| Re: [GMCnet] Optimum 455 Engine RPM at cruise speed. Is there one? [message #358879] |
Wed, 23 September 2020 15:27  |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Different story. 78 Royale by Coachman. Heavy coach. 403 cu in. Doug
Thorley headers. Reworked vacuum advance and mechanical advance weights.
Stock correct quadrajet carb, Manny Tranny with a torque converter that
locks up at a lower rpm than stock. 3:70 - 1 final drive. Stock
circumference radial tires, steel belted under treads/fabric sidewalls.
B.F. Goodrich brand. Eagle Alloys.
So, how does she run? Before the mods, it was a serious hazard to
navigation. Would not pull at all below about 2400 rpm. Gutless wonder.
Really unhappy with performance.
After the mods? Wow, night and day difference. Acceleration entering
freeway on ramps is great. No problems matching truck speed, now. I drive
with one eye on the tach, and vacuum gage. Generally run about 3200 rpm.
Coach is one happy camper now. Pulls hills with the 455's, has no problems
matching speeds with a 23 footer with a 500 cu in Cadillac engine.
Fuel economy? about the same. Keep the vacuum gage on about 12 - 15
inches and it is better than stock. 10 inches or less, mileage goes down.
Jim Hupy
Salem, Oregon
On Wed, Sep 23, 2020, 1:03 PM John Wright via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Scott,
>
> In our first coach we changed the final drive to a 3.55 ratio and it
> really woke up the 403 and made it peppy, when we want to pass I could pull
> out and it was ready to roll. I typically run at 64 ish MPH and the RPM
> typically was 2775-2800 RPM and the motor is up on the cam and not
> lugging. I also use the stock size 225 tire as I wanted to have the entire
> rpm range available. Before the final drive change the RPM was in the
> 2270-2300 RPM and it couldn’t get out of it own tracks, it was a real DOG!
>
> The present Stretch coach runs a 3.21 final with a 3.50 chain drive in the
> transmission for an effective ratio of 3.66. This coach also has a much
> peppier engine, 461 CU. IN., Accel Gen 6 MPFI, good headers, big mufflers
> and 3” exhaust system. I run about 2900 to 2950 RPM at 64 ish MPH.
>
> Both coaches had or have the HD Alison style converter which starts to
> lock at a lower RPM which it doesn’t slip as much. The first coach at a
> stop light would load the convert up so that you got instant acceleration.
> When I sold the coach the new owner left and drove the coach about 30 miles
> and then came back and said that there was something wrong with the coach.
> We all went for a short and when we were at the stop sign he said that the
> coach was surging. I said YEA! You have a HD Alison type Torque converter
> and it is pulling against the motor RPM. Then I put my foot in it and he
> finally understood, it was not a slush box!
>
> I will not get into a discussion of MPG. Everyone has their own thoughts
> on how good they get, NOT GOING THERE!!!! My personal experience over the
> years and 85+K miles is that I have got almost as high as 9 and as low as 6
> depending on load and speed. Personal opinion is that I really don’t care
> what the MPG is, because it is what it is. We used to take the coach to
> Tucson for a number of years and we always joked that we got better MPG
> going back to Michigan because it was down hill most of the way. (Think
> about it)
>
> J.R. Wright
> GMC Great Laker MHC
> GMCGL Tech Editor
> GMC Eastern States Charter Member
> GMCMI
> 78 GMC Buskirk 29.5’ Stretch
> 75 GMC Avion (Under Reconstruction)
> Michigan
>
>
>
>> On Sep 23, 2020, at 2:05 PM, Scott Nutter via Gmclist gmclist@list.gmcnet.org> wrote:
>>
>> Does anyone know what the best cruise engine RPM on a 455 would be?
> Where performance and fuel efficiency would meet on a graft. Basically on a
>> standard day, with no wind, on flat land, at sea level. It seems to me
> I’ve heard 2700 RPM’s might be the number. Or would a vacuum gauge be the
>> better indication?
>> Thanks, Scott.
>> --
>> Scott Nutter
>> 1978 Royale Center Kitchen, Patterson 455, switch pitch tranny, 3.21
> final drive, Quad bags, tankless water heater, everything Lenzi. Alex
> Ferrera
>> installed MSD Atomic EFI
>> Houston, Texas
>>
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