7A-FE Compression Ratio, Looking at changing the CR of my 7afe when I rebuild. Options

[PaukST]

Hey all, I'm trying to decide what to do with my 7AFE (have to take it apart due to a rod knock anyway). I've kicked around a few ideas (or maybe a lot ), and I'm currently researching what an increase in compression ratio might do. Does anyone have any info on this? I'm unsure of the potential difference in performance, and I can't seem to find any hard data on what pistons (4age, smallport 4age, 4agze) result in what CR. I've been trying to make sense of these tools and formulas - http://www.csgnetwork.com/compcalc.html http://s-86.com/s-cr.html and http://s-86.com/s-cr-chart.html - but I'm having a hard time digging out the relevant numbers. Doing the math myself or using the calculator I come up with significantly different numbers than their little handy chart, so I don't know what to think. Is this a futile exercise? Will I even be able to notice a performance difference over the stock 9.5:1 without having to use higher octane?

[Bitter]

I think it'll turn all the way to 25 but the gains pretty much end at 15 from what I was able to research.

[Special_Edy]

I think it'll turn all the way to 25 but the gains pretty much end at 15 from what I was able to research.

The more retarded you make it the more low end grunt you will get. The engine will run cooler and the power band will be shifted down with the engine producing less horsepower. The more advanced the engine is the sooner it will fire the spark plug on the compression stroke before the piston reaches top dead center and makes full compression. Firing sooner yeilds more power because the flame front of the combustion coming off the spark plug burns more of the vapor in the cylinder and compression is boosted by the combusting gas.
Because the flame front of the burning gas/air vapor travels at a certain speed(however many feet per second) and piston speed is relative to rpm, higher rpms need the spark sooner(more advanced) and lower rpms need it later(more retarded). If you advance it too much then the explosion will happen too early and compression will be increased so much that it will actually try to push the piston back down in reverse. This will make the engine impossible to start as every combustion trys to turn the engine backwards and it will cause detonation at higher rpm. Adding higher octane will combat this predetonation but at this point your not gaining anything. 15 degrees advanced will probably give your engine a little more hop but realize that the ignition is already electronically advanced so dont overdue it. If you hear any predetonation then dial the distributor back a hair and retest around the neighborhood. You will feel it if its too advanced.

Sorry for the long post but I hope its helpful

This post has been edited by Special_Edy: Apr 23, 2012 - 1:36 AM

[celica74]

I think it'll turn all the way to 25 but the gains pretty much end at 15 from what I was able to research.

The more retarded you make it the more low end grunt you will get. The engine will run cooler and the power band will be shifted down with the engine producing less horsepower. The more advanced the engine is the sooner it will fire the spark plug on the compression stroke before the piston reaches top dead center and makes full compression. Firing sooner yeilds more power because the flame front of the combustion coming off the spark plug burns more of the vapor in the cylinder and compression is boosted by the combusting gas.
Because the flame front of the burning gas/air vapor travels at a certain speed(however many feet per second) and piston speed is relative to rpm, higher rpms need the spark sooner(more advanced) and lower rpms need it later(more retarded). If you advance it too much then the explosion will happen too early and compression will be increased so much that it will actually try to push the piston back down in reverse. This will make the engine impossible to start as every combustion trys to turn the engine backwards and it will cause detonation at higher rpm. Adding higher octane will combat this predetonation but at this point your not gaining anything. 15 degrees advanced will probably give your engine a little more hop but realize that the ignition is already electronically advanced so dont overdue it. If you hear any predetonation then dial the distributor back a hair and retest around the neighborhood. You will feel it if its too advanced.

Sorry for the long post but I hope its helpful

Sometimes longer post yeild more information. And thus better understanding. Thankyou for a clear response. Gunna have to take an hour to ight to mess with it.