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Old 02-06-2010, 11:54 AM   #1 (permalink)
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Bogging/Lugging (How can you tell when you are doing the engine harm?)

How can you tell when you are doing the engine harm? I have a 2.5 L 170 hp Mazda3 and I usually shift at 2.5k at 3k then go straight to 6 and cruise on 6th gear in cities going 55. I don't accelerate then by stepping hard on the throttle but rather lightly if i ever needed to get to 65 which would take a pretty long time.

Anyways I just wanna know if 1200 rpm would be too low to cruise at? I do this too even if the engine is cold.

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Old 02-06-2010, 02:14 PM   #2 (permalink)
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I've heard, but can't verify the truthfulness of the claim, that modern engine control computers won't allow you to damage an engine. They supposedly shut the engine down, rather than allow it to run under too much load.

My SE-R's engine/transmission combo is similar to yours, a 2.5L, 170 hp, 6 speed. I use 6th gear from 40-45 mph upwards, on level ground. I have noticed my cruise control will kick me out when I'm going up steep hills at 50 or 55 mph. IIRC, the engine is still on, and I'm left in 6th gear under minimum load, with speed steadily dropping, forcing a downshift.

1200 rpm is probably too low an engine speed for cruising. Your engine's BSFC graph should point you to where you want to be. Usually, your best efficiency is between about 1500 and 2200 rpm for 2 liter class engines. Rather than cruise at 1200 rpm (30 mph in 6th?), I'd use a lower gear to accelerate past that speed, and coast in neutral or EOC through that 1200 rpm dead spot.

The way I'd handle a 30 mph cruise, I'd pulse from 25 mph to 35 mph in 4th gear (about 1500-2000 rpm), shift to neutral (either NICE or EOC), and glide back to 25. I wouldn't risk lugging the engine in 5th or 6th gear at 30 mph. 4th gear gives you more acceleration ability.

You might find this article interesting for ideas on how to wring best mileage out of your 3.
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Old 02-06-2010, 09:02 PM   #3 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
I've heard, but can't verify the truthfulness of the claim, that modern engine control computers won't allow you to damage an engine. They supposedly shut the engine down, rather than allow it to run under too much load.

My SE-R's engine/transmission combo is similar to yours, a 2.5L, 170 hp, 6 speed. I use 6th gear from 40-45 mph upwards, on level ground. I have noticed my cruise control will kick me out when I'm going up steep hills at 50 or 55 mph. IIRC, the engine is still on, and I'm left in 6th gear under minimum load, with speed steadily dropping, forcing a downshift.

1200 rpm is probably too low an engine speed for cruising. Your engine's BSFC graph should point you to where you want to be. Usually, your best efficiency is between about 1500 and 2200 rpm for 2 liter class engines. Rather than cruise at 1200 rpm (30 mph in 6th?), I'd use a lower gear to accelerate past that speed, and coast in neutral or EOC through that 1200 rpm dead spot.

The way I'd handle a 30 mph cruise, I'd pulse from 25 mph to 35 mph in 4th gear (about 1500-2000 rpm), shift to neutral (either NICE or EOC), and glide back to 25. I wouldn't risk lugging the engine in 5th or 6th gear at 30 mph. 4th gear gives you more acceleration ability.

You might find this article interesting for ideas on how to wring best mileage out of your 3.
Oh Okay, thanks for the reply. I hope it`s true your car wont let you damage your engine cause I`ve been doing this for a while, but usually I`m around 1400 and the engine seems pretty comfortable at 1200 unless i try to accelerate which then I`d down shift. and damn what mods do you have on your sentra thats a high mpg
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Old 02-06-2010, 11:27 PM   #4 (permalink)
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No mods. I drive slow - either cruise control at 55 mph, or pulse and glide between 52 and 65 mph on highways. In town I use the speed limit as the upper end of my pulse and glide range. I get my mpg gains by modifying my driving technique.
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Old 02-07-2010, 03:43 AM   #5 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
No mods. I drive slow - either cruise control at 55 mph, or pulse and glide between 52 and 65 mph on highways. In town I use the speed limit as the upper end of my pulse and glide range. I get my mpg gains by modifying my driving technique.
Some of the best advice you'll get here.
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Old 02-07-2010, 11:44 AM   #6 (permalink)
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Some of the best advice you'll get here.
Yep, now I'm just more carefull about my low revs. I really hope i did no harm to my engine by cruising at 1.2k sometimes my car still only has 3.5k km so i don't wanna mess it up!
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Old 02-07-2010, 06:47 PM   #7 (permalink)
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Regarding loading (given air/fuel ratios remain constant), as rpm's decrease turbulence within the combustion chambers decrease, flame front speed decreases, and required timing lead increases. The end result - more negative work applied to the reciprocating assembly, more wasted work than driving higher rpm. This is a general rule with 1000's of variables, but there is a window where your loading/rpm relationship yields the lowest bsfc. Finding that window is the hard part.

It's doubtful you've done any measurable damage to your engine unless you've been driving it like that when it's cold for a while.
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Old 02-07-2010, 07:08 PM   #8 (permalink)
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Quote:
Originally Posted by greasemonkee View Post
Regarding loading (given air/fuel ratios remain constant), as rpm's decrease turbulence within the combustion chambers decrease, flame front speed decreases, and required timing lead increases. The end result - more negative work applied to the reciprocating assembly, more wasted work than driving higher rpm. This is a general rule with 1000's of variables, but there is a window where your loading/rpm relationship yields the lowest bsfc. Finding that window is the hard part.

It's doubtful you've done any measurable damage to your engine unless you've been driving it like that when it's cold for a while.
I sometimes do, however I'm not always under 1.5km either. And if I am i don't really step on the gas pedals so I don't hear any vibration, im barely stepping on it, you think that should help?
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Old 02-08-2010, 03:05 PM   #9 (permalink)
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I regularly pulse starting as low as 1200 rpm. My driving is all pulse & glide, as well, so that happens often. Cruising is much lighter load - I sometimes cruise down below 1000 rpm with light load. 175,000 miles and my engine is doing fine.

I like to look at the rpm and gear selection from a miles / engine revolution perspective. The higher the gear you can run, the fewer engine cycles per distance traveled.
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Old 02-08-2010, 04:01 PM   #10 (permalink)
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If you are pushing in the gas and the engine is shuddering your RPM is too low. The oil film that hangs out in your bearings can get compressed out of the bearings and you will have increased wear. I lope along at 1000 rpm frequently l, 5th gear and mph, the engine is smooth so no worries. If I try to increase my speed and the engine vibrates it means I am applying too much pressure to the accelerator or I need to down shift.

An engine is a very robust thing, you really have to be abusive to do much damage. If you are being abusive to it, it will complain if you are listening for it.

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