ok so i found a 77 engine that i found in a parts car that was rebuilt by nissan it had the tag by the crank. So i pulled the head off and it had flat top pistons .010 over it (they were stamped on the piston). and then i put a 260 e-88 head on the was milled, ported, and a cam and fel-pro head gasket (the head was for a i.t.s or i.t.e scca race car). The person that i got the head from said that the compression ratio wound be 12.5 to 1 to 13.5 to 1. so i guess what i am asking is this really what the compression is ??? he said that i would have to run trick fuel so it wouldnt ping? and if this is true could i run 50/50. i am also using 3 srew su carbs that have be jetted for the 2.8 liter and a header for a 72 240z and this is all in my 81 280zx.

 

According to the LEngine Builder app from Sydney Z Car Club (circa 1999), that would give a compression ratio more like 10.17:1 Assuming they kept the L28 crank and rods. And it's odd that it says it's a 0.010 overbore. Smallest I've seen available is 0.020. Not to say they couldn't get pistons made for that, but there wouldn't be a point to it. And of course, if they did get custom pistons made, or had the combustion chamber altered, then there's no way to confirm actual volumes and CRs short of measuring it all yourself. Some folks say that LEngine app isn't entirely accurate, but then they never bother to say what specs they believe are off, and in the end it's at least a good indicator. Get some high octane gas and try it. If it pings, put some octane boost in and see if that stops it. If it still pings, you need to find a source for higher octane gas, or decide that the motor is impractical for anything other than track use. Also, you're in Albuquerque, so you're already somewhat more resistant to pinging then your lower altitude brethren.
good luck,
Dave

 

That guy was wrong. the most you can get with that head is about 9-1. To get 12+ you would need longer 240 rods. a stroker crank, higher pistons, deck milled, etc. If you want to get more exact you can got to zhome.com and get to the tech articles.

 

if it does ping, you could always take the hour it takes to swap in a 2mm head gasket and increase the comp ratio by 1 point.

 

use this to figure it out
its the ****!
http://www.ozdat.com/ozdatonline/enginedesign/