Home » Public Forums » GMCnet » Differential question for thought
| Differential question for thought [message #263825] |
Thu, 09 October 2014 07:48  |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
Karma: 15
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My 77 is a late model with the 403. Freshly rebuilt with Paterson carb and distributor, Manny tranny with PowerDrive and a 3:21. This gives a final drive ratio of 3.66. UNBELIEVABLE change over the original 3:07. Pulling my Saturn TOAD, I got 8.5 MPG on the recent 1900 miles to Wisconsin and back. On that trip, Ken Henderson was following us in his Caddy coach. Ken likes to run on his cruise control and I'm sure he was driven nuts by my speed changes since I don't have a cruise. I would hit the bottom of a big hill at 65 and be going slower at the top UNLESS I really got into the carb secondaries.
HOWEVER, I drive around 60-65 and the tach shows it's running just at 3000 RPM. This is right at the bottom of where the motor seems to want to run. While she pulls every hill we come to, I'm thinking that a cruising RPM around 3500 might make the motor happier and more responsive.
I doubt that I will do a differential change unless we decide to make some cross country jaunts but would like to hear from members who are running greater than 3:70s
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: [GMCnet] Differential question for thought [message #263832 is a reply to message #263825] |
Thu, 09 October 2014 08:37  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Kerry,
Considering that Ken's GMC is a 23' and has 100 cubic inches more in his Caddy 500 you're lucky he didn't run away from you or ram
you! ;-)
Too bad you didn't have Manny build you a switch pitch with the power drive and when you start up a hill switch pitch. IIRC Emery
Stora has reported that what he does and it works great.
Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
USA '75 Avion - Double Trouble TZE365V100426
AUS '75 Avion - The Blue Streak TZE365V100428
-----Original Message-----
From: Kerry Pinkerton
My 77 is a late model with the 403. Freshly rebuilt with Paterson carb and distributor, Manny tranny with PowerDrive and a 3:21.
This gives a final drive ratio of 3.66. UNBELIEVABLE change over the original 3:07. Pulling my Saturn TOAD, I got 8.5 MPG on the
recent 1900 miles to Wisconsin and back. On that trip, Ken Henderson was following us in his Caddy coach. Ken likes to run on his
cruise control and I'm sure he was driven nuts by my speed changes since I don't have a cruise. I would hit the bottom of a big
hill at 65 and be going slower at the top UNLESS I really got into the carb secondaries.
HOWEVER, I drive around 60-65 and the tach shows it's running just at 3000 RPM. This is right at the bottom of where the motor
seems to want to run. While she pulls every hill we come to, I'm thinking that a cruising RPM around 3500 might make the motor
happier and more responsive.
I doubt that I will do a differential change unless we decide to make some cross country jaunts but would like to hear from members
who are running greater than 3:70s
--
Kerry
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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: Differential question for thought [message #263834 is a reply to message #263825] |
Thu, 09 October 2014 09:14 |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
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Interesting Kerry.
I have a 455 in a my 26' with 3.70s I got 8.5 to and fro Chippewa falls ( but NOT towing ), but Western Wisconsin is very hilly and I was deep into the throttle a number of times. I think it will do better on flatter ground.
The 455 seems very happy at the 60 / 2800 rpm I'm running and small hills only cost a couple of inches of vacuum.
I am very glad I didn't go with the 3.55 and 4.11 might be a big much
I would bet the 370s in your coach with the 403 would be a minor upgrade but probably not worth the cost and effort.
Rob
I don't see the need for the switch pitch, it only helps on startup and I'm not racing from stoplight to stoplight. I only really care how it runs on the freeway at 60 where I spend most of me time. I mean if I get on it the thing is a rocket off the line, I don't need to accelerate any faster!
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: Differential question for thought [message #263835 is a reply to message #263834] |
Thu, 09 October 2014 09:57 |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
Karma: 15
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Keith V wrote on Thu, 09 October 2014 09:14...I would bet the 370s in your coach with the 403 would be a minor upgrade but probably not worth the cost and effort....
Certainly going from my current 3.66 to the 3.70 would be a waste of time and money. If I went to a 3.42 with the power drive (15%), that would end up with a 3.94 (3.42 * 1.15%). Wondering if someone is running something like this and how it works.
Keith the 403 likes to run a bit higher RPM than the 455 and the torque curve is a bit higher as well.
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: [GMCnet] Differential question for thought [message #263838 is a reply to message #263835] |
Thu, 09 October 2014 10:06 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Kerry, if you are looking to spend as few dollars as possible, and want the
most bang for your bucks, pull the front timing cover off, remove the stock
timing chain set, and replace it with a double row true roller chain set at
4° advance. That will move your torque peak down the rpm range 400-600 rpm
or so. If you want to go a bit further, replace the cam and lifters while
you are in there with a RV cam from Comp Cams. It will do almost as much
for you as the gearing change did.
Jim Hupy
On Oct 9, 2014 7:57 AM, "Kerry Pinkerton" wrote:
> Keith V wrote on Thu, 09 October 2014 09:14
>> ...I would bet the 370s in your coach with the 403 would be a minor
> upgrade but probably not worth the cost and effort....
>
>
> Certainly going from my current 3.66 to the 3.70 would be a waste of time
> and money. If I went to a 3.42 with the power drive (15%), that would end
> up with a 3.94 (3.42 * 1.15%). Wondering if someone is running something
> like this and how it works.
>
> Keith the 403 likes to run a bit higher RPM than the 455 and the torque
> curve is a bit higher as well.
> --
> Kerry Pinkerton
>
> North Alabama, near Huntsville,
>
> 77 Eleganza II, "The Lady", 403CI, Manny Brakes, 1 ton, tranny also a 76
> Eleganza to be re-bodied as an Art Deco car hauler
> _______________________________________________
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| Re: Differential question for thought [message #263941 is a reply to message #263825] |
Fri, 10 October 2014 19:35 |
Chr$
Messages: 2690
Registered: January 2004
Location: Scottsdale, AZ
Karma: 1
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Manny talked me out of the switch pitch, so I sold my SP core. WTF???
-Chr$: Perpetual SmartAss
Scottsdale, AZ
77 Ex-Kingsley 455 SOLD!
2010 Nomad 24 Ft TT 390W PV W/MPPT, EV4010 and custom cargo door.
Photosite: Chrisc GMC:"It has Begun" TT: "The Other Woman"
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| Re: Differential question for thought [message #264126 is a reply to message #263825] |
Mon, 13 October 2014 09:09 |
bobby5832708
Messages: 237
Registered: November 2006
Location: Winter Springs FL
Karma: 3
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What's a switchpitch trans and a 3.21 diff worth? I picked up a sp/3.21 a few years ago, used the 3.21 for a couple of years, put the 3.07 back in this past spring, and will probably never use the sp trans. I'm just thinking about getting rid of excess "stuff", the garage is too full.
Bob Heller
2017 Winnebago 29VE
Winter Springs FL
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| Re: Differential question for thought [message #264130 is a reply to message #263825] |
Mon, 13 October 2014 09:48 |
bobby5832708
Messages: 237
Registered: November 2006
Location: Winter Springs FL
Karma: 3
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"I'm thinking that a cruising RPM around 3500 might make the motor happier and more responsive."
Are you SURE that you want to drive down the highway with the engine running 3500 all the time? If so, why not just put the trans in 'S' when you feel you really need the extra revs.
Earlier this year Tony S and I were driving in my GMC and talking about gear ratios, engine revs, my then-current 3.21 diff, engine noise, and a bunch of other nonsense. The subject of 3100 rpm cruising revs came up so I slowed down a bit, put the trans in 'S', and brought the engine up to 3100 rpm. After about two miles we both decided there was no way we were going to modify our GMC's to rev that high while cruising on the highway. 3100 revs might be OK if we were towing and in the mountains but since we use our coaches mostly on flat roads in Florida the extra revs are just an added irritant. If I really wanted 3100-or-more revs I could just downshift.
I even went so far as to remove the 3.21 and reinstall the 3.07 in my GMC because at 70 mph the extra 250-or-so rpms were very noticeable (to me at least). To me, only 4 cyl buzz bombs need to be running at high rpms. I always liked a V8 because it didn't need lots of revs to get the job done. I would love to have something that could push the GMC down the highway at 2000 rpms, if the 6.5 diesel conversion ever becomes economically feasible that may be the answer.
In the early 80's I drove a Dodge VW-powered shitbox for a while. I hated the 3000 rpms at 60 mph but those old 4 cyls had to be revving to get any power out of them. Also at the time a coworker had a GTI VW that revved the same way even with a 5 speed box. Both cars were terrible at highway cruising due to the high engine rpms, they always seemed like they needed another gear or two. Maybe that's where I got my dislike of high-revving engines. There's nothing wrong with revving up while accelerating, I just don't want to hear the engine screaming for 4 hours while driving on the highway.
Just my opinion! I suggest you make sure you can live with the noise of 3500 rpms before spending money on another diff.
Bob Heller
2017 Winnebago 29VE
Winter Springs FL
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| Re: [GMCnet] Differential question for thought [message #264132 is a reply to message #264130] |
Mon, 13 October 2014 10:01 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Sochiro Honda said it best I think "All factors being equal, RPM are free".
He was speaking of the Mike "the bike" Hailwood 6 cylinder racers of the
60's. Came up on the cams about 17,500, and red lined at 23,000 or so. No
other engine that I ever heard of ran like those things did. Over square
engines can do that. Undersquare, not so well. Pays yer money, and takes
yer choices.
Jim Hupy
Salem, Or.
78 Gmc Royale 403
On Oct 13, 2014 7:48 AM, "Bob Heller" wrote:
>
>
> "I'm thinking that a cruising RPM around 3500 might make the motor happier
> and more responsive."
>
>
> Are you SURE that you want to drive down the highway with the engine
> running 3500 all the time? If so, why not just put the trans in 'S' when
> you feel
> you really need the extra revs.
>
> Earlier this year Tony S and I were driving in my GMC and talking about
> gear ratios, engine revs, my then-current 3.21 diff, engine noise, and a
> bunch
> of other nonsense. The subject of 3100 rpm cruising revs came up so I
> slowed down a bit, put the trans in 'S', and brought the engine up to 3100
> rpm.
> After about two miles we both decided there was no way we were going to
> modify our GMC's to rev that high while cruising on the highway. 3100 revs
> might be OK if we were towing and in the mountains but since we use our
> coaches mostly on flat roads in Florida the extra revs are just an added
> irritant. If I really wanted 3100-or-more revs I could just downshift.
>
> I even went so far as to remove the 3.21 and reinstall the 3.07 in my GMC
> because at 70 mph the extra 250-or-so rpms were very noticeable (to me at
> least). To me, only 4 cyl buzz bombs need to be running at high rpms. I
> always liked a V8 because it didn't need lots of revs to get the job done. I
> would love to have something that could push the GMC down the highway at
> 2000 rpms, if the 6.5 diesel conversion ever becomes economically feasible
> that may be the answer.
>
> In the early 80's I drove a Dodge VW-powered shitbox for a while. I hated
> the 3000 rpms at 60 mph but those old 4 cyls had to be revving to get any
> power out of them. Also at the time a coworker had a GTI VW that revved
> the same way even with a 5 speed box. Both cars were terrible at highway
> cruising due to the high engine rpms, they always seemed like they needed
> another gear or two. Maybe that's where I got my dislike of high-revving
> engines. There's nothing wrong with revving up while accelerating, I just
> don't want to hear the engine screaming for 4 hours while driving on the
> highway.
>
> Just my opinion! I suggest you make sure you can live with the noise of
> 3500 rpms before spending money on another diff.
> --
> Bob Heller
> 1974 X-Canyonlands 26ft
> Original 455, 134k miles
> Winter Springs FL
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| Re: [GMCnet] Differential question for thought [message #264181 is a reply to message #264132] |
Mon, 13 October 2014 22:12 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Jim,
I remember Mike and those bikes! When Honda turned up a race with one of those everybody else was racing for second place. If they
showed up with two it was third!
Regards,
Rob M.
-----Original Message-----
From: James Hupy
Sochiro Honda said it best I think "All factors being equal, RPM are free".
He was speaking of the Mike "the bike" Hailwood 6 cylinder racers of the
60's. Came up on the cams about 17,500, and red lined at 23,000 or so. No
other engine that I ever heard of ran like those things did. Over square
engines can do that. Undersquare, not so well. Pays yer money, and takes
yer choices.
Jim Hupy
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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] Differential question for thought [message #264185 is a reply to message #264132] |
Mon, 13 October 2014 22:55 |
mrgmc3
Messages: 210
Registered: September 2013
Location: W Washington
Karma: 2
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James Hupy wrote on Mon, 13 October 2014 10:01Sochiro Honda said it best I think "All factors being equal, RPM are free".
RPM may be free in terms of creating HP, but they are definitely not free when you are talking fuel economy. Motoring friction is directly proportional to RPM. This is why all modern cars and trucks run 5 or more speeds. Operating at a low cruise RPM is an effort to minimize engine friction in addition to all the design tricks like roller valve trains. For a given vehicle speed you get the added benefit of operating at a higher throttle opening which reduces pumping losses ( I realize that low vacuum is sacrilege in this group but you can't argue with thermodynamics).
I don't tow so I'll stick with the 3.07:1. It's the closest thing to overdrive, and the old 403 still manages to pull those 6% grades out west at 50 mph. If I were towing I might view it differently.
Chris Geils - Twin Cities / W Wa
1978 26' Kingsley w/ very few mods; PD9040, aux trans cooler, one repaint in stock colors, R134a, Al rad, Alcoas, 54k mi
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| Re: [GMCnet] Differential question for thought [message #264192 is a reply to message #264185] |
Mon, 13 October 2014 23:51 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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If you look back through this entire thread, you will find very few
references to fuel economy when I am referring to the package in question.
All of the balls are still in the air that effect economy. I probably
should not have digressed into the rpm area either, but that is water under
the bridge, now. We just got home from Treasure Island gathering with an
overnight stop in Redding, Ca. All Ca. driving was with my heavy 78 Royale
pulling a trailer with 4 one ton front ends in it. I did not drive faster
than 55 in California. 3:70 - 1 final drive, 403 with 118,000 miles. Lots
of busy traffic, but, went from sea level to 4300 feet elevation. Engine
struggled to pull the grades, because it was below it's torque peak at 55.
Oregon has a 65 mph speed limit. Ran all day today at 65 mph. I would
wager that there was not much difference in fuel economy in my case. 65 mph
in my case is 3000 rpm. My coach climbs grades much better without down
shifting at that rpm. Don't know about your 403 with 3:07 final drive, but
I spent a lot of time in second gear driving this same road when I had
3:07-1's in my heavy coach. Jus' sayin.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Oct 13, 2014 8:55 PM, "Chris Geils" wrote:
> James Hupy wrote on Mon, 13 October 2014 10:01
>> Sochiro Honda said it best I think "All factors being equal, RPM are
> free".
>
>
> RPM may be free in terms of creating HP, but they are definitely not free
> when you are talking fuel economy. Motoring friction is directly
> proportional to RPM. This is why all modern cars and trucks run 5 or more
> speeds. Operating at a low cruise RPM is an effort to minimize engine
> friction in addition to all the design tricks like roller valve trains.
> For a given vehicle speed you get the added benefit of operating at a higher
> throttle opening which reduces pumping losses ( I realize that low vacuum
> is sacrilege in this group but you can't argue with thermodynamics).
> I don't tow so I'll stick with the 3.07:1. It's the closest thing to
> overdrive, and the old 403 still manages to pull those 6% grades out west
> at 50
> mph. If I were towing I might view it differently.
> --
> Chris Geils - Twin Cities
> 1978 26' Kingsley w/ very few mods; Headers, Progressive Dynamics 9040,
> aux trans cooler, one repaint in stock colors, R134a, Al rad, 47k mi
> _______________________________________________
> GMCnet mailing list
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| Re: [GMCnet] Differential question for thought [message #264213 is a reply to message #264192] |
Tue, 14 October 2014 08:54 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Jim,
The 6.5 diesel has already demonstrated 14 mpg in Bill Hubler's and Marc Hogenboom's GMC. I don't think either one of them tows.
IIRC from listening to presentations made by EFI devotees the best they've ever gotten was 12 (I may be wrong) and when speaking
with Dave Lenzi about his 8.1 Vortec he noted he got 12mpg towing an HHR behind his Royale. He carries a LOT of stuff with him we
goes to conventions.
I have no idea what final drives any of the coaches above runs and since I don't have a dog in this hunt I ain't gonna research it.
;-)
Regards,
Rob M.
Paterson Carb'd 8 - 10 / 3.21 to 1 / 455
-----Original Message-----
From: James Hupy
If you look back through this entire thread, you will find very few
references to fuel economy when I am referring to the package in question.
All of the balls are still in the air that effect economy. I probably
should not have digressed into the rpm area either, but that is water under
the bridge, now. We just got home from Treasure Island gathering with an
overnight stop in Redding, Ca. All Ca. driving was with my heavy 78 Royale
pulling a trailer with 4 one ton front ends in it. I did not drive faster
than 55 in California. 3:70 - 1 final drive, 403 with 118,000 miles. Lots
of busy traffic, but, went from sea level to 4300 feet elevation. Engine
struggled to pull the grades, because it was below it's torque peak at 55.
Oregon has a 65 mph speed limit. Ran all day today at 65 mph. I would
wager that there was not much difference in fuel economy in my case. 65 mph
in my case is 3000 rpm. My coach climbs grades much better without down
shifting at that rpm. Don't know about your 403 with 3:07 final drive, but
I spent a lot of time in second gear driving this same road when I had
3:07-1's in my heavy coach. Jus' sayin.
Jim Hupy
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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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