Chris in MD

DemonRX-8 What's up with the electronic limit of 149 in the US? Does that mean the speedo only goes to 149 or an engine de-rich function to limit power above that?

Reeko

From what I have read, there is no electronic speed governor. I was under the impression that it was a horespower issue (the point where the engine power can't overcome the wind resistance).

If that is true, then someone should be able to go over 149 with aome power mods.

Britelight

Love the Theory but don't forget the electonic limite rkicks in at 148mph

RX8_Buckeye

I believe I can help clarify a few things I've read in this thread.


The numbers come close to real-life but I see a few MPH drop beginning at 3-4K due to drag. The numbers drop significantly as the car moves faster and drag builds up.

As others have pointed out, this is incorrect. There is a fixed relationship between engine speed and vehicle speed, or n/v as it is called in the auto industry. It is completely dependent upon transmission gear ratio (& turbine/impeller speed ratio for unlocked torque converter), final drive ratio, and tire rolling radius.



The 'Final Gear Ratio' then means that there will be 4.444 times FEWER revolutions of the Prop Shaft (and hence of the rear wheels).

This is incorrect. The final drive ratio, or differential gear ratio, is the ratio of prop shaft speed to axle / half shaft speed. So the ratio of 4.444 means that the prop shaft is always rotating 4.444 times faster than the axle shafts, and hence the rear wheels.



In a piston engine, I thought RPM's were defined as how many complete piston cycles occur in one minute, which I believe happens to be the same number as how many crankshaft turns occur in one minute (one cycle of the pistons = one revolution of the crankshaft).

Engine speed is always defined as the rotational speed of the crankshaft, regardless of the type of engine. In any 4-cycle piston-cylinder engine, each cylinder fires once for every 2 revolutions of the crankshaft. So in an I-4 engine, 2 cylinders fire for every 1 rotation of the crankshaft. In a V-8 engine, 4 cylinders fire for every 1 rotation of the crankshaft.


there would be slippage in the torque converter, unless it has a lockup ... i've really read nothing about the automatic on the 8.
so the automatic #'s wouldn't pan out the same way as the manual numbers

Even though the auto trans has a converter cluch, you are still right about the numbers not panning out the same way as the manual. The converter clutch in almost all 4-speed automatic transmissions only locks in 4th gear, and even then it may occur several seconds after the upshift. Therefore the ratio of engine speed to prop shaft speed is not completely defined by the transmission gear ratio. It will also depend on the turbine/impeller speed ratio in the torque converter, which varies from 0 at standstill to a max of about ~95% (without lockup). For this reason, it is not easy to map engine speed to vehicle speed for automatic transmissions unless you make a lot of assumptions (which for the most part are not valid).

What can be easily mapped for automatic transmissions is upshift/downshift schedule. The shift schedule depends almost completely on vehicle speed and throttle position (or pedal position for electronic throttle control applications). The PCM monitors vehicle speed and throttle position to determine when to shift. For example, when upshifting, a higher throttle position means that the PCM will simply wait until a higher vehicle speed is attained before giving the command to shift. At low throttle positions, the PCM gives the command to shift at much lower vehicle speeds. This is how the transmission knows what type of driving you want to do (fuel economy vs. performance).