I have recently noticed something odd about my car. I had always believed that when one floored the gas (as if one was drag racing from a stop) a car with an automatic transmission would accelerate in first gear from a stop up to a speed coinciding approximately with the horsepower peak in first gear and then shift into second gear, and so on. I have found that my car shifts from first gear to second gear at 47 MPH, which is the torque peak in first gear and a full 1000 RPM or 16 MPH below horsepower peak.
For reference (in case someone actually wants to do this math), I drive a 1992 Mercury Grand Marquis GS with a 4.6 Liter V-8 which makes 190 brake HP at 4200 RPM and makes 260 ft-lb of torque at 3200 RPM. It has a 2.73 rear end ratio and gear ratios of 2.00 (1st), 1.47 (2nd), 1.0 (3rd) and .67 (4th). Its tires are 225 mm with a 70% aspect ratio and 15 inch wheels.
Perhaps Mercury decided that it could decrease warranty costs by not permitting higher RPM operation of this engine. That’s the only reason I can think of, but it still seems kind of odd. Based on vehicle simulations performed using CarTest v4.5 (see cites for download info), this change in vehicle tuning raises my 0-60 times from 10.6 seconds to 12.1 seconds and is kind of annoying.
This raises two questions in my mind:
Is anyone else aware of other passenger vehicles with automatic transmissions or other vehicles that have shifting substantially below their horsepower peaks?
I think this is probably an intentional design trait of the vehicle, but could this be due to some defect in the car?
Why besides warranty cost concerns would Mercury use these kinds of shift points?
Cites:
Horsepower and torque information from The Stand Catalog of American Cars. Note that my car is single exhaust, and the peak RPM differs for the dual exhaust version of this vehicle.
CarTest is available from http://home.earthlink.net/~patglenn/ct.html
I suspect that many pieces of data go into deciding where to set the shift points. Some that come to mind are wear and tear, acceptable acceleration, smooth shifts, engine sound, and sort of a general “what feels right” to the target consumer.
While fast acceleration is one consideration, I doubt it is the highest factor for most car drivers. Most cars aren’t designed to maximize speed off the line.
Modern transmissions also can detect how hard the pedal is pushed and decide to delay the shift point for fastest acceleration. But if your car doesn’t have that it’s probably not optimized for drag racing.
Perhaps since the Grand Marquis is supposed to be a luxury car they decided to shift it low just to keep things quieter and smoother rather than the harsh change you get with a high rpm shift. If my WAG is true then someone may make a replacement chip that allows for a more powerful shifting sequence.
Older vehicles, at least domestics, utilized a device the name of which escapes me (modulator?) installed at the transmission which functions via engine vacuum, when you tromp on the accelerator the vacuum drops, triggering the shift, the modulator comes in different ‘flavors’ allowing some range in the shift point.
You may be able to sub in a different modulator if your car is so equipped. If not, not.
You may find that it’s torque peak rather than horsepower peak that is sought. You can be sure that passenger car designs consider drivability, smoothness, longevity, and other qualities besides acceleration. If you want maximum acceleration, you need to get a car designed to be a race car.
Having a 1-2 shift at 47 mph at full throttle sounds normal rather than sign of a defect. If you want to see if a later shift will improve 0-60 times, put it into first and shift manually. Having cash reserves for engine and/or transmission overhaul might be a wise policy before doing so, however.
Right, I have done so, but lack the facilities to time this acceleration properly. However, using a vehicle simulator cited in my first post I documented that a later 1st-2nd shift point would result in my 0-60 times being lowered by 1.5 seconds.
Certainly not a huge deal, more of a curiousity to me, actually.
But I’d still like to know if anyone else has noticed dramatically low shift points on their cars.
I can tell you that in later years, the base models of the Ford Crown Victoria on the same platform with a single-exhaust was programmed to shift at 4250 RPM, the dual exhaust (sport) versions shifted at 4500 RPM and the police package shifted at 4750 RPM.