Cams and Compression Ratios

[Kevin M]

I have a question that hopefully one of the more technically inclined members of this forum can answer.

When you increase the duration of a cam, you reduce the effective compression ratio in the engine.

Is there a rule of thumb about how much you can increase compression for every 10 deg. you increase the duration (everything else remaining constant of course)?

For example: Lets say I have a 347 with aluminum heads, a 230 deg. duration cam (@ .050 lift), and a 10.25:1 compression ratio (the most I can safely use with pump gas).
How much do I increase the compression ratio to stay equivalent if I go to a 240 deg. cam?

Thanks,

Kevin

 

[Gary Gibbs]

Best advise is talk the the cam manufacturer as they are very familiar with what works and does not through extensive testing, etc. Most big engine builders that do extensive dyno testing will also have a good feel for it (e.g. www.keithcraft.com) )