4th gear vs 5th gear

[wintersubaru]

What use's more gas, 4th gear at 3000 rpms 55 mph with the gas pedal barely down or 5th gear at 2800 rpms at 55 mph with the gas pedal at 1/2 down? 86 Ea 82 4x4 wagon carbed.

[Ross]

Thats a tricky question to answer - all engines are different in that respect and it will also depend on car weight, wind and so on.

Myself, i'd go with 4th, better for the engine, although it really wont make much difference at all.

 

You may just have to experement with both, and see if you can notice a difference in fuel consumption.

[KStretch55]

5th gear. Fewer engine RPM under less load, more driveline RPM for the same amount of fuel.

[wintersubaru]

5th gear. Fewer engine RPM under less load, more driveline RPM for the same amount of fuel.

I understand that, but if you have your foot more into the pedal isnt the carb putting more fuel into the engine using more fuel?

[edrach]

I know I'll get flack for this, but moving a car down the road at 55mph requires energy; the same speed requires the same amount of energy; so it doesn't matter which gear you use since you have to expend the same amount of energy.

[Ross]

5th gear. Fewer engine RPM under less load, more driveline RPM for the same amount of fuel.

 

Actually, with slower engine speed, you require more load to keep the car at the same speed.

 

As wintersuabru mentions, the carb lets more fuel mixture in when you push the gas pedal further, as you do when the engine is reving slower.

 

On the other hand, the engine of course "sucks" more at higher revs, and friction power will be higher.

 

Thats why you can't really say "the car will use less gas in 5th/4th" with any certainty without testing in the conditions it will be run in.

[NorthWet]

I know I'll get flack for this...

INCOMING!!!

 

Any increase in RPM requires more energy just to achieve it. If this were not the case, then the engine would be prone to overreving to destruction at "idle" throttle-opening (and would, in fact, not require any opening at all to idle). The amount of energy is actually quite considerable. This is the rationale behind "overdrive" gears.

 

Running an engine at low RPMS does not hurt it, assuming that the oil pressure is adequate. Damage from "lugging" can be the case with rolling-element bearings, but even then these bearings are supposed to be designed for the engine's maximum torque/BMEP and are well up to the task of handling "lugging" at half of the BMEP.

[Ross]

INCOMING!!!

 

Any increase in RPM requires more energy just to achieve it. If this were not the case, then the engine would be prone to overreving to destruction at "idle" throttle-opening (and would, in fact, not require any opening at all to idle). The amount of energy is actually quite considerable. This is the rationale behind "overdrive" gears.

 

Running an engine at low RPMS does not hurt it, assuming that the oil pressure is adequate. Damage from "lugging" can be the case with rolling-element bearings, but even then these bearings are supposed to be designed for the engine's maximum torque/BMEP and are well up to the task of handling "lugging" at half of the BMEP.

 

But, will the thermal efficiency of the engine not vary considerably with changing engine speed? ie the engine may be running more efficiently at 3000rpm than 2500rpm at a given load?

[NorthWet]

But, will the thermal efficiency of the engine not vary considerably with changing engine speed? ie the engine may be running more efficiently at 3000rpm than 2500rpm at a given load?

 

OK, the premise is steady-cruise at highway speed, correct? That requires a certain HP to maintain (torque is irrelevant). HP varies as a function of torque times RPM, so at a higher RPM you need to produce less torque, which means that you are running at a lower effective VE. This also produces a lower effective compression ratio, which would cause a lower thermal efficiency. At a lower RPM, you need to produce more torque in order to generate the required HP, so you are running at a higher effective VE, higher effective compression ratio, and higher thermal efficiency.

 

(This is assuming a few things regarding not being way off of the torque curve and/or into rich mixtures regions.)

 

Does this sound reasonable, or am I suffering from sleep deprivation?

[Ross]

OK, the premise is steady-cruise at highway speed, correct? That requires a certain HP to maintain (torque is irrelevant). HP varies as a function of torque times RPM, so at a higher RPM you need to produce less torque, which means that you are running at a lower effective VE. This also produces a lower effective compression ratio, which would cause a lower thermal efficiency. At a lower RPM, you need to produce more torque in order to generate the required HP, so you are running at a higher effective VE, higher effective compression ratio, and higher thermal efficiency.

 

(This is assuming a few things regarding not being way off of the torque curve and/or into rich mixtures regions.)

 

Does this sound reasonable, or am I suffering from sleep deprivation?

 

Sounds good to me, now that you mention it.

I'd agree that the assumptions you mention are probably quite valid for such a small change in engine speed (2800 - 3000). Could be different if we were talking about the difference between 2400 and 3000rpm.

[soobscript]

What use's more gas, 4th gear at 3000 rpms 55 mph with the gas pedal barely down or 5th gear at 2800 rpms at 55 mph with the gas pedal at 1/2 down? 86 Ea 82 4x4 wagon carbed.

Do those two situations both cruise, or are you accelerating in 5th with the pedal half down?

[LPGsuperchargedBrumby]

one thing that ppl never seem to take into consideration in discussions like this is that its only petrol vapor that burns not microscopic droplets of petrol.

 

petrol is like most liquids in that the more vacuum it is subject to the more it transforms into vapour

 

so 3000rpm at nearly closed throttle should be more economical that 2500rpm and half throttle...in theory it should work even better in an injected motor thats running closed loop and reading fuel/air on the O2 sensor.

[wintersubaru]

Do those two situations both cruise, or are you accelerating in 5th with the pedal half down?

That would be highway cruse at steady speed!

[azsubaru]

I agree with Northwet. I don't know all the theory, but driving a lot, you get bored and try things like this. Based on only what I found, 5th would seem to be better than 4th in this case.

[TurboSPFI]

4th uphill grade, 5th downhill grade.

[Numbchux]

well, I was asking myself the same thing, except in my car it's 70mph, full throttle in 5th, or 1/2-3/4 in 4th, don't remember rpm's but 1000-1500 difference...

 

anyway, my dad brought up a good point, cars with taller gear ratios, tend to get better gas mileage...our 3AT loyale got horrible mileage, considering it's a 4-cyl, FWD car...

 

the latest issue of Road & Track has a full-mile acceleration test of a number of sports cars. In it, they dest a Viper SRT-10, and note that it's top speed is in 5th gear, and that "6th gear must be geared for about 300mph," the reasoning behind this is it's "overdrive 6th gear is indeed a fuel economy ratio that bumps the EPA highway mileage figure up to a surprisingly good 20 mpg."

 

soooo, the assumption gathered from that, is a higher gear = lower rpm = less fuel consumption...right?

[TurboSPFI]

no, there becomes a point to where you start to waste gas running it around in lug mode, trust me i tried.

[Ross]

Yes, 5th will only be more economical under low load situations. When you start to need higher power, you should use a lower gear.

[NorthWet]

As in many things, it depends...

 

It depends on what induction system you have, and how it handles mixture under load. If we are talking carburetor, and that carburetor has a "power"/"economy" valve, then if you are loading the engine to the point where the manifold vacuum drops to the point where the power-valve opens and enriches the mixture, then specific fuel consumption will increase and fuel mileage will decrease. If it is an EFI system, it depends on how the ECU reacts to low manifold vacuum conditions.

 

I tried running my EA82T in 4th gear to stay off of boost (and resulting open-loop enrichment), and ended up with worse mileage then in 5th and on boost.

 

"Ya pays yer money and ya takes yer pick!"

[edrach]

Lets not confuse the issue with new variables like rear end ratios, gear ratios in 4th and 5th, and load or no load, etc. We're talking the same car (whatever it is) at a steady cruising speed of 55 mph on a level stretch of road. Whether you are in 4th or 5th gear, the same amount of work needs to be done to push the car through the air and overcome frictional aspects. That being said, the same amount of energy will be required to do that work. Now the question becomes, is the engine more efficient in 4th or 5th gear and can the car squeeze more energy out of the gasoline/air mixture. The simple way to tell that is to look at the torque developed by the engine at a given rpm. Most cars have the torque peak at around 3000 rpm; above that the torque drops off and the horsepower continues to increase. Forget horsepower, think torque. An engine is most efficient at the torque peak; so the answer to the 4th or 5th gear question is "what's the engine rpm of the car at 55 mph?" The engine rpm at 55 mph closest to the torque peak rpm will prove to be the more efficient and squeeze the greater amount of energy out of the fuel mixture and will determine whether 4th or 5th gear will use the lesser amount of fuel.

[NorthWet]

You can't forget horsepower. Steady state cruise uses the same amount of HP, not torque (except as measured where the rubber meets the road). Torque curves are somewhat irrelevant, as they measure MAXIMUM torque that the engine is able to generate at any given RPM. Maximum implies wide open throttle, a condition that is not the case at steady state cruise.

 

In different gears the generated HP will be the same but the torque requirement will vary inversely with the engine speed. The developed torque varies with the effective VE, blah, blah, blah.

[Ross]

Something not completely unrelated - a small deviation from proper ignition timing will change the engine efficiency dramaticly. The engine will be running most efficiently when the peak combustion chmber pressure occurs at roughly 5 degrees after top dead center.

Depending on how well the centrifugal advance is working, this may or may not be steady across the rev range.

So, engines will operate at different efficiencies than manufacturers "test" engine, making the use of their WOT torque etc. curves dubious at best.

[mcbrat]

Maximum implies wide open throttle, a condition that is not the case at steady state cruise.

 

unless your in a lifted ea81 with stock engine w/weber and 27" tires, and you have crappy Iowa gas....

 

then it's foot to the floor for 200 miles just trying to hold it at speed with the other vehicles on the interstate

 

 

[NorthWet]

unless your in a lifted ea81 with stock engine w/weber and 27" tires, and you have crappy Iowa gas....

Aw! C'mon! You've got the best gas additive that this Country can grow!