KiD-ViD Posted May 6, 2003 Share Posted May 6, 2003 ok here goes...I live in CA and the best pump octane we can get here is 91. I have heard 8.5:1 is the most dcr one can run on pump gas safly..and even then its borderline. is this like a hard number..no matter what type of engine or other characteristics an engine has 8.5 will always be the magic number? I ask this because I think I remember reading somewhere that volumectric pressure has to do with it too. if so what type of v/p numbers can you run on pump gas? and is there a program out there that will help me calculate an engines v/p like the dcr program does. that dcr calculator is a great help. thanks alot guys. Quote Link to comment Share on other sites More sharing options...
johnc Posted May 6, 2003 Share Posted May 6, 2003 While I don't agree that there is something called "Dynamic Compression Ratio" here's a short article describing what some folks think it is. I feel Cylinder Pressure is what people should be thinking about. http://cochise.uia.net/pkelley2/DynamicCR.html Quote Link to comment Share on other sites More sharing options...
KiD-ViD Posted May 7, 2003 Author Share Posted May 7, 2003 yes I saw that page a while ago...that is where I got the dcr calculator. I tried figuring out my V/P and I got ~145 and that doesent seem right cause the other motors done on the site got 250+ but then again thoes are v8s. if anyone has any ideas on figuring out what comp ratio to run I am all ears. Quote Link to comment Share on other sites More sharing options...
Guest Venture-Z Posted May 7, 2003 Share Posted May 7, 2003 I'm running 10:1 compression on 91 with no problems at all. Quote Link to comment Share on other sites More sharing options...
gene_w Posted May 7, 2003 Share Posted May 7, 2003 I've been running my '69 Mustang on 89 octane for quite some time with no negative results as far as I can tell. It's got about 9.3:1 compression. Gene Quote Link to comment Share on other sites More sharing options...
80LS1T Posted May 11, 2003 Share Posted May 11, 2003 Yeah but he's not talking about the static compression ratio(I think thats what its called) that you guys are talking about with 10:1 and 9.3:1. Dynamic Compression ration is complety different. Guy Quote Link to comment Share on other sites More sharing options...
strotter Posted May 11, 2003 Share Posted May 11, 2003 I did a search on this site for "dynamic AND compression"; grumpy posted this some time ago: http://cochise.uia.net/pkelley2/DynamicCR.html I think it may be what you're looking for =) Quote Link to comment Share on other sites More sharing options...
johnc Posted May 11, 2003 Share Posted May 11, 2003 Amazingly similar to the link posted above... Quote Link to comment Share on other sites More sharing options...
strotter Posted May 11, 2003 Share Posted May 11, 2003 If I could read, I wouldn't be writing, now would I? -or- I just wanted to make sure he saw it! -or- Well, my girlfriend says I'm good looking, I don't need brains! Quote Link to comment Share on other sites More sharing options...
KiD-ViD Posted May 12, 2003 Author Share Posted May 12, 2003 yeah I have read that page extensivly...thats how I know what I know about DCR. I wasent really asking how DCR works I was asking what the max DCR I could run on 91 octane gas was and if volumectric pressure has any affect on this "magic number" Quote Link to comment Share on other sites More sharing options...
Guest DaneL24 Posted May 12, 2003 Share Posted May 12, 2003 I went through the site and it said 7.5-8.5 DCR is good for a street car (since its a street car, it runs on 91 octane gas), and it said its a good idea to stay below 8.25 DCR. Also, what do you mean by volumetric pressure? Please explain... By the way, did you get my message? If not, check your messages (on Hybridz account, not email). Quote Link to comment Share on other sites More sharing options...
KiD-ViD Posted May 12, 2003 Author Share Posted May 12, 2003 I guess I will shoot for 8.0 DCR just to be safe. ok here is the link to the site that explains volumectric pressure. it is about half way down. im pretty sure I am doing the math right but it doesent seem to be right in comparison to the samples the guy gives at the bottom of the article. http://victorylibrary.com/mopar/cam-tech-c.htm Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 14, 2003 Share Posted May 14, 2003 Gentlemen; Well.....We have to have some way to relate to one another about meaningful values of.....in this case Compression. I have organized my own way of thinking about this subject if for no other reason than to keep my sanity! I will reinterate here..... 1. The statement "Compression Ratio" has no value in and of itself because no reference with respect to what kind is given. 2. Static Compression is a calculated mathematical function obtained by formula with the engine at rest; In other words STATIC. 3. Compression Pressure is a dynamic function obtained using a guage conected to one cylinder while the engine is rotated other than by it's own power. 4. Dynamic Compression can not only be expressed as a function the same as item three above but also as a function and result of actually running the engine while taking/recording pressure readings with the same guage and plotting a graph for the purpose of determining camshaft/ engine performance thruout the entire RPM range of the camshaft. 5. It is the intent of the method/function to be used which must be conveyed to another that demands these definitions be used in order to prevent confusion between us. Therefore, whenever we speak of compression we must always reference to one of the following in our speech: a. Static b. Pressure c. Dynamic Cranking d. Dynamic Running And there you have it. There is no reason to suppose that we can ever concurr on any of the above if we refuse to knuckle down to the facts of reality. We must all agree on a logical order of definitions for any discussion of the subject at hand, in order for the converastion to bear fruit, whatever the subject may be. Quote Link to comment Share on other sites More sharing options...
johnc Posted May 14, 2003 Share Posted May 14, 2003 Compression Pressure is commonly referred to as Cylinder Pressure. Cylinder Pressure is what folks should be focusing on, not static or dynamic compression (something I don't think exists in the real world) ratio. You should be concerned about absolute pressures in the combustion chamber, not ratios. Check out this article at SDS: http://www.sdsefi.com/meltdown.htm Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 15, 2003 Share Posted May 15, 2003 John Coffey say's..... Cylinder Pressure is what folks should be focusing on, not static or dynamic compression (something I don't think exists in the real world) ratio. You should be concerned about absolute pressures in the combustion chamber, not ratios. --------------------------------- SPIIRIT say's..... Absolute Pressure: The total pressure exerted on a system, equal to the gauge pressure plus atmospheric pressure. The formula used to determine "ABSOLUTE SYSTEM PRESSURE" is: Total Pressure = Guage Pressure + Atmospheric Pressure. So, if we have a Guage with a dial calibrated at a value of Zero at sea level, we have a measure of Absolute Pressure, where 0 = 14.7 psi. I fail to see any difference when applied under normal field conditions, to engines using normal aspiration. However, If we speak of exotic fuels and pressures outside the norm, then there is a necessity for additional information and Absolute Pressure would be the answer. But as you well kinow, I was staying within the realm of "Field Testing" and normal aspiration. As long as we remain within the area of field testing, what I stated in my previous post holds true. However, once we switch to a synthetic means of aspiration and it's associated fuel/pressure, we effectively have stepped into the laboratory where much heavier calculations are demanded. My text leaves off at this piont because I am not interested in turbos and superchargers and therefore have no need for any such calcukations to determine the value of what you say does not exist. (Smile). If you are interested in such things as Absolute Pressure, then I would like to see you give us all a "run down" by example so we can share your thoughts on the matter. It won't hurt me to hear it and I would like to see how you understand it, and also, I am sure many readers are into that area and would be very interested in what you have to say. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 15, 2003 Share Posted May 15, 2003 KiD ViD; Once given your "Build Data", Cam manufactures will in no wise grind you a street cam that yields in excess of 8.5:1. The info they require is so they can determine (among other things) how many degrees ABDC of crankshaft rotation they should cause the intake vale to close or nearly close. This setting will determine the "DCR" necessary to achieve that "Magic Number" of 8.5:1. Because they have a reputation to protect, they go about this process of also protecting you along with their own good name. Use the following as a point of interest..... *** Below are two websites with formulas for both Static and Dynamic comoression ratios. Static = http://www.rbracing-rsr.com/compstaticcalc.html Using the static one first, take that answer and factor it into the dynamic one. Dynamic = http://www.rbracing-rsr.com/comprAdvHD.htm While in the Dynamic program, also introduce the arbitrary value of say 50 for openers where the "Intake closes ABDC" value is requested. Once filled out, click for the results and observe the Dynamic Compression Ratio (should be 8.5:1) and the Dynamic compression pressure (Should be right at 170). The trick now is to go back to the calculator by clicking the bottom of the results box and raising or lowering the intake closing ABDC until you have the correct DCR of 8.5:1 showing in the results box. Once you have this value, look at the DCP and you will observe that it is very close (if not right on) the 170 value. By the time you have done all this you will nave your answer and be better versed on what the cam manufactures are doing in order to grind your cam. And, you will see the truth about true dynamic street compression ratio and it's value and function as related to the intake valve timing. Quote Link to comment Share on other sites More sharing options...
KiD-ViD Posted May 15, 2003 Author Share Posted May 15, 2003 cam being the brain of an engine I decided on a cam first. I picked one that would suit my needs best considering rpm range and streetablity etc...now I am looking for how much I should mil the head to achive the ideal compression ratio and/or pressure. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 16, 2003 Share Posted May 16, 2003 KiD ViD; I wasn't going to get into this but what the heck..... It has come before me that for every 0.005" removed from an aluminum head, you reduce the chamber volume by one (1) cc./cylinder. And, for the cast iron:, for every 0.004" removed, you reduce the chamber volume by one (1) cc./cylinder. Since you want to raise your compressio, this would be one way to figure it (if you already knew what your chamber cc's were). Otherwise you would have to use a Burret to get your cc's as a base, do all the calculations to discover your current static and dynamic ratios, and then with those figures, re-calculate to determine the difference in what you got and what you want (in cc's), and using that difference, you would further determine how much material to remove according to the values first mentioned. You're going to have to bite a bullet an learn some more here if you want to get any where positive. Perhaps you can find a local racecar Mechanic/Machinist that can take your numbers and your heads and do the final crunching together for you. Good Luck! Quote Link to comment Share on other sites More sharing options...
KiD-ViD Posted May 16, 2003 Author Share Posted May 16, 2003 right I was planning on doing all the measuring and get accurate reading and such when I do a rebuild and some head work...right now I am just trying to get some rough numbers to keep me satisfied till I get some more money together. putting over a grand a month in the bank now though so shouldent be too long. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted May 16, 2003 Share Posted May 16, 2003 Say!..... I wonder how much of that grand could I expect to be transferred into my savings account if I were to offer you my prayers for your quick success?.....LOL! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.