Compression Ratio
Thread Starter
TECH Resident
Joined: Sep 2009
Posts: 859
Likes: 2
From: Frozen Tundra, WI
Compression Ratio Does anyone have a "rule of thumb" formula for compression ratio?? I am trying to figure out an LQ9 shortblock with 66cc chambers?? Factory was 10.08:1 with 71.06cc chambers. Does 10.6 sound right for 66cc chambers?? Am I OK with pump gas yet??
Thread Starter
TECH Resident
Joined: Sep 2009
Posts: 859
Likes: 2
From: Frozen Tundra, WI
OK...so I did a little homework and found that each cc of change is around .125 of compression on a 6.0...a little less change (ie .120-) on less cubes and a little more (ie .130+) on more cubes. This is a ballpark figure that should get you close if you do not know the exact specs of a motor. Each combo will be different and this is no substitute for doing a formal calculation. But, if you have someone thinking about different heads this should get you within .1 or .2 most of the time on a typical 6.0 in a 60cc to upper 70cc application. Hopefully someone will find it useful and not too far off after their formal calculation. 
Now we need someone to tell us how much a full point of compression is worth in HP.
Now we need someone to tell us how much a full point of compression is worth in HP.
Last edited by Paul57; Nov 25, 2011 at 02:15 AM.
In order to be able to safely run 93 octane, dynamic compression ratio needs to taken into account. http://www.wallaceracing.com/dynamic-cr.php This is found by knowing the rod length and the ABDC value of the intake valve. If you are below 8.9 DCR you are good to go with 93 octane from what I've read.
Raising compression by itself does not yield as much hp as 1 would think . If You have an engine that produces 350hp at 10 to 1 scr and You increase the scr to 12 to 1 it only increases hp by something like 16.28 hp . Going from 8 to 1 on up to 10 to 1 with the same engine pretending it still had 350 hp would increase hp by 23ponies. Plus the added heat is harder on the engine .
OP if You google "Wallace Racing engine calculators" They have any calculator You could ever want 1/4 mile/hp/tire size/gear ratio/ compression ratio both scr dcr
OP if You google "Wallace Racing engine calculators" They have any calculator You could ever want 1/4 mile/hp/tire size/gear ratio/ compression ratio both scr dcr
