CobraSix":av0o2a0c said:
Now, I couldn't find good heat and drag numbers, but to me it seems better to spin as M90 at 8500RPM than a M62@12000. Shouldn't the M90 make less heat at 8500 and have less parasitic drag since it's spinning at 66% of the M62 for the same boost level?
You're logic is sound, I followed the same path for a while. Something about the design of the 2 SC's allows the M62 to achieve a greater efficiency over the M90 up to around 9psi. And yes, you will be spinning the fool out of it. However, the M62 favors the higher rpm's over the M90. I found Temp and Drag charts at Magnuson Superchargers. They're the only aftermarket company for the Eaton roots SC's.
Something may be askew in your math, I suspect the volumetric efficiency assumption. Looking at your SC rpm's and outputs, your engine rpm's may be closer to 4000 to 4500 and the rpm spread doesn't quite compare head to head. One point to note, and this is where some theory and guestimation comes in : volumetric efficiency (85%, a sound assumption) really applies to how well an engine draws fuel and air naturally. Once you pressurize the intake tract, vacuum falls out of the equation as it is no longer drawing from atmospheric pressure, but some multiple of it. Certainly, the greater the natural VE, the better it'll breathe under pressure, but VE has a lessened effect against pressurized flow than when the intake tract is naturally fed. Sources I have read indicate that with mild and higher boost rates, you're safe assuming 100% VE. However, greater natural VE's translates to lesser boost measures given the drive ratio, meaning a 70% VE engine may yield 10psi, when a 90% engine may yield 9psi at the same drive ratio and flow rate. A SC is a high volume, low pressure air pump where pressure is a result of flow resistance. I know this is splitting hairs, but ultimately, which ever one moves the greater volume with the greater density for the application is the winner. Your goals are skirting right on the edge between the 2 SC's, where as mine favors the M62.
Broncitis":av0o2a0c said:
One more thing is if you run much past 8.9-1, on pump gas you will not be able to run more than 7 psi on premium gas with about 4 degrees static and 25 degrees total timing
This is a great point! And, can never be stressed enough. The greater your static compression ratio, the less you'll need to, or can, boost the engine before you'll need to take serious steps towards pre-detonation prevention. You can SC any engine, but how far depends on where you're starting from and what you're willing to do to help the situation. A higher static CR means less boost required to reach a goal, where as a lesser CR requires more. Neither is really "safer", because both lead to the same end result : higher combustion chamber pressure and temperature due to an increased volume of intake charge at the start of the compression stroke.