Home » Public Forums » GMCnet » Another newb question
| Another newb question [message #187832] |
Sun, 21 October 2012 00:45  |
gibsongo
Messages: 116
Registered: October 2012
Location: Montreal West, Quebec, Ca...
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Thanks to those who answered my "newb" questions about water heaters and the GMC.
I have another fact finding question related to the transmission.
If my understanding is correct, the "stock" transmission for a GMC is a three speed.
I have a three speed on my Empress Triple E (powered by a big block Ford - 460 I think).
In the interests of bringing cruising RPM down to save gas and reduce noise, I installed a Gearmasters overdrive (it bolts on after the regular transmission and requires shortening the drives-shaft). Once installed, it did a great job, bring my revs down to about 2,000 at 60 mph.
Is there a "doable" swap that would put a 4 speed or a 3 speed with overdrive tranny in a GMC? I assume a solution like the Gearmasters overdrive is out because of FWD.
The purpose would be the same: fuel economy and more relaxed highway cruising.
Thanks in advance
Gordon Gibson
Montreal West, Quebec
Canada
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
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| Re: Another newb question [message #187833 is a reply to message #187832] |
Sun, 21 October 2012 01:34  |
George Beckman
Messages: 1085
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Location: Colfax, CA
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| gibsongo wrote on Sat, 20 October 2012 22:45 |
Thanks to those who answered my "newb" questions about water heaters and the GMC.
I have another fact finding question related to the transmission.
If my understanding is correct, the "stock" transmission for a GMC is a three speed.
Is there a "doable" swap that would put a 4 speed or a 3 speed with overdrive tranny in a GMC? I assume a solution like the Gearmasters overdrive is out because of FWD.
The purpose would be the same: fuel economy and more relaxed highway cruising.
Thanks in advance
Gordon Gibson
Montreal West, Quebec
Canada
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Would that it were possible. There are a couple of problems. First, there is almost no room for any other transmission. Second, our transmission spins backwards from other transmission. The case is built to take the stress of turning backwards.
What I am about to say goes against what all GMC wisdom says. I have a 2.73 final drive. I didn't mean to end up with one, but anyway, I did. On our recent trip, Oregon coast, western Washington, Montanna, Banff, Dakotas, Iowa and then back on 80 to CA, we improved our MPG by over one mpg.. We got 10.7 at the pump vs actual miles traveled. I cruise at 62 at 2175 RPMs and it is quiet and I have to say I may not undo the mistake right away. I was not towing. (I also have a well tuned EFI with spark control and EBL which allows lean cruise, but I have had that when I got 9.8 and less. I do hav a switch pitch transmission which essentially gives me another gear of sorts.
I like you have always run my vehicles with high gears. ('37 Ford pickup with a Mitchell 36% overdrive)
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
[Updated on: Mon, 22 October 2012 08:43] Report message to a moderator |
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| Re: Another newb question [message #187899 is a reply to message #187832] |
Sun, 21 October 2012 19:45 |
kingd
Messages: 592
Registered: June 2004
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Gordon in Montreal, Hi from David down the 401 in Toronto. The stock "final drive" ratio as from the factory is 3.07:1. This is MUCH taller than the final drive ratio that comes "stock" on almost (probably all) all other motorhomes built on front engine rear wheel drive gas chassis. I know there are a number of people on here with various final drive ratios and tachometers. Perhaps some will chime in with RPM, final drive ratio and speed. Gordon, I think you'll be surprised at the low RPM of a stock GMCMH at normal highway speeds. (not counting the states with 75 MPH limits)
DAVE KING
lurker, wannabe
Toronto, Ontario, Canada
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| Re: Another newb question [message #187922 is a reply to message #187832] |
Sun, 21 October 2012 22:39 |
homer
Messages: 73
Registered: April 2011
Location: arizona/ BC canada
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Being some-what new to my GMC,and not knowing the gear ratio in my tranny,on a standard 2.73 what rpm should you be running at 60 mph, and what is a switch pitch transmission?. Homer
homer
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| Re: [GMCnet] Another newb question [message #187925 is a reply to message #187922] |
Sun, 21 October 2012 22:55 |
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USAussie
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Registered: July 2007
Location: Sydney, Australia
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G'day,
GMC Motorhomes came with 3.07 to 1 final drives that were the same as those in a Olds Toronado. AFAIK they did not come with a 2.37
to 1 from the factory. It is possible that a PO put one in.
The switch pitch transmission came on 1966 and 1967 Olds Toronados. It has a torque convertor with vanes that move and when they do
the car does too! ;-)
Basically they provide extra torque when the vanes switch. The downside is that the transmission generates a huge amount of heat
when in they do.
Regards,
Rob M.
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of les holmes
Sent: Sunday, October 21, 2012 9:40 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Another newb question
Being some-what new to my GMC,and not knowing the gear ratio in my tranny,on a standard 2.73 what rpm should you be running at 60
mph, and what is a switch pitch transmission?. Homer
--
homer
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Another newb question [message #187942 is a reply to message #187832] |
Mon, 22 October 2012 07:44 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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Unless you have a lightweight 23' like mine and drive in the flat country, you're going to find the stock (3:07-1) final ratio is really a bit too relaxed. There are many setups to increase the numeric ratio, both by new innards in the drive case and by playing with the drive chain sprockets. Speak to JimK of Applied and Manny of Manny's Trannys for the scoop on the ways to do this. Without major machine work, replacing the THM425 isn't going to happen. It works, hang on to it. There isn't any place under there to fit an overdrive, nor does the coach need one anyway. If it were easy. both of mine would fit TorqueFlites as fitted behind the 426CID engine.
--johnny
'76 23' transmode norris
'76 palm beach
From: Gordon Gibson <gordon.gibson@videotron.ca>
To: gmclist@temp.gmcnet.org
Sent: Sunday, October 21, 2012 1:45 AM
Subject: [GMCnet] Another newb question
Thanks to those who answered my "newb" questions about water heaters and the GMC.
I have another fact finding question related to the transmission.
If my understanding is correct, the "stock" transmission for a GMC is a three speed.
I have a three speed on my Empress Triple E (powered by a big block Ford - 460 I think).
In the interests of bringing cruising RPM down to save gas and reduce noise, I installed a Gearmasters overdrive (it bolts on after the regular transmission and requires shortening the drives-shaft). Once installed, it did a great job, bring my revs down to about 2,000 at 60 mph.
Is there a "doable" swap that would put a 4 speed or a 3 speed with overdrive tranny in a GMC? I assume a solution like the Gearmasters overdrive is out because of FWD.
The purpose would be the same: fuel economy and more relaxed highway cruising.
Thanks in advance
Gordon Gibson
Montreal West, Quebec
Canada
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Another newb question [message #187943 is a reply to message #187832] |
Mon, 22 October 2012 07:48 |
Steven Ferguson
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Registered: May 2006
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No.
On Sat, Oct 20, 2012 at 9:45 PM, Gordon Gibson
<gordon.gibson@videotron.ca>wrote:
>
>
> Thanks to those who answered my "newb" questions about water heaters and
> the GMC.
>
> I have another fact finding question related to the transmission.
>
> If my understanding is correct, the "stock" transmission for a GMC is a
> three speed.
>
> I have a three speed on my Empress Triple E (powered by a big block Ford -
> 460 I think).
>
> In the interests of bringing cruising RPM down to save gas and reduce
> noise, I installed a Gearmasters overdrive (it bolts on after the regular
> transmission and requires shortening the drives-shaft). Once installed, it
> did a great job, bring my revs down to about 2,000 at 60 mph.
>
> Is there a "doable" swap that would put a 4 speed or a 3 speed with
> overdrive tranny in a GMC? I assume a solution like the Gearmasters
> overdrive is out because of FWD.
>
> The purpose would be the same: fuel economy and more relaxed highway
> cruising.
>
> Thanks in advance
>
> Gordon Gibson
> Montreal West, Quebec
> Canada
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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health insurance......but not everyone has to prove they're a citizen.
Steve Ferguson
Sierra Vista, AZ
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| Re: Another newb question [message #187948 is a reply to message #187922] |
Mon, 22 October 2012 08:40 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
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| homer wrote on Sun, 21 October 2012 20:39 |
Being some-what new to my GMC,and not knowing the gear ratio in my tranny,on a standard 2.73 what rpm should you be running at 60 mph, and what is a switch pitch transmission?. Homer
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The switch pitch is a variable stall torque converter. I believe that the original was a stall of about 2200, meaning if yo lock the brakes an give it lots of gas, it goes to 2200. The SP is a stall of 1800 or a tad less and 2600 when in "switch".
Yeah, other guys have told me it should run slower RPMs with a 2.73. All I know is I finally had to get an 11% speedo increase to get the speedo back to close. My speedo has always been about 3 mph slow at 60 when it had the 3.07 stock final drive and 16.5" wheels. Now I have 16s and equivalent tires. The numbers I gave were with the gps speed and digital tach.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: Another newb question [message #187949 is a reply to message #187832] |
Mon, 22 October 2012 08:41 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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Oh Boy,
There is so much smoke here that I am not sure where to throw the water....
When out coaches were designed and first manufactured, the interstate speed limits were 70 in lots of places and people were talking about increasing them. Then came the first arab oil embargo and the Carter 55 energy speed limit. The thought in the industry was that this would be more temporary than it turned out to be. Keep that in mind.....
First, an automatic transmission should not be run close to the stall speed of the torque converter. If it is run below the stall speed, most of the excess horsepower gets turned into heat and this is wasteful. That heat also shortens the life of the operating fluid. I am sure that this is why the owner's manual requires a transmission fluid change at 25K mile when passcars of the era were all at a minimum of 50K.
Second, for best fuel rate most SI engines don't want to cruise very far from 10% below the torque peak. The torque peak is where the engine's volumetric efficiency is highest. (That means the most air is pumped at wide open throttle.) This applies even with reduced manifold pressure (less than WOT) operation. If you get far from that either way, the airflow gets worse and you pay for it with fuel. The information I have puts the torque peak in the 2200~2400 region with a pretty flat peak.
Then, there is the poor engine itself to consider. Bearings do not work because oil is pumped into them, they work because the oil gets pulled into the loaded area by the rotational velocity of the shaft in the journal (shell). The motion builds an "hydraulic wedge" that is what actually supports the load. If the rotational velocity is too little, the lube oil will not be effectively pulled into the load area and bearing damage may result. Translation: Lugging any engine is a really bad idea.
I have a very light (9400#) coach. At 60 MPH, the cheezy little tach says it is running about 2000 RPM. If we slow down to 55, the engine drops to about 1900 RPM, but as soon as you touch the throttle, it is back to 2000+. That tells me that the converter was below the stall region (not near lockup). So, we try to run 60+. If I can ever afford to do so, I will install a 3.55 for the benefit of both the engine and the transmission.
It is just a guess, but I believe that the road load of our coaches is in the range of 30HP (A large passcar of the period was about 20 - today's cars are 12~15). That means one could run with an engine speed of 1750~1800 on flat level ground with no wind with the OE converter. But, even Ohio isn't that flat and that calm. Any grade or headwind would start the converter slipping and that is just not a good place on the curve to be running for any length of time.
Knowing all that and reading your post, I can tell you that if you were hoping the overdrive will get you a quieter cab, you would be very disappointed. If you really want a quieter cab, first, get a working fan clutch. It is amazing the difference that will make. Next, put sound insulation either under the cab floor or under the carpet.
When things are right, we can carry on a conversation in the cab at living room levels. Well, I can, but Mary still has to shout a little, but that is true of the living room too and there isn't anything to be done about that.
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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| Re: Another newb question [message #187952 is a reply to message #187922] |
Mon, 22 October 2012 08:47 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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| homer wrote on Sun, 21 October 2012 20:39 |
Being some-what new to my GMC,and not knowing the gear ratio in my tranny,on a standard 2.73 what rpm should you be running at 60 mph, and what is a switch pitch transmission?. Homer
|
Homer , about 2100. I looked back and I said I was doing 1275 at 63 and that was a transposed number... 2175 . I have corrected the original post. But as noted a 2.73 was not originally put in motor homes.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: Another newb question [message #187986 is a reply to message #187970] |
Mon, 22 October 2012 14:33 |
habbyguy
Messages: 896
Registered: May 2012
Location: Mesa, AZ
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| Harry wrote on Mon, 22 October 2012 09:46 |
There is nothing "normal" about our GMC's.
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... or most of us, for that matter. Seems like a match made in heaven!
Mark Hickey
Mesa, AZ
1978 Royale Center Kitchen
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| Re: Another newb question [message #187995 is a reply to message #187832] |
Mon, 22 October 2012 15:45 |
homer
Messages: 73
Registered: April 2011
Location: arizona/ BC canada
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Thanks for the feed back, I am still trying to figure out what I really have for gear ratio, the original owner from Florida spent a lot-lots of bucks on this coach.I am running on 16.5 rims&tyers clock speed at 60 is 68.5 on GPS Tack is off the wall need to replace,climbs hills like a stallion.Great coach. Lester
homer
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| Re: Another newb question [message #188014 is a reply to message #187949] |
Mon, 22 October 2012 20:05 |
Larry
Messages: 2875
Registered: January 2004
Location: Menomonie, WI
Karma: 10
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| Matt Colie wrote on Mon, 22 October 2012 08:41 |
Oh Boy,
There is so much smoke here that I am not sure where to throw the water....
When out coaches were designed and first manufactured, the interstate speed limits were 70 in lots of places and people were talking about increasing them. Then came the first arab oil embargo and the Carter 55 energy speed limit. The thought in the industry was that this would be more temporary than it turned out to be. Keep that in mind.....
First, an automatic transmission should not be run close to the stall speed of the torque converter. If it is run below the stall speed, most of the excess horsepower gets turned into heat and this is wasteful. That heat also shortens the life of the operating fluid. I am sure that this is why the owner's manual requires a transmission fluid change at 25K mile when passcars of the era were all at a minimum of 50K.
Second, for best fuel rate most SI engines don't want to cruise very far from 10% below the torque peak. The torque peak is where the engine's volumetric efficiency is highest. (That means the most air is pumped at wide open throttle.) This applies even with reduced manifold pressure (less than WOT) operation. If you get far from that either way, the airflow gets worse and you pay for it with fuel. The information I have puts the torque peak in the 2200~2400 region with a pretty flat peak.
Then, there is the poor engine itself to consider. Bearings do not work because oil is pumped into them, they work because the oil gets pulled into the loaded area by the rotational velocity of the shaft in the journal (shell). The motion builds an "hydraulic wedge" that is what actually supports the load. If the rotational velocity is too little, the lube oil will not be effectively pulled into the load area and bearing damage may result. Translation: Lugging any engine is a really bad idea.
I have a very light (9400#) coach. At 60 MPH, the cheezy little tach says it is running about 2000 RPM. If we slow down to 55, the engine drops to about 1900 RPM, but as soon as you touch the throttle, it is back to 2000+. That tells me that the converter was below the stall region (not near lockup). So, we try to run 60+. If I can ever afford to do so, I will install a 3.55 for the benefit of both the engine and the transmission.
It is just a guess, but I believe that the road load of our coaches is in the range of 30HP (A large passcar of the period was about 20 - today's cars are 12~15). That means one could run with an engine speed of 1750~1800 on flat level ground with no wind with the OE converter. But, even Ohio isn't that flat and that calm. Any grade or headwind would start the converter slipping and that is just not a good place on the curve to be running for any length of time.
Knowing all that and reading your post, I can tell you that if you were hoping the overdrive will get you a quieter cab, you would be very disappointed. If you really want a quieter cab, first, get a working fan clutch. It is amazing the difference that will make. Next, put sound insulation either under the cab floor or under the carpet.
When things are right, we can carry on a conversation in the cab at living room levels. Well, I can, but Mary still has to shout a little, but that is true of the living room too and there isn't anything to be done about that.
Matt
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One additonal consideration is Oil Pump speed. One must not forget that the oil pump is driven off of the camshaft...half the speed of the crank. So when you are running at 2000 rpm, that pump is turning at 1000rpm. If you have ever watched oil pressure at different rpm's, you will note that...especially when the engine is hot...as the engine slows down, oil pressure drops...sometimes below 20psi at an idle. If you run the motor at low RPM, and clearances on the engine are wide, pressure will drop indicating that the pump is not reaching its full capacity. If this happens under a heavy load, you may not be getting enough oil to properly protect bearing surfaces. JMHO
Larry 
78 Royale w/500 Caddy
Menomonie, WI.
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| Re: Another newb question [message #188044 is a reply to message #188029] |
Tue, 23 October 2012 08:16 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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| Hal Kading wrote on Tue, 23 October 2012 00:24 |
Matt,
Please clarify your statement:
"Second, for best fuel rate most SI engines don't want to cruise very far from 10% below the torque peak. The torque peak is where the engine's volumetric efficiency is highest. (That means the most air is pumped at wide open throttle.) This applies even with reduced manifold pressure (less than WOT) operation. If you get far from that either way, the airflow gets worse and you pay for it with fuel."
Are you saying don't cruise very far from 10% below torque peak, or torque peak is best for cruise, but it is okay to go up to about 10% below?
What do you do for best economy when you have a very flat, torque curve like my Ramjet 502? Shoot for the rpm in the high end or middle of the peak or go with the rpm at the low end to minimize the friction losses?
I really appreciate your explanations that often shoot holes in the oft repeated myths.
Thanks,
Hal Kading 78 Buskirk Las Cruces NM
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Hal,
Though I may not write these things to cause questions, I actually appreciate those that come up. Thank You for asking.
Now, the explanation in depth.
Simple things that engine engineers all know:
Volumetric efficiency (VE)is a big part of overall efficiency. (The less work used to pump air, the more power to the crank.)
Air equals horsepower. (The amount of fuel that can be burned is completely limited by the amount of air available to burn it.)
So, since the torque peak is always the region where the volumetric efficiency is highest, you get the most air for the least work. Hence, that will be where the least fuel makes the most power. If you look at any engines complete data set (not often available to the public), along with the WOT HP and Torque you will see a line called BSFC - Brake Specific Fuel Consumption. That curve will always dip at the torque peak for the reasons already explained. If the torque has a long flat, so will the BSFC. Close examination will usually reveal that the lowest is just as the torque goes flat as the crankshaft speed increases. This is because the engine's internal friction loads also increase with speed. In the case of engines that have a very flat torque curve around the peak, the peak is usually accepted as the center of the flat span.
Now, in a vehicle, you create additional drag with speed, this drag requires more power. The combination (product - they are multipliers) of this increasing power demand and the even so slight trend of the BSFC curve may make notable change in the fuel consumption.
To give a clear and definitive answer to your questions.
- Yes, the most fuel efficient cruise will be in the speed range from the torque peak to about 10% below the torque peak.
- If you can arrange the gearing, make your target the area below the torque peak, but not very much below. And yes, this is to minimize friction loses.
It also just happens that this will typically get you the best "driveablity" as it only takes a small increase of throttle position (MAP) to increase speed and the speed increase with change in throttle is very predictable.
If this is not a clear and effective answer, then please try again.
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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| Re: Another newb question [message #188045 is a reply to message #188044] |
Tue, 23 October 2012 08:42 |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
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Thanks Matt, that is a fantastic explanation and clarifies quite a few things.
Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com
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| Re: Another newb question [message #188053 is a reply to message #188044] |
Tue, 23 October 2012 10:42 |
A Hamilto
Messages: 4508
Registered: April 2011
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| Matt Colie wrote on Tue, 23 October 2012 08:16 |
Hal,
Though I may not write these things to cause questions, I actually appreciate those that come up. Thank You for asking.
Now, the explanation in depth.
Simple things that engine engineers all know:
Volumetric efficiency (VE)is a big part of overall efficiency. (The less work used to pump air, the more power to the crank.)
Air equals horsepower. (The amount of fuel that can be burned is completely limited by the amount of air available to burn it.)
So, since the torque peak is always the region where the volumetric efficiency is highest, you get the most air for the least work. Hence, that will be where the least fuel makes the most power. If you look at any engines complete data set (not often available to the public), along with the WOT HP and Torque you will see a line called BSFC - Brake Specific Fuel Consumption. That curve will always dip at the torque peak for the reasons already explained. If the torque has a long flat, so will the BSFC. Close examination will usually reveal that the lowest is just as the torque goes flat as the crankshaft speed increases. This is because the engine's internal friction loads also increase with speed. In the case of engines that have a very flat torque curve around the peak, the peak is usually accepted as the center of the flat span.
Now, in a vehicle, you create additional drag with speed, this drag requires more power. The combination (product - they are multipliers) of this increasing power demand and the even so slight trend of the BSFC curve may make notable change in the fuel consumption.
To give a clear and definitive answer to your questions.
- Yes, the most fuel efficient cruise will be in the speed range from the torque peak to about 10% below the torque peak.
- If you can arrange the gearing, make your target the area below the torque peak, but not very much below. And yes, this is to minimize friction loses.
It also just happens that this will typically get you the best "driveablity" as it only takes a small increase of throttle position (MAP) to increase speed and the speed increase with change in throttle is very predictable.
If this is not a clear and effective answer, then please try again.
Matt
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So maybe the best replacement engine for a GMC is a 3.0L port fuel injected DOHC V6 with a 2:1 underdrive unit between in and the THM425? If that doesn't hit the "sweet spot" for the V6, maybe change the underdrive or FD ratio to one that does?
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| Re: Another newb question [message #188056 is a reply to message #188053] |
Tue, 23 October 2012 11:12 |
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mike miller
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Location: Hillsboro, Oregon
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| ahamilto wrote on Tue, 23 October 2012 08:42 |
So maybe the best replacement engine for a GMC is a 3.0L port fuel injected DOHC V6 with a 2:1 underdrive unit between in and the THM425? If that doesn't hit the "sweet spot" for the V6, maybe change the underdrive or FD ratio to one that does?
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Kind of the idea behind JimB's "Tin Soldier." A 23 foot coach with 350ci Olds and 4.11 gearing.
I am thinking that a small block Chevy (due to availability of engines and parts) might be an acceptable replacement engine sometime in the future. (When Olds engines get harder to find.)
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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