What would my compression be on a 2nd gen b16a if the head is milled .030" with a stock head gasket?
I know its 10.4:1 stock.

 

im thinking perhaps 10.6 or 10.7.

 

Well do the math damit (kidding). Compression is the ratio of all the space in the cylider when the piston is at bottom dead center (big number) to what ever space is left when it is at top dead center (small number). Then, to make things easy, this is usually converted into terms of "1" - (10:"1", 10.4:"1", etc.).

When calculating the ratio obviously you need to know how much room is in the cylinder when the piston is at the bottom, and then when its the top. To do this you just gotta break it down into sections, find out the room of each section, then add them up.

At bottom dead center (big number):

1) Head volume,
2)+ Inner head gasket volume,
3)+ Deck height volume (space left when the piston is highest in cylinder, but not quite level with the block deck, literally the volume ("up to the") the deck height ),
4)+ Stroke volume (bore x stroke),
-Add thoes up;
5) If you have a dish type piston, add the dish volume to the total; If you have a dome type piston, subtract it from the total.
---------------------------------------
= "Big number"

(Pistons have a crown or a dish (possibly flat too, zero volume), so this volume can be postive or negative or 0 - think of a crown as a negative number because it takes space away from the rest of the cylinder, A dish adds volume.)


At top dead center (small number):

1)Head volume,
2)+ Inner head gasket volume,
3)+ (To the) Deck height volume
-Add thoes up
4) If you have a dish type piston, add the dish volume to the total; If you have a dome type piston, subtract it from the total.
---------------------------------------
= "Small number"


Take these totals, big# and small# and put them in a fraction -> "big#/small#". Put this fraction in terms of 1 (so there is a 1 at the bottom).

In terms of 1 because "25:2.5" and "10.4:1" are a little hard to compare. Divide the top into the bottom and the result is the new/final big number, then use "1" as the new/final small number.

example: 25/2.5 (25:2.5)

25 "divided by" 2.5 = 10. So, 10:1 is the compression in terms of 1.

Now you need to use a little geometry to figure out some of these volumes. All the engine specs for these engines are avaible on the net, you just have to look for them. Stock head volume and piston volumes are specs you can easly find online. You'll need to calulate head gasket and deck height volume. Think of the head gasket volume as a short cylinder, same for the deck height.

If you want to get really crazy you can take into account piston rings, flow through rates and bore taper but that isnt practical.

If you're really a lazy focker, heres a link to a great site that has a form you can input all these volumes to and have it calculate compression for you! As to calculating the other volumes, there are forms on this site for calculating the volume of a cylinders as well.

http://www.csgnetwork.com/compcalc.html <-compression calulator
http://www.csgnetwork.com/volumecalc.html <- cylinder volume calculator

Also here is a great article on calculating engine compression

http://hotrod.com/techarticles/54258/

Last tip, You're going to need to convert all your length measurments (mm, 1/1000ths", inch) to the same unit before you start calculating volumes. Also remember that: 1 cc (cubic centimeter) is NOT= 1 centiliter (1cl = 10ml). 1cc = 1ml

http://www.onlineconversion.com <- good conversion calculator

Hope that helped, cuse i know you had a thirst for the raw knowledge and didnt just expect someone else to do your math for you.

 

this might help you

http://www.c-speedracing.com/h...c.php

 

wow. that is most cool. i always knew that comp was determined by vloume in cyl, but this really puts it into perspective; nice post!

 

revive thread! i have a question for you VTEC gurus.. what's the highest compression ratio for a B series engine you've ever heard/built?