There are no inexpensive options to raise the compression for my neon's motor, which is getting a new head/cam/intake over thanksgiving weekend. Therefore, I'd like to deck the head to achieve 10.5 to 11.0 compression to accomodate the new cam (.355/.345 lift, 262/262 duration). However, the wonderful folks on the neon boards don't seem to know how much decking the head actually helps. Care to check my math?

Bore: 87.5mm
Stroke: 83.0mm
Stock CR: 9.8:1

((8.75/2)^2)*pi*8.3 = 499.096 cc per cylinder

9.8:1 Compression, so the the combustion volume is 50.928cc (499.096/9.8)

Now, assuming the first .1 or so of the cylinder head is perfectly cylindrical and we deck it .040:

.040 inches * 2.54 = .1016 cm
.1016 * ((8.75/2)^2)*pi = 6.109cc taken out of the combustion volume

50.928-6.109 = 44.819 combustion volume

499.096/44.819 = 11.135:1 Compression Ratio

Tell me where my math is wrong, because I sure hope it isn't.

 

you should seriously cc it when you are reassembling the motor. your math is right, but you're assuming the combustion chamber is the same size throughout. not sure if i am wording this right. but you know your combustion chamber isn't shaped like a beer can. it's got a gradual tape to it. so essentially, the more you mill, the more cc's each thousandth take from the total cc volume. am i making sense? i am drunk right now.
look at it this way, .010 off a stock head and you math should be right, but after you go past .010, the chamber starts to taper, and any milling from there will give slightly different results. also, different spark plugs can change the ratio a little bit. compare bosch 4427 plugs to ngk zfr5f11. HUGE difference. now, it might only be .05 on total compression, but if you didn't cc your chamber and just went on hard math numbers, your compression could be off by .2 or .3 or even more. just do it right and cc it.

 

I figured out my error, I didnt use the right formula to figure combustion volume.

It's more like 10.8:1. Thanks guys!