Which gears in my rear end?

[hat_man]

After reading some more I do have another question.

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.

Maybe a 3.08 would be bad in the lower gears/speeds but how about the 3.45? I mean, we look at grill blocks, pizza pans, removing passenger side mirrors, etc. all to gain a couple of mpg. Is this a viable (though expensive) option to gain 1-2 mpg? It's not like I can afford to do it right now, but after doing some little things (and getting back to work) maybe someday I could afford it. I'm just looking at future options. The truck is paid for and in great shape. I think I'll be driving it for a long time.

Thanks

[comptiger5000]

Also, keep in mind that the more aeromods and such you do, you will benefit more from taller (lower number) gears.

[Olympiadis]

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.

[comptiger5000]

That sounds like a pretty good explanation. However, wouldn't accessory losses count more at high rpm, where they're spinning faster, and frictional losses, as well as pumping losses (in the power steering pump) are greater?

[Olympiadis]

Originally Posted by comptiger5000

That sounds like a pretty good explanation. However, wouldn't accessory losses count more at high rpm, where they're spinning faster, and frictional losses, as well as pumping losses (in the power steering pump) are greater?

That is a common belief because it is more intuitive and easier to measure the changes at WOT & high RPM - (racing or dyno'ing). Even though a % gain in power is similar, people tend to bias their judgment based on the difference in raw numbers.

The situation with judging the effectiveness of under-drive pulleys is further confused because they tend to show such poor gains in power during steady-state testing. Where the reduction of accessory drag really shines is during positive engine speed transitions. An under-drive pulley set that shows only 10 horsepower gain in a chassis-dyno test can show a solid performance gain equivalent to 20+ horsepower during a quarter mile pass.
A large chunk of this extra performance gain is during the first 60 feet of the run where average engine power output is the lowest, and the engine speed transition rate is the highest. Hard for some to imagine, but the vehicle acceleration rate is also the highest during the first 60 feet of a quarter mile pass.


Though the % power gain isn't exactly linear in relation to RPM, it is close enough for practical theory. It may help the intuitive understanding to realize that at lower RPM the engine speed can transition at a faster rate. Going from 1000 RPM up to 2000 RPM is doubling the engine speed, the same as 3000 RPM up to 6000 RPM. The same % of engine speed transition is concentrated into a visually smaller range of engine operation.

At the drag strip an under-powered vehicle may experience some drive-ability problems when launching at a low engine RPM. The load on the engine may be so much that tuning changes aren't enough to resolve the drive-ability problem without also lowering the vehicle performance. In a case like this, the reduction of accessory drag can make an unusually large improvement in vehicle performance (acceleration) by unloading the engine to the point that the low RPM drive-ability problem is either resolved, or then can be resolved with a change to the tune. Unloading the engine by reducing accessory drag can change the engine speed transition rate to the point that a given spark advance curve can be used without experiencing lug or knock.

This effect would not be so pronounced in a situation where the engine was already over-powered, or the launch RPM was significantly increased.

Under-drive pulleys perform in this way because it takes much more energy to accelerate the rotating mass of the accessories than it takes to spin them at a steady speed. The relative % energy required to accelerate this mass is proportionally larger when compared to the net engine power output when at very low RPM (especially at part-throttle). During normal driving the rate of RPM transition is also proportionally larger for a lower RPM range of operation, than it is for a higher RPM range.

[bhazard]

My dads 94 2wd 5sp splash is a 3.73 8.8, and in my experience 2.3's dont like to be lugged under 1800 rpm or so, and really hate being under 1500.

Taller gears may help you on the highway but you will find that in town your top two gears are useless.

Wheres most of your driving at? City or highway? If highway I might consider a 3.27 at most. If in town I would stick with what you have, I wouldnt go any taller than 3.55...

[hat_man]

Thanks for all the excellent information.

The majority of my driving is on 2 lane blacktops at 55mph. Probably about 80% of it. This is broken up at about 12-15 mile intervals because of small towns I pass through. Now I could take the interstate (tollway) to work but then any $$ saved in the tank would go towards paying the tolls. Approx. $7/round trip unless you use Illinois' I-Pass then it's about $4 and is 5 miles longer each direction. I was just looking at it as an future option.

As a side point...I just added my first grille block over the weekend. It's a full upper grille block and I partially blocked the lower grille with a new license plate mounting. I haven't had the chance to run a tank through yet but If I knew how to upload pics I would put them on here for opinions. Can anyone help me out with how to get my pics on here?

Thanks again for all the great info.