WOT and High Load

[buster]

Forgive me for my stupidity when it comes to this stuff. I'm not as savy as many of you when it comes to engines.

I'm trying to understand when the greatest load is put on an engine? I've been reading some posts on various forums about engine break-in and load. I always wondered how you can give the engine high load, but keep the rpms below 4k or so to comply somewhat with the owner's manual.

I have a manual transmission so would flooring it in 2nd gear until the rpms hit 4k be considered "high load"? I thought increasing the rpms through the gears was high load but according to some it's best to start with the rpms low, and floor it until about 4k rpms. With an automatic, if you WOT you automatically hit the redline almost. With a manual, it seems much easier to WOT without raising the rpms. Does this make any sense?

 

[labman]

Lugging the engine, trying to accelerate or climb a hill at such a low RPM that the engine has little power puts large loads on the bearings. You need to down shift and bring the RPM up to where the engine can handle the load. On the other hand, even an unloaded engine has high loads on the bearings at higher RPM's. Perhaps best to confine WOT to accelerating down hill at mid range.

 

[brianl703]

Perhaps the easiest way to think about engine load is to consider how far down the gas pedal needs to be pushed to maintain a given engine speed.

At low engine loads, the extreme of which would be when the trans is in neutral, it takes very little throttle to achieve high engine speeds.

At high engine loads, the extreme of which would be when the trans is in top gear and you are going uphill, it takes a greater throttle opening to achieve high engine speeds. If you are driving an automatic, pushing the gas pedal beyond a certain point will make it downshift. (The logic of an automatic transmission is always to downshift once the load exceeds a certain point, that is why it is easier to WOT without raising the RPMs when you have a manual. Some automatics may allow you to override the automatic downshifting when load exceeds a certain amount, but that capability seems very rare).

You could also think about load in terms of manifold vacuum. Lower manifold vacuum=higher load. At WOT, the manifold vacuum is as low as it's going to get, so going WOT puts a heavy load on the engine.

You could also get a Scanguage..it'll tell you the engine load among a bunch of other things.

Finally, I should mention that back in the old days of non-electronically controlled transmissions, there were two main ways that the automatic knew to downshift due to higher load: Lower intake manifold vacuum (sensed through a "vacuum modulator"), and a throttle valve cable which tells the transmission when the gas pedal has been pushed beyond the point at which it should invoke a downshift.

 

[buster]

Great responses. Thanks. I was always confused on this topic.