I want to check my numbers. What do you get if you calculate a compression ratio for the following:

<table border="0" cellpadding="0" cellspacing="0" width="508"><colgroup><col style="mso-width-source:userset;mso-width-alt:13897;width:285pt" width="380"> <col style="width:48pt" span="2" width="64"> </colgroup><tbody><tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;width:285pt" height="20" width="380">Bore</td> <td class="xl65" style="border-left:none;width:48pt" align="right" width="64">4.03</td> <td class="xl65" style="border-left:none;width:48pt" width="64">in</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Stroke</td> <td class="xl65" style="border-top:none;border-left:none" align="right">3.5</td> <td class="xl65" style="border-top:none;border-left:none">in</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Head Gasket Compressed Thickness</td> <td class="xl65" style="border-top:none;border-left:none" align="right">0.055</td> <td class="xl65" style="border-top:none;border-left:none">in</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Head Gasket Bore</td> <td class="xl65" style="border-top:none;border-left:none" align="right">4.06</td> <td class="xl65" style="border-top:none;border-left:none">in</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Deck Height (piston below deck)</td> <td class="xl65" style="border-top:none;border-left:none" align="right">0.021</td> <td class="xl65" style="border-top:none;border-left:none">in</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Piston Top Volume (flat top w/2 valve reliefs)</td> <td class="xl65" style="border-top:none;border-left:none" align="right">6.5</td> <td class="xl65" style="border-top:none;border-left:none">CCs</td> </tr> <tr style="height:15.0pt" height="20"> <td class="xl65" style="height:15.0pt;border-top:none" height="20">Combustion Chamber</td> <td class="xl65" style="border-top:none;border-left:none" align="right">65</td> <td class="xl65" style="border-top:none;border-left:none">CCs</td> </tr> </tbody></table>



I get 9.3555:1

 

I get 9.3555:1

Yep so do I.

 

Thanks Dan

 

Your very welcome and just to be clear I didn't manually do the math, I could have but didn't, I did it the easy way instead.

Engine Compression Ratio (CR) Calculator

 

Well I guess my real question would be what do others trust and use to calculate this? I have seen many and used several on the internet but I decided to make my own in excel so I know what goes into the calcs. I've compared mine to those and I think mine is more accurate than a few of them. At the time I asked I was asking because I had a problem with the 351W I just built, but since this post I took out the motor and found it was a problem with the head gasket and not the compression.

 

I calculate it by hand, using the same info, just like you do. To calculate cylinder volume, I use (boreXboreXstrokeX.7854). It is more accurate and easier to use than the old (pi X radius X radius X stroke), especially since most people only use 3.14 for "pi".
In terms of accuracy, it all depends on how/where you get the info! Most cast combustion chambers can vary in volume from head to head (and even within the same head). I've even found that many new pistons vary in compression height and dish volume, sometimes even different than they list in their catalogs. The best way to get a truely accurate figure is to "cc" the head with the plugs and valves that will be used, "cc" the piston top, measure the deck clearance w/dial indicator, etc.. I guess you could "cc" the cylinder with the piston @ TDC to account for the small volume between the top ring and top of the piston (if you wanted to get dead-accurate). Anyway, I never trust any calculations if the input info is coming from catalogs and not actual measurements....unless I'm just trying to get "in the ballpark".

 

You really want to know for sure best bet do this/similar to both head and cylinders with each piston at TDC.

Cylinder Head Combustion Chamber Volume - Circle Track Magazine

Other then that yea I'd just use an on line calculator for a basic idea where it'd end up at, maybe go as far use two different ones make sure they agree. Verify one I picked didn't have the math right!

 

Its the same damn equation. Enter: 3.14159265358979323846264338327950288419716939937 for pi and now cylinder volume is calculated more accurately using (pi x radius x radius x stroke).

Or use a decent calculator that just has a pi button on it. 0.7854 is 3.1416/4, not pi/4. Volume of a cylinder is correctly calculated using (pi x radius x radius x height). My dad used the equation you presented when I was a kid and it confused the hell out of me until I thought about it later in life and realized its just something old guys use because they didn't have ultra dorky math teachers that taught students pi to at least 3.1415926.

Shoot I guess I do miss school

 

LOL, you are correct. It's the same exact thing as using 3.1416 for pi, just easier to use if you have a cheap calculator that doesn't have a pi function. Plus, depending on the bore measurment, you don't have to divide to get the radius, remember it, then multiply that out. Just less over-all keystrokes, I guess. Once I started using it, the more I liked it. Like you said, if you carry pi out a few more places it is more precise, but most people don't use (or even know) more than "3.14" and going past more than .xxxx is probably way more precise than any of the other input values. Using the .7854 (or 3.1416 for Pi) is simple and plenty precise for the application. Like a fart in a whirlwind, its all relative. In fact, the decimal value of pi truncated to 11 decimal places is precises enough to calculate the circumference of circle that fits within the earth's boundaries with <1.0mm of error! One of the other arguments I've heard is that by using 3.1416 to calculate cylinder volume on an engine, you more accurately account for the cylinder wall taper. I suppose it could be true in theory, since you are "rounding up" but I've never actually measured and compared to test this "old mechanics" tale.

Your wrong about the older math teachers though....the teachers I had for Calculus & Differential Equations was plenty "dorky"! Not as bad as my old Fortran teacher, but he still wore pocket protectors, smoked a pipe, and listened to NPR radio