In a standard transmission vehicle, is it bad to be in a high gear at low speeds?
If you don't need to accelerate, it is fine, but if you try to accelerate and it lurches, it is not good for your engine.
Yes, if the engine RPMs are too low in the higher gear you are 'lugging' the engine.
Lugging occurs when you are driving the car at a lower speed than the engine is turning. Basically you are making the engine work much harder than it needs to.
I'm not sure what kind of vehicle you are referencing, however I think as a rule of thumb you should always drop to a lower gear if your RPMs drop below around 1200 or so.
Some strong higher torque producing V8 engines that I have driven can handle a high gear with RPMs that are a little lower than that.
The smaller the engine, the harder it will struggle.
Every engine has a sweet spot where it produces the most power/runs the most efficiently. This sweet spot is function of the engine's power band as well as things like pumping losses in the air intake system. Generally speaking, for efficiency, you will want to be in the highest speed that still provides the required power. I would check the owner's manual for more information.
Manual transmissions physically connect the engine and transmission to one another with a clutch, automatics use a fluid coupling via the torque converter. In the standard, if the engine's RPM is faster than the wheels are turning, the engine will either slow down to match the wheel speed via friction within the clutch and run poorly, or the resistance in the drivetrain will stall the engine.
In a simple, practical example, take a V6 engine with an idle RPM of 1200, and a gear ratio between the differential and tranny that is 3.08:1, meaning the engine rotates 3.08 turns for every turn of the wheel (this is the highest gear ratio in my Ford Ranger and the idle speed of its 3.0 V6, albeit it's an auto, not a manual. That's not important for discussion, we'll pretend it's a manual). With a 31" diameter tire, the idle speed will make the wheels of the truck go around approximately 390x in one minute. Multiply that by the circumference of the tire, and the truck would cover 3,162 feet in a minute at idle RPM, which is 36 mph. To not drag down the engine or burn out the clutch, the truck would need to go at least that fast in overdrive with a standard transmission to run normally. If you're going slower than that, either the engine will force you up to this speed, or it'll stall/ run really bad from you riding the brake to maintain the slower speed than it "wants" to go. Drop in a motor with a lower idle RPM with the same gearing, and the speed will reduce. Clutches and engines are expensive, so generally you don't want to do this. You really never shift into high gear in RPM ranges like this, because the engine isn't putting out much torque or horsepower at such a low RPM.
The different speeds aren't a problem in the fluid coupling of an automatic's torque converter, as the engine isn't physically connected to the transmission. The crankshaft spins an impeller in hydraulic fluid, which spins another impeller on the transmission input shaft. The engine doesn't care what the wheels are doing, and vice versa, because They're not physically connected. It can actually enhance torque delivery, but that's a whole different discussion.
Very in depth answer. Thank you.
Isn't 1200 a fairly high idle? I don't think any of my V6's ever did.
It idles a hair above 1,000 when it's warmed up, may be closer to 1100 or 1050. It's definitely around or above 1000. I've never hooked up a scan tool to see what the computer actually reads. I might have to do that.
When I was in engineering school, I took a class called Theory of Machines, which was a crash course in hydraulics, gearing and things like that. This was where I first got to look inside a Honda manual transmission the professor had made a cut-away out of. I thought that was one of the coolest things ever. Haha. He liked gearing and would give problems like this as homework. Differentials were also a thing we talked about, but not as in-depth.
interesting. Is it a small V6?
My Vortec 4.3 idled at about half the speed of yours
My V6's only a 3.0L, it was meant for the 1st generation Taurus originally, and is a pretty high revving engine. The torque and power bands are up in the 3000-4500 range. It runs like a sedan, not a low speed, torque-y truck V6 like your 4.3 or Ford's bigger 4.0 Cologne V6.
Makes sense. Smaller displacement will need higher RPM to make enough torque to maintain smooth idle. I've never had anything smaller than 3.1L
I wouldent do it for hours. say if you were on a road trip about 4-7 hour trip and you do it half way whatre I dont see it as a bright ideah for hours at a time.
I’d like to echo the sentiments stated here. Every car RPM range is different, but the principle is the same. Find the “sweet spot” of RPM range for your car. It’s the spot where the engine isn’t working too hard, but also where it is NOT struggling.
