Posted By: emarine01
Static VS Dynamic compression - 07/18/09 01:13 PM
Is there a ratio between the two that works best?
Static VS Dynamic compression
Posted By: 602heavy
Re: Static VS Dynamic compression - 07/18/09 01:52 PM
Good question , would'nt this be the 'holy grail'? , am i right in thinking the calculators for determining DC are hit & miss? , the way i see it static CR is meaningless , it all hinges on IVC , so are we looking for a specific DCR across the board regardless of SCR?
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 02:22 PM
Well I waited till the weekend to throw this one up here, I know its a very general question to a very involved process but there are a lot of swift guys on here that have have probably plowed thru the math and have come up with some base line results, A lot of it is trial and error or field test results but some one must have come up with sort of a rule of thumb that has value 
Posted By: 67_Satellite
Re: Static VS Dynamic compression - 07/18/09 02:53 PM
Trial and error is right. Just think of all the variables that effect the amount of air coming into the cylinder before that intake valve touches the seat.Carb.size plenum volume,runner distribution variance,runner entry shape,taper&size,valve size,lift rate,seat angles,port shape& flow,chamber shape,valve shrouding, piston acceleration rate&velocity,brand of beer consumed while assembling the engine,etc.etc. It boggles the mind. 
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 03:13 PM
I see your point, I was leaning more toward fuel requirement at the inception of a build with a some what general VE target
Posted By: Dodgem
Re: Static VS Dynamic compression - 07/18/09 03:42 PM
This one has been pretty close from my experiences
http://www.rbracing-rsr.com/comprAdvHD.htm
I think many engine builders aim at 185 max with aluminum heads for 93 octane? I ran my old motor at 195 (193 to 202) on sunoco 94??
Iron heads maybe 160 and attention to any sharp high spots that can glow is a must!
Tryng to get this JWD closer to 190 I'm at 160 low now and 170 high was at 150 low and 160 high so advanced the cam 4 more degs??
http://www.rbracing-rsr.com/comprAdvHD.htm
I think many engine builders aim at 185 max with aluminum heads for 93 octane? I ran my old motor at 195 (193 to 202) on sunoco 94??
Iron heads maybe 160 and attention to any sharp high spots that can glow is a must!
Tryng to get this JWD closer to 190 I'm at 160 low now and 170 high was at 150 low and 160 high so advanced the cam 4 more degs??
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 03:57 PM
Is there a chart or calc for dynamic ratio and fuel requirement?
Posted By: Dodgem
Re: Static VS Dynamic compression - 07/18/09 04:11 PM
There must be somewhere as depending on how brave you are 200 with aluminum heads seems to be the upper limit with pump gas but most seem to say 180 to 185 with pump gas and aluminum heads.
You TQ is also a factor as the higher the RPMs the less octane needed as air fuel mixture burns at a constant rate and things happen quicker at rpm.
So as an example a car with a 2500 stall loads at 2500 and may ping till 4500. put a 5000 stall in the same combo and it loads at 5000 beyond the detination as the piston gets over the top much quicker before air/fuel can expand enough to detonate/ pre ignight~!!!
You TQ is also a factor as the higher the RPMs the less octane needed as air fuel mixture burns at a constant rate and things happen quicker at rpm.
So as an example a car with a 2500 stall loads at 2500 and may ping till 4500. put a 5000 stall in the same combo and it loads at 5000 beyond the detination as the piston gets over the top much quicker before air/fuel can expand enough to detonate/ pre ignight~!!!
Posted By: Dodgem
Re: Static VS Dynamic compression - 07/18/09 04:34 PM
KB calculator similar results on the dynamic compression but not exactly the same heard this is most acurate
http://www.kb-silvolite.com/calc.php?action=comp
Found this number
DCR of 7.8:1 - 8.5:1 for pump gas with aluminum heads
think it said 7.5 to 8.0 for iron heads?
http://www.kb-silvolite.com/calc.php?action=comp
Found this number
DCR of 7.8:1 - 8.5:1 for pump gas with aluminum heads
think it said 7.5 to 8.0 for iron heads?
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 04:51 PM
Correct , it seems everyone's looking at static compression for fuel requirement when most race engines dont operate at low rpm, I make 220 psi static with 12 to 1 and according to the calcs I range between 9.8 and 10.2 DCR , the engine load starts at 5k to 7k , I am more interested in fuel requirement using DCR , it seems that could be done on a chart?
Posted By: Dodgem
Re: Static VS Dynamic compression - 07/18/09 05:25 PM
I searched and could not find one??
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 05:44 PM
This is around the point where the swift guys are suppose to help out 
Posted By: 64Post
Re: Static VS Dynamic compression - 07/18/09 07:45 PM
Quote:
This is around the point where the swift guys are suppose to help out
Well, I'm jumping in anyway even though I'm not qualified...
This sorta ties in with the "Intake Closing Angle" question that was posted yesterday here and in the Q&A.
So is everyone using the ICP @.050" numbers from the cam card when calc'ing their DCR? Or, are you adding a few (5, 10, 15*) degrees -- per some online calculators -- because the valve is still off its seat?
Another calculator with altitude correction thrown in the mix. This one makes no mention of an additional 15* ICA.
http://www.wallaceracing.com/dynamic-cr.php
Posted By: emarine01
Re: Static VS Dynamic compression - 07/18/09 09:22 PM
Good question, in theory the intake valve has to be closed, so the on line calcs vary , but what about valve lash, lobe ramps, solid compared to hyd cams it all effects DCR by exactly when the intake seals the chamber 
Posted By: Dodgem
Re: Static VS Dynamic compression - 07/18/09 09:29 PM
They vary slightly but it is a good general guide. tight lash compression goes down loose it goes up ever so slightly!!
Posted By: 64Post
Re: Static VS Dynamic compression - 07/18/09 10:33 PM
Quote:
Good question, in theory the intake valve has to be closed, so the on line calcs vary , but what about valve lash, lobe ramps, solid compared to hyd cams it all effects DCR by exactly when the intake seals the chamber
I made essentially the same point in the other thread.
I guess the smart guys took the day off, which leaves us.
I'll be corrected, but I think Crane came up with the .050" measurements originally, or at least started listing their cams @ .050 first. I think the story was that, empirically through their research, cylinder and charge pressure loss/gain were negligible at .050, and that air really didn't start moving until the valve was at .050". If you subscribe to that line of thinking then use the 50 number to establish ICP and calc from there.
The numbers in my case at 5300 ft.
Static CR = 12.2
ICP = 58* @.050"
Dynamic CR = 9.0
Dynamic CP =185
V/P = 176
Posted By: goldmember
Re: Static VS Dynamic compression - 07/18/09 10:57 PM
I used some online calcs using a known combo for giggles. If I use the instructions my DCR is 9.44,when I sustitute the the more accurate closing point I get 9.14 DCR. I don't spend much time playing with DCR calcs,I haven't found any magic in them.
Posted By: 451Mopar
Re: Static VS Dynamic compression - 07/19/09 12:12 AM
Quote:
Is there a ratio between the two that works best?
"Dynamic Compression" is just a description of re-calculating the compression ratio from the point where the intake valve closes.
The assumption is that at low engine RPM, like cranking, the intake valve will close at a point after BDC "trapping" the standard atmospheric pressure, and then compress the trapped air (with no losses) to estimate the cranking cylinder pressure.
Usually if the calculated cranking pressure is around 160 psi on run on pump gas. Below 160 psi, usually means you are leaving power on the table by not having enough static compression. Above 160 psi is still doable on pump gas, but you would want a good quench chamber and aluminum heads.
This is just a rough guide, as there are many variables that determine what octane level a specific engine will need.
This is the Victory Library tech article on this:
http://victorylibrary.com/tech/cam-c.htm
Posted By: polyspheric
Re: Static VS Dynamic compression - 07/19/09 03:58 PM
in theory the intake valve has to be closed
Actually, the stoppage can be from the closed valve, or simply the difference in pressure between the inertia of the intake flow vs. the lower static pressure of the charge in the cylinder ATDC.
At low speeds, there isn't enough inertia to get flow to enter after the piston changes direction, and flow not only stops at the valve but reverses.
At higher speed, inertia keeps it going until IVC.
DCR is useful to estimate how knock-prone an engine will be a low to mid speeds, but (as said above) at high RPM the full stroke length is captured and compressed (not just the stroke after IVC) at the beginning of the torque range and DCR becomes irrelevant.
Actually, the stoppage can be from the closed valve, or simply the difference in pressure between the inertia of the intake flow vs. the lower static pressure of the charge in the cylinder ATDC.
At low speeds, there isn't enough inertia to get flow to enter after the piston changes direction, and flow not only stops at the valve but reverses.
At higher speed, inertia keeps it going until IVC.
DCR is useful to estimate how knock-prone an engine will be a low to mid speeds, but (as said above) at high RPM the full stroke length is captured and compressed (not just the stroke after IVC) at the beginning of the torque range and DCR becomes irrelevant.
Posted By: emarine01
Re: Static VS Dynamic compression - 07/19/09 04:50 PM
So if your stall is below peek torque the DCR numbers will help at lower rpm & if your stall is high in the rpm range DCR is not too inportant , ok I was just trying to figure a better way of setting total advance with different fuels without pulling the plugs and looking all the time, a chart would be nice but I now see 
Posted By: 451Mopar
Re: Static VS Dynamic compression - 07/20/09 03:02 AM
Oops, I originally posted the link to the motorcycle version, Here is the link to the Mopar version with examples:
http://victorylibrary.com/mopar/cam-tech-c.htm
http://victorylibrary.com/mopar/cam-tech-c.htm