Originally Posted by 67-4-fun
my questin is this, to figure dynamic compression acuratly, would you use duration numbers @ .050 example KB calculator or use the advertise duration numbers? my thoughts are you would want to used the advertise duration numbers and not the 50 numbers because the intake valve is not completly closed at 50 which i would think would make your compression number inacurate... please correct me if im wrong on my thoughts and explain if so..
Sometimes all you have is the .050 duration number. While not absolute for calculating DCR is better than nothing. Many calculator sites recommend adding 15 degrees to the .050 closing point; this is closer but for rampy cams usually still not there. For the new modern fast acting cams it's pretty good. When you get into muscle car era cams they are very rampy and may need 20 to 25 degrees added to the .050 closing point. One also need to consider that reversion through the valve is happening in the same flow ratios as you'd see on the bench by lift amount, so at .050 you're not getting a lot past the valve.
I think Pat Kelly is pretty close on his numbers of 7.5 to 8.5 on the street though I like to push the high side to 9 with the right combinations of parts and I try not to get under 8 for closed chamber heads but will fall to 7.5 for open chambers. One of his calculations is based off the 929 cam used by GM for the 300 horse 327, this cam is extremely rampy, I'm not convinced that it's .006 closing numbers are all that meaningful on this type cam as it stays almost but not quite closed for a lot of degrees. So while it isn't air tight, I just don't think it's losing as much ratio as the calculations would indicate. Still arithmetic models are all we have here so you've got to go with something.
This whole concept is varies greatly with chamber and piston design, bore diameter, squish/quench clearance, spark plug location, head material, intake air temp and engine operating temp, fuel mixture, ignition timing are all in play against the compression ratio.
- Smaller bores are more tolerant of high compression.
- Tighter and or larger surface area of the squish/quench is more tolerant of high compression.
- The closer the spark plug is to the bore center the more compression can be tolerated.
- Aluminum's higher rate of heat transfer than cast iron will allow it to take more compression ratio.
- Cooler temps on inlet air and engine temp will allow a higher compression ratio.
- A richer mixture is more tolerant of higher compression ratios.
- Ignition timing is a huge player; this is a place where backing down the compression ratio will cost less power loss than having to back down the optimum ignition advance to stay under the detonation limit.
For the street I like to hit about a DCR of 8 to 1 for iron and regular fuel. 8.5 with iron and premium fuel, or with aluminum and regular fuel. About 9 with aluminum and premium fuel. These being for Vortec type chambers. For open chambers these may need to be backed off half a ratio. This thought would also include using the 305 head on a 350. The 305 has the plug pretty far from the bore center which isn't much of a problem on the smaller bore 305. But on a 4 inch bore these will act a lot like the open chamber head that puts the spark plug way off the bore centerline. These type heads have a longer burn time to get across the bore and can be a detonation maker on a large bore engine if you don't compensate somewhere else.