Originally posted by MichaelDelaney on Apr/28/03 at team-integra.net
Get your intake cam's ABDC valve closing spec.

Convert the bore and stroke to in.

Find out the altitude of where you live in ft. If you can't find it then enter 400 or zero for sea level.

Go to the dynamic calculator link in the MD Forum Common Topics.

Enter all the relevant data for your stock or baseline setup (including your current CR) and record the cranking pressure.

Change the CR. in the calculator..enter the CR you want or go midway . When you go past a 10-15% change in cranking pressure, you've gone too high on the CR...your limit.

If you want more than this CR limit, then you need to use bigger cams to lower the cranking pressure back to within that 15% range near the baseline. If the cranking pressure is too high you're undercamming the engine.

Bigger N/A cams (not FI cams) have longer durations. The longer the duration, the later the intake valve closes. The later the intake valve closes the more chance that the pressure in the cylinder may leak out into the intake port and reduce your cranking pressure with that higher CR that you wanted.

Usually we work the other way around...we upgrade the cams after porting to get more air in to make more power and then see what CR on the pistons we need to ensure there's enough cranking pressure to turn the crank beyond the force of the combustion and to get a good burn to ensure all of the air we put in gets burned.

You want to increase the CR first...that's ok but now you know that there's a working relationship between cams and the CR (piston dome size) and must find the limit based on whether you will spend cams with that or not.

Everything is a thought out planned decision when you build an engine. It isn't get this part , then get that part, then get another part.

You look at the goals and then find the specs that deliver those goals. This way you control your engine's destiny rather than the engine dictating your spending. Most people who aren't tapped into the racing world do it the other way because they don't understand (or no-one has taught them) the relationships between parts and how those relationships work. So they rely on piecemeal purchases and focus in on what the part does by itself.
We're trying to eliminate that blindspot for the members of the TI community here.