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Unread 01-03-2011, 11:59 AM   #1
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Default A couple questions

I was talking to someone about old car 'wives tales' and a couple things came up that I've always wondered.

1. Is it bad for your motor to drive it hard and rev high when recently started in the morning?

2. Is it bad to drive high speed in 4wd? Obviously not regularly, but I often drive country roads engaged because some spots have drifts. Is a little bare pavement that bad?
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Unread 01-03-2011, 02:05 PM   #2
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1. Yes. Cold metal is brittle metal, also, the tolerances are much tighter when it's cold. It's harder for the much thicker cold oil to get into the bearings and protect everything. Some engines will literally explode if you beat on them when cold, most will experience a shortened life. Doing this is a good way to get a 350 to throw a rod before 200,000 miles.

2. That is what broke my Dakota the first time. Someone drove it on pavement in 4-hi. It didn't like this. The reason is the front and rear axles want to spin at different speeds due to minor variations in tire diameter. Off road or on snow/ice, one axle slips a bit and all is fine. But on pavement there's too much grip for that, so something else has to go. That something else can be anything from the CV joints to the bellhousing, whatever part of the driveline is weakest. AWD is different in that it has a center diff, which allows both axles to spin at differing speeds while also driving them. That's why Subies, lambos, etc etc don't have exploding drivetrain issues that pickup trucks do.
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Unread 01-03-2011, 02:19 PM   #3
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Well said both. My jeep says in the Manuel 4wd not for on road use only for loose gravel or wet slippery roads.
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Unread 01-03-2011, 02:31 PM   #4
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Agree with above, and I might add some more.

The piston rings expand to the designed gap when the engine is up to temp. When the engine is cold the ring gap is much greater allowing blow-by to happen much more easily. As the load increases blow-by increases sending combustion waste into your crankcase. This contaminates the oil much sooner and is hard on engine bearings.

Also oil moves much slower through the small oil galleries when cold and may starve critical parts when the RPM is increased.

Also the head gasket to block & head is designed to seal its best at operating temp. If you push the engine when cold it can shorten the life of the head gasket as the head may warp as you increase the load suddenly, creating hot spots, before the whole thing is up to temp.

I don't condone idling an engine to get it up to temp before driving, but I run it long enough to get the oil circulating before I move off, about 30 seconds is all that's needed when it's quite cold. Driving it easy until it's up to temp warms the engine up much quicker than idling anyways.

Driving on a hard surface in 4wd is a no-no as described above. Driving at high speed, say over 30 mph is not good unless you have U-joints on the front drive shaft that are designed for high speed. Most 4wd vehicles use a simple cross type U-joint for the front. These are not good for going fast. AWD vehicles use a CV type joint on the front shaft, these are OK. When I did my conversion to AWD I had to adapt an AWD front shaft to my vehicle.
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Unread 01-03-2011, 04:48 PM   #5
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Is a cross type u-joint different from what's on the rear?

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Unread 01-03-2011, 07:48 PM   #6
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That's about what i figured.

About the 4x4, i don't drive it on dry pavement in 4x4, that would be redundant. But sometimes there are bare spots on the road, i didn't think that would hurt it.
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Unread 01-03-2011, 08:03 PM   #7
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Well the cross type U-joint on the rear drive shaft works because it is aligned correctly so the 2 U-joints cancel each other, no vibration.

The front drive shaft on an S-truck is NOT aligned because the U-joint angles are out because of the layout, it will vibrate when the shaft is run too fast.

The drive shaft of an AWD S-truck like mine works because it uses a double-cardon joint at the transfer case and a CV joint at the diff.

Skipping over a short section of dry pavement is OK, but what for shuddering or that the truck doesn't want to roll easily, binding. If you keep the wheels straight and not cut a tight turn you will be OK.
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Unread 01-03-2011, 08:39 PM   #8
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Just crossing short patches? Yeah you'll be fine. The damage comes when you drive for a prolonged amount of time on dry pavement with 4WD engaged. Several miles.
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Unread 01-04-2011, 01:06 AM   #9
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For number one I can fully back up what Kenny and Car Guy said. Like said, a cold engine is a fragile engine, so revving the crap out of it is not a good idea at all. Probably not good for the transmission either. My neighbor busted his car like that and had to spend a ton of money to fix it because he'd drive it hard before it warmed up. Normal driving on cold start is acceptable, especially on more modern cars, but yeah, above is correct.

For number two, well its been said and it makes a lot of sense as to why you shouldn't drive on dry pavement in 4WD. Which actually I did not know myself so good info to be learned here .


Also if its okay I'd like to ask a question myself, answer only if it wouldn't be considered hijacking the topic. My question is, what exactly does an engine have to go through to overheat? For instance, would keeping it at a low gear like second while going like 35mph for like 10 minutes cause damage and overheating. I'm asking because I've noticed that when cars go fast they stay at highest rpm for a split second between shifting, so what if this was prolonged?
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Unread 01-04-2011, 01:39 AM   #10
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I'm assuming that you mean what would it take to overheat an engine with no issues.

The manufacturer designs the vehicle to be moving a minimum speed for the work it is doing. Increase either the load or the speed beyond what is designed into the vehicle and overheating is very possible.

Road speed is needed for enough air flow for adequate cooling. Running an engine at high RPM at too low a road speed will increase the load without enough air flow to cool. Increase the load without enough RPM increases load with inadequate coolant flow to cool.
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