View Single Post
  #35  
Old 10-14-2006, 02:43 AM
ChevyHighPerformance ChevyHighPerformance is offline
Hummer Professional
 
Join Date: Jul 2005
Posts: 278
ChevyHighPerformance is off the scale
Default Re: Any feel like having a tuning discussion?

Here is some new fuel economy test data.

First, the goal was to test my modified tranny shift table versus the one supplied from Alvin in his tune.

There are two elements to fuel economy: acceleration efficiency and cruise efficiency.

CRUISE EFFICIENCY

For highway traveling, the goal is to maximize the cruise efficiency. For example, for a typical vehicle the amount of gas it takes to accelerate from 0 to 60 MPH is about the same amount required to keep the vehicle traveling 60 MPH for about a mile. So you can see that when you are traveling many miles the amount of gas used to accelerate you is very small compared the amount of gas you?ll use to keep you at speed for many miles. This sets up my first test for which everyone probably already knows the results. I ran the H3 at two fixed speeds (40 MPH and 44 MPH) in both 3rd and 4th gear and measured the rate at which the gas is consumed. (The gas volume was measured and since the temperature was constant for all tests the volume is reflective of the mass of gas used.)

3rd gear:

40 MPH - fuel utilization rate = 3.38u/s

44 MPH - fuel utilization rate = 3.92u/s

4th gear:

40 MPH - fuel utilization rate = 2.90u/s

44 MPH - fuel utilization rate = 3.05u/s

u/s is a scaled volume of gas used per second

The conclusion here is that if you are cruising you want to be in the highest gear with the torque converter locked. My guess is that even though the drivetrain efficiency is higher in 3rd than 4th gear, the higher engine parasitic losses associated with the higher RPMs in 3rd than 4th gear dominated the fuel rate utilization.

ACCELERATION EFFICIENCY

For city traveling, the goal is to maximize the acceleration efficiency. If you are starting and stopping every block then the amount of gas used to accelerate the vehicle will dominate the fuel consumption and MPG. The trick the hybrids do is absorb energy and store it during braking; then use this stored energy to accelerate the hybrid. This is why the hybrids do proportionally better in the city than the highway.

I ran the H3 at three different constant throttle positions (35%, 45%, and 55%) and measured the total fuel volume used when going from 20 to 50 MPH. (The gas volume was measured and since the temperature was constant for all tests the volume is reflective of the mass of gas used.) I ran this test for both my tranny shift tables and Alvin?s tranny shift tables.

My tranny table

35% throttle: time=28.8 sec, total gas used = 3583u total

45% throttle: time=15.8 sec, total gas used = 2945u total

55%+ throttle: time=13.1 sec, total gas used = 2567u total

Alvin?s tranny table

35% throttle: time=29.3 sec, total gas used = 3803u total

45% throttle: time=12.0 sec, total gas used = 2473u total

55% throttle: time=11.1 sec, total gas used = 2508u total

u total is a scaled amount that represents the total gas volume used

Because it was difficult to precisely hold the throttle in a fixed position there is some variance (maybe +/- 5%) about each time and total fuel used data point. There are two conclusions from this test: 1) Alvin?s table resulted in less fuel consumption than mine and 2) the more throttle you use to accelerate (within reason) the less fuel you use. This goes against the EPA?s advice to not accelerate fast when pulling out because you?ll waste gas. I think the reason why the higher throttle resulted in less total fuel being used is due to the lower pumping losses associated with larger throttle positions thereby increasing the engine?s overall efficiency. Basically, your engine doesn?t have to work as hard to suck air into the engine when the throttle body is open more.

I think the key is learning how to drive based on how the tranny tables are set up. I think my tables are better for cruise (highway) and Alvin's are better for acceleration (city).
Reply With Quote