Home » Public Forums » GMCnet » Differences between 403 and 455
| Re: [GMCnet] Differences between 403 and 455 [message #279130 is a reply to message #279128] |
Fri, 05 June 2015 10:58   |
James Hupy
Messages: 6806
Registered: May 2010
Karma:
|
Senior Member |
|
|
I have a heavy 78 Royale with a 403. One of the first things I did to it
was to change the final drive gear ratio from a 3:07 to a 3:70. Monday on
the way home from a rally in Washington state, we ran into a headwind down
the Columbia Gorge that I estimate was 50 to 70 mph. I could easily run
over 65 miles per hour against that wind, but prudently slowed down to 55
to save fuel. I made up for it on the way south from Portland. I saw 80 on
the speedometer a couple of times. My 403 has over 120,000 miles on the
clock. It runs as well now as it did when I bought it with 80,000 miles. It
will run with the 455 very well, with the exception being at the bottom of
a hill when I let the RPM's fall below 2000. In the 1800 to 2200 rpm range,
the 455 will out torque the 403. But not above 2800 - 4000 rpm. No prudent
person will drive a GMC above 4000 rpms on highways with traffic anyway.
Either way, both engines are reliable as an anvil if you maintain them
well. If I stay out of the 4 barrel, I consistently get over 10 miles per
gallon.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Fri, Jun 5, 2015 at 8:42 AM, Randy Van Winkle wrote:
> George Beckman wrote on Fri, 05 June 2015 00:27
>> One oddity about the 403 is that it has siamese cylinders, meaning there
> are no water jackets between 2&4, and 6&8. (other side as well)
>> Apparently this is not a problem. I forget the mileage he has on the
> coach... I am guessing 78K or something like that. We could feel no ridge at
>> the top of the cylinders. Cylinders looked perfect.
>>
>> If I ran on to a nice 403, I might just consider swapping. (hate to
> think about finding all the brackets.)
>
>
> What George said! I have a little over 100K miles. It has been a good
> running engine. As George said, we pulled the head last winter because of a
> Tuliped intake valve. The cause is a little uncertain, but I believe it
> was caused by a failed spark plug. It appeared the electrode on the plug
> came loose and welded itself to the ground lug. Replaced valve and all
> seems to be good after around 5,000 miles.
>
> I love the 403 when mated to a 3.70 final drive. I would not let the type
> of engine be a deciding factor in making a coach decision.
> --
> Randy & Margie
> '77 Eleganza II '403'
> Battlefield, MO
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
Current Time: Sat May 25 09:03:13 CDT 2024
Total time taken to generate the page: 1.26480 seconds
|
GMCforum
Search
Help
Register
Login
Home
