Home » Public Forums » GMCnet » Re: [GMCnet] GMC killers- Steep-Long Grades
| Re: [GMCnet] GMC killers- Steep-Long Grades [message #94545] |
Wed, 04 August 2010 00:04  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
|
Senior Member |
|
|
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
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
| Re: [GMCnet] GMC killers- Steep-Long Grades [message #94576 is a reply to message #94545] |
Wed, 04 August 2010 09:34  |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
|
|
With a greater volume of air/fuel mixture in the cylinder/combustion chamber
at low revs wide open throttle, the loads on the piston crowns, rings and
lands, thrust faces, piston pin bearings, connecting rods, inserted rod and
main bearings, crankshaft journals, main bearing webs and so forth are
substantually greater than when the throttle is not wide open. All these
loads can be handled by the components mentioned for short durations quite
easily and engineered to do just that. When long, steep grades are
encountered by a heavily laden coach, the thermal load on lubrication and
cooling systems weighs heavily into this equation. If torque converter
slippage is greater at lower rpms, then the heat produced by this action
also has to be factored in. Now figure in the problem of low octane fuel and
detonation and the problem comes into focus. Reduce the load by changing
gears from high to second and increase the coolant flow with the added rpms
and the problem becomes manageable. 3:07 gears in the final drive were great
in a 4000# Toronado but are too tall for a 12,000+lb vehicle. Shorter
gearing in the final drive helps reduce the load, but can have the effect of
lowering the top speed of the coach somewhat depending on ratio. My 78
Royale with a 403 and stock gearing on flat level I 5 will easily go faster
than 80 mph. How much faster do we need to drive these things? Shift em
down, and gear the final drive correctly for the engine and total combined
vehicle weight, the engine life will not be adversely affected though fuel
economy could be. Just one mans opinion who has rebuilt many of these 455s
for service in fishing boats. They don't like it when you lug them, even
when you've got the whole damn ocean to cool them with.
Jim Hupy
Salem, Or
78 Royale 403
On Tue, Aug 3, 2010 at 10:04 PM, Gary Casey <casey.gary@yahoo.com> wrote:
> 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
>
>
>
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
| Re: [GMCnet] GMC killers- Steep-Long Grades [message #94580 is a reply to message #94576] |
Wed, 04 August 2010 10:41 |
gmcrv1
Messages: 839
Registered: August 2007
Location: Memphis
Karma: -1
|
Senior Member |
|
|
Jim,
My coach (73 - 26') has the 455 with the 3.07. At what speed on a climb
should I be looking to safely downshift? Or engine RPMs? Coach is not on
the road yet (interior work delay) I live in west Tennessee and can safely
say my trips should be reasonably level and less then 500 miles until the
Glacier proves it's reliability.
Will not be towing anything for a while.
Thanks,
Tom Eckert N2VWN
Oakland, TN
On Wed, Aug 4, 2010 at 9:34 AM, James Hupy <jamesh1296@gmail.com> wrote:
> With a greater volume of air/fuel mixture in the cylinder/combustion
> chamber
> at low revs wide open throttle, the loads on the piston crowns, rings and
> lands, thrust faces, piston pin bearings, connecting rods, inserted rod and
> main bearings, crankshaft journals, main bearing webs and so forth are
> substantually greater than when the throttle is not wide open. All these
> loads can be handled by the components mentioned for short durations quite
> easily and engineered to do just that. When long, steep grades are
> encountered by a heavily laden coach, the thermal load on lubrication and
> cooling systems weighs heavily into this equation. If torque converter
> slippage is greater at lower rpms, then the heat produced by this action
> also has to be factored in. Now figure in the problem of low octane fuel
> and
> detonation and the problem comes into focus. Reduce the load by changing
> gears from high to second and increase the coolant flow with the added rpms
> and the problem becomes manageable. 3:07 gears in the final drive were
> great
> in a 4000# Toronado but are too tall for a 12,000+lb vehicle. Shorter
> gearing in the final drive helps reduce the load, but can have the effect
> of
> lowering the top speed of the coach somewhat depending on ratio. My 78
> Royale with a 403 and stock gearing on flat level I 5 will easily go faster
> than 80 mph. How much faster do we need to drive these things? Shift em
> down, and gear the final drive correctly for the engine and total combined
> vehicle weight, the engine life will not be adversely affected though fuel
> economy could be. Just one mans opinion who has rebuilt many of these 455s
> for service in fishing boats. They don't like it when you lug them, even
> when you've got the whole damn ocean to cool them with.
> Jim Hupy
> Salem, Or
> 78 Royale 403
>
> On Tue, Aug 3, 2010 at 10:04 PM, Gary Casey <casey.gary@yahoo.com> wrote:
>
> > 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
> >
> >
> >
> >
> > _______________________________________________
> > GMCnet mailing list
> > List Information and Subscription Options:
> > http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
> >
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
|
|
|
|
|
|
|
|
Goto Forum:
Current Time: Sun Sep 22 18:32:56 CDT 2024
Total time taken to generate the page: 0.00757 seconds
|
GMCforum
Search
Help
Register
Login
Home
