| Re: [GMCnet] GMC killers- Steep-Long Grades [message #94545] |
Wed, 04 August 2010 00:04  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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I agree with most of what Rick said, but maybe for different reasons.
Certainly the main and rod bearings operated with less minimum clearance at
lower rpms - the hydrodyamic forces are better able to keep the journal centered
at high rpm. But on the other hand, the capacity of the bearings are plenty
adequate at low rpms. Many of these similar engines could be run at full
throttle at less than 1,000 rpm with no problem. And all the oil pump has to do
is deliver enough oil to make up for what is "squished" out from the bearings -
it has far more capacity to that than what is required.
Yes, the transmission line pressure is increased, probably from about 120 psi to
about 150 when in a manually-selected lower gear. But the clutches have plenty
of capacity to hold the torque without slippage, so the extra pressure doesn't
do much. It does slightly increase the cooling oil flow, and that is a good
thing. However, it increases the potential wear on the pump, and that's a bad
thing. Not enough to worry about, though.
Yes, I also don't depend on the automatic shift schedule to downshift the
transmission at the right time. As I showed on the shift schedule I published,
these transmissions have a very inhibited downshift schedule. Manually
downshifting is a regular thing with me.
Perhaps the most important reason to keep the engine speed up is to increase the
torque converter efficiency. With the engine running at 2,000 rpm and nearly
full throttle I would be surprised if the torque converter efficiency would be
above 90%. Putting 10% of the engine's power into the transmission oil is a
real drag. Downshift to second and the efficiency is likely going to jump to
95%.
Gary Casey, a repeat downshifter
>
> I don't understand Roger's statement "don't let the RPM's drop below
> 2500-2700 even if you go to a lower gear manually".
>
> I can understand overrevving is bad but what is the danger in low rev's?
>
Remember that the oil pump is only turning at half the RPMs of the
crankshaft.
At lower RPMs, you are having to apply more torque--force on the piston
heads and the connecting rods (and the bearings). At higher RPMs, you are
applying less force more frequently, and with a better supply of oil.
Ride a bicycle up a steep hill in a high gear, versus riding it in a lower
gear, and your knees will explain it to you.
I keep my engine at 2000 RPMs when applying lots of power. I do not depend
on the kick down to do so on a long hill. I downshift manually. That has the
added benefit of maintaining a higher pressure on the bands in the
transmission.
On my semiannual climb up to Flagpole knob, the roughness of the road
prevents going fast enough to maintain 2000 RPMs. But that roughness also
prevents applying full power. On the paved bit before the jeep trail,
though, I am in L by the time I get to the top, with the engine still able
to accelerate from the middle 2000's.
Rick "not scared to downshift" Denney
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