Camshaft discussion: CFM requirements by RPM.
04-25-2004, 09:30 AM
#101
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Beavis5.3,I believe nascar engines are about12.5 cr the point Chris was making is they're running 260ish at .050 lift also these are flat tappet lifters that don't have near the lobe ramp accelaration rates as a good hydralic roller cam.Also you lose some duration when you consider the actual valve lash needed in the solid cam valvetrain.These engines are limited to 355 cubes I believe but they can be any bore x stroke combination. 
Also the more cr you have the bigger duration you can and need to use as you can have too much cylinder pressure which equals Detonation not a good thing!! 
Also the more cr you have the bigger duration you can and need to use as you can have too much cylinder pressure which equals Detonation not a good thing!! 
04-25-2004, 10:37 AM
#102
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Originally Posted by FASTONE
I can't beleive it , DenzSS you made me think alot about that formula and the 280 cfm , then it hit me out of the blue this morning.I kept trying to see it as a volume needed to fill the cylinder and not the speed at which it is filled, cfm is a speed measurement and a volume measurement. 
04-25-2004, 02:56 PM
#104
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Hey critter is MPH a speed measurement or mileage measurement??DefineCFM:CF:CUBIC FEET =VOLUME,M:MINUTE=TIME =SPEED measurement!!so CFM is a volume measurement over a measured amount of time.
04-25-2004, 09:39 PM
#105
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Originally Posted by Bink
cfm is a measurement of flow. Volume per unit time. Depending on tube, pipe, orifice size velocity can be changed..ie larger runners less velocity for a given flow.
04-25-2004, 09:53 PM
#106
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Originally Posted by FASTONE
Hey critter is MPH a speed measurement or mileage measurement??DefineCFM:CF:CUBIC FEET =VOLUME,M:MINUTE=TIME =SPEED measurement!!so CFM is a volume measurement over a measured amount of time.
04-26-2004, 08:43 AM
#108
Beavis, the cup engines are limited to 12 to1 and 350 to 358 CID. So CID and compression are very similar. Camshaft wise a 250ish flat tappet lobe is eqivelant to around a 230ish roller lobe. Now as you state if the cup teams can run 7000 to 9000 rpm with this size lobe, why do we 347CID trying to run larger lobes for less rpm?
Okay, back to the thread...
Chris
Okay, back to the thread...
Chris
04-26-2004, 09:18 AM
#109
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Critter--
You're getting a little too far ahead in your thinking at this point. This is just a base requirement, disregarding any orifice sizes, runner volume, or anything else. After we know how much air we NEED, we can go on to figuring out if we CAN feed it that much. Does that make sense.
You're getting a little too far ahead in your thinking at this point. This is just a base requirement, disregarding any orifice sizes, runner volume, or anything else. After we know how much air we NEED, we can go on to figuring out if we CAN feed it that much. Does that make sense.
04-26-2004, 09:39 AM
#110
6600 rpm clutch dump of death Administrator
Ok, how about someone answer me. 
Would y'all agree that once you determine your overlap "budget" that it is most optimal to do it with the least ammount of duration you can?
Now, my question that I asked above is simple. Its the whole question of fast lobes vs slower lobes. Obviously we know faster lobes are harder on valvetrain and springs, etc... So, if you want the same area under the curve, and you have a slower lobe, would you use a lobe with more duration (say @ .050) , or do you use the same duration.
Like my example above the 232@.050 is the same @.200 as a slower 242 lobe of course then the 232 stille nds up having more lift. But, suppose they were about the same lift. Would you worry as much about the .006-.200, or would you focus on your area under the curve?
Would y'all agree that once you determine your overlap "budget" that it is most optimal to do it with the least ammount of duration you can?
Now, my question that I asked above is simple. Its the whole question of fast lobes vs slower lobes. Obviously we know faster lobes are harder on valvetrain and springs, etc... So, if you want the same area under the curve, and you have a slower lobe, would you use a lobe with more duration (say @ .050) , or do you use the same duration.
Like my example above the 232@.050 is the same @.200 as a slower 242 lobe of course then the 232 stille nds up having more lift. But, suppose they were about the same lift. Would you worry as much about the .006-.200, or would you focus on your area under the curve?
04-26-2004, 09:53 AM
#111
Originally Posted by J-Rod
Ok, how about someone answer me.
Would y'all agree that once you determine your overlap "budget" that it is most optimal to do it with the least ammount of duration you can?
Yes, anymore duration then what is optimal for the intened powerband is a waste.
Now, my question that I asked above is simple. Its the whole question of fast lobes vs slower lobes. Obviously we know faster lobes are harder on valvetrain and springs, etc... So, if you want the same area under the curve, and you have a slower lobe, would you use a lobe with more duration (say @ .050) , or do you use the same duration.
I am going to get ahead of Denzss here alittle. Each engine combo, the complete deal, will have a "lift sweet spot", the point at which maximum cylinder fill is reached. Based on the head numbers, this "spot" is higher for poor flowing heads, lower for better heads. This "spot" also relates to duration at .100,.150, .200" and so on. The "agressiveness" of the cam is dictated by the flow numbers.
Like my example above the 232@.050 is the same @.200 as a slower 242 lobe of course then the 232 stille nds up having more lift. But, suppose they were about the same lift. Would you worry as much about the .006-.200, or would you focus on your area under the curve?
Would y'all agree that once you determine your overlap "budget" that it is most optimal to do it with the least ammount of duration you can?
Yes, anymore duration then what is optimal for the intened powerband is a waste.
Now, my question that I asked above is simple. Its the whole question of fast lobes vs slower lobes. Obviously we know faster lobes are harder on valvetrain and springs, etc... So, if you want the same area under the curve, and you have a slower lobe, would you use a lobe with more duration (say @ .050) , or do you use the same duration.
I am going to get ahead of Denzss here alittle. Each engine combo, the complete deal, will have a "lift sweet spot", the point at which maximum cylinder fill is reached. Based on the head numbers, this "spot" is higher for poor flowing heads, lower for better heads. This "spot" also relates to duration at .100,.150, .200" and so on. The "agressiveness" of the cam is dictated by the flow numbers.
Like my example above the 232@.050 is the same @.200 as a slower 242 lobe of course then the 232 stille nds up having more lift. But, suppose they were about the same lift. Would you worry as much about the .006-.200, or would you focus on your area under the curve?
Chris
04-26-2004, 11:59 AM
#113
6600 rpm clutch dump of death Administrator
So, when you come up with a target goal of lift+duration+ramp=lobe , if you need to reduce one of those threee, then to get the same result, then you increase one of the others to get the same area.
Now, when you do that, what do you do with the valve events.
Do you shoot for the same IVO or IVC on the intake side since something has to move one way or another to make up for the bigger lobe.
On the exhaust do you shoot for EVO or EVC? leave them the same i.e. a 232@ .050 XE-R is the same as a slower 242 @ .050 XE lobe.
Now, when you do that, what do you do with the valve events.
Do you shoot for the same IVO or IVC on the intake side since something has to move one way or another to make up for the bigger lobe.
On the exhaust do you shoot for EVO or EVC? leave them the same i.e. a 232@ .050 XE-R is the same as a slower 242 @ .050 XE lobe.
04-26-2004, 12:10 PM
#114
J-rod,
I leave them the same.. . .90% of the time. If it warrants it, I may change to decrease torque on a small tire class or increase cylinder pressure for a low comp. engine. There are variables.. I put your fella on the right direction for what he was looking for. . .
Chris
I leave them the same.. . .90% of the time. If it warrants it, I may change to decrease torque on a small tire class or increase cylinder pressure for a low comp. engine. There are variables.. I put your fella on the right direction for what he was looking for. . .
Chris
04-26-2004, 12:14 PM
#115
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I need to dig up an example I used to use. I had three different lobes out of the catalog. 1 street/strip hydraulic flat, one street hydraulic roller, one pure race solid roller. All different durations, all with the same EVO.
04-26-2004, 12:43 PM
#116
6600 rpm clutch dump of death Administrator
Ok, so in our little example above of an agressive 232 vs a milder 242.
242/242 108/108
Intake Valve opens - IVO 13
Intake Valve closes - IVC 49
Exhaust Valve Opens - EVO 49
Exhaust Valve Closes - EVC 13
Exhaust Centerline - ECL 108
Overlap 26
232/232 108/108
Intake Valve opens - IVO 8
Intake Valve closes - IVC 44
Exhaust Valve Opens - EVO 44
Exhaust Valve Closes - EVC 8
Exhaust Centerline - ECL 108
Overlap 16
IVO delta 5
IVC delta 5
EVO delta 5
EVC delta 5
ECL delta 0
Overlap Delta 10
So, you wouldn't worry as much about keeping the overlap the same, or shooting for the same valve event?
BTW, thanks for the help.
242/242 108/108
Intake Valve opens - IVO 13
Intake Valve closes - IVC 49
Exhaust Valve Opens - EVO 49
Exhaust Valve Closes - EVC 13
Exhaust Centerline - ECL 108
Overlap 26
232/232 108/108
Intake Valve opens - IVO 8
Intake Valve closes - IVC 44
Exhaust Valve Opens - EVO 44
Exhaust Valve Closes - EVC 8
Exhaust Centerline - ECL 108
Overlap 16
IVO delta 5
IVC delta 5
EVO delta 5
EVC delta 5
ECL delta 0
Overlap Delta 10
So, you wouldn't worry as much about keeping the overlap the same, or shooting for the same valve event?
BTW, thanks for the help.
04-26-2004, 12:44 PM
#117
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Originally Posted by DenzSS
Critter--
You're getting a little too far ahead in your thinking at this point. This is just a base requirement, disregarding any orifice sizes, runner volume, or anything else. After we know how much air we NEED, we can go on to figuring out if we CAN feed it that much. Does that make sense.
You're getting a little too far ahead in your thinking at this point. This is just a base requirement, disregarding any orifice sizes, runner volume, or anything else. After we know how much air we NEED, we can go on to figuring out if we CAN feed it that much. Does that make sense.
04-26-2004, 01:33 PM
#119
6600 rpm clutch dump of death Administrator
Originally Posted by Cstraub
J-rod,
No. The engine tells you what it needs.
Chris
No. The engine tells you what it needs.
Chris
I don't have the actual lobe areas, but I am using these as strictly an example
Lets say a 220 duration lobe with a .615 lift vs a 242 with .560 lift. if the area under the curve is the same, then you could use a more agressive lobe as long as the lobe area is the same. Now, I know, there are drawbacks to using a lobe that is hyper-agressive on a hyd. roller valvetrain.
All things being equal do you prefer a slower lobe to alllow the air "time" to get moving, or a cam with lobe with agressive lobes to get open and in the power quicker?
04-26-2004, 01:44 PM
#120
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Originally Posted by critter
I understand what you are saying, in that you are talking about how much air we need and that it is a base requirement before we try to figure out how to feed it that much. What I don't understand is how you get 279 CFM as "how much you need" at 6800. One cylinder is 0.025 ft^3 and filling it 3400 times per minute requires 85 CFM if the flow is constant. So, you are using some condition(s) and/or some method that you have not revealed to arrive at 279. That is why I asked for the code - to try to understand what is going on. If you don't wish to reveal that, I guess I will have to take it on faith that 279 is the number and wait until you are further on with your discourse.
I'll get it to you, just may take a bit.