Camshaft discussion: CFM requirements by RPM.
05-20-2004, 10:49 AM
#341
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Originally Posted by verbs
Interesting how spinning my car an extra 200rpms creates a jump from a 218 to a 224 cam.
. Seems like a big jump to me.
. Seems like a big jump to me.
Originally Posted by Cstraib
Verbs,
Your 200 rpm. I refer this to the "Threshold". Any engine has a point at which the induction goes from a positive, easily fill the CID, to a Negative, start to strain to fill the CID. When you hit this "threshold" number the camshaft numbers jump dramaticlly because now the cam is working beyond the capabilities of the induction system. This is common in NHRA stock, limited circle track, restrictor plate engines, etc. To keep the effiecency up, the cam has to grow.
Your 200 rpm. I refer this to the "Threshold". Any engine has a point at which the induction goes from a positive, easily fill the CID, to a Negative, start to strain to fill the CID. When you hit this "threshold" number the camshaft numbers jump dramaticlly because now the cam is working beyond the capabilities of the induction system. This is common in NHRA stock, limited circle track, restrictor plate engines, etc. To keep the effiecency up, the cam has to grow.
cam_duration = CID * RPM / CFM / 38.4
That is linear - there are no steps. If displacement and head flow are held constant, predicted cam duration is proportional to RPM. Using the example in question, with 290 CFM, 7200 RPM yields 223.7*, 7000 RPM yields 217.5*, and 6500 yields 201.9*.
I don't know about the validity of the method, but the math is clear.
05-20-2004, 10:55 AM
#342
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Originally Posted by 99 Black Bird T/A
290 thru the intake is very serious airflow...I agree if you get that type of flow it calculates out to a much smaller cam than I listed.
I also agree it's pretty amazing how much more cam is needed for an extra 200 rpm based on the ghetto formulas.
Looking forward to seeing the flow data with the LSX intake.
I also agree it's pretty amazing how much more cam is needed for an extra 200 rpm based on the ghetto formulas.
Looking forward to seeing the flow data with the LSX intake.
Lift Head LS6 Ported LS6 Ported LSX
.100 69.7 66.1 67.5 68.1
.200 138.6 133.0 135.2 135.6
.300 201.2 190.4 193.1 194.6
.400 254.0 232.5 235.7 239.0
.500 291.7 261.5 265.4 268.3
.550 309.0 273.0 277.2 280.7
.600 318.7 282.0 288.2 290.3
.700 xxx.x 292.4 295.9 296.0
05-20-2004, 11:29 AM
#344
FormerVendor
Originally Posted by Cstraub
Well Erik, I was talking about in the 80's and the intro of "plates". Said nothing about what has transpired since.
Chris
Chris
05-20-2004, 03:04 PM
#345
6600 rpm clutch dump of death Administrator
I have updated my VE calulator spreadsheet to also have a tab for putting that info in, and getting DCR out of it. I also put in a header length calculator.
Are the .006, .050, and .200 enough, or would folks like to see anything else?
Also, I will put the port to duration calculation in there if folks will agree on which formula is really right...
http://users3.ev1.net/~black_ops/cam...calculator.xls
Are the .006, .050, and .200 enough, or would folks like to see anything else?
Also, I will put the port to duration calculation in there if folks will agree on which formula is really right...
http://users3.ev1.net/~black_ops/cam...calculator.xls
05-20-2004, 05:16 PM
#346
Originally Posted by Cstraub
Verbs,
Your 200 rpm. I refer this to the "Threshold". Any engine has a point at which the induction goes from a positive, easily fill the CID, to a Negative, start to strain to fill the CID. When you hit this "threshold" number the camshaft numbers jump dramaticlly because now the cam is working beyond the capabilities of the induction system. This is common in NHRA stock, limited circle track, restrictor plate engines, etc. To keep the effiecency up, the cam has to grow.
Case in point, when Nascar first inacted the "plate racing", camshafts grew and springs broke. The air flowing through a plate was only around 600 cfm, not even close to feed 358cid at 7500 rpm. So engine builders through duration at it to keep the intake valve open and "suck" up as much air as they could get. Takes about 750 to 780, ( these are figures) to keep that 358 CID at 100% VE. Well 1 engine builder thought different. He new he could make 110% VE, turn more rpm, and all with a small cam that would give good responsiveness out of the pits. So Ernie Elliot built a 308 CID engine. The restrictor plates allowed enough air for this engine to make big power. . .needless to say, Bill Elliot kicked butt and Nascar now has a minumum rule of 350 CID and a max of 358 CID.
Again matching the air with camshaft. . . critcal.
Chris
Your 200 rpm. I refer this to the "Threshold". Any engine has a point at which the induction goes from a positive, easily fill the CID, to a Negative, start to strain to fill the CID. When you hit this "threshold" number the camshaft numbers jump dramaticlly because now the cam is working beyond the capabilities of the induction system. This is common in NHRA stock, limited circle track, restrictor plate engines, etc. To keep the effiecency up, the cam has to grow.
Case in point, when Nascar first inacted the "plate racing", camshafts grew and springs broke. The air flowing through a plate was only around 600 cfm, not even close to feed 358cid at 7500 rpm. So engine builders through duration at it to keep the intake valve open and "suck" up as much air as they could get. Takes about 750 to 780, ( these are figures) to keep that 358 CID at 100% VE. Well 1 engine builder thought different. He new he could make 110% VE, turn more rpm, and all with a small cam that would give good responsiveness out of the pits. So Ernie Elliot built a 308 CID engine. The restrictor plates allowed enough air for this engine to make big power. . .needless to say, Bill Elliot kicked butt and Nascar now has a minumum rule of 350 CID and a max of 358 CID.
Again matching the air with camshaft. . . critcal.
Chris
joel
joel
05-27-2004, 12:52 PM
#348
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Originally Posted by J-Rod
I have updated my VE calulator spreadsheet to also have a tab for putting that info in, and getting DCR out of it. I also put in a header length calculator.
Are the .006, .050, and .200 enough, or would folks like to see anything else?
Also, I will put the port to duration calculation in there if folks will agree on which formula is really right...
http://users3.ev1.net/~black_ops/cams/VE%20calculator.xls
Are the .006, .050, and .200 enough, or would folks like to see anything else?
Also, I will put the port to duration calculation in there if folks will agree on which formula is really right...
http://users3.ev1.net/~black_ops/cams/VE%20calculator.xls
05-27-2004, 01:26 PM
#349
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Originally Posted by 99 Black Bird T/A
J-Rod, do you know what the heads on the car mentioned above flow with the intake in place?
I think the key point that often gets missed is very seldom do people have flow numbers with the LS6/LSX intake in place. These intakes can cut as much as 50 cfm off the intake flow numbers depending on the cylinder head.
My guess is the Absolutes on the car above flow ~325 without intake and 270 to 280 with the intake in place. These would be excellent flow numbers with the intake. I've had an older set of LS6 heads that would only flow 239 with the intake in place.
7000 rpm seems reasonable for the C5 above...lets do a little ghetto math...
346/2*7000 divided by 1728 = 707 cfm needed for the engine
707/8 = 88.4 cfm need per cylinder
Lets say the head flows 275 WITH INTAKE for this calculation
88.4*X=275 solve for X and X is .321
the cam need .321 of 720 degrees of crank rotation to get the air in the cylinder
so a 231.4 degree cam is needed
we know the ghetto math isn't exact so a couple of degrees either way might need to be tweaked. A low 230 something cam say between 231 and 235 sounds like a good starting point.
John speculated the G cams are low 230's as the ghetto math suggests they should be.
I think the key point that often gets missed is very seldom do people have flow numbers with the LS6/LSX intake in place. These intakes can cut as much as 50 cfm off the intake flow numbers depending on the cylinder head.
My guess is the Absolutes on the car above flow ~325 without intake and 270 to 280 with the intake in place. These would be excellent flow numbers with the intake. I've had an older set of LS6 heads that would only flow 239 with the intake in place.
7000 rpm seems reasonable for the C5 above...lets do a little ghetto math...
346/2*7000 divided by 1728 = 707 cfm needed for the engine
707/8 = 88.4 cfm need per cylinder
Lets say the head flows 275 WITH INTAKE for this calculation
88.4*X=275 solve for X and X is .321
the cam need .321 of 720 degrees of crank rotation to get the air in the cylinder
so a 231.4 degree cam is needed
we know the ghetto math isn't exact so a couple of degrees either way might need to be tweaked. A low 230 something cam say between 231 and 235 sounds like a good starting point.
John speculated the G cams are low 230's as the ghetto math suggests they should be.
CFM Needed
Engine Cylinder
610.7060185 76.33825231
LS1 heads w/ LS6 intake Intake
lift flow x of 720 Deg. Cam Duration?
.200" 136 0.561310679 404.1436887
.300" 186 0.410420711 295.5029122
.350" 206 0.37057404 266.8133091
.400" 223 0.342324001 246.473281
.450" 227 0.33629186 242.1301395
.500" 236 0.323467171 232.896363
.550" 241 0.316756234 228.0644882
.600" 242 0.315447324 227.122073
This means I need a 230/226 or so Cam with .55" lift (that exhaust is a complete guess)? That's as high of lift as I want to go cause I want to stay stock heads for a while (about 5 or so years). I know this is ghetto math and I'm gonna get Al Futral to grind it for me, so he'll have the experience and the know-how that I don't, I just want to know how to ask for the right thing.
Thanks!
05-27-2004, 03:34 PM
#351
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Originally Posted by 99 Black Bird T/A
I was only calculating fo the peak flow for the head with intake in place. We've not got into the detailed discussion beyond that. Allan will do an excellent job of selecting a cam for you.
05-27-2004, 04:40 PM
#352
6600 rpm clutch dump of death Administrator
Originally Posted by critter
J-Rod - the piston position numbers are wrong. I tried to understand the formula (not one I have seen/used before) but finally figured out that Excel wants angles in radians. DST is still wrong, but RD and RR are now correct.
I'll have a look see, and see if I can fix it. Oops.
06-02-2004, 12:04 PM
#355
6600 rpm clutch dump of death Administrator
Ok, so now we sort of have an understanding of CFM rquirement by RPM. I say sort of based on the fact that we have a "ghetto math" formula, and we also have a integral with time and frequency components being solved for, but as for what you do with that (the integral) I am still a bit fuzzy as on how to apply it...
Now, assuming we can solve for CFM and RPM, here is the next question. How do we solve for lift. What is the determination in deciding how much lift is enough vs how much is too much.
Now, assuming we can solve for CFM and RPM, here is the next question. How do we solve for lift. What is the determination in deciding how much lift is enough vs how much is too much.
06-03-2004, 02:16 PM
#357
6600 rpm clutch dump of death Administrator
Originally Posted by Cstraub
Seat diameter of the intake valve.
Chris
Chris
Ok, so a valve has a diameter of 2.05". How do we determine max lift from that, and what happens if you have too much lift is it a mater of velocity being hurt, or is there no gain to be seen from too much lift?
06-04-2004, 09:40 AM
#358
6600 rpm clutch dump of death Administrator
Originally Posted by critter
I've got it straightened out. Holler if you what help.
Radians = Degrees × .017453, or Degrees × Pi ÷ 180
06-04-2004, 09:52 AM
#359
6600 rpm clutch dump of death Administrator
I thought folks might like this...
http://www.mid-lift.com/TECH/TECH-Definitions.htm
MEF: MEAN "EFFICIENCY FACTOR" A formula we developed for analyzing the efficiency of a cylinder head's port flow based from valve size. The MEAN Efficiency Factor establishes cylinder head port quality by quantifying the head's real doorway value (valve size) by it's greatest handicap (the smallest cross sectional area of the port feeding it), through using real data derived from flow analysis, then compares that value to a physical formula based on the head's port and the valve's diameter. The real secret that makes this novel and unprecedented (sort of the "MID-LIFT" secret to port flow) is it develops a formula for a floating FACTOR based from the two mechanical limiters (the valve and port area, respectively) to determine what value of airflow data to use in comparing to the valve's diameter. (Also see: PORT VOLUME, below.) These FOUR (4) STEPS ARE AS FOLLOWS:
1. FIND, MEASURE and USE the port's smallest cross sectional area DIVIDED into the same value for the VALVE DIAMETER. This will give a ratio value in our 3rd step and be used in step #2 below.
2. MEASURE the AIRFLOW of the port in question at a known standard of FLOW DEPRESSION, finding the peak LAMINAR FLOW Air Flow point. (NOTE: I always preferred 1.5" of H2O, which is Holley's standard, and I was always comparing to entire system flow-through. This equates to 21.3" of water depression at Sea Level, but most flow at the arbitrarily chosen 28" of water.) Whatever, everyone needs to agree on a standard, then a cross-reference can be used for the rogues.
3. DIVIDE the PEAK Airflow of Step #2 by our RATIO VALUE of Step #1 for a CFM value.
4. DIVIDE the CFM answer of Step #3 by the VALVE DIAMETER. ^
The above reduces all cylinder heads to their real measure of "size," which is the valve. It reduces the "efficiency" of the valve to what the head's port has allowed it to be, and this is what we really want to know when comparing cylinder heads. For an exercise of the above, see the following:
A 2.100" Diameter Intake valve would have a square inch area of 2.72". A SBC Intake Port might have a minimum cross sectional area found at the pushrod point of the port, at approximately 3/4" past the port's entrance, of 80% this size, or 2.176" (square). Therefore: 2.72/2.176= 1.250. REMEMBER this "1.250" is NOT inches, it is a "VALUE" based on a calculation, nothing more. But the smaller this number gets, the lesser is the difference between the cross-sectional area and the valve's area; which reduces upstream velocity in the port, elevates the torque curve and increases volume of airflow; but ONLY if the "quality" of flow is maintained. The smaller this number, the more horsepower potential and the more cam that is required, but so too is the risk of turbulence, unresponsive lower RPM's and overall horsepower loss. This value should never be less than "1.00," and my personal "warning point" would be to limit this to 1.15 to 1.20 on any serious, naturally aspirated engine. NOTE: The part of the port where smallest cross-sectional area is usually found on a PROFESSIONAL race head where the intake side of the head has unlimited induction (no "restrictor plates") is usually found at the short side radius and valve bowl area. This area of the head will typically be 1.10 to 1.12 (for Hemi style engines) and 1.13 to 1.15 (for wedge style combustion chambers). It is the UPSTREAM port design that I am talking about, which should be careful not to increase in port size so much that you have a "reduced" value less than 1.15 to 1.20 - unless you really know what you're doing.
In Step #3, our PEAK laminar airflow might be 260 cfm @ 28", so the equation would look like this: 260/1.25= 208 (cfm). Now this cfm value is divided by the valve diameter, and the equation would look like this: 208/2.100= 99.04. this "99.04" is the MEF. One last point: The greater the port is enlarged to meet more ideal "maximum effort" flow, the smaller the difference between the valve diameter area and port area will be (sum of Step #1). So when you go to step #3, you'll be using a higher standard of Ratio Value for a higher standard of cylinder head (more rpm intentions). There is a FIFTH equation to be used for selecting this head for the right ENGINE SIZE versus RPM's INTENDED. But this FOUR STEP formula still provides a very effective snapshot of the head's real merits. ^
http://www.mid-lift.com/TECH/TECH-Definitions.htm
MEF: MEAN "EFFICIENCY FACTOR" A formula we developed for analyzing the efficiency of a cylinder head's port flow based from valve size. The MEAN Efficiency Factor establishes cylinder head port quality by quantifying the head's real doorway value (valve size) by it's greatest handicap (the smallest cross sectional area of the port feeding it), through using real data derived from flow analysis, then compares that value to a physical formula based on the head's port and the valve's diameter. The real secret that makes this novel and unprecedented (sort of the "MID-LIFT" secret to port flow) is it develops a formula for a floating FACTOR based from the two mechanical limiters (the valve and port area, respectively) to determine what value of airflow data to use in comparing to the valve's diameter. (Also see: PORT VOLUME, below.) These FOUR (4) STEPS ARE AS FOLLOWS:
1. FIND, MEASURE and USE the port's smallest cross sectional area DIVIDED into the same value for the VALVE DIAMETER. This will give a ratio value in our 3rd step and be used in step #2 below.
2. MEASURE the AIRFLOW of the port in question at a known standard of FLOW DEPRESSION, finding the peak LAMINAR FLOW Air Flow point. (NOTE: I always preferred 1.5" of H2O, which is Holley's standard, and I was always comparing to entire system flow-through. This equates to 21.3" of water depression at Sea Level, but most flow at the arbitrarily chosen 28" of water.) Whatever, everyone needs to agree on a standard, then a cross-reference can be used for the rogues.
3. DIVIDE the PEAK Airflow of Step #2 by our RATIO VALUE of Step #1 for a CFM value.
4. DIVIDE the CFM answer of Step #3 by the VALVE DIAMETER. ^
The above reduces all cylinder heads to their real measure of "size," which is the valve. It reduces the "efficiency" of the valve to what the head's port has allowed it to be, and this is what we really want to know when comparing cylinder heads. For an exercise of the above, see the following:
A 2.100" Diameter Intake valve would have a square inch area of 2.72". A SBC Intake Port might have a minimum cross sectional area found at the pushrod point of the port, at approximately 3/4" past the port's entrance, of 80% this size, or 2.176" (square). Therefore: 2.72/2.176= 1.250. REMEMBER this "1.250" is NOT inches, it is a "VALUE" based on a calculation, nothing more. But the smaller this number gets, the lesser is the difference between the cross-sectional area and the valve's area; which reduces upstream velocity in the port, elevates the torque curve and increases volume of airflow; but ONLY if the "quality" of flow is maintained. The smaller this number, the more horsepower potential and the more cam that is required, but so too is the risk of turbulence, unresponsive lower RPM's and overall horsepower loss. This value should never be less than "1.00," and my personal "warning point" would be to limit this to 1.15 to 1.20 on any serious, naturally aspirated engine. NOTE: The part of the port where smallest cross-sectional area is usually found on a PROFESSIONAL race head where the intake side of the head has unlimited induction (no "restrictor plates") is usually found at the short side radius and valve bowl area. This area of the head will typically be 1.10 to 1.12 (for Hemi style engines) and 1.13 to 1.15 (for wedge style combustion chambers). It is the UPSTREAM port design that I am talking about, which should be careful not to increase in port size so much that you have a "reduced" value less than 1.15 to 1.20 - unless you really know what you're doing.
In Step #3, our PEAK laminar airflow might be 260 cfm @ 28", so the equation would look like this: 260/1.25= 208 (cfm). Now this cfm value is divided by the valve diameter, and the equation would look like this: 208/2.100= 99.04. this "99.04" is the MEF. One last point: The greater the port is enlarged to meet more ideal "maximum effort" flow, the smaller the difference between the valve diameter area and port area will be (sum of Step #1). So when you go to step #3, you'll be using a higher standard of Ratio Value for a higher standard of cylinder head (more rpm intentions). There is a FIFTH equation to be used for selecting this head for the right ENGINE SIZE versus RPM's INTENDED. But this FOUR STEP formula still provides a very effective snapshot of the head's real merits. ^
06-04-2004, 04:31 PM
#360
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Originally Posted by J-Rod
I think I found what I need.
Radians = Degrees × .017453, or Degrees × Pi ÷ 180
Radians = Degrees × .017453, or Degrees × Pi ÷ 180
OT - it is hard to stop grinning. This is the first lumpy cam I've had in a long, long time. I took it in for an alignment and the whole shop collected around the car. It is only 232* but fairly narrow LCA.