Home » Public Forums » GMCnet » New Manny Tranny on hills (Question for Matt Colie re: engine "lugging")
| Re: New Manny Tranny on hills [message #195339 is a reply to message #195317] |
Thu, 10 January 2013 09:36   |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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| gbarrow wrote on Wed, 09 January 2013 21:03 |
Back in August I posted the following:
"Coach, 1976 Palm Beach has 455 w/carb and 3:70. coach weighs~11,000 lbs.
Northbound on I-5 between the Oregon border and Canyonville (about 100 miles) you encounter Siskiyou Summit 4310' (highest point on I-5) 7 miles of 6% grade on both sides,
Sexton Pass (named for GMCer Dennis Sexton?) 1960', Smith Hill Summit 1730', Stage Road Pass 1830', Canyon Creek Pass 2020' the approaches to each of these passes have one to three miles of 6% grade
All of these hills except Canyon Creek Pass required downshifting to second for part of the climb. .
When speed drops below 50 mph and rpm drops below 2500 and vacuum is 5" or less I manually shift to 2nd."
In early November my old tranny died and I installed a new Manny Tranny and have now driven it almost 2500 miles, including a trip from Red Bluff, Ca. to Hillsboro, Ore. and back over the above mentioned grades/passes.
With the new Manny Tranny the coach runs "stronger' feels "lighter", peppier, more responsive to the throttle. Not my imagination.
On the trip to Oregon with one exception the coach made all these grades without falling below below 48 mph or 2400 rpm in 3rd gear. So I didn't manually downshift
The exception was the southbound 7 mile 6% grade.
Near the top of the hill the tranny automatically downshifted to 2nd at about 42mph and 2200 rpm.
So the question for Matt Colie or any one else who knows is this: At what speed/rpm at WOT on a long uphill grade does "lugging" occur and become detrimental to the engine or transmission? Or: Given the ~2700 rpm most efficient power point for the 455, what is the lowest "safe" rpm for the engine/tranny under heavy load?
Any Oregon Netters want to report their experience over these passes?
Thanks to all for your input.
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Gene,
First I have to say that most of what has gone by is correct. The fact that you have a 3.70 is really not going to change much in a real sense, but your road speeds will be much more real world than most. In the current world 3.07 is just plain wrong.
Executive Summary:
Pull it down to stay out of the secondaries and you will be alright. If you get near 4K, let it back up.
George's remark that the torque converter is a big part of the issue is very correct. And that stall test is something you should do. (It takes 30 seconds the next time the engine is well warmed up.) Avoiding running below stall is quite important. With a 3.07, this can be tough. Mine is right there at 60MPH.
Generally speaking, it is tough to lug an engine into a damaging region with the calibration of most automatic transmissions. It is true that you do not want to go into the secondaries if you can avoid it for two reasons, the first is straight economy - the secondaries are usually set up for rich for max power and second is that this calibration is not intended for extended operation. (We used to call it WOT-ZOOM mode.)
The "automatic" downshift happens one of two ways, either the "kick-down" switch attached to the throttle linkage or the vacuum modulator. I am told that neither is the best mode and manually pulling the transmission down is a far better way to get things in the correct band.
If we can get back to the engine speeds and related things, let me start by saying that I very little real data to work from here. I have to go the power curve data that I have seen and that indicates a in the 24~2600RPM region for both the 455 and 403. With the torque converter going start to slip (mine starts a ~2K) and with real lugging probably down below 1500 range, there is just no reason at all to run the engine at those speeds. While the timing is still on the centrifugal advance, there is no bet that it is close to best at this load.
Damage from lugging an occurs two ways, either from the fuel/spark calibration being way out for the load/speed or the cylinder pressure being too high for the bearings to work correctly. With the 455, the later is just not going to be an issue. The relatively large bearing for the class is why this was a preferred industrial conversion for many years. But mixture and timing are another bet. If you are climbing and you hear it cackle, back out and pull it down to second right then.
There a lot here, and I hope the questions were answered. If not, well I'm here a lot. (When I am not in/under the coach.)
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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