Educated Discussion- How do you pick a camshaft based on flow-numbers from heads?
06-03-2006, 10:22 AM
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Educated Discussion- How do you pick a camshaft based on flow-numbers from heads?
I have always wondered this and so i thought i might pick yalls brain. How do you figure what lifts would be best based off of flow numbers?
06-03-2006, 12:36 PM
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http://www.mx6.com/forums/archive/in.../t-134786.html
Lift:
How much lift should I get in my cam? Well that will depend on your heads' flow characteristics. To choose the correct turbo camshaft, you really need to know how your cylinder heads flow. Reason is if your cylinder head flows X amount of air at X amount of lift, choosing a cam that has a lift much greater then that will gain you nothing except extra heat and premature wear of the valve spring. Airflow through a head reaches a peak as the valve is opened, then starts to drop off as the valve is lifted beyond that peak. Most of this of this will hold true to definition, but with a forced induction motor, valve lift is not as critical since the incoming air is pressurized.
A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point.
So be the definition above if your head flows best at 0.500" of lift, use a cam that will lift the valve between 0.600" and 0.625". The reasoning behind this is, if you lift the valve only to its peak flow point, then the valve only flows best when it's wide open. The cycle is brief and would only happen once per stroke. So to benefit from you peak flow the most, you want to lift the valve past its peak. That way the valve will pass its peak flow twice in the cycle. The result is more flow during the opening and closing event of the valve. You do not want to raise the valve much past the peak flow though, or you lose total flow by going too high.
Calculating the best lift:
0.500 X 1.20 = 0.600
0.500 X 1.25 = .0625
http://superchevy.com/technical/engi...ht_cam_choice/
It's important to look at all aspects of you cylinder head's, and total engine's, performance. I tend to focus heavily on the cylinder heads Intake to Exhaust (IvE) flow percentage. That's the amount of air the exhaust port can flow vs. the intake port. A head with a higher percentage can use a cam with more closely matched intake and exhaust lobe figures. Conversely, a head with a poor IvE ratio needs more exhaust duration to work well. The differences here a minimal. A small change in head flow is usually worth more of a power increase than a large change in cam timing. The following chart represents the same big-block Chevy head, before and after the exhaust ports were worked on. The stock head would respond better to a cam with about 10 degrees more exhaust timing, but no more lift. While the ported head would work well with a cam that has its intake and exhaust figures closely matched, with maybe only 2-4 degrees more duration on the exhaust lobes. And since exhaust flow improved more dramatically as lift increased in the ported head, then its cam could make better use of more lift on the exhaust side as well.
While most cylinder head companies will be happy to give you their flow figures, you'll probably have to do the percentage calculations yourself, so here's the formula: (exhaust flow / intake flow).
And to compare heads, take the larger exhaust flow from one head, subtract the smaller ex flow from the other and then divide that by the smaller figure.
Here's an example from .200-lift above: (107-99) = 8/99 = 0.08
Move the decimal two points to the right and you've got your percentage of improvement: 8% I've found this to be a very good method of comparing cylinder heads and choosing the proper camshaft to go along with it. Of course, this method really only applies to naturally aspirated cylinder heads and as soon as you add nitrous or a blower or turbo, cam selection changes dramatically.
Ive always wondered about this and wanted to find an exact formula regarding heads flow specs and cam for an rpm range but there is alot of gray area.
Id like to hear from some of the head manufacturers but, I doubt they'll jump in here. Go on tech and look at Tony's posts from AFR there is alot of good info from him, he will actually tell you his ideas behind design philosophy one of the few manufacturers that actively participates.
Lift:
How much lift should I get in my cam? Well that will depend on your heads' flow characteristics. To choose the correct turbo camshaft, you really need to know how your cylinder heads flow. Reason is if your cylinder head flows X amount of air at X amount of lift, choosing a cam that has a lift much greater then that will gain you nothing except extra heat and premature wear of the valve spring. Airflow through a head reaches a peak as the valve is opened, then starts to drop off as the valve is lifted beyond that peak. Most of this of this will hold true to definition, but with a forced induction motor, valve lift is not as critical since the incoming air is pressurized.
A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point.
So be the definition above if your head flows best at 0.500" of lift, use a cam that will lift the valve between 0.600" and 0.625". The reasoning behind this is, if you lift the valve only to its peak flow point, then the valve only flows best when it's wide open. The cycle is brief and would only happen once per stroke. So to benefit from you peak flow the most, you want to lift the valve past its peak. That way the valve will pass its peak flow twice in the cycle. The result is more flow during the opening and closing event of the valve. You do not want to raise the valve much past the peak flow though, or you lose total flow by going too high.
Calculating the best lift:
0.500 X 1.20 = 0.600
0.500 X 1.25 = .0625
http://superchevy.com/technical/engi...ht_cam_choice/
It's important to look at all aspects of you cylinder head's, and total engine's, performance. I tend to focus heavily on the cylinder heads Intake to Exhaust (IvE) flow percentage. That's the amount of air the exhaust port can flow vs. the intake port. A head with a higher percentage can use a cam with more closely matched intake and exhaust lobe figures. Conversely, a head with a poor IvE ratio needs more exhaust duration to work well. The differences here a minimal. A small change in head flow is usually worth more of a power increase than a large change in cam timing. The following chart represents the same big-block Chevy head, before and after the exhaust ports were worked on. The stock head would respond better to a cam with about 10 degrees more exhaust timing, but no more lift. While the ported head would work well with a cam that has its intake and exhaust figures closely matched, with maybe only 2-4 degrees more duration on the exhaust lobes. And since exhaust flow improved more dramatically as lift increased in the ported head, then its cam could make better use of more lift on the exhaust side as well.
While most cylinder head companies will be happy to give you their flow figures, you'll probably have to do the percentage calculations yourself, so here's the formula: (exhaust flow / intake flow).
And to compare heads, take the larger exhaust flow from one head, subtract the smaller ex flow from the other and then divide that by the smaller figure.
Here's an example from .200-lift above: (107-99) = 8/99 = 0.08
Move the decimal two points to the right and you've got your percentage of improvement: 8% I've found this to be a very good method of comparing cylinder heads and choosing the proper camshaft to go along with it. Of course, this method really only applies to naturally aspirated cylinder heads and as soon as you add nitrous or a blower or turbo, cam selection changes dramatically.
Ive always wondered about this and wanted to find an exact formula regarding heads flow specs and cam for an rpm range but there is alot of gray area.
Id like to hear from some of the head manufacturers but, I doubt they'll jump in here. Go on tech and look at Tony's posts from AFR there is alot of good info from him, he will actually tell you his ideas behind design philosophy one of the few manufacturers that actively participates.
Last edited by 02sierraz71_5.3; 06-03-2006 at 02:03 PM.
06-03-2006, 02:20 PM
#4
found something from TonyM
Guys...
There is no "best cam" for AFR's or any other cylinder head for that matter. Cams determine the driving manners and power curve an engine generates and there is always a "give and take" when it comes to cam design. A bigger cam usually makes more power and always has worse driving manners than a smaller cam all else being equal.
The proven 224/228 cam I ran for awhile is a great cam capable of producing solid power and good TQ while still retaining stock like driving manners. It has minimal chop at an idle and is simply a fantastic choice for guys looking for a very well rounded package....not just big peak numbers. With the right complimenting components and alot of attention to detail spent, the power my car made is a testimony of what it is capable of. What's funny is you could hand the keys to your wife and have her go get groceries with the exact same combination....unbeknownst to her she is piloting a vehicle producing in the vicinity of 550 flywheel horsepower (unless she nails the throttle of course!).
Is that the "best" cam for AFR's?? Only if your goals lie within some of the parameters I just mentioned of course....its all a subjective thing. For some of the guys that didn't mind a little more lope and were willing to trade a little grunt on the bottom for a little more upstairs (but wanted to stay close to that "recipe"), I have been spec'ing a few 228/232 grinds which, BTW, is the biggest I would recommend without notching pistons, and 114 LSA is recomended because it has a little more P to V than a 112 does. I have gotten alot more aggressive with customers willing to notch pistons and bump their CR into the mid elevens or higher....they don't care about idle quality and primarily drive their cars on Friday nights and at the track. They are looking for solid power from 4500 - 7000 and coudn't care less what was going on below that point....a 224/228 is the absolute wrong cam for that individual.
The point I'm making is simply that there is no "best" cam for a particular cylinder head....only the best cam for your personal needs....the key is to accurately access those needs or goals, and then be able to pick the right cam to achieve them (assuming they are realistic)....thats where you have to rely on the input of a professional if you dont have enough backround yourself and its always better to err on the conservative side when picking one....the bad driving characteristics of a too big a cam quickly out-weigh the race car idle quality...even though your buddies that might hear it think your car sounds bad-*** (they don't have to drive it everyday where it's shortcomings are readily apparent). One of the reasons the packages I like to feature are always conservatively cammed is because the car is more enjoyable to drive on a day to day basis with a smaller cam (but more challenging to put up bigger numbers of course). Even the big cam guys will tell you that if they are willing to come clean about it....LOL
Anyway...just some food for thought.
Regards,
Tony M.
__________
Guys...
There is no "best cam" for AFR's or any other cylinder head for that matter. Cams determine the driving manners and power curve an engine generates and there is always a "give and take" when it comes to cam design. A bigger cam usually makes more power and always has worse driving manners than a smaller cam all else being equal.
The proven 224/228 cam I ran for awhile is a great cam capable of producing solid power and good TQ while still retaining stock like driving manners. It has minimal chop at an idle and is simply a fantastic choice for guys looking for a very well rounded package....not just big peak numbers. With the right complimenting components and alot of attention to detail spent, the power my car made is a testimony of what it is capable of. What's funny is you could hand the keys to your wife and have her go get groceries with the exact same combination....unbeknownst to her she is piloting a vehicle producing in the vicinity of 550 flywheel horsepower (unless she nails the throttle of course!).
Is that the "best" cam for AFR's?? Only if your goals lie within some of the parameters I just mentioned of course....its all a subjective thing. For some of the guys that didn't mind a little more lope and were willing to trade a little grunt on the bottom for a little more upstairs (but wanted to stay close to that "recipe"), I have been spec'ing a few 228/232 grinds which, BTW, is the biggest I would recommend without notching pistons, and 114 LSA is recomended because it has a little more P to V than a 112 does. I have gotten alot more aggressive with customers willing to notch pistons and bump their CR into the mid elevens or higher....they don't care about idle quality and primarily drive their cars on Friday nights and at the track. They are looking for solid power from 4500 - 7000 and coudn't care less what was going on below that point....a 224/228 is the absolute wrong cam for that individual.
The point I'm making is simply that there is no "best" cam for a particular cylinder head....only the best cam for your personal needs....the key is to accurately access those needs or goals, and then be able to pick the right cam to achieve them (assuming they are realistic)....thats where you have to rely on the input of a professional if you dont have enough backround yourself and its always better to err on the conservative side when picking one....the bad driving characteristics of a too big a cam quickly out-weigh the race car idle quality...even though your buddies that might hear it think your car sounds bad-*** (they don't have to drive it everyday where it's shortcomings are readily apparent). One of the reasons the packages I like to feature are always conservatively cammed is because the car is more enjoyable to drive on a day to day basis with a smaller cam (but more challenging to put up bigger numbers of course). Even the big cam guys will tell you that if they are willing to come clean about it....LOL
Anyway...just some food for thought.
Regards,
Tony M.
__________
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06-04-2006, 10:24 PM
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Originally Posted by 02sierraz71_5.3
I would really like to hear from some of the aftermarket head porters on here, itd be nice to know they design thier heads with a specific range of power and rpm band in mind.
on that! I really would like to see what Richard from WCCH has to say about the subject.
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