower. A long stroke engine tends to respond better to increased duration. Increased duration also moves the volumetric efficiency curve up the rpm range... along with torque and hp curves.



If the head you are using has good low lift flow then the engine won't require as much duration. At the same time, if the ports are large and lazy use less duration and overlap to increase cylinder pressure and volumetric efficiency.



If the exhaust-to-intake flow is low then you can increase exhaust timing to compensate.



Restrictive exhausts usually benefit from having more exhaust duration.



The better the exhaust port and system the later the exhaust valve can be opened. You may have to shorten duration to keep overlap at a good level though.



The lower the compression ratio the earlier the intake valve should close. Same goes for the other direction, higher compression needs a later closing intake. 12:1 mechanical compression can give you 10:1 corrected compression but at high rpms the dynamic compression could be 17:1. This is because volumetric efficiency is high from intake charge inertia.



Match valve lift to port flow and valve head diameter.



An engine with a high rod-to-stroke-ratio needs max airflow at a later degree because piston speed occurs later.



The faster the valve is lifted the more exposed area throughout the intake event. If you lift a valve quicker in the same duration you don't have the negative effects of increased duration. At the same time, it requires better valve train components. More often then not, a cam with more aggressive rate of lift will accelerate the bike faster.
 

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what is reversion?

is duration the total amount of time the valve is opened?

is lift the difference in open to closed? as in a distance?



this one is hard to put into words.... really hard

im guessing the distance from closed to open and then open to closed can be expressed as an integral of a cylinder's surface area (because its moving over time, and not fixed) (the circumference of the outter edge of the valve traveling up and down), does the speed at which the valve goes from closed to open and then open to close matter?? and also does the total distance that it travels make a difference? or is that what duration and lift are?

i hope you know what that is trying to ask. ask me, i might be able to explain it better after i think about it more....
 
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Discussion Starter · #3 ·
Reversion is when slow moving gases along the short side radius of the exhaust port reverse flow and move back into the compression chamber before the exhaust valve closes. Or when new charge that has moved into the compression chamber reverses flow and goes back out into the intake port before the intake valve closes.



Lift on the cam is the total height of the cam lobe minus the base circle diameter (how tall the cam lobe is). Valve lift is the cam lift times your rocker ratio (total distance away from the seat that the valve moves). If the valve lifts .250" that means at peak lift the valve is .250" off the seat. That would mean the valve moves a total distance of .500" from when it first opens then finally closes.



Duration is the time (in crank degrees) that the valve is off of the seat a certain distance. This is measured with no valve lash. There are three main duration numbers.



1. Duration @ .040" lift. This is the duration (in crank degrees) the valve is lifted .040" off the seat.

2. Duration @ .050" lift. This is the duration (in crank degrees) the valve is lifted .050" off the seat.

3. Seat to Seat duration. This is the duration (in crank degrees) the valve is not contacting the seat.



Duration includes the full motion of the valve. Meaning, from when it starts to lift, gets to peak lift, starts to close, then closes. If you are looking at duration @ .040" lift that means 0 degrees is when the valve has already lifted .040" and the final degrees is when the valve is .040" from the seat on its way back down.



The speed at which the valve lifts is very crucial. That is what the rate of lift is. The faster you move the valve open the more area under the curve. The more area under the curve the more total flow area the engine is given in that period of time. If you can lift the valve faster without breaking stuff then you don't need a lot of duration to have the same flow area. Look at my last consideration in the first post.
 

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ok, thats what that last comment meant. i thought it had something to do with like a different surface area being touched when the stem was farther exposed or something.



wow, reversion sucks.



.040" lift. is that where the counting for duration begins? why not at .000", as in the moment it begins to open? or is that number so small that nothing can get by it at that measurement?



this is very interesting. ill check this in the morning. sleepy time



thankyou jordan
 
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Discussion Starter · #5 ·
There are different points in which to start reading the cams duration. The most common are...



1. Duration @ .040" lift.

2. Duration @ .050" lift.

3. Seat to Seat duration.



Every cam has all three of those. Most cam manufactures only provide one or two though. Every cam also has a ramp rate. That is the rate at which you start to open the valve and the rate you slow it down and shut it. You can't just instantly start opening the valve or just slam it shut because that would be extremely hard on valvetrain components.



The "perfect" cam lobe would be square. You instantly open the valve to full lift then instantly have it shut. That isn't possible of course because the acceleration rate of the valve has to be kept down. To instantaniously move even a small mass would require an infinant amount of force.



We have to accelerate the valve as we first start to lift it, decellerate it when we reach peak lift, accelerate it back down toward the seat, then decellerate it right before it touches the seat.



The ramp rate of the cam does the first and last of the above.



Example...



Cam A has 285 degrees seat-to-seat duration and 250 degrees @ .040" lift.

Cam B has 290 degrees seat-to-seat duration and 245 degrees @ .040" lift.



Cam B has more seat-to-seat timing but it is actually less at a lift. This is because it has a slower ramp rate. That keeps the acceleration of the valve low. Cam A has more duration at a higher lift which adds a lot more area under the lift curve then the extra degrees cam B has at a very low lift.



The higher lift degrees make a bigger difference to the cam profile. For that reason, it is more common practice to compare camshafts at .040" or .050" lift. The best way to do it is to compare the actual lift graphs. Those are the graphs you see with degrees of crank rotation along the x axis and valve lift along the y axis. With those graphs you can compare all the characteristics of the camshaft.



Below is an example of lift graphs and the valve acceleration rates they provide. Study it and try to put together what I'm saying with what you can see in the picture.



 

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Thanks Cyclerider57, but can you tell us what all this means in regards to our most common cams, G2, Z40 etc... because most of us won't ever get a custom grind. Typical applications would be 125-146cc with big valve heads. You don't need to reveal secrets, just that one cam likes this, the other that etc...

Thanks for indulging our curiosities.
 

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Tygen1 said:
Thanks Cyclerider57, but can you tell us what all this means in regards to our most common cams, G2, Z40 etc... because most of us won't ever get a custom grind. Typical applications would be 125-146cc with big valve heads. You don't need to reveal secrets, just that one cam likes this, the other that etc...

Thanks for indulging our curiosities.


Yes a little bit more in english please
 

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this is great info but you guys need to realise that the choice of cam you use is decided by a bunch of things. 1st thing you need to determine is what and where do you want your power at (in the rpm range) then from there you can deterimine what cam, port shape, valve size and size of port you need. the sooner you figuire out what you want the eaiser it is for you or the engine builder to get started.
 

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he put it all into terms that i understand



thats why i asked specifically on terms and what this or that means.





285 degrees that you are talking about is on the crank right? the cam turns 1:2 with the crank



why does one of the graphs start at .275, and then go up to around .425, and then go down to around .150??



and are those bumps where it bounces whet it hits the seat and where it jumps on the very top of the lobe?
 
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Discussion Starter · #12 ·
Yes, that's crank degrees.



The one that starts at .275 lift is not lift at all. That is valve acceleration rate. Look at the right side of the graph for the corresponding numbers. .275 on the left side is actually 0 acceleration. Then when the valve starts to open the acceleration goes up. It reaches peak acceleration about mid lift then starts decelerating. That's why it goes back down toward the bottom. That is deceleration rate. Then it accelerates again then decelerates right before touching the seat. Once it contacts the seat it decelerates at a faster rate which is why it has a slight bump at the end. The "bumps" in the graph in the middle at the bottom are just the way the acceleration behaves as the nose of the cam lobe.



As Hack said, it's hard to say cam A will always work with application A because unless you are sending them off an assembly line every application is different. As far as a more simple description, that would be pretty difficult. Just think critically and it will eventually come to you.
 
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