| [GMCnet] Advancing cam timing [message #260486] |
Fri, 29 August 2014 20:00 |
glwgmc
Messages: 1014
Registered: June 2004
Karma:
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I want to jump in here to give Jim a lot more credit than he is willing to give himself. When we went through the engine replacement trauma on the Royale (ten trips either in or out of the coach to finally get a great one), Jim suggested advancing the cam timing 4 degrees on the S & J remanufactured 455 engine. S & J says they build to the same specs GM originally used except the bore and pistons are machined for a more precise fit and are rounder than they were coming out of the factory due to more precise computer controlled tooling. They say the cam is the same grind GM used. Intake manifold is stock with cross overs blocked, Patterson carb and distributor. Final drive is an effective 3.67 (355 power chain Manny tranny plus a 3.21 planetary gear final drive). I had them install the Mondello oil restrictors and used a Mondello high volume standard pressure oil pump. I installed the double roller cam chain 4 degrees advanced.
That coach is a heavy 78 Royale with lots of battery capacity (four 6 volt golf cart batteries) and nearly every dodad one can do to modernize a GMC. When we first purchased it it had a Mark McNeal (Dynao Shop in Santee, CA) new dyno tuned engine that was very strong. This S & J engine with the advanced cam timing is considerably stronger and also considerably stronger than the stock 455 in the 77 Clasco which has exactly the same gearing but weighs less. Both coaches have 16 alloy wheels and the same tires so the only real difference is the cam timing. The Royale will climb the passes in Southern Oregon in third gear without breaking a sweat, the Clasco will not. I lost interest in trying to measure mileage a long time ago (always 8 to 9 in both coaches no matter how geared or which engine), but even my wife can tell the difference from the passenger seat as we traverse the long distances we often do going from Canada to Mexico. The Royale just seems "less strained" in
her words.
When I get around to changing engines in the Clasco it will be another S & J with the cam timing set 4 degrees advanced. There is that much difference, at least from our experience.
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR
Visitors always welcome!
glwork@mac.com
http://jerrywork.com
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Message: 7
Date: Fri, 29 Aug 2014 14:17:13 -0700
From: James Hupy
Subject: Re: [GMCnet] Advancing cam timing
To: gmclist@temp.gmcnet.org
Message-ID:
Content-Type: text/plain; charset=UTF-8
You MIGHT gain a slight increase in vacuum by advancing 4?. Fuel economy is
largely determined by how far open the throttle plates are. Obviously,
steady state 60 mph will achieve a bit more fuel mileage than 70 mph. Wheel
alignment, tire pressures, terrain, road surface, all play a part as well.
"Jack Rabbit" starts where the secondaries are open are a no no if you seek
mileage. If you average 8+ mpg currently, I think you are in the ball park
as far as engine tune up is concerened. You might try sneaking up
(advance) ignition timing by a couple of degrees. If you or your
significant other detect any pinging under steady hard pull, retard it
until you can not. Pinging is a big no no. Remember this, if you run no
alcohol fuel, you will see an increase in fuel economy.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
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Jerry & Sharon Work
78 Royale
Kerby, OR
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