Quote:
Originally Posted by hat_man
It seems some forums are saying that going to a different rear end (3.45 or 3.08) will lower my mileage. Not finding any math or reasoning to back their statements up though. I thought a drop in gears would help mileage.
|
It's not an absolute either way as stated, but they do have a valid point. It depends heavily on how you intend to drive, and if your vehicle is already under-powered when in its top gear.
When under-powered, there is the distinct possibility that you will have to use a lower gear more often, or for a larger percentage of your driving time, or that your driving will require more AE - acceleration enrichment fuel, or that you would be using PE mode (power-enrichment) more often during your normal driving.
There are countermeasures that you can take like changing your driving habits, and doing some modifications to your vehicle, and/or tuning parameters that will net your a certain increase in fuel efficiency.
Of course driving slower and being lighter on your throttle modulation come to mind, but that's not to say that you're not already doing that.
Dropping your final drive ratio puts you at a lower engine RPM for a larger percentage of your driving time. If your engine's power output is very low in this range, then it can cause driveability problems like lugging, and knocking. The reaction of your ECM/PCM by pulling spark advance (KR- knock-retard), while saving your engine from damage, can also result in even less power and a drop in fuel economy.
In a situation like this, you want to focus on improving your low RPM power, and reducing the tendency for lugging and knock.
First, to address knocking you want to keep tight control over the temperatures in your combustion chamber. Running a WAI in this situation would be counterproductive, especially with a MPFI fueling system. You'd want to keep your intake charge cooler because you're loading your engine more with the highway gears. In some situations a slightly lower temperature thermostat would be in order as well.
Also, in situations like this I have used a colder heat-range of spark-plug, but unless you have full control over your tune, you probably don't want to go more than one to two steps colder. No matter the heat range, running a tighter gap can often be helpful in preventing misfires, which are a primary component of lugging.
To improve power at low RPM you want to focus on reduction of intake charge contamination, mainly from PCV and EGR systems. The less contamination of the charge, the more power your engine can make. At the very least, add an extra filter to your PCV system before the point the vapors are delivered back into the intake charge.
Accessory losses count the most at low RPM. This makes modifications like under-drive pulleys and manual steering very effective in this situation.
As was said before, things like aero-mods, weight reduction, better alignment specs, and low rolling resistance will be more effective in a situation with an under-powered vehicle.
In some cases, re-timing the camshaft(s) to improve efficiency at the lower RPM can be helpful. If your cams already have a good bit of (absolute) negative overlap, then simply advancing the intake side can be helpful in shifting peak efficiency to a lower engine speed. Of course if you only have one camshaft, then you'd be advancing both intake and exhaust by the same amount, which may or may not help. For low RPM part-throttle power, you generally want the least amount of overlap possible.
Although it can become complicated, modifications that increase the length of the intake manifold runners, and/or reduce the volume of the intake manifold plenum can shift efficiency to a lower RPM, often at the expense of high RPM power potential.