Static Compression vs Dynamic Compression Ratio
01-17-18, 04:02 PM
#1
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Static Compression vs Dynamic Compression Ratio
WARNING: Little bit of a read, but was curious if someone knows the DCR for the Silver & Blue top 2UR-GSE motors by chance??? Great read though!
7. DYNAMIC COMPRESSION RATIO (DCR) VS STATIC COMPRESSION RATIO (SCR)Most every gearhead believes that he understands compression ratio numbers, and simply takes them at face value. The normal compression ratio that everyone talks about and see’s on spec sheets is technically called “STATIC” compression ratio (SCR). That is always “THE” compression ratio being discussed unless otherwise specified.
And it of course comes from:
The total cylinder/head gasket/combustion chamber volume at BDC (bottom dead center), which we will call “V large”.
Then divide that total volume at BDC by the combustion chamber volume at TDC (top dead center), call that “V small”. So, you have (V large / V small) = Static compression ratio. As the name implies, it is a ratio of the max total volume divided by the small volume.
The 4 strokes are of course:
1. Intake
2. Compression
3. Power
4. Exhaust
And that’s all well and good for textbook learning, but in a real running engine, things aren’t so cut and dried. The “problem” is that an engine never “see’s or feels” the static compression ratio number. So, that makes the static compression ratio more or less a theoretical reference tool.
The difference in a running engine is that the cylinder volume needed to determine a running or Dynamic compression ratio (DCR), is not calculated with the piston at BDC. It is calculated with the piston at the position where the intake valve just closes. It is only at this point, that true compression can actually begin.
Here are the Intake and Exhaust valve timing events at .050” tappet lift (meaning lobe lift or lifter lift, NOT valve lift), per my 540ci BBC engine’s cam card:
In. opens at 25* BTDC (before top dead center)
In. closes at 61* ABDC (after bottom dead center) = 266* duration at .050 tappet lift
Ex. opens at 64* BBDC (before bottom dead center)
Ex. closes at 28* ATDC (after top dead center) = 272* duration at .050 tappet lift
As you can see, there is overlapping everywhere. This is done to optimize engine performance by making use of dynamic intake charge ramming effects and dynamic exhaust gas scavenging effects. So, actual running engine specs don’t fit neatly into the basic idea of the simple and separate 4 strokes. In order to calculate DCR from a useful intake valve closing point, rather than the .050” tappet lift timing shown above from the cam card, you need to use the Cam maker’s advertised tappet lift value.
For my Comp Cams steel billet solid roller cam, the advertised duration specs are given at .015” tappet lift. But my cam card does not provide the actual intake and exhaust timing events at that .015” tappet lift spec. So, I manually measured and calculated the piston/crankshaft position at the intake valve closing point based on .015” tappet lift of my actual engine. I did this during engine assembly mock up, where I could also take into account valve lash, rocker arm geometry, and rocker arm ratio. By doing it this way, I ended up with very precise numbers, which were used to get the most accurate final results. But, to get numbers this precise, it required that I determine the actual DCR after the fact, rather than determining it before buying any parts. So, I had to make some careful calculations earlier, in order to end up as close to my target as possible.
I ended up with my intake valves closing at 80.5* ABDC (or only 99.5* from TDC, rather than the theoretically ideal of BDC or 180* BTDC). This position had the piston 2.805” from TDC.
And considering that my stroke is 4.250”, this means that my piston had traveled 34% up the cylinder before the intake valve had closed, and compression could finally begin. I have a fairly large bad boy street/strip cam, and the larger the cam’s duration, the later the intake valve will close.
Then to do the calculations for DCR, it’s from the total cylinder/head gasket/combustion chamber volume at the point of intake valve closing. Call that value “DV large”. Then divide all that by the combustion chamber volume at TDC, the same value that was used above to calculate the SCR, which was called simply “V small”. So, you have (DV large / V small) = Dynamic compression ratio. As the name implies, it is a ratio of the large volume divided by the small volume. It is of course the same process that is used to determine SCR, except for the DCR, the large volume (DV large) is a much smaller value. And since the TDC volume (V small) was used for both SCR and DCR calculations, you can see how changing that TDC volume will change both types of compression ratio’s. They are linked by that “V small” value.
After crunching all the numbers, I ended up with an actual running engine compression ratio, or dynamic compression ratio (DCR) of 7.43 to 1. So, my two compression ratio numbers are:
Static Compression Ratio (SCR) = 10.75:1, which is the one seen on spec sheets
Dynamic Compression Ratio (DCR) = 7.43:1, which is the one the engine actually see’s/feel’s
You can see that the dynamic compression ratio is a far cry from the more commonly referenced static compression ratio of 10.75 to 1. This 7.43 DCR is what the engine actually see’s/feel’s and is what primarily determines its octane requirement. And as you have seen by now, the cam and its intake valve closing point, is the primary factor for determining an engine’s DCR. Change your cam, change you DCR.
If your Hotrod is on the ragged edge of detonating/pinging, you could switch to a cam with more duration, which will reduce your DCR and make the engine less sensitive to the octane it requires, because of a later intake closing point. That is just the opposite of what some folks might think. Because they’d likely think if their Hotrod was on the ragged edge of detonating/pinging, they’d need a milder cam. But, that would be going the “wrong” direction. Because a milder cam with less duration, would close the intake valve sooner, increasing the DCR. And that would make the engine even “more sensitive” to the octane it requires.
As an example, my cam has a 108* LSA (lobe separation angle), and the narrower this is, the sooner the intake valve closes, thus upping the DCR. And my cold cranking compression checked out to be 175 psi. But another very similar BBC engine with the same displacement and the same SCR, but with a wider 112* LSA, checked out to have only 165 psi cold cranking compression, due to its later intake valve closing, and thus lower DCR.
General approximate guidelines for DCR, though not absolute, are that a DCR of 7.5 to 8.5 will make best power for a street engine running 91 octane or higher. And the lower the DCR is in that range the better, for avoiding detonation problems.
Note: Race engines using race gas, can tolerate higher DCR’s up to about as high as 9:1.
As you can see, my 7.43 DCR came in quite close to the conservative 7.5 DCR number I had been targeting. I wanted to stay at the lower end of the recommended range so that my engine could tolerate California’s winter blend of pump premium, which has been known to fall below the octane number that we see with the summer blend. Call it adding a bit more margin of safety. Because detonation can cause ugly failures that you must avoid at all cost.
On top of that, I wanted to run a lot of ignition timing advance at low rpm, for crisp and quick throttle response. And staying at the lower end of the DCR range, allows me to do that without issue. It’s also no secret that larger engines, say upper 400 cubic inches and above, are big enough that they can absorb a low DCR and/or big cams with ease, so that you won’t even notice it.
BOTTOM LINE: The critical compression ratio that really counts, is the Dynamic Compression Ratio (DCR). OEM’s of course design their engines based on DCR. That’s why a lot of high performance, high rpm, factory stock engines with more cam duration and/or wider LSA’s (which results in a later intake valve closing), are running higher SCR’s, because that brings the DCR back into the desired range.
This lowering of the DCR, due to the late closing of the intake valve, is the reason why aftermarket Hotrod and Racing cam manufacturers spec a higher static compression ratio for their larger cams, because that gets the DCR into the proper range.
NOTE: HP = (Torque x rpm)/5252.
Little engines can make big HP, if you spin them to a high rpm. And in order to spin them to a high rpm, you need a large duration cam for the engine to breathe. But of course a large duration cam means a later closing intake valve, thus a lower DCR. So, you adjust the static compression ratio (SCR) to set the DCR to right where you want it. That allows you to have a very high performance engine that runs on ordinary pump gas. Here’s an example of just that:
The 2011 Yamaha YZF-R6 (600cc in-line 4 cylinder Sport Bike)
Its short 1.673” stroke allows it to rev to a 16,000 rpm redline, with only a 74.4 ft/sec average piston speed, while still being under the OEM limit of 80 ft/sec.
And it’s large duration cam that allows it to breathe enough to rev to 16,000 rpm, would have lowered the DCR unacceptably, except for the amazingly high 13.1 to 1 SCR which brings the DCR back up to an acceptable level. And the DCR is still set low enough so that even with the 13.1 SCR, it can still operate safely on ordinary pump premium gas.
After reading this, you may never look at the commonly referenced static compression ratio (SCR) the same way again. What is REALLY the most important compression ratio, is the Dynamic Compression Ratio (DCR). Because that is one of the primary factors determining how well your engine will run, and what its octane requirement will be.
540 RAT
7. DYNAMIC COMPRESSION RATIO (DCR) VS STATIC COMPRESSION RATIO (SCR)Most every gearhead believes that he understands compression ratio numbers, and simply takes them at face value. The normal compression ratio that everyone talks about and see’s on spec sheets is technically called “STATIC” compression ratio (SCR). That is always “THE” compression ratio being discussed unless otherwise specified.
And it of course comes from:
The total cylinder/head gasket/combustion chamber volume at BDC (bottom dead center), which we will call “V large”.
Then divide that total volume at BDC by the combustion chamber volume at TDC (top dead center), call that “V small”. So, you have (V large / V small) = Static compression ratio. As the name implies, it is a ratio of the max total volume divided by the small volume.
The 4 strokes are of course:
1. Intake
2. Compression
3. Power
4. Exhaust
And that’s all well and good for textbook learning, but in a real running engine, things aren’t so cut and dried. The “problem” is that an engine never “see’s or feels” the static compression ratio number. So, that makes the static compression ratio more or less a theoretical reference tool.
The difference in a running engine is that the cylinder volume needed to determine a running or Dynamic compression ratio (DCR), is not calculated with the piston at BDC. It is calculated with the piston at the position where the intake valve just closes. It is only at this point, that true compression can actually begin.
Here are the Intake and Exhaust valve timing events at .050” tappet lift (meaning lobe lift or lifter lift, NOT valve lift), per my 540ci BBC engine’s cam card:
In. opens at 25* BTDC (before top dead center)
In. closes at 61* ABDC (after bottom dead center) = 266* duration at .050 tappet lift
Ex. opens at 64* BBDC (before bottom dead center)
Ex. closes at 28* ATDC (after top dead center) = 272* duration at .050 tappet lift
As you can see, there is overlapping everywhere. This is done to optimize engine performance by making use of dynamic intake charge ramming effects and dynamic exhaust gas scavenging effects. So, actual running engine specs don’t fit neatly into the basic idea of the simple and separate 4 strokes. In order to calculate DCR from a useful intake valve closing point, rather than the .050” tappet lift timing shown above from the cam card, you need to use the Cam maker’s advertised tappet lift value.
For my Comp Cams steel billet solid roller cam, the advertised duration specs are given at .015” tappet lift. But my cam card does not provide the actual intake and exhaust timing events at that .015” tappet lift spec. So, I manually measured and calculated the piston/crankshaft position at the intake valve closing point based on .015” tappet lift of my actual engine. I did this during engine assembly mock up, where I could also take into account valve lash, rocker arm geometry, and rocker arm ratio. By doing it this way, I ended up with very precise numbers, which were used to get the most accurate final results. But, to get numbers this precise, it required that I determine the actual DCR after the fact, rather than determining it before buying any parts. So, I had to make some careful calculations earlier, in order to end up as close to my target as possible.
I ended up with my intake valves closing at 80.5* ABDC (or only 99.5* from TDC, rather than the theoretically ideal of BDC or 180* BTDC). This position had the piston 2.805” from TDC.
And considering that my stroke is 4.250”, this means that my piston had traveled 34% up the cylinder before the intake valve had closed, and compression could finally begin. I have a fairly large bad boy street/strip cam, and the larger the cam’s duration, the later the intake valve will close.
Then to do the calculations for DCR, it’s from the total cylinder/head gasket/combustion chamber volume at the point of intake valve closing. Call that value “DV large”. Then divide all that by the combustion chamber volume at TDC, the same value that was used above to calculate the SCR, which was called simply “V small”. So, you have (DV large / V small) = Dynamic compression ratio. As the name implies, it is a ratio of the large volume divided by the small volume. It is of course the same process that is used to determine SCR, except for the DCR, the large volume (DV large) is a much smaller value. And since the TDC volume (V small) was used for both SCR and DCR calculations, you can see how changing that TDC volume will change both types of compression ratio’s. They are linked by that “V small” value.
After crunching all the numbers, I ended up with an actual running engine compression ratio, or dynamic compression ratio (DCR) of 7.43 to 1. So, my two compression ratio numbers are:
Static Compression Ratio (SCR) = 10.75:1, which is the one seen on spec sheets
Dynamic Compression Ratio (DCR) = 7.43:1, which is the one the engine actually see’s/feel’s
You can see that the dynamic compression ratio is a far cry from the more commonly referenced static compression ratio of 10.75 to 1. This 7.43 DCR is what the engine actually see’s/feel’s and is what primarily determines its octane requirement. And as you have seen by now, the cam and its intake valve closing point, is the primary factor for determining an engine’s DCR. Change your cam, change you DCR.
If your Hotrod is on the ragged edge of detonating/pinging, you could switch to a cam with more duration, which will reduce your DCR and make the engine less sensitive to the octane it requires, because of a later intake closing point. That is just the opposite of what some folks might think. Because they’d likely think if their Hotrod was on the ragged edge of detonating/pinging, they’d need a milder cam. But, that would be going the “wrong” direction. Because a milder cam with less duration, would close the intake valve sooner, increasing the DCR. And that would make the engine even “more sensitive” to the octane it requires.
As an example, my cam has a 108* LSA (lobe separation angle), and the narrower this is, the sooner the intake valve closes, thus upping the DCR. And my cold cranking compression checked out to be 175 psi. But another very similar BBC engine with the same displacement and the same SCR, but with a wider 112* LSA, checked out to have only 165 psi cold cranking compression, due to its later intake valve closing, and thus lower DCR.
General approximate guidelines for DCR, though not absolute, are that a DCR of 7.5 to 8.5 will make best power for a street engine running 91 octane or higher. And the lower the DCR is in that range the better, for avoiding detonation problems.
Note: Race engines using race gas, can tolerate higher DCR’s up to about as high as 9:1.
As you can see, my 7.43 DCR came in quite close to the conservative 7.5 DCR number I had been targeting. I wanted to stay at the lower end of the recommended range so that my engine could tolerate California’s winter blend of pump premium, which has been known to fall below the octane number that we see with the summer blend. Call it adding a bit more margin of safety. Because detonation can cause ugly failures that you must avoid at all cost.
On top of that, I wanted to run a lot of ignition timing advance at low rpm, for crisp and quick throttle response. And staying at the lower end of the DCR range, allows me to do that without issue. It’s also no secret that larger engines, say upper 400 cubic inches and above, are big enough that they can absorb a low DCR and/or big cams with ease, so that you won’t even notice it.
BOTTOM LINE: The critical compression ratio that really counts, is the Dynamic Compression Ratio (DCR). OEM’s of course design their engines based on DCR. That’s why a lot of high performance, high rpm, factory stock engines with more cam duration and/or wider LSA’s (which results in a later intake valve closing), are running higher SCR’s, because that brings the DCR back into the desired range.
This lowering of the DCR, due to the late closing of the intake valve, is the reason why aftermarket Hotrod and Racing cam manufacturers spec a higher static compression ratio for their larger cams, because that gets the DCR into the proper range.
NOTE: HP = (Torque x rpm)/5252.
Little engines can make big HP, if you spin them to a high rpm. And in order to spin them to a high rpm, you need a large duration cam for the engine to breathe. But of course a large duration cam means a later closing intake valve, thus a lower DCR. So, you adjust the static compression ratio (SCR) to set the DCR to right where you want it. That allows you to have a very high performance engine that runs on ordinary pump gas. Here’s an example of just that:
The 2011 Yamaha YZF-R6 (600cc in-line 4 cylinder Sport Bike)
Its short 1.673” stroke allows it to rev to a 16,000 rpm redline, with only a 74.4 ft/sec average piston speed, while still being under the OEM limit of 80 ft/sec.
And it’s large duration cam that allows it to breathe enough to rev to 16,000 rpm, would have lowered the DCR unacceptably, except for the amazingly high 13.1 to 1 SCR which brings the DCR back up to an acceptable level. And the DCR is still set low enough so that even with the 13.1 SCR, it can still operate safely on ordinary pump premium gas.
After reading this, you may never look at the commonly referenced static compression ratio (SCR) the same way again. What is REALLY the most important compression ratio, is the Dynamic Compression Ratio (DCR). Because that is one of the primary factors determining how well your engine will run, and what its octane requirement will be.
540 RAT
Last edited by MileHIFcar; 01-17-18 at 04:11 PM.
01-17-18, 04:47 PM
#2
Tech Info Resource
iTrader: (2)
He's wrong. Dynamic compression ratio is calculated using volumetric efficiency and actually varies based on rpm because the cylinders fill different amounts based on the things he touched on - inertia of the air coming in and inertia of the air going out along with the timing of the valves open and closing. OEMs measure this by creating a pressure/volume map using special sparkplugs (Kistler makes these along with the instrumentation necessary to read them) so they know precisely what the cylinder pressure is at any given rpm for the engine under test. The pressure/volume map will tell you everything you really need to know and won't be based on guessing (which, despite his attempt to use mathematics, he failed at miserably) which is what we have above.
The biggest reason for the epic fail above is VE can and does exceed 100% on a properly designed and tuned NA engine, and always exceeds 100% on a boosted engine. There is way more math involved in this than what Rat has above.
Don't take this as anything other than he's wrong about the term "dynamic compression ratio." His oil stuff is dead on the money.
The biggest reason for the epic fail above is VE can and does exceed 100% on a properly designed and tuned NA engine, and always exceeds 100% on a boosted engine. There is way more math involved in this than what Rat has above.
Don't take this as anything other than he's wrong about the term "dynamic compression ratio." His oil stuff is dead on the money.
01-17-18, 04:59 PM
#4
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Thread Starter
Interesting???............Yea its started with his oil testing that you posted the link to in the GSF section/thread and I went down a rabbit hole with the other reading and stumbled across that and thought I'd post it because I hadn't heard anyone mentioning about DCR before??
Thx
Thx
01-17-18, 05:11 PM
#5
Tech Info Resource
iTrader: (2)
01-17-18, 06:13 PM
#6
Tech Info Resource
iTrader: (2)
Interesting???............Yea its started with his oil testing that you posted the link to in the GSF section/thread and I went down a rabbit hole with the other reading and stumbled across that and thought I'd post it because I hadn't heard anyone mentioning about DCR before??
Thx
Thx
Standard compression pressure: 1400 kPa (14.3 kgf/cm 2 , 203 psi) or more
Minimum pressure: 1000 kPa (10.2 kgf/cm 2 , 145 psi)
The gas laws tell you something different than what Rat tells you is happening here, and that's just at starter motor speed (200 - 300 rpm).
01-17-18, 07:19 PM
#7
Tech Info Resource
iTrader: (2)
This is a far better discussion of how dynamic compression ratio is, and how useless both static and the "classic" dynamic compression ratios are:
http://www.matrixgarage.com/content/...nearly-useless
It's all about VE, and yes, it is possible to have more than 100% VE in a well tuned NA engine!
kitabel is dead on about the influence of cam lift and duration. I've spent a whole lot of time playing with cam timing on DOHC engines and it's really surprising how you can change the way an engine makes power with just small variations in exhaust cam timing.
And if you want some real fun, compare thermal efficiency (torque peak) and volumetric efficiency (horsepower peak). Those two things can tell you an awful lot about an engine's design and tuning.
http://www.matrixgarage.com/content/...nearly-useless
It's all about VE, and yes, it is possible to have more than 100% VE in a well tuned NA engine!
kitabel is dead on about the influence of cam lift and duration. I've spent a whole lot of time playing with cam timing on DOHC engines and it's really surprising how you can change the way an engine makes power with just small variations in exhaust cam timing.
And if you want some real fun, compare thermal efficiency (torque peak) and volumetric efficiency (horsepower peak). Those two things can tell you an awful lot about an engine's design and tuning.
Last edited by lobuxracer; 01-17-18 at 07:28 PM.
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01-17-18, 09:19 PM
#8
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Thread Starter
Very interesting!.............Thanks for the additional info and link
01-18-18, 08:11 AM
#9
As stated you have to take into account all of the elements of the intake and exhaust as they dramatically impact cylinder filling. Some see the concept of for example exhaust scavenging and how it impacts a cylinders volume during compression but many do not understand what happens those spent gasses when they gut a CAT and allow those exhaust gasses to expand loosing heat and velocity. I'd like to find a Solidworks 3D model showing these details. Anyways...
The article didnt touch on current technology at all.
Although big rat motors make huge torque our little 3 and 4 liter engines with vvt across the whole rpm range are pretty impressive. The ability to adjust valve timing, fueling, and ignition timing on demand for say, launch, cruise, and max hp are in the hands of the ECU maps. That said I'm still looking for the white papers on what our Yoda engines are doing with all these nested maps.
The article didnt touch on current technology at all.
Although big rat motors make huge torque our little 3 and 4 liter engines with vvt across the whole rpm range are pretty impressive. The ability to adjust valve timing, fueling, and ignition timing on demand for say, launch, cruise, and max hp are in the hands of the ECU maps. That said I'm still looking for the white papers on what our Yoda engines are doing with all these nested maps.
01-18-18, 08:29 AM
#10
And the second example of the Yamaha R6 engine. From the factory it left with I 105 / E 105° lobe centers and has this midrange lull where torque falls off. A tiny shift of I102° / E104° and the torque curve becomes nearly linear and it makes more power. So why did they ship it that way? Oh, because the rider feels a punch about 8k which is perceived as more power....
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