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Melting down the engine

Okay, so during the show you guys said that lugging the engine won’t wear out the clutch, but it will melt down the engine. How does it damage the engine? Also, what does driving at low RPMs (1500), just above lugging, do to gas mileage?

Thanks! Love your show. You guys are like huggy bears with large wrenches behind your backs!

When you lug the engine, it causes the engine to run hotter than normal.
It also places extra stress on the connecting rods and the rod bearings because the engine is not running in its proper torque range for the speed of the vehicle. On some engines, the oil pump may not put out enough pressure, and the end result of all of these situations can be expensive engine damage.

As to the effect on gas mileage, you did not tell us what gear you would be in–theoretically, at least–when operating the engine at 1500 RPMs. On my car, I can be in 5th gear at 40 mph, with RPMs at ~ 1500 RPMs, and my mileage readout will be in the range of 27-28 mpg, which is the top of the gas mileage possibilities for this car. However, the speed at which you are driving your car while at 1500 RPMs, and the gear that you have it in, might yield very different results.

Thank you for your reply. I’m understanding this better now.

Yes, for example, lugging the engine in 5th gear, regarding gas mileage. In other words, do you get best gas mileage just above lugging the engine, or at higher RPMs? Is it better for gas mileage to be driving in a 35 mph zone in 5th gear or, say, 3rd gear at a little above 3000 rpm?

35 mph in 5th will likely take you close to lugging. The best mpgs will come, as you say, in the highest gear possible without lugging.

“do you get best gas mileage just above lugging the engine”

Generally yes. Less energy is wasted in friction and “pumping losses”.

It won’t melt down the engine - unless you are at full load and cannot accelerate for some time.

But what speed is best - it depends. There are a lot of variables to consider - so there is no blanket answer. Here are a few things:

Is the terrain flat - or hilly?
How many cylinders are there (3, 4, 5, 6, 8, 10, 12, or a Wankel)?
Does the engine have variable valve timing? If so - what type - a two step or true variable?
Will the ECU cut out some cylinders when not at full load?
Does the ECU cut all fuel when at no load and in gear going downhill?
Does it have a turbo or is it NA?

In days of yore, before hardened valve seats and unleaded gas, lugging a small displacement engine was very likely to burn exhaust valves. I don’t understand the physics of the situation, but I know it happened to a lot of MGs.