Optimum 455 Engine RPM at cruise speed. Is there one?

[scott nutter1]

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

 

[powwerjon]

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

>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[Jim Hupy]

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 <

> 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 <

> >
> > 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
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[George Z]

So you have raced Ken H, eh?

George Zhookoff
78 EL II
Atlanta

> “ has no problems
> matching speeds with a 23 footer with a 500 cu in Cadillac engine.”
> Jim Hupy
> Salem, Oregon
>
>> On Wed, Sep 23, 2020, 1:03 PM

 

[scott nutter1]

I don’t have a power problem with my Patterson 455 at all. It runs all day long at 70 mph at 2700 rpm’s. And if I don’t watch it it will sneak
up to 75 mph. I was just wondering if there was a “optimum cruising rpm” for the 455.
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

 

[Jim Hupy]

Somewhere under 3000 rpm. Kinda depends upon cam, headers, heads, gearing,
as to where the 455 is a happy camper. I would think 2600 or so.
Jim Hupy
Salem, Oregon

On Wed, Sep 23, 2020, 3:35 PM Scott Nutter via Gmclist <

> I don’t have a power problem with my Patterson 455 at all. It runs all day
> long at 70 mph at 2700 rpm’s. And if I don’t watch it it will sneak
> up to 75 mph. I was just wondering if there was a “optimum cruising rpm”
> for the 455.
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[zandr milewski]

> 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.

There isn't one. What you're looking for here is BSFC (Brake Specific
Fuel Consumption), and that's a 3-dimensional map. It's *probably* the
lowest RPM that makes the power you need to overcome mechanical
(driveline losses, rolling resistance, brake drag) and aerodynamic drag
and maintain a constant speed. With a coastdown and a dyno chart, you
could make a good guess at that RPM, but you'd just discover that you
want more than three gears.

 

[jim kanomata1]

Using a VAC gage to know where the engine is running in the sweet spot.
One need to realize that you are not always tacking around 2,700rpm as
you'll hit small hills.
I have been running my 403 with 4.10 ratio.
It works well when I get tied up in slower traffic.
We have heard good feed back from several,
Walt Halley loves it on his 403 and tows and drives it around 65mph.
I have ran 3.55 and 3.70 on my other coach with 540 Cad with Twin Turbo and
found that the 3.70 works best,
When it comes to fuel economy, 60% will say it improves, 10% say decrease
and rest say they cannot notice any dif.
If you get worse mileage, check the mechanical timing or a bad carb.
We are selling lot of Limited Slip differential as it works.

On Wed, Sep 23, 2020 at 4:45 PM Zandr Milewski via Gmclist <

> > 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.
>
> There isn't one. What you're looking for here is BSFC (Brake Specific
> Fuel Consumption), and that's a 3-dimensional map. It's *probably* the
> lowest RPM that makes the power you need to overcome mechanical
> (driveline losses, rolling resistance, brake drag) and aerodynamic drag
> and maintain a constant speed. With a coastdown and a dyno chart, you
> could make a good guess at that RPM, but you'd just discover that you
> want more than three gears.
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502

 

[Russell]

Scott,
I've also wondered the same thing about our 403. With the factory specs for the 403 at 185 hp @ 3600 RPM and 320 ft/lbs torque @ 2200 RPM; is the
engine running "happier" with rpm's closer to max torque, or at higher rpms?

Related question; when hill climbing in "Super", do you optimally want to keep the rpm's near the max torque range? or near max hp?

Russell

--
Russell Keith,
1978 E2 "Harry" 403 (still carbureted), Danny Dunn Tranny, Thorley, Stock Brakes w/Remote Vacuum Brake Booster, Quad Bags, Dakota Digital Dash, 6.5 kW
Onan, Dunedin, Florida

 

[jim kanomata1]

You can run it at top or bottom, but you must consider your not going to be
driving at the rpm constantly.
So you need to find a happy medium .
That is why selecting the lower is not preferred as when your speed drop,
your hurting.
High end is ok,but cruising above 3,500 in my estimation is not wise

On Mon, Sep 28, 2020 at 4:41 AM Russell Keith via Gmclist <

> Scott,
> I've also wondered the same thing about our 403. With the factory specs
> for the 403 at 185 hp @ 3600 RPM and 320 ft/lbs torque @ 2200 RPM; is the
> engine running "happier" with rpm's closer to max torque, or at higher
> rpms?
>
> Related question; when hill climbing in "Super", do you optimally want to
> keep the rpm's near the max torque range? or near max hp?
>
> Russell
>
>
> --
> Russell Keith,
> 1978 E2 "Harry" 403 (still carbureted), Danny Dunn Tranny, Thorley, Stock
> Brakes w/Remote Vacuum Brake Booster, Quad Bags, Dakota Digital Dash, 6.5 kW
> Onan, Dunedin, Florida
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502

 

[Matt Colie]

> 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,

the thing I can tell you is that there are too many variables to write down a number. You have a Royale (Heavy), a Paterson 455 (set up as best they
can be) and a 3.21 final. A stock 455 has a torque peak at about 23~2400. So, you want to run a little over that because most engines have a sweet
spot that is just above the dyno torque peak. I can tell you why, but only if we meet somewhere and you buy the beer. This will be a summary of 40
years in dynoland. Things used to be different, but with the technical expertise that has gone into make Otto (spark ignition) engine more effective
and cleaner, a lot has been learned.

I would venture that if you are running between 25 & 2700 most of the time, you are probably about as close as you can be.

In the case that a 403 owner is also reading this, they should only be a little different - like 26 ~ 2800. Years ago, it was believed that if you
wanted an engine to last, you wanted it to run slow. We now know that this was wrong because the higher BMEP (cylinder pressure) causes more wear
than does the miles of piston travel. But, we still know that cold starts, fuel injected or no, are still a big issue. We call them thermal cycles
and those are the only thing that they still count in aircraft service.

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