bgd73 Posted November 18, 2006 Share Posted November 18, 2006 I had this algorithm in an old chiltons manual. It was "right on the money" with variables to fill, such as the "cc's" of combustion chamber, and where the piston met at tdc, the dips and grooves in the pistons cc'd, and even a thick head gasket or a thin one. Does anyone have this algorithm? The title of this thread was typed into google and yahoo search engines quoted- and once again I am treated like a retard with no replies.... I am hoping a soob engine builder has this math, I am wanting it again for a soob engine. Guessing really sucks without it. Link to comment Share on other sites More sharing options...
mikeshoup Posted November 18, 2006 Share Posted November 18, 2006 You need an equation, not an algorithm. That'll probably help you further. Here's a quick article that I pulled up on google. Perhaps this will help you a bit: http://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm The general equation though is: CR = V_BDC/V_TDC Link to comment Share on other sites More sharing options...
bgd73 Posted November 19, 2006 Author Share Posted November 19, 2006 You need an equation, not an algorithm. That'll probably help you further. Here's a quick article that I pulled up on google. Perhaps this will help you a bit: http://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm The general equation though is: CR = V_BDC/V_TDC awesome! thank you Link to comment Share on other sites More sharing options...
NorthWet Posted November 19, 2006 Share Posted November 19, 2006 ...The general equation though is: CR = V_BDC/V_TDC Ding!!! Ding!!! Ding!!! We have a winner, Ipso facto!!! BTW, this is theoretical/static CR, and can vary greatly from actual/effective CR (which can be affected by valve timing, et al). Link to comment Share on other sites More sharing options...
bgd73 Posted November 19, 2006 Author Share Posted November 19, 2006 It is instructive to remember that the static compression ratio that your engine displays on paper does not translate directly to higher cylinder pressures. The cylinder pressure (prior to ignition) during engine operation is dependent on what can loosely be called "dynamic compression ratio". The pressure is greatly affected by the timing of your valve events - i.e. cam duration and timing. Specifically, the intake valve closing point is intimately related to an engine's dynamic, or "effective" compression ratio. I have never agreed completely with a web article with free knowledge given, but this one is really good. Excellent find. Link to comment Share on other sites More sharing options...
bgd73 Posted November 19, 2006 Author Share Posted November 19, 2006 example for SPFI ea82 C.R. = 1 + 61.64 x 6.7 cm (413.988) ______________________________ == 9.5 43.577684 Here is an example to work with to figure oem cc's for a 9 to 1 carbed,413.988 /9 == 45.9986 cc chamber Thanks again . As you can see it is just 2 cc's for a 9 to 9.5:1 soob ea82. An example of the the spark plug taking a dive 2 cc's more into the head, than a 9.5 == 9.96 to 1 (wow) It is exactly what I had churning in my head as a guess after taking the cyl head off and seeing how much they cram it all already for the little stroke- it is very responsive to every little change in combustion chamber. That means a dirty piston could be cramming it way past 10:1 just by letting the crud build up on valves and pistons, hence the knocking on some high mileaged ea82s. My latest spfi was a good example of going way beyond factory numbers, but still ran ok on 87 octane. 2cc's is a mere "few drops of water" (not exactly- but I hope you get my point). I want to achieve a clean 10:1 without doing anything dramatic to the engine. I am thinking head gasket thinner like copper and deeper plugs by a mere 2-3 mm (quite safely) Link to comment Share on other sites More sharing options...
mikeshoup Posted November 19, 2006 Share Posted November 19, 2006 Just need to correct your math a little (sorry, I'm a wannabe math teacher, its habit). Vbdc == Volume of chamber with piston at Bottom Dead Center Vtdc == Volume of chamber with piston at Top Dead Center Vdis == Individual piston displacement. Vdis = pi*(Bore/2)^2*stroke = 445 cm^3 (makes sense, 445 * 4 = 1781) CR = Vbdc/Vtdc Vbdc = Vtdc + Vdis CR = (Vtdc + Vdis)/Vtdc CR = 1 + (Vdis/Vtdc) Vtdc = Vdis / (CR-1) In a 9.5:1 EA82, Vtdc (V2 in the article) is 52.3 cc In a 9.0:1 EA82, Vtdc is 55.6 cc You were close, but that's the correct number. Still, same thing. 3CC's in the TDC combustion chamber makes the difference for 9.5 to 9.0. 3 CCs is actually quite large though. If you have an 18mm spark plug, the cross sectional area is 2.5cm^2. In order to subtract 2ccs, the spark plug would have to protrude nearly a centimeter, which from a picture you posted earlier does not look like it. The washer is at most, 3 mm thick would you say? That's only subtracting 0.7ccs from Vtdc. On the 9.5:1 motor, that bumps the compression up to 9.62. Yes, a gain, but not all that significant. Link to comment Share on other sites More sharing options...
Hodaka Rider Posted November 19, 2006 Share Posted November 19, 2006 Measure the end of your spark plug (diameter) and the entire threaded length. I'm willing to bet that the math would put the entire threaded end of the plug at maybe a little more than 1cc - maybe 2cc. Plus, it's partly hollow. So, here's what you do: Drill the plug holes in the head out to the same diameter as the plug where the hex for the socket is. Now hold the plug in place in the head, as far in as you can, and glue it there with crazy glue, epoxy, or bubblegum or something. You've just reduced your combustion chamber by approximately 1cc! Yay for you! Link to comment Share on other sites More sharing options...
bgd73 Posted November 19, 2006 Author Share Posted November 19, 2006 Just need to correct your math a little (sorry, I'm a wannabe math teacher, its habit). Vbdc == Volume of chamber with piston at Bottom Dead Center Vtdc == Volume of chamber with piston at Top Dead Center Vdis == Individual piston displacement. Vdis = pi*(Bore/2)^2*stroke = 445 cm^3 (makes sense, 445 * 4 = 1781) CR = Vbdc/Vtdc Vbdc = Vtdc + Vdis CR = (Vtdc + Vdis)/Vtdc CR = 1 + (Vdis/Vtdc) Vtdc = Vdis / (CR-1) In a 9.5:1 EA82, Vtdc (V2 in the article) is 52.3 cc In a 9.0:1 EA82, Vtdc is 55.6 cc You were close, but that's the correct number. Still, same thing. 3CC's in the TDC combustion chamber makes the difference for 9.5 to 9.0. 3 CCs is actually quite large though. If you have an 18mm spark plug, the cross sectional area is 2.5cm^2. In order to subtract 2ccs, the spark plug would have to protrude nearly a centimeter, which from a picture you posted earlier does not look like it. The washer is at most, 3 mm thick would you say? That's only subtracting 0.7ccs from Vtdc. On the 9.5:1 motor, that bumps the compression up to 9.62. Yes, a gain, but not all that significant. the 9.62 is exactly what I came up with for a plug 1/8 th inch into cylinder head deeper (+.13 on c.r.) .I am not confident with the math, but it did definately knock the c.r. up noticably. Knowing the actual chamber size wasn't even necessary, the goal was to encapsulate the math of the compression so when adding or removing from chamber it can still be figured exactly.having the bore and stroke and knowing oem compression ratio was all that was necessary. Things could change from the different variables slightly to get the same C.R., but where bore and stroke are very much static for me the math worked great.The actual volume is so tight on a 67mm stroke... this math is good to have. 3 cc's is indeed huge area to take up in the ea82 c chamber Thanks again, it helped me figure out the spark plug going in longer and a simple engine I would like to put together. Link to comment Share on other sites More sharing options...
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