Dan, also consider not only the cooling effects of the increased flow at higher rpm with the bigger cam, but think what I said before, how long do you run your car "on" the cam at a time?
Also, I used to have a Baja( don't tell anybody). I wanted lots of low end, so I ran 9-1 and an engle 100 cam. Not good at all. The motor melted a hole in a pistion.
My point is, you can't say so-so CR is correct for any given Octane. A lot of other factors wiegh in, like cam duration, head flow, exhaust size, car wieght, gear ratios, etc.
A heavy car with a 3.88 gear is not going to able to run the same CR as the guy with a light wieght dunebuggy and 4.37 R&P. Even if all other variables are the same.
cams vs. compression
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MASSIVE TYPE IV
- Posts: 20132
- Joined: Fri Jul 07, 2000 12:01 am
cams vs. compression
Wow a 15.05, in a 914....I bet he never got out of third in the QTR mile....gotta love that 901 tranny.
IMy best time was only 2 second faster, and I spun completely through first with street tires. 2nd and 3rd were the power gears, since 4th was for cruising..
I stated above in my earlier post that more headwork and bigger carbs must be used in conjunction with the higher CR to make the combination work. I do believe that John was making the same point..
This is one reason why I like a type 4, 8.5:1 is a street engine for us, with a mild cam and upright kit, it cools like crazy..But that is beside the point..
The cam, heads and carbs, must be configured in unison, or a nightmare is coming your way, and Freddy Krueger is administering the pain...I do believe he is my HERO..
Now back to listening to Charlie, and slamming another fifth of Jack, then maybe I will go back out to the shop and build a High compression grenade....Who knows, somebody might "squeal like a pig" before the night is over!!!(If I get lucky)
IMy best time was only 2 second faster, and I spun completely through first with street tires. 2nd and 3rd were the power gears, since 4th was for cruising..
I stated above in my earlier post that more headwork and bigger carbs must be used in conjunction with the higher CR to make the combination work. I do believe that John was making the same point..
This is one reason why I like a type 4, 8.5:1 is a street engine for us, with a mild cam and upright kit, it cools like crazy..But that is beside the point..
The cam, heads and carbs, must be configured in unison, or a nightmare is coming your way, and Freddy Krueger is administering the pain...I do believe he is my HERO..
Now back to listening to Charlie, and slamming another fifth of Jack, then maybe I will go back out to the shop and build a High compression grenade....Who knows, somebody might "squeal like a pig" before the night is over!!!(If I get lucky)
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JohnConnolly
- Posts: 3336
- Joined: Thu Apr 13, 2000 12:01 am
cams vs. compression
"yesss! more cam may = higher dynamic compression = higher octane requirements."
NONONONONONONO!
I'm going to say this one more time, then I'm going to shut up (you are coming at this "discussion" with too closed a mind Dan).
I want you to ANSWER every question I pose, not just the ones you feel like answering.
1) what RPMs is the stock camshaft good for on a 2.0L engine with mild 40 X 35.5 heads and mild porting?
2) why do those heads have that top end limitation? What does this limitation do to the airflow at high RPMs (from mid powerband up)?
3) How much TIME does atmospheric pressure have to fill the cylinder at 2K RPMs? How much TIME does it have at 6K RPMs? At full throttle, do you think (stock cam) the cylinder is filled as much at 6K as it is at 2K?
4) 286 duration cam (110, C-35, etc). Is this engine filling the cylinder any more at 4K RPMs than the stock cammed engine is at 2K RPMs? (hint: the answer is NO)
5) FK-87: Is this engine filling the cylinder any more at 7K than the stock cam engine is at 2K RPMs? (hint: the answer is No)
What does this all mean? Look at an engine's torque curve. Look at the TORQUE #s (not HP). This tells you how full the cylinders are (assuming the engine is timed correctly).
HP is a formula that is directly derived from torque. Normally, torque drops off at higher RPMs, because the cylinder isn't getting filled up enough, AND/OR combustion doesn't have time to get a full PUSH on the piston and crank. The combination of these two things is why torque falls off at higher RPMs. The fact that you get more pusheS (cylinder firings) at more RPMs is why power increases (power, not torque). When the cylinder is getting filled less than you are getting more pushes, power drops off. If you can maintain torque, but move the powerband up, you get the same torque, but more piston firings (pushes), and more power. So, torque is the measure of how full the cylinder is, and how well the engine is translating this pressure to a twisting force.
An engine that achieves 150 lb/ft of torque at 2500 is filling the cylinder THE SAME as another engine that's getting 150lb/ft at 7500 RPMs, but the latter is producing 3X the HP!
ANY engine maximizes it's power output (and efficiency) the more compression you run in it, UP TO THE POINT OF DETONATION. This is very important! If you can keep detonation at bay, you will make more power every time you bring the compression up (the power stroke is only one reason why, but that's another discussion). Once you get detonation, all bets are off (and your engine is a pile of parts (broken ones) pretty fast). The key with higher compression is to raise it and bleed off the "higher compression" at low RPMs by running more cam overlap. How does this lower compression? In a stock cam (for example), the cam closes the intake valve close to BDC.
Dynamic compression is close to static compression at lower RPMs. At high RPMs the dynamic compression gets LOWER because the port and intake system can't flow enough air to FILL the cylinder in the amount of time the valve is open.
Let's look at a hotter cam now. We bump up the "static" compression # (which is nothing more than a guide on a performance camshaft). WHY? Because the intake valve is STILL OPEN well after BDC.
6) Low RPMs, do you really think the cylinder is still filling up with the intake valve open, and the piston MOVING UP? (answer this one)
As engine speed increases, IF you match the compression to the cam selected, the intake system (port, carb, etc) fills the cylinder, and FLOW drops off (you have less time to fill it up as RPMs increase). But, you are getting a "supercharged" effect as the intake charge's momentum continues to fill the cylinder. This RARELY exceeds 100% VE on a 4 stroke in our form, it's exceptional on tuned 2 strokes, but that's another discussion entirely. So, what we are trying to achieve is matching the cam's "extra filling" capability at higher RPMs, to the intake system's "falling off" of flow. If you think of the system as the "sum" of the two, as one is rolling off, the other is picking up, keeping the SUM (torque) at a constant. If it's mismatched, you get too much cylinder pressure somewhere (8:1 with a stock cam = BOOM), or not enough 8:1 with FK-89 = LAME). In the latter case, you can get a lot of power, but not as much as you would if you matched the cam to the compression. I'm not saying it won't work, it will "work", but not as well as it could.
"this is where we diverge... it's a known fact that a motor running higher compression than the octane will support is one that will NOT live a long lifetime... the gasoline faq clearly spells that out."
Octane is a measurement of resistance to detonation. This is related to charge temperature, dynamic compression (
7)How much dynamic compression would your engine have if you ran 340 degrees of duration, running at 2K RPMs?) Why is the engine's torque output so low? If it's so low, WHY? (hint: cylinder isn't full)
If it's this low, what does this tell you about dynamic compression with a large cam?
"so if anything, bigger cams could require less compression, if you want the motor to live a long life."
Please elaborate!
"i can certainly see some truth in that, but it also sounds like a function of air speed thru the intake system to me... if you throw low c.r. at hogged-out ports and a giant cam, the motor could easily be a pig."
AGREED! You have two choices, go milder on the cam to bring the lower RPM torque #s up, OR run a hotter cam along with higher compression. Either way will work! The difference is where the torque curve lies! With one it's down low, and with the other it's up high. Torque is the same, power is NOT.
"i'm just interested in disproving this "theory" that people can always get away with running more compression because of a bigger cam."
Logically, please send your arguments my way and let's here your point of view on why you can NOT run more compression with a larger cam.
"i don't care how the motor "feels" by the seat of your pants, if it's detonating and you can't hear it, where is the common sense?"
Where's the common sense in running a engine at 60% VE? Build quality/detail aside, it lasts forever because the thing isn't making any respectable power!
NONONONONONONO!
I'm going to say this one more time, then I'm going to shut up (you are coming at this "discussion" with too closed a mind Dan).
I want you to ANSWER every question I pose, not just the ones you feel like answering.
1) what RPMs is the stock camshaft good for on a 2.0L engine with mild 40 X 35.5 heads and mild porting?
2) why do those heads have that top end limitation? What does this limitation do to the airflow at high RPMs (from mid powerband up)?
3) How much TIME does atmospheric pressure have to fill the cylinder at 2K RPMs? How much TIME does it have at 6K RPMs? At full throttle, do you think (stock cam) the cylinder is filled as much at 6K as it is at 2K?
4) 286 duration cam (110, C-35, etc). Is this engine filling the cylinder any more at 4K RPMs than the stock cammed engine is at 2K RPMs? (hint: the answer is NO)
5) FK-87: Is this engine filling the cylinder any more at 7K than the stock cam engine is at 2K RPMs? (hint: the answer is No)
What does this all mean? Look at an engine's torque curve. Look at the TORQUE #s (not HP). This tells you how full the cylinders are (assuming the engine is timed correctly).
HP is a formula that is directly derived from torque. Normally, torque drops off at higher RPMs, because the cylinder isn't getting filled up enough, AND/OR combustion doesn't have time to get a full PUSH on the piston and crank. The combination of these two things is why torque falls off at higher RPMs. The fact that you get more pusheS (cylinder firings) at more RPMs is why power increases (power, not torque). When the cylinder is getting filled less than you are getting more pushes, power drops off. If you can maintain torque, but move the powerband up, you get the same torque, but more piston firings (pushes), and more power. So, torque is the measure of how full the cylinder is, and how well the engine is translating this pressure to a twisting force.
An engine that achieves 150 lb/ft of torque at 2500 is filling the cylinder THE SAME as another engine that's getting 150lb/ft at 7500 RPMs, but the latter is producing 3X the HP!
ANY engine maximizes it's power output (and efficiency) the more compression you run in it, UP TO THE POINT OF DETONATION. This is very important! If you can keep detonation at bay, you will make more power every time you bring the compression up (the power stroke is only one reason why, but that's another discussion). Once you get detonation, all bets are off (and your engine is a pile of parts (broken ones) pretty fast). The key with higher compression is to raise it and bleed off the "higher compression" at low RPMs by running more cam overlap. How does this lower compression? In a stock cam (for example), the cam closes the intake valve close to BDC.
Dynamic compression is close to static compression at lower RPMs. At high RPMs the dynamic compression gets LOWER because the port and intake system can't flow enough air to FILL the cylinder in the amount of time the valve is open.
Let's look at a hotter cam now. We bump up the "static" compression # (which is nothing more than a guide on a performance camshaft). WHY? Because the intake valve is STILL OPEN well after BDC.
6) Low RPMs, do you really think the cylinder is still filling up with the intake valve open, and the piston MOVING UP? (answer this one)
As engine speed increases, IF you match the compression to the cam selected, the intake system (port, carb, etc) fills the cylinder, and FLOW drops off (you have less time to fill it up as RPMs increase). But, you are getting a "supercharged" effect as the intake charge's momentum continues to fill the cylinder. This RARELY exceeds 100% VE on a 4 stroke in our form, it's exceptional on tuned 2 strokes, but that's another discussion entirely. So, what we are trying to achieve is matching the cam's "extra filling" capability at higher RPMs, to the intake system's "falling off" of flow. If you think of the system as the "sum" of the two, as one is rolling off, the other is picking up, keeping the SUM (torque) at a constant. If it's mismatched, you get too much cylinder pressure somewhere (8:1 with a stock cam = BOOM), or not enough 8:1 with FK-89 = LAME). In the latter case, you can get a lot of power, but not as much as you would if you matched the cam to the compression. I'm not saying it won't work, it will "work", but not as well as it could.
"this is where we diverge... it's a known fact that a motor running higher compression than the octane will support is one that will NOT live a long lifetime... the gasoline faq clearly spells that out."
Octane is a measurement of resistance to detonation. This is related to charge temperature, dynamic compression (
7)How much dynamic compression would your engine have if you ran 340 degrees of duration, running at 2K RPMs?) Why is the engine's torque output so low? If it's so low, WHY? (hint: cylinder isn't full)
If it's this low, what does this tell you about dynamic compression with a large cam?"so if anything, bigger cams could require less compression, if you want the motor to live a long life."
Please elaborate!
"i can certainly see some truth in that, but it also sounds like a function of air speed thru the intake system to me... if you throw low c.r. at hogged-out ports and a giant cam, the motor could easily be a pig."
AGREED! You have two choices, go milder on the cam to bring the lower RPM torque #s up, OR run a hotter cam along with higher compression. Either way will work! The difference is where the torque curve lies! With one it's down low, and with the other it's up high. Torque is the same, power is NOT.
"i'm just interested in disproving this "theory" that people can always get away with running more compression because of a bigger cam."
Logically, please send your arguments my way and let's here your point of view on why you can NOT run more compression with a larger cam.
"i don't care how the motor "feels" by the seat of your pants, if it's detonating and you can't hear it, where is the common sense?"
Where's the common sense in running a engine at 60% VE? Build quality/detail aside, it lasts forever because the thing isn't making any respectable power!
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MASSIVE TYPE IV
- Posts: 20132
- Joined: Fri Jul 07, 2000 12:01 am
cams vs. compression
maybe what is needed here, is just what we wanted in the type 4 thread, lets put it on the DYNO...and run them both till they explode, with hall semsors all over, and read numbers the whole time, as well as cylinder head temps and oil temps, and the amount of fuel used by each...
Dan, I do believe that you would be amazed...
Now back to the bootlegger down the road, Jack is hollerin' again, and the beer joint already closed..And I still got heads to bolt on...
Dan, I do believe that you would be amazed...
Now back to the bootlegger down the road, Jack is hollerin' again, and the beer joint already closed..And I still got heads to bolt on...
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JohnConnolly
- Posts: 3336
- Joined: Thu Apr 13, 2000 12:01 am
cams vs. compression
the EASY solution is to get a cylinder pressure gauge (pricey, but we are considering getting one and renting it out). Detonation is easily measurable. In addition, we could chart the WOT cylinder pressure at all RPMs with different cams/heads, perhaps that would make it easier to digest?
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type11969
- Posts: 745
- Joined: Mon May 07, 2001 12:01 am
cams vs. compression
John-
I'm trying to get this whole cams vs compression thing straight, and I guess my big problem is, what exactly is the dynamic compression ratio? Is it the exact compression ratio at a given rpm considering the engine setup? If that is the case, how can the dynamic compression ratio ever get above the static compression ratio? Or is the dynamic compression ratio more along the lines of the amount of air/fuel mixture being compressed? If that's the case, I can see how the dynamic compression ratio can get above the static compression ratio, but the term "dynamic compression ratio" sucks (I guess no one asked me though). Compression ratio is the max volume divided by the min volume, so in the case of bleed-off the compression ratio changes, but otherwise it remains constant, regardless of how much vapor is in the cylinder. Or are both my suggestions for the dynamic cr wrong?
Also, with long duration cams, does the overlap cause the burned fuel/air mixture to get sucked back into the cylinder through the exhaust valve during the intake stroke? Does this cause a problem, like extra heat or leaning out of the mixture?
Finally, wouldn't an engine with a high cr and a long duration cam running at high rpms (thus giving the engine an even higher dynamic compression) melt due to all the added heat? I've caught bits and pieces of the idea that the intake charge helps cool the engine (if I remember correctly), but it doesn't seem like the intake charge would be a large enough sink to absorb some, or even any of the heat generated by the previous cycle, especially if the cr is so high. It also seems like the intake charge would have very little time to absorb any heat due to the high rpms. If you have time, could you try to explain this theory.
Finally, how does one even begin to measure the dynamic cr? If it is based only on static cr and the amount of bleed off at a given rpm, then its not a problem, but if it is based on the amount of vapor in the cylinder at a given rpm, it seems like a very difficult amount to measure without some very expensive equipment.
Well any light you can shed on this subject would be much appreciated. You don't have to dumb down the explaination too much because I just finished my sophmore year as a mechanical engineer (yep, I sure am looking forward to next year's work load) and I took Thermo 1 too.
Thanks,
Chris
I'm trying to get this whole cams vs compression thing straight, and I guess my big problem is, what exactly is the dynamic compression ratio? Is it the exact compression ratio at a given rpm considering the engine setup? If that is the case, how can the dynamic compression ratio ever get above the static compression ratio? Or is the dynamic compression ratio more along the lines of the amount of air/fuel mixture being compressed? If that's the case, I can see how the dynamic compression ratio can get above the static compression ratio, but the term "dynamic compression ratio" sucks (I guess no one asked me though). Compression ratio is the max volume divided by the min volume, so in the case of bleed-off the compression ratio changes, but otherwise it remains constant, regardless of how much vapor is in the cylinder. Or are both my suggestions for the dynamic cr wrong?
Also, with long duration cams, does the overlap cause the burned fuel/air mixture to get sucked back into the cylinder through the exhaust valve during the intake stroke? Does this cause a problem, like extra heat or leaning out of the mixture?
Finally, wouldn't an engine with a high cr and a long duration cam running at high rpms (thus giving the engine an even higher dynamic compression) melt due to all the added heat? I've caught bits and pieces of the idea that the intake charge helps cool the engine (if I remember correctly), but it doesn't seem like the intake charge would be a large enough sink to absorb some, or even any of the heat generated by the previous cycle, especially if the cr is so high. It also seems like the intake charge would have very little time to absorb any heat due to the high rpms. If you have time, could you try to explain this theory.
Finally, how does one even begin to measure the dynamic cr? If it is based only on static cr and the amount of bleed off at a given rpm, then its not a problem, but if it is based on the amount of vapor in the cylinder at a given rpm, it seems like a very difficult amount to measure without some very expensive equipment.
Well any light you can shed on this subject would be much appreciated. You don't have to dumb down the explaination too much because I just finished my sophmore year as a mechanical engineer (yep, I sure am looking forward to next year's work load) and I took Thermo 1 too.
Thanks,
Chris
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James2
- Posts: 3148
- Joined: Fri Mar 16, 2001 12:01 am
cams vs. compression
Dynamic CR can never be greater then static, UNLESS Volumetric Efficiecy (VE) exceeds 100%.
VE is a measure of how the motor fills it's cylinders. It is the mathmatical realation ship between the displacement and the volume of gas that enters the cylinder. AT 100% VE the cylinder would be 100% filled. ( note the chamber does NOT need to be filled to reach 100% VE, thus it is possible to reach over 100% VE by filling both the cylinder and chamber in a NA motor)
Most "HOT" street engines do good to reach 85-90% VE with out forced induction, so it's unlikley that Dynamic CR will ever exceed static CR with out running a turbo or supercharger.
I'm sure John can explain it more.
VE is a measure of how the motor fills it's cylinders. It is the mathmatical realation ship between the displacement and the volume of gas that enters the cylinder. AT 100% VE the cylinder would be 100% filled. ( note the chamber does NOT need to be filled to reach 100% VE, thus it is possible to reach over 100% VE by filling both the cylinder and chamber in a NA motor)
Most "HOT" street engines do good to reach 85-90% VE with out forced induction, so it's unlikley that Dynamic CR will ever exceed static CR with out running a turbo or supercharger.
I'm sure John can explain it more.
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bad62bug
- Posts: 288
- Joined: Sun Dec 31, 2000 12:01 am
cams vs. compression
1
[This message has been edited by bad62bug (edited 06-06-2001).]
[This message has been edited by bad62bug (edited 06-06-2001).]
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Cpt Zoomy
- Posts: 427
- Joined: Sun Oct 08, 2000 12:01 am
cams vs. compression
???? What is wrong with this discussion? Compared to the other ones we have had to endure, I am acually getting something worth while out of this.
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http://www.sgvwa.20m.com
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http://www.sgvwa.20m.com
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type11969
- Posts: 745
- Joined: Mon May 07, 2001 12:01 am
cams vs. compression
James-
Thanks for the info, I thought someone posted that their dynamic cr was greater than their static cr, I guess I must have misread. Regardless, the rest of my questions still stand, can anyone else help?
Thanks,
Chris
Thanks for the info, I thought someone posted that their dynamic cr was greater than their static cr, I guess I must have misread. Regardless, the rest of my questions still stand, can anyone else help?
Thanks,
Chris
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JohnConnolly
- Posts: 3336
- Joined: Thu Apr 13, 2000 12:01 am
cams vs. compression
OK, now this is getting good (it's NOT getting out of hand either).
"I'm trying to get this whole cams vs compression thing straight, and I guess my big problem is, what exactly is the dynamic compression ratio? Is it the exact compression ratio at a given rpm considering the engine setup?"
consider that a cylinder on a 2.0L engine sweeps .5L of displacement. Dynamic Compression is the compression actually SEEN in the cylinder. It is mostly dependent upon throttle position (at idle, the cylinder ain't fillin' much, for example), carbs, cam, and RPMs. AT high RPMs (too high), the engine's cam and carbs can't let enough air in, so dynamic compression drops (it's like a restrictor plate), no matter what the static compression is.
"If that is the case, how can the dynamic compression ratio ever get above the static compression ratio?"
Ever hear of a tuned exhaust? You use the momentum of what's coming out of one pipe to PULL the combustion gasses out of the cylinder. Well, the same thing can be done on the intake. When it's done right, the cylinder can fill MORE than it's 100% displacement (tuned system). 2 strokes really have it down, but it can be done on 4 strokes too.
" Or is the dynamic compression ratio more along the lines of the amount of air/fuel mixture being compressed? "
It's a combination of static compression, along with how much the cylinder is getting filled. For example, a 10:1 static compression engine that's running at 90% VE is running 9:1 dynamic compression. A 9:1 static engine running at 110% VE is at 9.9:1 dynamic.
Of course, this is also where cam duration comes in. HUGE cams at low RPMs have the intake valve (for example) open even as the piston has started it's compression stroke. WHY? While you lose power (compression) at low RPMs, you gain at high RPMs! See, at higher RPMs things are happening fast. By having the valve open MORE (you can only open and close the valve at a certain rate before you start having problems), you shove more in early on. Though you still have reversion, you get "more in early on than you get out later", so it's a win situation if you gain more than you lose. This is why hot cams need more RPMs to work. There is a point where you are shoving in more than is coming back out, and that's where the "cam comes on".
"If that's the case, I can see how the dynamic compression ratio can get above the static compression ratio, but the term "dynamic compression ratio" sucks (I guess no one asked me though). Compression ratio is the max volume divided by the min volume, so in the case of bleed-off the compression ratio changes, but otherwise it remains constant, regardless of how much vapor is in the cylinder. Or are both my suggestions for the dynamic cr wrong?"
How much is the cylinder getting filled at idle? You think you are running 10:1 at idle dynamic on a 10:1 static engine? STATIC means just do the math. Dynamic means you have to measure how much air is coming in and do some math (not easy). Dynamic is easy to theorize, but beyond that, it's tough unless you can measure cylinder pressure while the engine is running.
"Also, with long duration cams, does the overlap cause the burned fuel/air mixture to get sucked back into the cylinder through the exhaust valve during the intake stroke? "
A little bit. Also, some of the fresh fuel/air goes right back out the exhaust without a compression/combustion cycle (up goes your HCs)
"Does this cause a problem, like extra heat or leaning out of the mixture?"
NO, just emmisions go up. Actually, think about what happens if you send fuel and air thru the engine without burning it. Your mileage goes down, your emissions go up, but you COOL OFF the cylinder (no burn).
"Finally, wouldn't an engine with a high cr and a long duration cam running at high rpms (thus giving the engine an even higher dynamic compression) melt due to all the added heat?"
Why? torque is the same, you are only firing the cylinders faster! You are creating more heat, because you are making more power at high RPMs, but you are also spinning the fan faster. Additionally, if you take Thermodynamics, you'll find that efficiency increases as you increase compression.
Think about this! If you are burning a fixed amount of fuel and air, you create (unleash) a certain amount of energy, right? What do you do with it? We try to create mechanical energy with it. What's LEFT OVER is heat, and sound, which we get out the exhaust, or have to use the cooling system for. IF YOU MAKE MORE POWER FROM THE SAME AMOUNT OF AIR/FUEL, IT MEANS LESS HEAT IS LEFT OVER TO HEAT UP YOUR ENGINE AND EXHAUST.
Try this, retard the timing in your engine 6 degrees, then drive it. See how cool it runs (you aren't turning the energy into power)? you'll burn your hand on the exhaust and the engine will be one hot mother). THIS is also why the additional advance of a vacuum advance makes your engine run cooler on the highway.
" I've caught bits and pieces of the idea that the intake charge helps cool the engine (if I remember correctly), but it doesn't seem like the intake charge would be a large enough sink to absorb some, or even any of the heat generated by the previous cycle, especially if the cr is so high. It also seems like the intake charge would have very little time to absorb any heat due to the high rpms. If you have time, could you try to explain this theory."
I hope that helped some.
>Finally, how does one even begin to measure the dynamic cr? If it is based only on static cr and the amount of bleed off at a given rpm, then its not a problem, but if it is based on the amount of vapor in the cylinder at a given rpm, it seems like a very difficult amount to measure without some very expensive equipment. "
you got that right. But, some guys (like me and Jake R) learned the hard way you can't run 8:1 on a MILD cam engine (BOOM). You CAN run 9:1 on a 296 duration cam, and it works very well. 10-11:1 on a FK-87 or so, on pump gas.
" Well any light you can shed on this subject would be much appreciated. You don't have to dumb down the explaination too much because I just finished my sophmore year as a mechanical engineer (yep, I sure am looking forward to next year's work load) and I took Thermo 1 too."
ok, since you took thermo, what did you guys learn (look it up if you can't remember) about compression and the Otto engine Chris?
Homework: excluding detonation, what happens to engine efficiency if you go from 7:1-9:1 compression, you have to answer since I spent this time answering your Qs!
John
"I'm trying to get this whole cams vs compression thing straight, and I guess my big problem is, what exactly is the dynamic compression ratio? Is it the exact compression ratio at a given rpm considering the engine setup?"
consider that a cylinder on a 2.0L engine sweeps .5L of displacement. Dynamic Compression is the compression actually SEEN in the cylinder. It is mostly dependent upon throttle position (at idle, the cylinder ain't fillin' much, for example), carbs, cam, and RPMs. AT high RPMs (too high), the engine's cam and carbs can't let enough air in, so dynamic compression drops (it's like a restrictor plate), no matter what the static compression is.
"If that is the case, how can the dynamic compression ratio ever get above the static compression ratio?"
Ever hear of a tuned exhaust? You use the momentum of what's coming out of one pipe to PULL the combustion gasses out of the cylinder. Well, the same thing can be done on the intake. When it's done right, the cylinder can fill MORE than it's 100% displacement (tuned system). 2 strokes really have it down, but it can be done on 4 strokes too.
" Or is the dynamic compression ratio more along the lines of the amount of air/fuel mixture being compressed? "
It's a combination of static compression, along with how much the cylinder is getting filled. For example, a 10:1 static compression engine that's running at 90% VE is running 9:1 dynamic compression. A 9:1 static engine running at 110% VE is at 9.9:1 dynamic.
Of course, this is also where cam duration comes in. HUGE cams at low RPMs have the intake valve (for example) open even as the piston has started it's compression stroke. WHY? While you lose power (compression) at low RPMs, you gain at high RPMs! See, at higher RPMs things are happening fast. By having the valve open MORE (you can only open and close the valve at a certain rate before you start having problems), you shove more in early on. Though you still have reversion, you get "more in early on than you get out later", so it's a win situation if you gain more than you lose. This is why hot cams need more RPMs to work. There is a point where you are shoving in more than is coming back out, and that's where the "cam comes on".
"If that's the case, I can see how the dynamic compression ratio can get above the static compression ratio, but the term "dynamic compression ratio" sucks (I guess no one asked me though). Compression ratio is the max volume divided by the min volume, so in the case of bleed-off the compression ratio changes, but otherwise it remains constant, regardless of how much vapor is in the cylinder. Or are both my suggestions for the dynamic cr wrong?"
How much is the cylinder getting filled at idle? You think you are running 10:1 at idle dynamic on a 10:1 static engine? STATIC means just do the math. Dynamic means you have to measure how much air is coming in and do some math (not easy). Dynamic is easy to theorize, but beyond that, it's tough unless you can measure cylinder pressure while the engine is running.
"Also, with long duration cams, does the overlap cause the burned fuel/air mixture to get sucked back into the cylinder through the exhaust valve during the intake stroke? "
A little bit. Also, some of the fresh fuel/air goes right back out the exhaust without a compression/combustion cycle (up goes your HCs)
"Does this cause a problem, like extra heat or leaning out of the mixture?"
NO, just emmisions go up. Actually, think about what happens if you send fuel and air thru the engine without burning it. Your mileage goes down, your emissions go up, but you COOL OFF the cylinder (no burn).
"Finally, wouldn't an engine with a high cr and a long duration cam running at high rpms (thus giving the engine an even higher dynamic compression) melt due to all the added heat?"
Why? torque is the same, you are only firing the cylinders faster! You are creating more heat, because you are making more power at high RPMs, but you are also spinning the fan faster. Additionally, if you take Thermodynamics, you'll find that efficiency increases as you increase compression.
Think about this! If you are burning a fixed amount of fuel and air, you create (unleash) a certain amount of energy, right? What do you do with it? We try to create mechanical energy with it. What's LEFT OVER is heat, and sound, which we get out the exhaust, or have to use the cooling system for. IF YOU MAKE MORE POWER FROM THE SAME AMOUNT OF AIR/FUEL, IT MEANS LESS HEAT IS LEFT OVER TO HEAT UP YOUR ENGINE AND EXHAUST.
Try this, retard the timing in your engine 6 degrees, then drive it. See how cool it runs (you aren't turning the energy into power)? you'll burn your hand on the exhaust and the engine will be one hot mother). THIS is also why the additional advance of a vacuum advance makes your engine run cooler on the highway.
" I've caught bits and pieces of the idea that the intake charge helps cool the engine (if I remember correctly), but it doesn't seem like the intake charge would be a large enough sink to absorb some, or even any of the heat generated by the previous cycle, especially if the cr is so high. It also seems like the intake charge would have very little time to absorb any heat due to the high rpms. If you have time, could you try to explain this theory."
I hope that helped some.
>Finally, how does one even begin to measure the dynamic cr? If it is based only on static cr and the amount of bleed off at a given rpm, then its not a problem, but if it is based on the amount of vapor in the cylinder at a given rpm, it seems like a very difficult amount to measure without some very expensive equipment. "
you got that right. But, some guys (like me and Jake R) learned the hard way you can't run 8:1 on a MILD cam engine (BOOM). You CAN run 9:1 on a 296 duration cam, and it works very well. 10-11:1 on a FK-87 or so, on pump gas.
" Well any light you can shed on this subject would be much appreciated. You don't have to dumb down the explaination too much because I just finished my sophmore year as a mechanical engineer (yep, I sure am looking forward to next year's work load) and I took Thermo 1 too."
ok, since you took thermo, what did you guys learn (look it up if you can't remember) about compression and the Otto engine Chris?
Homework: excluding detonation, what happens to engine efficiency if you go from 7:1-9:1 compression, you have to answer since I spent this time answering your Qs!
John
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James2
- Posts: 3148
- Joined: Fri Mar 16, 2001 12:01 am
cams vs. compression
>>>>>>Finally, wouldn't an engine with a high cr and a long duration cam running at high rpms (thus giving the engine an even higher dynamic compression) melt due to all the added heat? I've caught bits and pieces of the idea that the intake charge helps cool the engine (if I remember correctly), but it doesn't seem like the intake charge would be a large enough sink to absorb some, or even any of the heat generated by the previous cycle, especially if the cr is so high. It also seems like the intake charge would have very little time to absorb any heat due to the high rpms. If you have time, could you try to explain this theory.
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The engine would INDEED overheat if ran "on the cam" at full throttle for any amount of time. The reason I feel that this doesn't happen is simple, the average street motor is not ran wide open throttle for more then a few seconds.( even when driven hard at the strip, it's only for 12-14 seconds) This gives the cooling system time to catch up.
John C. pointed out that the type 1 cooling system can only cool about 90 hp. The reason people run 200 plus HP with stock cooling systems, is for most of the time they are producing less then 200 hp.
Even when driving at high speeds less then 90 mph, they are only using 70-75 hp. The throttle is not fully open at those HP levels, thus VE is way down, because the cylinder is not allowed to fill completly. Thus the dynamic CR is way down also.
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The engine would INDEED overheat if ran "on the cam" at full throttle for any amount of time. The reason I feel that this doesn't happen is simple, the average street motor is not ran wide open throttle for more then a few seconds.( even when driven hard at the strip, it's only for 12-14 seconds) This gives the cooling system time to catch up.
John C. pointed out that the type 1 cooling system can only cool about 90 hp. The reason people run 200 plus HP with stock cooling systems, is for most of the time they are producing less then 200 hp.
Even when driving at high speeds less then 90 mph, they are only using 70-75 hp. The throttle is not fully open at those HP levels, thus VE is way down, because the cylinder is not allowed to fill completly. Thus the dynamic CR is way down also.
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Thorkhild
- Posts: 394
- Joined: Mon Mar 19, 2001 12:01 am
cams vs. compression
So if i had a cb cam with these specs:
Adv. Duration 270° / Dur. @ .050" 236° / Lift @ cam .374" / Lift w/1.1:1 Rocker Arms .411"
what kind of compression should i run considering that i will have 85.5 p&c,s and CB 044 heads, and dual 34 icts and soon to have a merged exhaust (have regular header now).
Or should i hold off using that cam for my 1776 or 90.5x78?
Its these kind of discussions that i really like. I haven't the best of luck with building engines and i really don't wanna screw up next time, especially cause i want to build a scat stroker engine for my fasty and the price tag isn't cheap.
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Thorkhild
1968 VW Blue Fastback
My Car Page
Adv. Duration 270° / Dur. @ .050" 236° / Lift @ cam .374" / Lift w/1.1:1 Rocker Arms .411"
what kind of compression should i run considering that i will have 85.5 p&c,s and CB 044 heads, and dual 34 icts and soon to have a merged exhaust (have regular header now).
Or should i hold off using that cam for my 1776 or 90.5x78?
Its these kind of discussions that i really like. I haven't the best of luck with building engines and i really don't wanna screw up next time, especially cause i want to build a scat stroker engine for my fasty and the price tag isn't cheap.
------------------
Thorkhild
1968 VW Blue Fastback
My Car Page
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JohnConnolly
- Posts: 3336
- Joined: Thu Apr 13, 2000 12:01 am
cams vs. compression
with that cam in a T-3, you better not exceed 7.5:1!
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danimal
cams vs. compression
>>>Dynamic compression is close to static compression at lower RPMs. At high RPMs the dynamic compression gets LOWER because the port and intake system can't flow enough air to FILL the cylinder in the amount of time the valve is open.<<<
phew! thank you, now that i'm sober
i can see where i got off into left field.
john, your example above would be dependent on cam design; you have described a lower-rpm jake cam, one with short duration, and it doesn't appear that you are going to see it any other way.
i'm oversimplifying, but by increasing the duration, the torque band can be moved up the rpm scale radically, so that the engine can breath at higher rpm's... h.p. is torque x rpm... that long duration will definitely increase the cylinder pressure at high rpm's, and decrease the cylinder pressure at low rpm's... that is exactly the point that david reher made, and yes, it's relevant to comparing cams only.
so then we raise the static c.r. to make up for the loss of dynamic compression at low rpm's, caused by the long duration... i'm with you there.
yes, i know that this is how it's done in the v-8 world, as darren pointed out on the clf, the v-8 cam mfg.'s will many times recommend a static c.r. to be run with their cams... apparently there are no vw cam manufacturers who do that.
i think we are in some agreement up to this point; where it goes south for me is in calculating the dynamic compression at high rpm's with a long duration cam.
high static c.r. will increase that high rpm dynamic c.r. figure(long duration cam), but i admit that it will be offset somewhat by the increased piston speed(flow losses).
about the only thing relative to actual dynamic c.r. figures that i know of is that some v-8 people claim that 200 psi is the limit for pump gas, without detonation.
my claim was that the dynamic c.r. with a radical cam could be unacceptable for pump gas; jake gave us one example(still don't know if i believe it!) but it's really a question of how far out in left field i am with long duration cams and dynamic c.r...
i'd sure hate to be proven wrong!
dan
oceanstreetvideo.com
phew! thank you, now that i'm sober
i can see where i got off into left field.john, your example above would be dependent on cam design; you have described a lower-rpm jake cam, one with short duration, and it doesn't appear that you are going to see it any other way.
i'm oversimplifying, but by increasing the duration, the torque band can be moved up the rpm scale radically, so that the engine can breath at higher rpm's... h.p. is torque x rpm... that long duration will definitely increase the cylinder pressure at high rpm's, and decrease the cylinder pressure at low rpm's... that is exactly the point that david reher made, and yes, it's relevant to comparing cams only.
so then we raise the static c.r. to make up for the loss of dynamic compression at low rpm's, caused by the long duration... i'm with you there.
yes, i know that this is how it's done in the v-8 world, as darren pointed out on the clf, the v-8 cam mfg.'s will many times recommend a static c.r. to be run with their cams... apparently there are no vw cam manufacturers who do that.
i think we are in some agreement up to this point; where it goes south for me is in calculating the dynamic compression at high rpm's with a long duration cam.
high static c.r. will increase that high rpm dynamic c.r. figure(long duration cam), but i admit that it will be offset somewhat by the increased piston speed(flow losses).
about the only thing relative to actual dynamic c.r. figures that i know of is that some v-8 people claim that 200 psi is the limit for pump gas, without detonation.
my claim was that the dynamic c.r. with a radical cam could be unacceptable for pump gas; jake gave us one example(still don't know if i believe it!)
but it's really a question of how far out in left field i am with long duration cams and dynamic c.r...i'd sure hate to be proven wrong!
dan
oceanstreetvideo.com