| Stock 403 timing. 3.55 final drive [message #263165] |
Thu, 02 October 2014 11:46  |
77Royale
Messages: 461
Registered: June 2014
Location: Mid Michigan
Karma: 6
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Im looking to have the timing set / adjusted on my coach, It has the 403, with stock carb and dizzy. I have the manual and know whats listed for timing.
However, I do have a 3.55 final drive installed. I do not plan on towing anything.
What would you all say as to recommendations for setting the timing. Im not doing this myself but have a competent shop who has worked on GMCs before.
Im looking for suggestions if there are any given the final drive change.
Thank you in advance. Again I have the manual, but want to know what others have done or where they have found the best settings.
77 Royale, Rear Dry Bath. 403, 3.55 Final Drive, Lenzi goodies, Patterson carb and dizzy.
Mid Michigan
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| Re: Stock 403 timing. 3.55 final drive [message #263166 is a reply to message #263165] |
Thu, 02 October 2014 11:57  |
mrgmc3
Messages: 210
Registered: September 2013
Location: W Washington
Karma: 2
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A change to the final drive ratio does not really impact the timing needs of the engine. With a 3.55:1 compared to a 3.07:1 you will be operating at a higher rpm but lower load (lower man P or higher vac depending on your preference) for a given road speed. The conventional distributor accounts for this change with the vacuum and centrifugal advance. Make sure both are working correctly and set your base timing at 12. If you are feeling adventurous you can add 2 degrees to base timing and listen for audible knock. Retard back if any is heard. Engines tend to be less knock sensitive in cooler weather so be prepared to readjust next spring / summer as well.
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: Stock 403 timing. 3.55 final drive [message #263168 is a reply to message #263165] |
Thu, 02 October 2014 12:28 |
77Royale
Messages: 461
Registered: June 2014
Location: Mid Michigan
Karma: 6
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Thank you, I will provide this info to the mechanic.
I was not sure if the final drive would make a difference so Im glad I asked.
77 Royale, Rear Dry Bath. 403, 3.55 Final Drive, Lenzi goodies, Patterson carb and dizzy.
Mid Michigan
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| Re: [GMCnet] Stock 403 timing. 3.55 final drive [message #263171 is a reply to message #263168] |
Thu, 02 October 2014 12:56 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Wayne,
In my opinion setting the timing at anything other than the spec called for in the manual will not provide any significant gain that
you will feel in the seat of your pants, we're not talking about a hot rod we're talking about a motorhome!
It will increase the possibility of detonation and that ain't good!
Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
USA '75 Avion - Double Trouble TZE365V100426
AUS '75 Avion - The Blue Streak TZE365V100428
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Wayne Rogewski
Sent: Thursday, October 02, 2014 12:28 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Stock 403 timing. 3.55 final drive
Thank you, I will provide this info to the mechanic.
I was not sure if the final drive would make a difference so Im glad I asked.
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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: Stock 403 timing. 3.55 final drive [message #263172 is a reply to message #263165] |
Thu, 02 October 2014 13:31 |
77Royale
Messages: 461
Registered: June 2014
Location: Mid Michigan
Karma: 6
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Thanks Rob,
Im certainly not looking for ultra high performance, hot rods, seat of the pants or 1/4 mile time improvements. That wasnt the crux of the question. My concern was the 3.55 final drive and if that addition to the equation would make a difference at all in properly setting the timing, which it does not appear that it will. The manual doesnt account for, nor address different final drive gear ratios, So thats why I wanted to ask.
77 Royale, Rear Dry Bath. 403, 3.55 Final Drive, Lenzi goodies, Patterson carb and dizzy.
Mid Michigan
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| Re: [GMCnet] Stock 403 timing. 3.55 final drive [message #263243 is a reply to message #263172] |
Fri, 03 October 2014 01:12 |
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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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Wayne,
You're welcome.
The manual doesn't address different final drive ratios because it doesn't matter what the ratio is.
Advancing the timing can improve performance which is why I spoke of hot rods and some guys do it. I can't see the point as my
objective is reliability and longevity out of the 455 in Double Trouble.
I just can't see why one should run the risk of detonation.
I believe what kills an engine and transmission is jackrabbit starts. I do my best to be in the right lane when I come to a stop.
When it's time to go I slowly depress the gas pedal and get Double Trouble up to whatever the speed limit is as I've found that with
a 3.21 to 1 final drive I can drive at 75 without any problem at all. My oil pressure runs 40 - 50 psi, water temp sits on the
thermostat setting, trans temp the same.
Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
USA '75 Avion - Double Trouble TZE365V100426
AUS '75 Avion - The Blue Streak TZE365V100428
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Wayne Rogewski
Thanks Rob,
Im certainly not looking for ultra high performance, hot rods, seat of the pants or 1/4 mile time improvements. That wasnt the crux
of the question.
My concern was the 3.55 final drive and if that addition to the equation would make a difference at all in properly setting the
timing, which it does not appear that it will. The manual doesnt account for, nor address different final drive gear ratios, So
thats why I wanted to ask.
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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] Stock 403 timing. 3.55 final drive [message #263311 is a reply to message #263243] |
Fri, 03 October 2014 21:42 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Why mess with it in the first place? The static timing does not normally change unless the distributor in shot. Now if you want to pull the distributor cap and check the advance weight pivots for rust and lubrication, that would make more sense to me.
If you are dead set on adjusting the timing then set it to the book number. If you have a rebuilt distributor, then set it to the rebuilder's specification. Dick Paterson says 10 degrees base (static) timing on his rebuilts. If you know what you are doing, 12 degrees is a good starting point IF YOU HAVE GOOD HEARING and can hear ignition ping. Two people, who I will not name have lost GMC engines from setting the timing to far advanced. Other's have had drive ability problems for advance weights sticking.
My suggestion is leave the timing alone and check the advance weights for wear and lubrication.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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