Static Compression vs. Dynamic Compression Ratio
Static Compression vs. Dynamic Compression Ratio 1.First, how in the hell do you calculate "dynamic compression ratio"?
2.What is the target dynamic compression ratio that I should be "looking" for running 91 octane?
Thanks for your help, fellas...
2.What is the target dynamic compression ratio that I should be "looking" for running 91 octane?
Thanks for your help, fellas...
Well, I'm not too familiar with dynamic compression ratio (DCR), but there is a spreadsheet stickied above that will help you calculate it. As far as what octane to run, the autoignition of any fuel is a function of temperature and pressure. You would have to know what pressure and temperature a certain octane rating can take to know whether a cetain compresion ratio plus boost will be ok.
Last edited by Drivn2Fast; Jan 2, 2010 at 03:41 PM.
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From: Honolulu, HI
I know on Watercraft i.e. Ski's they rate max cranking PSI for any given octane. This may be a poor mans way of matching your DCR to fuel used, but I am sure there are many other variables such as plugs, operating temp head design and so on which are fixed quantities on most Ski builds....
If engine builders would tell you, ask them what pumping or cranking PSI they see with certain setups (heads, plugs, temps) may get you insight and direction... Then it is a matter of trail and errors with cams/ quench i suppose oh and I would love to see the results of testing
>..
If engine builders would tell you, ask them what pumping or cranking PSI they see with certain setups (heads, plugs, temps) may get you insight and direction... Then it is a matter of trail and errors with cams/ quench i suppose oh and I would love to see the results of testing
>.. http://www.rbracing-rsr.com/comprAdvHD.htm
i used this to figure it. just use 0 for boost. and you can look up your elevation on google.
i used this to figure it. just use 0 for boost. and you can look up your elevation on google.
read here if you already haven't
http://www.empirenet.com/pkelley2/DynamicCR.html
the important thing to remember is that DCR just like SCR does not change with rpm, unless you have variable valve timing then DCR will change. Regardless of that, from the fuel's point of view it does not care what the compression ratios are, it cares what the cylinder pressure is because it's the cylinder pressure which creates the heat which causes the fuel to detonate, and cylinder pressure varies with rpm with max pressure happening at the rpm max torque occurs. So i would not solely focus on DCR.
I've read that cylinder pressures near 200 psi and above will cause 93 octane fuel to detonate, hence the ~180psi cranking compression which is what most street engines are. And other factors will influence the detonation point of the fuel... primarily combustion chamber design and piston to head (quench) clearance- the tighter the quench the better detonation resistance for a given octane allowing for higher cylinder pressures (or compression ratios), and the converse being true where if you lowered compression ratio to 9:1 versus greater than 10:1 but the engine would still knock and detonate because of too large a quench area.
http://www.empirenet.com/pkelley2/DynamicCR.html
the important thing to remember is that DCR just like SCR does not change with rpm, unless you have variable valve timing then DCR will change. Regardless of that, from the fuel's point of view it does not care what the compression ratios are, it cares what the cylinder pressure is because it's the cylinder pressure which creates the heat which causes the fuel to detonate, and cylinder pressure varies with rpm with max pressure happening at the rpm max torque occurs. So i would not solely focus on DCR.
I've read that cylinder pressures near 200 psi and above will cause 93 octane fuel to detonate, hence the ~180psi cranking compression which is what most street engines are. And other factors will influence the detonation point of the fuel... primarily combustion chamber design and piston to head (quench) clearance- the tighter the quench the better detonation resistance for a given octane allowing for higher cylinder pressures (or compression ratios), and the converse being true where if you lowered compression ratio to 9:1 versus greater than 10:1 but the engine would still knock and detonate because of too large a quench area.
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From: Frozen Tundra, WI
from the fuel's point of view it does not care what the compression ratios are, it cares what the cylinder pressure is because it's the cylinder pressure which creates the heat which causes the fuel to detonate, and cylinder pressure varies with rpm with max pressure happening at the rpm max torque occurs. So i would not solely focus on DCR.
I've read that cylinder pressures near 200 psi and above will cause 93 octane fuel to detonate, hence the ~180psi cranking compression which is what most street engines are.
I've read that cylinder pressures near 200 psi and above will cause 93 octane fuel to detonate, hence the ~180psi cranking compression which is what most street engines are.
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lol 
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I can tell you I'm running 12.5:1 scr with a dcr of 8.22:1 on 92-93 octane in my GTO and have 0 knock at full timing. Dcr if figured from the effective compression stroke after the intake valve has closed...my intake valve starts to close at 85 degrees abdc, so until then there is no compression.
I run all my motors at 8.6 to 8.8 DCR. I am in NC with 100+ degree temps and the usual of over 60% humididty and have 0 issues with 93 pump gas with over 30* timing in some combos. I do back the timing off from that since it doesn't produce more power usally. I just ran a full tank of 87 though my 8.7 DCR truck to get rid of it, after I did the heads and gasket, with no issues what so ever. I just checked my cranking compression today and it was 215 psi on all the cylinders except one that was almost 225 psi (got to figure that out). My stock C5 Z06 cranking compression with a 224/228 114 cam was around 210psi. I added 10lbs of boost to that motor and it ran just fine. Just think about how high stock compression is now and how small the cams are. Once you cam something you are just loosing DCR i.e. torque. SCR makes it back. I love all these "safe" myths that no one puts to the test. All of my motors produce more power than normal on PUMP gas. Start with the DCR and work your way back for best all around power. The only car I ever had detonate, someone put the wrong head gasket in it, and it ended up having 9.2 DCR and 100,000+ miles, I could still run 22*-23* total timing on that combo. Most LS motors make best power from 24*-26* total so that motor was pretty close. I think a lot of these "numbers" came from the Iron headed, carburated days. With all aluminum engines and EFI you can take things up a notch.
Last edited by Pray; Mar 1, 2012 at 07:43 PM.
I can tell you I'm running 12.5:1 scr with a dcr of 8.22:1 on 92-93 octane in my GTO and have 0 knock at full timing. Dcr if figured from the effective compression stroke after the intake valve has closed...my intake valve starts to close at 85 degrees abdc, so until then there is no compression.
http://www.rbracing-rsr.com/comprAdvHD.htm
i used this to figure it. just use 0 for boost. and you can look up your elevation on google.
i used this to figure it. just use 0 for boost. and you can look up your elevation on google.
http://kb-silvolite.com/calc.php?action=comp
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From: PA/MD
Since I set it up for daily driving I didn't want to push it too much, and had worked with my machine shop and the few "peak" safe numbers we could find (8.5ish dcr) and backed off a little from there....the setup was actually very easy to drive on the street and with the 3" dual exhaust at 8,000 rpms it sounded like God was pissed off and coming after you
I've even had the thought of dropping two big turbos on it to kind of build in lag, so its not really building boost til just over 4,000 rpms (engine spinning too fast for detonation to occur) might even be able to hit the magic 1,000hp number then
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From: PA/MD
I would have been tearing it back down to open up the chambers some to drop the compression back down if had found it was too high to run on pump gas lol and i would not have been happy.
But i must say, even with the huge cam it is very peppy even just off idle
Like I said in my earlier post the only motor I had detonate with over 23* timing had 9.2 DCR. Your compression is a full point over what I suggested and your DCR is also .5 higher than what I suggested at max. Just posting what I run over and over at sea level. Have you tried 93 in your combo, which is sick by the way. I wonder what kind of timing you could get into it. Do you daily drive it or is it a track only car?
It's a 408 stroker with a 255/263 .624/.624 115lsa in my daily driver 04 GTO
Since I set it up for daily driving I didn't want to push it too much, and had worked with my machine shop and the few "peak" safe numbers we could find (8.5ish dcr) and backed off a little from there....the setup was actually very easy to drive on the street and with the 3" dual exhaust at 8,000 rpms it sounded like God was pissed off and coming after you
I've even had the thought of dropping two big turbos on it to kind of build in lag, so its not really building boost til just over 4,000 rpms (engine spinning too fast for detonation to occur) might even be able to hit the magic 1,000hp number then
Since I set it up for daily driving I didn't want to push it too much, and had worked with my machine shop and the few "peak" safe numbers we could find (8.5ish dcr) and backed off a little from there....the setup was actually very easy to drive on the street and with the 3" dual exhaust at 8,000 rpms it sounded like God was pissed off and coming after you
I've even had the thought of dropping two big turbos on it to kind of build in lag, so its not really building boost til just over 4,000 rpms (engine spinning too fast for detonation to occur) might even be able to hit the magic 1,000hp number then
Like I said in my earlier post the only motor I had detonate with over 23* timing had 9.2 DCR. Your compression is a full point over what I suggested and your DCR is also .5 higher than what I suggested at max. Just posting what I run over and over at sea level. Have you tried 93 in your combo, which is sick by the way. I wonder what kind of timing you could get into it. Do you daily drive it or is it a track only car?
What I stated above was my experience with LSX cars at sea level. I had a L98 383 that was at 210 cranking psi that I never had a problem with and that was way before I knew what DCR was. I worked very hard in those days for the power I had and was actually on the cutting edge of performance back then. Pushing the limits of what was "streetable". I think I was the first one to run AFR 210's and the TPIS Miniram after dealing with the Super Ram. I have all the respect in the world for you guys that can make those engines haul the mail. I am sure you have a pretty aggressive spark curve in your LTX that wouldn't work with pump no matter what. I was running 87octane in my truck at 8.2 DCR with no issues. Could be in the tune, could be in the fuel here, I don't know. I am sure it also has a lot to do with the runner degree going from 23 to 15, the advances in sequential port fuel injection and a straighter shot at the intake valve for the injector now. Not to mention combustion chamber design. I am willing to bet that the stock LS2/3/7 DCR is higher than I am recommending. Any way, you have a bad *** ride and keep pushing it. You know what they say, "It's good to be the King".
