Cost saving and lighter weight.
If you plug in 70.5 and 114.5, the actual range of IVC commanded, the DCR goes from 9.1 to 4.9. That’s a huge range and I don’t see how you could justify assuming the cam timing is static when it’s not. And as I pointed out earlier, later IVC would indicate lower DCR and less chance of detonation but we know for a fact that this isn’t actually what happens due to ram tuning.
The OP wanted to know a ballpark max SCR they can possibly get away with. If you consider that the rpm range where detonation is more likely the dynamic effects are less. Simplifying it down to just the effective stroke based on IVC you can get a good enough approximation of what "dynamic" compression ratio you have at low speeds. Assume a target of 8:1 "DCR" and work backwards based on the cam timing to see what "static" compression ratio is required. You are correct that the simple DCR calculation would show that the DCR is going down, but in reality the true DCR could actually be way higher. But, at high speeds we don't have to worry as much with staying at 8:1. If it is on the edge timing and fuel can be tweaked. At 3000 rpm and lower is where I would be most concerned with having the detonation. And an averaged DCR below 3000 rpm gives fairly close results to the simplified DCR calculation based on effective stroke length. I use software to build the engine first and look at all the cylinder pressures, timing, cam timing and SCR to find the right balance regarding, power requirements, fuel availability, and overall manners on how it drives. With our VVT it is easy to find that balance. I am currently working with a friend and we've built a 347 for his 93 cobra. It made 627 HP on the engine dyno on 91 octane with a custom solid roller grind we came up with so that it idles like a stock 5.0. This is with 10:1 static compression.
