Let me also clarify what IZwack was talking about, VLIM, or Ford also calls it IMRC (Intake Manifold Runner Control).
This system is a computer controlled variable intake runner, during normal operation air is routed through a longer intake runner for low RPM torque, under 3/4 to WOT the comptuer opens a butterfly at around 3k RPM's to allow air to be routed through a shorter intake runner, for high RPM horsepower.
Cross section cuttaway of intake manifold
This is
NOT as you would call VTEC (Honda), VANOS (BMW), or VVTi (Toyota) which infact is variable valve timing.
Ford is not currently using variable valve timing on any of it Exploder's at this time.
Variable Cam Timing:
Variable Cam Lift:
Lets also note that there may be two engines we're all talking about here, the first is the SOHC, the heads of this motor contain the cam shaft which actuates the valavetrain, a much more effecient design especially for high RPM usage and power.
The OHV motor, also a 4.0 has a single cam in the block/or lower portion of the engine using lifters and push rods to actuate rocker arms to open and close the intake and exhaust valves, still a very funtional design (look at the Chevy LS engines) but none the less a lower reving engine, plauged with valve float because of mass, at high RPM's and usally really great at making off idle, to near off idle torque and power quickly but not much past 4300rpm's unless some head and cam work has been done, remember this is a truck not a race car, and the engine was never designed for this.
Now as for fast and quick, we'll go baised on common sense here:
Quick = Time it takes to get from point A to point B
Fast = The top speed of a vehicle
And there is a certian amount of give and take for each, you can be quick but topend suffers, you can be fast, but the time it takes to get to fast suffers.
Rear Gearing:
Axle ratio: 4.10 = quick
Axle ratio: 3.27 = fast
Understand??
A chassis dynomometer, or dyno's as the slang term goes, is used to show where the horsepower and torque patterns are in an rpm range using (hopefully) a 1:1 gear ratio, usally run it 3rd or 4th gear pending on what's closer to a 1:1 ratio, and the opterator will normally start the "run" at 3k rpms to rediline or what the customer specify's, same with the starting point if it can be applicable (usally manual transmission, as an automtic will down shift to the next lowest gear).
Ah which brings us to "Passing Gears", in your automatic transmission there are 4 or 5 gears, baised on which transmission you have, when you step on the gas to pass, baised on caculated engine load, vehicle speed, rpm's, and the gear the transmission is currently in, the computer may command the trans mission to down shift one or two gear to give ratio of grearing for what the compter thinks you trying to accomplish (rashionality/control strategy), there is not a mystical "passing gear" that it shifts into, sorry.
And releasing a depressing the gas so to speak as you mentioned, it quite hard on the transmission, these
ARE the weakest links of these vehcles, and not too cheap to have worked on
In a layman's term:
Torque, is what gets the mass of the vehicle moving.
Horsepower, is how fast it;s going to makeit move.
Looking at a dyno sheet will let you know where the torque come into pay and how fast this happens in the rpm band/range.
Same with horsepower, but once the horsepower falls off, that's it, spinning the motor past this area/range/or point is worthless, you need to shift 200rpm's before this happens to be effective.
In this case this motor is making max torque at 2700rpms and slowly tappers off, max horsepower is at 5750rpms, to be effective in a drag race he would need to be shifting at around 5500rpm's or just slightly sooner.
Jeff - :navajo:
