Cam help
#11
07-10-2007, 07:44 PM
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#13
08-16-2007, 06:49 PM
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RE: Cam help
Do you have the piston specifications handy? Most people comment about compression, in reference to a stock FL or FLH cam. I do it, too. Really, knowing the piston dome volume and knowing the intake closing angle are the two best pieces of information to work from. Combustion chamber volume can be treated as a constant, as very few heads are modified on stock machines. Swept volume can be treated as another constant, unless you want to recalculate for an increase in bore or stroke. If you haven't bought your pistons yet, be sure to request the piston spec sheet/info, if it is not included. I like to calculate as close as possible the actual compression ratio and cylinder pressure. Makes it easier to decide if you are chosing the right cam for pump gas, performance, fuel mileage, etc.. I find all the nominal values, which are way high. These are calculated from BDC. Then, I calculate the effective values, taken from intake valve closure ABDC.
There are plenty of online programs to do all the math for you. But, after doing it for years, beginning with a sliderule (am I old, or what?), and moving to a programmable calculator, I began writing my own (after learning Borland Pascal). I use Delphi v5 these days, just for the simple graphics that I can add. Anyway, I run the numbers on engines that I build, just to see how they will turn out. I'm not as up on all the cams available, as I was back in the 70's. I do only restorations these days, and stock doesn't require a whole lot of math. Still like to compare some of the grinds that I hear people talk about. Andrews is one brand that I've used for a long time. I'm tending towards Leineweber, though, as his grinds seem to be a departure from the cookie-cutter grinds of Andrews, Crane, and such. I wish the old man would write a book on his methods and theory. He's getting ancient.
Jack
There are plenty of online programs to do all the math for you. But, after doing it for years, beginning with a sliderule (am I old, or what?), and moving to a programmable calculator, I began writing my own (after learning Borland Pascal). I use Delphi v5 these days, just for the simple graphics that I can add. Anyway, I run the numbers on engines that I build, just to see how they will turn out. I'm not as up on all the cams available, as I was back in the 70's. I do only restorations these days, and stock doesn't require a whole lot of math. Still like to compare some of the grinds that I hear people talk about. Andrews is one brand that I've used for a long time. I'm tending towards Leineweber, though, as his grinds seem to be a departure from the cookie-cutter grinds of Andrews, Crane, and such. I wish the old man would write a book on his methods and theory. He's getting ancient.
Jack
#14
08-16-2007, 07:32 PM
#15
08-18-2007, 03:44 PM
RE: Cam help
ORIGINAL: Jack Hester
You will do just fine with the '1'-grind Andrews. A good friend has a '74 Police Special, and is running low (7.5:1) compression pistons on a stock 'H' cam. Very good performer for what he wants. Road cruiser. I advised him to go with the '1'-grind if he ever wanted to get a bit more pep out of it. The '1'-grind raises the effective compression back up to somewhere around 8:1 - 8.5:1.
My advice to those who want to these bolt on/in mods to their Shovels, is to actually go with higher-domed pistons, for a bit higher static compression. But then, install a cam that reduces the effective down to something very near to stock. My reasoning behind this is that the Shovelhead ports are way to big for low-end performance, combined with basically the same domed pistons of the previous generation of Big Twins. They just don't make the vacuum for a smooth idle, unless you reduce the intake size (manifold, carburetor). If you look at the Panhead setup, they have small intake ports, a short intake manifold, and a long carburetor (Linkert) with a small venturi. Makes for higher intake velocity, smoother vacuum at low RPMs, and a strong torque engine on bottom end. Of course they run out of steam at around 100 mph. So what. They're cruisers. But, back to my reason for the hi-domed pistons. These will make for higher vacuum at low RPMs, but with the right cam, near stock compression, able to run on pump gas with no problem. Plus, you will have the crisp throttle response of the old Pans with Linkerts.
Jack
P.S. - I'm not advocating tearing down a perfectly good running engine to do such mods. Wait until it really needs that build. Then, you have something else to consider. You have options with low or high domed pistons. More with high, I'm thinking.
You will do just fine with the '1'-grind Andrews. A good friend has a '74 Police Special, and is running low (7.5:1) compression pistons on a stock 'H' cam. Very good performer for what he wants. Road cruiser. I advised him to go with the '1'-grind if he ever wanted to get a bit more pep out of it. The '1'-grind raises the effective compression back up to somewhere around 8:1 - 8.5:1.
My advice to those who want to these bolt on/in mods to their Shovels, is to actually go with higher-domed pistons, for a bit higher static compression. But then, install a cam that reduces the effective down to something very near to stock. My reasoning behind this is that the Shovelhead ports are way to big for low-end performance, combined with basically the same domed pistons of the previous generation of Big Twins. They just don't make the vacuum for a smooth idle, unless you reduce the intake size (manifold, carburetor). If you look at the Panhead setup, they have small intake ports, a short intake manifold, and a long carburetor (Linkert) with a small venturi. Makes for higher intake velocity, smoother vacuum at low RPMs, and a strong torque engine on bottom end. Of course they run out of steam at around 100 mph. So what. They're cruisers. But, back to my reason for the hi-domed pistons. These will make for higher vacuum at low RPMs, but with the right cam, near stock compression, able to run on pump gas with no problem. Plus, you will have the crisp throttle response of the old Pans with Linkerts.
Jack
P.S. - I'm not advocating tearing down a perfectly good running engine to do such mods. Wait until it really needs that build. Then, you have something else to consider. You have options with low or high domed pistons. More with high, I'm thinking.
The thing to remember is that piston companies that advertise a specific compression ratio for their pistons are on a best case situation. If you've decked your block, shaved your heads, and even run thin gaskets, or no gaskets, is all factors in you CR. For instance, if you run a Keith Black 9.5:1 pistons in a stock motor with stock gaskets with stock heads, you ARE NOT going to get 9.5:1.
For what the guy is describing, a stock cam, or Andrews "A" is perfect, with cheap gas.
#16
08-18-2007, 04:45 PM
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RE: Cam help
AlCherry -
You need to go back and read my post. You will find that cams have everything to do with compression ratios. Read again. Static compression ratiois the calculated value from BDC. This is pure math. Effective compression ratiois calculated from the time that the intake valve closes ABDC. That's why these specs are given about any particular cam. Your true compression (effective) will not begin until the intake closes. And, is completed when the piston reaches TDC. The two are never the same. Static becomes the constant value foran engine, once it's built. But, the minute you factor the cam in, orstart changing cams, the effective becomes the compression ratio that the engine is using. I don't know, right off hand, of any engine that has a cam with the intake closingtime equal toBDC.I don't doubt thatthere are. Speaking of OHV engines. I know of Flatheads that have intake closing times close to BDC. Then, the effective would be close to the static value.
As to all the other factors that you mentioned, that come into play for calculating compression ratios, I'm not arguing that fact. I totally agree. As a matter of fact, I have all the equations to use any of these values. But for the sake of conversation about evaluating cams, you will rarely see anyone mention any of these. We can split hairs and talk about compressed gasket thicknesses, or shave heads, or decked cylinders. That's fine. But, it's really not the point of this thread.
Now, I'm curious. Why would you take offense to my comments, to the point of being insulting:
"Glad your not building my motors. Google "compression ratio", and take a class."
Wouldn't it be better to say something like 'I disagree, and this is why'? I have taken 'a class'.Quite a few of them. And, I've got the books to back it up.I don't depend on just what Ihave read, or what I've been told. I do it, first hand. Then, I can say 'I agree/disagree, and this is why'? Ain't my first rodeo.I've profiled plenty of unknown cams, to find their characteristics, just so I would know what effect they would have if I wanted to use them in an engine. Try being nice, for a change. Maybe it's a stretch. People will like you for it. Have a nice day.
Jack
You need to go back and read my post. You will find that cams have everything to do with compression ratios. Read again. Static compression ratiois the calculated value from BDC. This is pure math. Effective compression ratiois calculated from the time that the intake valve closes ABDC. That's why these specs are given about any particular cam. Your true compression (effective) will not begin until the intake closes. And, is completed when the piston reaches TDC. The two are never the same. Static becomes the constant value foran engine, once it's built. But, the minute you factor the cam in, orstart changing cams, the effective becomes the compression ratio that the engine is using. I don't know, right off hand, of any engine that has a cam with the intake closingtime equal toBDC.I don't doubt thatthere are. Speaking of OHV engines. I know of Flatheads that have intake closing times close to BDC. Then, the effective would be close to the static value.
As to all the other factors that you mentioned, that come into play for calculating compression ratios, I'm not arguing that fact. I totally agree. As a matter of fact, I have all the equations to use any of these values. But for the sake of conversation about evaluating cams, you will rarely see anyone mention any of these. We can split hairs and talk about compressed gasket thicknesses, or shave heads, or decked cylinders. That's fine. But, it's really not the point of this thread.
Now, I'm curious. Why would you take offense to my comments, to the point of being insulting:
"Glad your not building my motors. Google "compression ratio", and take a class."
Wouldn't it be better to say something like 'I disagree, and this is why'? I have taken 'a class'.Quite a few of them. And, I've got the books to back it up.I don't depend on just what Ihave read, or what I've been told. I do it, first hand. Then, I can say 'I agree/disagree, and this is why'? Ain't my first rodeo.I've profiled plenty of unknown cams, to find their characteristics, just so I would know what effect they would have if I wanted to use them in an engine. Try being nice, for a change. Maybe it's a stretch. People will like you for it. Have a nice day.
Jack
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#17
08-18-2007, 06:04 PM
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#18
08-18-2007, 06:24 PM
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#19
08-19-2007, 02:02 AM
RE: Cam help
Jack, just to let you know, your terminoligy is a little off............... what you are talking about "compression ratio" in relatrion to the cam, is actually volumetric effeciency...... which also takes into account the flow charictoristics of the head as well as the cam, carb, air cleaner and intake valves.
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08-19-2007, 05:38 AM
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RE: Cam help
Actually, I'm not talking about Volumetric Effeciency. I'm talking about a simple 'ratio'. And, one about 'compression'. I'm talking about the ratio of when the fuel/air mixture in the cylinder is at a non-compressed state, to that of when the fuel/air mixture is at it's maximum compressed state. Just a ratio.
Let me qualify a bit further, so that the difference is a bit clearer. Static stroke (actually, I like to use the word 'nominal', but static is what popped in mind first) is the full/maximum stroke of an engine. BDC to TDC. Effective stroke is from some point ABDC, the very moment the intake valve closes. So, you have static compresion calculated from static stroke. And, effective compression calculated from effective stroke. Think about it. Your mixturebegins to compress, only from the moment that the intake valve closes. This is some point in the cylinder where the piston is higher than BDC. So, 'effective compression ratio' calculated from this point, only. Look at the cam spec sheet. Usually, the 2nd number on the intake valve data is the point of closure, in degreesABDC. You use this second number to calculate 'effective stroke' of the engine. This, in turn, is used to calculate 'effective compression'. By the way. It can also be used to calculate 'effectivecranking pressure', which comes out to be a value in 'absolute pressure' as opposed to 'gauge pressure' (which is what the general public knows, understands, and uses). My calculations convert to 'gauge'. But, we won't go there. Lord knows what someone will think I'm talking about, then.
There areso many other things thattake place with cam changes, piston changes, combustion chamber changes, port changes, exhaust pipe length changes (wish someone would start a thread on that one: can of worms, for sure), etc., etc.. Ain't what I've been talking about. A ratio, pure and simple. Not effeciency. Not volume. Not the combination of the two. 'Ratio'. A comparisson of no compression to max compression, be it 'static' or 'effective'. But, just a ratio.
I would like to stop, at this point, and apologize to the original author of this thread. This is so way off your topic that it really should be made another topic, altogether. I'm not hijacking your thread, and hope that you do get the answer you are looking for.
In the meantime, everyone else please read my posts with care. 'Compression ratio' is all I'm talking about.
By the way Al. The volume of the cylinder is just that. Cylinder volume, or displacement as most know it. And, it is calculated from the points that you stated.(Cyl Vol or Cyl Displacement = Bore squared× Stroke × .7854). This can be in cubic centimeters (for the metric folks), or cubic inches. But, not a 'ratio'. Only displacement. And, I agree. Moving fuel/air in and out of the cylinder is affected by the cam. That's one of it's many purposes. Intake valveopens, draws fuel/air in. Intake valvecloses, and compression begins.
Anyway, again, sorry for going so far off topic.
Jack
P.S. - Here is a really excellent writeup of what has been discussed here: http://victorylibrary.com/tech/cam-c.htm. I have great respect for this fellow, and all the work he has put into making his info available.
P.S. (again) - Please, if anything I say offends anyone, don't take it so. I, like anyone else, can get a bit frustrated at times.
Let me qualify a bit further, so that the difference is a bit clearer. Static stroke (actually, I like to use the word 'nominal', but static is what popped in mind first) is the full/maximum stroke of an engine. BDC to TDC. Effective stroke is from some point ABDC, the very moment the intake valve closes. So, you have static compresion calculated from static stroke. And, effective compression calculated from effective stroke. Think about it. Your mixturebegins to compress, only from the moment that the intake valve closes. This is some point in the cylinder where the piston is higher than BDC. So, 'effective compression ratio' calculated from this point, only. Look at the cam spec sheet. Usually, the 2nd number on the intake valve data is the point of closure, in degreesABDC. You use this second number to calculate 'effective stroke' of the engine. This, in turn, is used to calculate 'effective compression'. By the way. It can also be used to calculate 'effectivecranking pressure', which comes out to be a value in 'absolute pressure' as opposed to 'gauge pressure' (which is what the general public knows, understands, and uses). My calculations convert to 'gauge'. But, we won't go there. Lord knows what someone will think I'm talking about, then.
There areso many other things thattake place with cam changes, piston changes, combustion chamber changes, port changes, exhaust pipe length changes (wish someone would start a thread on that one: can of worms, for sure), etc., etc.. Ain't what I've been talking about. A ratio, pure and simple. Not effeciency. Not volume. Not the combination of the two. 'Ratio'. A comparisson of no compression to max compression, be it 'static' or 'effective'. But, just a ratio.
I would like to stop, at this point, and apologize to the original author of this thread. This is so way off your topic that it really should be made another topic, altogether. I'm not hijacking your thread, and hope that you do get the answer you are looking for.
In the meantime, everyone else please read my posts with care. 'Compression ratio' is all I'm talking about.
By the way Al. The volume of the cylinder is just that. Cylinder volume, or displacement as most know it. And, it is calculated from the points that you stated.(Cyl Vol or Cyl Displacement = Bore squared× Stroke × .7854). This can be in cubic centimeters (for the metric folks), or cubic inches. But, not a 'ratio'. Only displacement. And, I agree. Moving fuel/air in and out of the cylinder is affected by the cam. That's one of it's many purposes. Intake valveopens, draws fuel/air in. Intake valvecloses, and compression begins.
Anyway, again, sorry for going so far off topic.
Jack
P.S. - Here is a really excellent writeup of what has been discussed here: http://victorylibrary.com/tech/cam-c.htm. I have great respect for this fellow, and all the work he has put into making his info available.
P.S. (again) - Please, if anything I say offends anyone, don't take it so. I, like anyone else, can get a bit frustrated at times.