Dynamic CR vs Static CR
10-26-2004, 07:05 PM
#21
TECH Junkie
Join Date: Aug 2002
Location: Fort Collins, CO
Posts: 3,726
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by critter
So the density is greater when the intake closes, in effect packing 43 ci into the 35 ci? So by PV=NRT the pressure is higher at the start of real compression, and the net effect is the same as a higher compression ratio that the dynamic ratio?
10-27-2004, 07:11 PM
#22
It is my understanding that you can change your DCR by holding the intake valve open longer. This would also have a pretty good effect on P/V clearence because there is only so long that you can hold the valve open before they make contact. How long you would actually have to hold the valve open to lower the DCR enough to get you back to where you need to be is also a question. How many more degrees of open intake valve does it take to realistically lower DCR? Also, is there actaully a formula to figure DCR because I would like to know myself.
EDIT: I found this http://www.rbracing-rsr.com/comprAdvHD.htm
According to this I will have a dynamic compression of 9:6:1 off of a static compression of 11:1. What is considered to be a high DCR?
EDIT: I found this http://www.rbracing-rsr.com/comprAdvHD.htm
According to this I will have a dynamic compression of 9:6:1 off of a static compression of 11:1. What is considered to be a high DCR?
Last edited by Beast96Z; 10-27-2004 at 09:20 PM.
10-28-2004, 05:07 PM
#24
Originally Posted by John B
A stock LS6 DCR is in the ~7.7 range so 9.6:1 is sky high (too high for pump gas!).
EDIT: I looked around some more and found that other calculators for DCR make you add 15* to your .050" ABDC number. When I put that in, my number for DCR is 8.6.1. That makes much more sense. Wonder why this calculator dosen't inform you to add the 15*?
Last edited by Beast96Z; 10-28-2004 at 05:31 PM.
11-02-2004, 04:47 PM
#26
10 Second Club
Thread Starter
iTrader: (9)
Join Date: Jun 2003
Location: Goshen, IN
Posts: 1,001
Likes: 0
Received 0 Likes
on
0 Posts
I haven't really found anything definitive yet, but I have learned a little.
The point at which you should figure DCR is fuzzy. I can't find any concrete information in the literature, but if you look at flow data for a set of stock LS6 heads, they still flow abut 13% of capacity at 0.050" lift, so that surely isn't the correct point. That flow would obviously decrease as the valve approaches the seat, but without instrumentation it is hard to guess at what point it is effectively closed. My cam has about 25 degrees between the 0.050" spec and the 0.006" spec. Playing with one of the netland DCR calculators shows about 1.5 points difference between the two.
Now, to muddy the waters, I found pressure traces in a couple of texts that show considerably above atmospheric in the cylinder as the intake closes in an engine running at torque peak. This is the effect of the acoustic and inertial phenomena. That implies that the point used to figure DCR really is back toward the 0.050 point or maybe even beyond. The graphs I saw are too small and lack enough detail to determine exactly where the point might be, and it would surely be specific to that motor.
This just confuses me more than before. I think I will forget theory and use reality ... if it pings, you need more octane. I still don't know if a longer duration cam would help, but if some money falls into my lap I can try it.
The point at which you should figure DCR is fuzzy. I can't find any concrete information in the literature, but if you look at flow data for a set of stock LS6 heads, they still flow abut 13% of capacity at 0.050" lift, so that surely isn't the correct point. That flow would obviously decrease as the valve approaches the seat, but without instrumentation it is hard to guess at what point it is effectively closed. My cam has about 25 degrees between the 0.050" spec and the 0.006" spec. Playing with one of the netland DCR calculators shows about 1.5 points difference between the two.
Now, to muddy the waters, I found pressure traces in a couple of texts that show considerably above atmospheric in the cylinder as the intake closes in an engine running at torque peak. This is the effect of the acoustic and inertial phenomena. That implies that the point used to figure DCR really is back toward the 0.050 point or maybe even beyond. The graphs I saw are too small and lack enough detail to determine exactly where the point might be, and it would surely be specific to that motor.
This just confuses me more than before. I think I will forget theory and use reality ... if it pings, you need more octane. I still don't know if a longer duration cam would help, but if some money falls into my lap I can try it.
11-07-2004, 10:13 PM
#27
10 Second Club
Thread Starter
iTrader: (9)
Join Date: Jun 2003
Location: Goshen, IN
Posts: 1,001
Likes: 0
Received 0 Likes
on
0 Posts
I found this at http://victorylibrary.com/mopar/cam-tech-c.htm. It confirms my suspicions about VE and cylinder pressure at torque peak. I need to read it a couple more times, but it seems to bring closure to this thread.
Dynamic Compression Ratio (DCR) is offered (elsewhere) as a method of approximating how much power will be lost by extended intake closure (i.e., cylinder pressure is reduced). DCR is very useful in predicting what octane is needed at lower speeds to help setting idle (initial) spark vs. advance curve length, stall speed, &c. However, this is only partially accurate due to several errors and misconceptions.
1. The actual power loss is only present below the points where both full capture (where intake reversion ABDC stops) and positive flow-through at overlap occur (although these may not be close together). Above that point power is increased (over the milder cam), either by improved volumetric efficiency (and cylinder pressure, since a larger percentage is trapped in the cylinder), higher RPM reached @ the same efficiency level, or both.
2. The power loss is not solely due to reduced cylinder pressure (which is the output of the Kelley, RSR &c. DCR calculators), since the lower pressure is also acting through a shorter effective stroke (measured from intake valve closure ABDC) and therefore suffers from two reductions. My own method factors in the effective cylinder displacement as well, for a closer approximate at how much power is developed at low speed.
3. DCR is widely considered to be an accurate barometer of knock resistance (e.g., up to 8-1 DCR can be used with XX octane, &c.).
This is not true, and not safe, since (in a high compression motor with its DCR reduced by late intake closure) after the early low-pressure period expires (at the beginning of the torque curve) actual combustion pressure will be at least as high with the bigger cam than it was previously - even though the DCR is lower, and therefore suggests that lower octane is safe. However, in a motor with 14-1 static CR the gas pressure at its torque peak is not at all reduced by a very late intake closing point, although the (lower) DCR may indicate that 92 octane &c. is sufficient.
Since the knock will only occur at high speeds it may not be audible, and will reduce power (lower MPH) even if no damage can be detected. In my opinion, the power sometimes gained by retarding spark in high gear is actually an attempt to recover some of this loss - but would be better served by reducing the static ratio slightly.
DCR is a curve or slope of cylinder (not combustion) pressure, with Position 0 (the absolute low end) at cranking speed, then a small rise to idle speed, then another rise to the capture point &c. After this point (and especially near the torque peak) the static CR becomes more important, since almost the full stroke length is captured and compressed, regardless of the intake closing point.
Any cam will determine the slope (or rate of rise) of the DCR curve. A long-duration cam with its attendant late intake closing point will have a high degree of rise, a mild cam less, &c. A longer cam will also extend (stretch) the range of RPM that the slope covers, sometimes over several thousand more RPM.
The static ratio determines the height of the cylinder pressure line at Position 0 (cranking speed). With high static ratio the entire curve is higher, with the curve's upward intensity being governed by the intake closing point.
It's possible to design a DCR that looks promising, but will not provide any more power, by assuming that there is no limit to either static ratio or intake closure - and, of course, neither is true. Some motors cannot turn fast enough (due to stroke length, weak valve gear, high reciprocating weight, &c.) to reach their capture point if the intake closure is too late, and will produce more power with more conservative cam timing. A motor with limited static ratio (flathead) must conserve cylinder pressure by limiting intake closure for the same reasons.
Another error in use of DCR calculations for low-speed power prediction lies in the fact that a smaller volume of mixture being compressed to a higher ratio. Even though the pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure times the actual mixture volume captured may still be lower (compared to the original milder cam and moderate compression ratio).
To sum up: DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics.
1. The actual power loss is only present below the points where both full capture (where intake reversion ABDC stops) and positive flow-through at overlap occur (although these may not be close together). Above that point power is increased (over the milder cam), either by improved volumetric efficiency (and cylinder pressure, since a larger percentage is trapped in the cylinder), higher RPM reached @ the same efficiency level, or both.
2. The power loss is not solely due to reduced cylinder pressure (which is the output of the Kelley, RSR &c. DCR calculators), since the lower pressure is also acting through a shorter effective stroke (measured from intake valve closure ABDC) and therefore suffers from two reductions. My own method factors in the effective cylinder displacement as well, for a closer approximate at how much power is developed at low speed.
3. DCR is widely considered to be an accurate barometer of knock resistance (e.g., up to 8-1 DCR can be used with XX octane, &c.).
This is not true, and not safe, since (in a high compression motor with its DCR reduced by late intake closure) after the early low-pressure period expires (at the beginning of the torque curve) actual combustion pressure will be at least as high with the bigger cam than it was previously - even though the DCR is lower, and therefore suggests that lower octane is safe. However, in a motor with 14-1 static CR the gas pressure at its torque peak is not at all reduced by a very late intake closing point, although the (lower) DCR may indicate that 92 octane &c. is sufficient.
Since the knock will only occur at high speeds it may not be audible, and will reduce power (lower MPH) even if no damage can be detected. In my opinion, the power sometimes gained by retarding spark in high gear is actually an attempt to recover some of this loss - but would be better served by reducing the static ratio slightly.
DCR is a curve or slope of cylinder (not combustion) pressure, with Position 0 (the absolute low end) at cranking speed, then a small rise to idle speed, then another rise to the capture point &c. After this point (and especially near the torque peak) the static CR becomes more important, since almost the full stroke length is captured and compressed, regardless of the intake closing point.
Any cam will determine the slope (or rate of rise) of the DCR curve. A long-duration cam with its attendant late intake closing point will have a high degree of rise, a mild cam less, &c. A longer cam will also extend (stretch) the range of RPM that the slope covers, sometimes over several thousand more RPM.
The static ratio determines the height of the cylinder pressure line at Position 0 (cranking speed). With high static ratio the entire curve is higher, with the curve's upward intensity being governed by the intake closing point.
It's possible to design a DCR that looks promising, but will not provide any more power, by assuming that there is no limit to either static ratio or intake closure - and, of course, neither is true. Some motors cannot turn fast enough (due to stroke length, weak valve gear, high reciprocating weight, &c.) to reach their capture point if the intake closure is too late, and will produce more power with more conservative cam timing. A motor with limited static ratio (flathead) must conserve cylinder pressure by limiting intake closure for the same reasons.
Another error in use of DCR calculations for low-speed power prediction lies in the fact that a smaller volume of mixture being compressed to a higher ratio. Even though the pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure times the actual mixture volume captured may still be lower (compared to the original milder cam and moderate compression ratio).
To sum up: DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics.
35ci, area, compression, cr, cranking, cylinder, dcr, difference, dynamic, efficiency, octane, port, runner, static, volumetric
