ronchinoy wrote:Briliant guys.
Though you lost me when I read this
Below 100 kPa, we add 0.3° per 1 kPa drop. So for example, if our total
spark at 100kPa and 4000 rpm was 36°, the advance at 50 kPa would be:
36° + 0.3° x (100-50) = 51°
I dont see how an engine can run 51° under any conditions.
I have an air cooled motor, (Corvair), with good squish area, (cut out the step from the heads, and the piston comes to within .040 or the squish), I have measured 10.5:1 CR on 5 holes with almost 10.75:1 on the sixth hole. I use Sunoco 260 (94 octane), and have NO detonation problems running 40 degrees under max load, and 57 degrees max under low MAP. I have ALL my advance in by 2400 rpm, and could run MORE advance if I were racing. I drive the car on the street so I have the motor under load at 2000 rpm and need to keep a little timing out to prevent knock. If I were racing, I would not have the engine under load below 3500 rpm, and could probably dial in 3 degrees more timing. Once I have my MS1 3.0 built, I will use that to control my timing and will therefore be able to dial in a little more timing for racing, and keep it as is for street.
Cylinder head preperation has a LOT to do with how much timing you can run. If I had more "open chamber" design, and did not have the squish area I do, all things being equal I would not be able to run as much timing. If I had domed pistons, that were further down the bore, away from the squish area, I would not be able to run as much timing, (This is assuming that the CR remained as it is now, 10.5:1). The table that is posted here should keep you well out of trouble. Always better safe than sorry. Still, if you know how to build motors, and how to take advantage of the squish area offered by wedge heads, you can run possibly a lot more timing. Also, remember that aluminum heads will allow more timing than cast iron ones will.