questions on manual driving

[j.reed]

Hey...ive just got some simple questions. Basically...regarding starting in first gear vs 2nd gear

Is it more efficient to use 1st gear for a minimum of time, and short shift(1800 rpm) into 2nd? or drag first gear out a little longer and shift to 3rd?

or...Is starting in 2nd better, because of the taller ratio? but with more load on the engine it may not be worth it?

and finally...is there a "minimum" rpm at where you will be using less fuel? for hypothetical example...if i shift to 5th at 25 mpg, the engine chugs a little bit around 900 rpms. Or if i stay in 4th at 25mph...it runs smoother but at 1200 rpms. Which version is better?

[SentraSE-R]

It depends on the gear ratios in your transmission, and your transaxle's final drive ratio. Some cars can start in second gear fine. Some can skip gears without a beat. Some can lug at low rpm in 5th gear. Others can't. Generally speaking, lugging your engine at too low an rpm is a bad thing to do. I'd rather err on the side of wasting a little gas, rather than damaging an engine.

[PaleMelanesian]

I regularly start in 2nd gear. I shift early and "semi-lug" my engine, down to about 1000 rpm in some cases. I hate driving above 50 mph, because that takes me above 2000 rpm. You can see the results below.

[Old Tele man]

...too much lugging for too long duration can "eat" main bearings, epecially if the engine's already old and parts are obviously worn.

[Drive Stick]

As previously mentioned, there is no benefit to lugging your engine for the sake of saving fuel. You will replace a $3,500 engine once you spin a bearing, or snap a connecting rod and burn up many times the amount of money replacing your engine, or vehicle.

Row through the gears at 60% throttle or so and you will see the best combination of acceleration, and gas mileage in my opinion. Skipping gears is ok if you feel guilty about shifting through all 5 speeds, but only if you are not dropping below 1,800 rpm or so in any upshift. (unless traveling downhill when you won't lug the engine.)

Some people have mentioned reaping excellent gas mileage from never going over 2,000 rpms but every vehicle is different. My V8 truck can handle shifting early, its got 8 cylinders to power me along effortlessly. My 4 cylinder car can not be shifted too early with out lugging, it all depends on the vehicle and engine, play it by ear and gas receipt.

you'll find that winning combo of fuel efficiency, and longevity of your engine.

[dcb]

you really have to define "lugging" better.

yes there is a minimum rpm to keep the oil distributed, but I don't know that it is 1800rpm, or even 1000 rpm.

Bearing wear and low RPM high/load operation.

Lugging the engine - good or bad for economy?

Key concepts:
1. the rpm must be sufficient to keep the oil layer intact, i.e. above idle.

2. Bearing wear is a function of rpm and load at the low end. extremely low rpm is usually pretty low torque for an ice, so it is somewhat self-regulating.

3. if you are centering your shifts on bsfc peak, you don't really have anything to worry about anyway.

I just don't want folks racing their engines out of an irrational fear of a $3500 engine repair bill.

[PaleMelanesian]

Originally Posted by dcb

3. if you are centering your shifts on bsfc peak, you don't really have anything to worry about anyway.

Bingo.

[j.reed]

well i didnt mean to totally lug the engine around. But more like.... for example, 1st, 2nd, 3rd, all using 10% throttle to maintain safe acceleration. Or, start in 2nd and use say 14% throttle and 10% in third. Any advantage either way? Its just rpms vs load im asking about basically. And please explain bsfc peak? or better yet...how do i find it for my car? i know that is something like brake specific fuel consumption...but i thought that was for figuring out proper AFRs on the dyno?

[Christ]

10% throttle isn't safe, especially in NJ. 40-45% is safer, but doesn't load the engine as much as you could.

If you go 100%, or close to it, you're probably running in open loop, dumping fuel. Try 70-75% and see how you feel about it. If it's too much, back it off a bit.

[dcb]

Originally Posted by j.reed

And please explain bsfc peak? or better yet...how do i find it for my car? i know that is something like brake specific fuel consumption...but i thought that was for figuring out proper AFRs on the dyno?

here are some:
The BSFC chart thread (post 'em if you got 'em)

Basically, if you know (or make a reasonable guess) under what load/rpm your engine makes the most power for a given amount fuel, you can control the throttle and shifting to maximize your efficiency during acceleration.

The big trick is knowing when to stop accelerating and start coasting, so you don't turn all that gas energy into brake heat.

In absence of actual data, I usually shift at 2700 rpm and keep the throttle at 3/4 open. My tdi is different, it is wide open throttle and shift at 2000 rpm.

here is the tdi graph, the peak "oval" is at the top of the torque curve centered at about 1800 rpm. When I shift at 2000 rpm it drops down to about 1600 rpm and so I accelerate from 1600 to 2000 rpm at the top of the graph where it is most efficient (plus I don't brake much)