Home » Public Forums » GMCnet » Timing
| Timing [message #193321] |
Wed, 19 December 2012 06:11  |
skip2
Messages: 544
Registered: September 2011
Location: Winter Haven,FL (center o...
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Awhile back someone posted how they set the timing going down the road with the copilot driving and actually setting it under real loads and inclines. I just don't recall the details so if if someone recalls how to do it I'd appreciate it being explained again.
Thanks
Skip Hartline
74 Canyon Lands, FiTech,
3.7 FD LSD, Manny Tranny,
Springfield Distributor,
2001 Chevy Tracker Ragtop Towd
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| Re: [GMCnet] Timing [message #193331 is a reply to message #193321] |
Wed, 19 December 2012 08:43  |
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"I really don't remember where I learned this trick or I would love to give
credit. I don't even remember how long I have known about it but I have used
this method for several years when tuning a GMC Motorhome.
This method, which I call "dynamic timing while underway", will allow you to
set the timing for maximum efficiency matching your driving habits and the
fuel you use.
We are fortunate to be able to access the engine from the inside of the
coach. You do not want to try this hanging off the fender of a car. The
results would be unpredictable at best.
It takes two people to do this so you may need to get your navigator
involved but I would not recommend telling her that you need help working on
the coach. Those results will be equally unpredictable. Just tell her that
we are going for a little ride. In this case the navigator will probably be
doing the driving.
Ahead of time you should scout out a road that is on an incline and has a
long straight stretch. The one I use is 6 miles from my shop, about 2 miles
from start to the crest of the hill and on a 6 to 8 percent grade. I would
also recommend vacuuming the top of the engine to get rid of the acorn
shells and other types of debris deposited by our furry forest friends.
Initially set the timing to the specifications in the maintenance manual.
Now you are ready for your little drive. This is also a good time to note
the direction you turn the distributor to advance the timing. Tighten the
hold down bolt to the point that it is just snug.
Just before entering your test run, with the engine up to normal operating
temp remove the engine cover making sure you secure the carpet if it is not
attached to the cover. You don't want the carpet, the dog and your drivers
shoes sucked into the fan. Remove the air cleaner and the vacuum line that
operates the pre-heater damper door. Plug the port in the intake manifold.
Loosen the distributor hold down bolt so that you can just move the
distributor.
Now enter the test run and get up to the speed you normally drive. Assure
your driver that all that noise is really OK. You want to be in third gear
with the throttle open but not to the point that the secondary's are open.
Half throttle is fine. Advance the timing until you hear a little hard
pinging then retard the timing until you can hear occasional light pinging.
Light pinging is not a bad thing.
Secure the distributor hold down bolt. Pull over to the side of the road and
with the engine idling you can check the timing with your timing light.
Record the timing for future tune-ups.
This may be as much as 12 to 14 degrees advance. I record things like firing
order, direction of rotation, direction to advance (CW or CCW) and timing on
the brace across the back of the engine compartment access hole with a felt
tip marker for easy reference.
Re-install the air cleaner, vacuum line and engine access cover.
Take the driver to the DQ for an ice cream."
DJ Eberhart
-----Original Message-----
From: On Behalf Of Skip Hartline
Sent: Wednesday, December 19, 2012 6:11 AM
Subject: [GMCnet] Timing
Awhile back someone posted how they set the timing going down the road with
the copilot driving and actually setting it under real loads and inclines. I
just don't recall the details so if if someone recalls how to do it I'd
appreciate it being explained again.
Thanks
Skip Hartline
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bdub.net
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| Re: [GMCnet] Timing [message #193333 is a reply to message #193331] |
Wed, 19 December 2012 09:18 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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| Billy Massey wrote on Wed, 19 December 2012 08:43 |
"I really don't remember where I learned this trick or I would love to give
credit. I don't even remember how long I have known about it but I have used
this method for several years when tuning a GMC Motorhome.
This method, which I call "dynamic timing while underway", will allow you to
set the timing for maximum efficiency matching your driving habits and the
fuel you use.
We are fortunate to be able to access the engine from the inside of the
coach. You do not want to try this hanging off the fender of a car. The
results would be unpredictable at best.
It takes two people to do this so you may need to get your navigator
involved but I would not recommend telling her that you need help working on
the coach. Those results will be equally unpredictable. Just tell her that
we are going for a little ride. In this case the navigator will probably be
doing the driving.
Ahead of time you should scout out a road that is on an incline and has a
long straight stretch. The one I use is 6 miles from my shop, about 2 miles
from start to the crest of the hill and on a 6 to 8 percent grade. I would
also recommend vacuuming the top of the engine to get rid of the acorn
shells and other types of debris deposited by our furry forest friends.
Initially set the timing to the specifications in the maintenance manual.
Now you are ready for your little drive. This is also a good time to note
the direction you turn the distributor to advance the timing. Tighten the
hold down bolt to the point that it is just snug.
Just before entering your test run, with the engine up to normal operating
temp remove the engine cover making sure you secure the carpet if it is not
attached to the cover. You don't want the carpet, the dog and your drivers
shoes sucked into the fan. Remove the air cleaner and the vacuum line that
operates the pre-heater damper door. Plug the port in the intake manifold.
Loosen the distributor hold down bolt so that you can just move the
distributor.
Now enter the test run and get up to the speed you normally drive. Assure
your driver that all that noise is really OK. You want to be in third gear
with the throttle open but not to the point that the secondary's are open.
Half throttle is fine. Advance the timing until you hear a little hard
pinging then retard the timing until you can hear occasional light pinging.
Light pinging is not a bad thing.
Secure the distributor hold down bolt. Pull over to the side of the road and
with the engine idling you can check the timing with your timing light.
Record the timing for future tune-ups.
This may be as much as 12 to 14 degrees advance. I record things like firing
order, direction of rotation, direction to advance (CW or CCW) and timing on
the brace across the back of the engine compartment access hole with a felt
tip marker for easy reference.
Re-install the air cleaner, vacuum line and engine access cover.
Take the driver to the DQ for an ice cream."
DJ Eberhart
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I suppose that after a couple of tuning experiences your navigator might encourage you to buy an adjustable electronic distributor 
Dennis
Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] Timing [message #193334 is a reply to message #193333] |
Wed, 19 December 2012 09:36 |
sgltrac
Messages: 2797
Registered: April 2011
Karma: 1
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Haha. Funny sh;t Dennis. I think you know my navigator!
Sully
77 royale
Seattle
Sent from my iPhone
On Dec 19, 2012, at 7:18 AM, Dennis Sexton <dennisfsexton@aol.com> wrote:
>
>
> Billy Massey wrote on Wed, 19 December 2012 08:43
>> "I really don't remember where I learned this trick or I would love to give
>> credit. I don't even remember how long I have known about it but I have used
>> this method for several years when tuning a GMC Motorhome.
>> This method, which I call "dynamic timing while underway", will allow you to
>> set the timing for maximum efficiency matching your driving habits and the
>> fuel you use.
>> We are fortunate to be able to access the engine from the inside of the
>> coach. You do not want to try this hanging off the fender of a car. The
>> results would be unpredictable at best.
>> It takes two people to do this so you may need to get your navigator
>> involved but I would not recommend telling her that you need help working on
>> the coach. Those results will be equally unpredictable. Just tell her that
>> we are going for a little ride. In this case the navigator will probably be
>> doing the driving.
>> Ahead of time you should scout out a road that is on an incline and has a
>> long straight stretch. The one I use is 6 miles from my shop, about 2 miles
>> from start to the crest of the hill and on a 6 to 8 percent grade. I would
>> also recommend vacuuming the top of the engine to get rid of the acorn
>> shells and other types of debris deposited by our furry forest friends.
>> Initially set the timing to the specifications in the maintenance manual.
>> Now you are ready for your little drive. This is also a good time to note
>> the direction you turn the distributor to advance the timing. Tighten the
>> hold down bolt to the point that it is just snug.
>> Just before entering your test run, with the engine up to normal operating
>> temp remove the engine cover making sure you secure the carpet if it is not
>> attached to the cover. You don't want the carpet, the dog and your drivers
>> shoes sucked into the fan. Remove the air cleaner and the vacuum line that
>> operates the pre-heater damper door. Plug the port in the intake manifold.
>> Loosen the distributor hold down bolt so that you can just move the
>> distributor.
>> Now enter the test run and get up to the speed you normally drive. Assure
>> your driver that all that noise is really OK. You want to be in third gear
>> with the throttle open but not to the point that the secondary's are open.
>> Half throttle is fine. Advance the timing until you hear a little hard
>> pinging then retard the timing until you can hear occasional light pinging.
>> Light pinging is not a bad thing.
>> Secure the distributor hold down bolt. Pull over to the side of the road and
>> with the engine idling you can check the timing with your timing light.
>> Record the timing for future tune-ups.
>> This may be as much as 12 to 14 degrees advance. I record things like firing
>> order, direction of rotation, direction to advance (CW or CCW) and timing on
>> the brace across the back of the engine compartment access hole with a felt
>> tip marker for easy reference.
>> Re-install the air cleaner, vacuum line and engine access cover.
>> Take the driver to the DQ for an ice cream."
>> DJ Eberhart
>
>
> I suppose that after a couple of tuning experiences your navigator might encourage you to buy an adjustable electronic distributor :)
>
> Dennis
> --
> Dennis S
> 73 Painted Desert 230
> Germantown, TN
> _______________________________________________
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Sully
77 Royale basket case.
Future motorhome land speed record holder(bucket list)
Seattle, Wa.
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| Re: Timing [message #193338 is a reply to message #193321] |
Wed, 19 December 2012 12:35 |
skip2
Messages: 544
Registered: September 2011
Location: Winter Haven,FL (center o...
Karma: 3
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I've just got to find some valium, for me, so I can let my navigator drive.
Thanks
Skip
74 Canyon Lands, FiTech,
3.7 FD LSD, Manny Tranny,
Springfield Distributor,
2001 Chevy Tracker Ragtop Towd
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| Re: [GMCnet] Timing [message #193364 is a reply to message #193358] |
Wed, 19 December 2012 18:04 |
rcjordan
Messages: 1913
Registered: October 2012
Location: Elizabeth City, North Car...
Karma: 1
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>let my navigator drive
I got lucky, she's just ended an 8 yr tour of duty as a church bus driver. It was one of those 22' wide-body Econoline shuttlebus styles so the Birchaven should feel about right --well, except for the cab. She had her CDL until the recent renewal.
SOLD 77 Royale Coachmen Side Dry Bath
76 Birchaven Coachmen Side Wet Bath
76 Eleganza
Elizabeth City, NC
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| Re: [GMCnet] Timing [message #193365 is a reply to message #193331] |
Wed, 19 December 2012 18:13 |
gbarrow2
Messages: 765
Registered: February 2004
Location: Lake Almanor, Ca./ Red Bl...
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I would try this , but I don't think I could hear the pinging unless it was as loud and evident as a rod knocking.
Is the result going to be significantly better than static timing as it's done in thousands of shops on millions of vehicles
around the world?
Gene Barrow
Lake Almanor, Ca.
1976 Palm Beach
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| Re: [GMCnet] Timing [message #193368 is a reply to message #193365] |
Wed, 19 December 2012 18:52 |
Bruce Hart
Messages: 1501
Registered: October 2011
Location: La Grange, Wyoming
Karma: 5
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Gene, my hearing is also bad but I would think you could use a long
screw driver and use it like a stethoscope to hear the pinging.
On 12/19/12, gene barrow <barrowgene@gmail.com> wrote:
>
>
> I would try this , but I don't think I could hear the pinging unless it was
> as loud and evident as a rod knocking.
>
> Is the result going to be significantly better than static timing as it's
> done in thousands of shops on millions of vehicles
> around the world?
> --
> Gene Barrow
> Lake Almanor, Ca.
> 1976 Palm Beach
> _______________________________________________
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--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
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Bruce Hart
1976 Palm Beach
1977 28' Kingsley
La Grange, Wyoming
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| Re: Timing [message #193369 is a reply to message #193321] |
Wed, 19 December 2012 19:04 |
Darryl
Messages: 144
Registered: December 2011
Location: Northern California
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"Advance the timing until you hear a little hard
pinging then retard the timing until you can hear occasional light pinging.
Light pinging is not a bad thing."
So when I hear light pinging when climbing a hill at 50mph or so in third gear I don't need to back off? I never hear any pinging in second gear at full throttle at 35-40mph but I do hear some at higher speeds in third. I assmue it's light but I thought I needed to adjust the timing.
Darryl Meyers
1978 Eleganza II
El Dorado Hills, CA
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| Re: Timing [message #193372 is a reply to message #193321] |
Wed, 19 December 2012 19:33 |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
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My Dad had a mechanic shop in Chicago in the late 40's. Knew his way around motors. He always told me to time a car by ear. His opinion was that the best overall performance/mileage happened when the engine was timed to ping a LITTLE under heavy load. I've had several cars where the timing mark was not correct and I timed them like this. Worked good...lasted a long time.
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: Timing [message #193385 is a reply to message #193321] |
Wed, 19 December 2012 20:58 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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| skip2 wrote on Wed, 19 December 2012 04:11 |
Awhile back someone posted how they set the timing going down the road with the copilot driving and actually setting it under real loads and inclines. I just don't recall the details so if if someone recalls how to do it I'd appreciate it being explained again.
Thanks
Skip Hartline
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You could buy an MSD timing advance unit for the HEI then all you would do is turn a calibrated knob for advance or retard. MSD also can supply a knock sensor for the folks that have trouble hearing. I can advance mine to 20 degree initial and I don't notice any difference or pinging so I just leave it at 12 degrees. I checked it with a timing light so I know it works.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: Timing [message #193436 is a reply to message #193321] |
Thu, 20 December 2012 11:33 |
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John Heslinga
Messages: 632
Registered: February 2011
Location: Edmonton, Alberta, Canada
Karma: 4
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Re: "So when I hear light pinging when climbing a hill at 50mph or so in third gear I don't need to back off? I never hear any pinging in second gear at full throttle at 35-40mph but I do hear some at higher speeds in third. I assmue it's light but I thought I needed to adjust the timing."
At part throttle you have higher engine vacuum and the vacuum advance pot is adding more advance. At full throttle you will have little or no engine vaccum and the vac advance will not be adding advance to the total advance.
At Amana this fall, Jim Pattison mentioned that the stock vac advance actually adds too much to the total. He said he had vac advance pots that did not add so much which would allow more initial advance, and a better balance at part throttle.
I would speak to him directly to see if this applies to you.
Best regards
John
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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| Re: Timing [message #193439 is a reply to message #193321] |
Thu, 20 December 2012 12:56 |
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John Heslinga
Messages: 632
Registered: February 2011
Location: Edmonton, Alberta, Canada
Karma: 4
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Re: Can we assume that you mean Dick Paterson?
YES!! My appologies to Dick!!
The Other person is a well known western Canadian businessman and I obviously interchanges their names.
John
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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| Re: [GMCnet] Timing [message #193450 is a reply to message #193428] |
Thu, 20 December 2012 16:33 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Matt,
Reading your email brings me to the conclusion that the engineers that designed the engine put the timing light procedure in the
Maintenance Manual to assure if used on any engine it would result in an engine timed "safely". By "safely" I mean the engine would
not be advanced so far as to cause damage if all the components involved were working properly.
From what you note below following that procedure may or may not result in an engine timed optimally. I assume to accomplish that
you have to "do the standard dyno procedure to set best timing."
Could you please describe that procedure?
Thanks,
Rob M.
-----Original Message-----
From: Matt Colie
Gene,
I am in your position as to hearing the ping, but let me assure you that you can just about always do better that the specified
timing. Engines coming off an assembly line are all set to the timing deemed safe by someone long ago. If you do the standard dyno
procedure to set best timing on a collection of stock engines, you will learn that they are all different. Five degrees was common,
but as the tooling aged even that could show more scatter.
I have known more than a few engines that could accept as much a 10° more than book timing and some even more than that.
This is all out the window once you have a computer and a knock sensor to set best timing on a moment by moment basis.
Matt
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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: [GMCnet] Timing [message #193452 is a reply to message #193450] |
Thu, 20 December 2012 16:47 |
gbarrow2
Messages: 765
Registered: February 2004
Location: Lake Almanor, Ca./ Red Bl...
Karma: 3
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Senior Member |
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Matt,
From what you note below following that procedure may or may not result in an engine timed optimally. I assume to accomplish that
you have to "do the standard dyno procedure to set best timing."
Could you please describe that procedure?
And while you're at it can you also 'splain the proceedure for setting time using a vacuum gauge?
Thanks
Gene Barrow
Lake Almanor, Ca.
1976 Palm Beach
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| Re: [GMCnet] Timing [message #193547 is a reply to message #193452] |
Fri, 21 December 2012 09:46 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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| Robert Mueller wrote on Thu, 20 December 2012 17:33 |
Matt,
Reading your email brings me to the conclusion that the engineers that designed the engine put the timing light procedure in the Maintenance Manual to assure if used on any engine it would result in an engine timed "safely". By "safely" I mean the engine would not be advanced so far as to cause damage if all the components involved were working properly.
From what you note below following that procedure may or may not result in an engine timed optimally. I assume to accomplish that you have to "do the standard dyno procedure to set best timing."
Could you please describe that procedure?
Thanks,
Rob M.
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| gbarrow wrote on Thu, 20 December 2012 17:47 |
Matt,
From what you note below following that procedure may or may not result in an engine timed optimally. I assume to accomplish that you have to "do the standard dyno procedure to set best timing."
Could you please describe that procedure?
And while you're at it can you also 'splain the proceedure for setting time using a vacuum gauge?
Thanks
|
Are you two trying to gang up on me??
I don't mind explaining the process in detail, but I won't go into the complete detail simply because it is nothing that can be done outside of a completely equipped and well instrumented test facility. Start by controlling the intake air quality to 68°F+/-3 and RH 50%+/-5 the coolant in at Spec (typically 160°F) and a fuel temperature also at 68°F. We have (for years) had cell readouts that return actual timing and I like to do a "pull down", so I would have (or the tech if BU) start at the engine's max rev and set WOT. Then we keep incrementing the timing forward one degree at a time, and when it is set, then do a fuel loop. That means incrementally increasing the fuel flow until the torque “flattens out. (Going past that serves only to carbon foul the engine.) We would look for the “knee” in the curve and mark that one LBT (Lean for Best Torque) at that timing. Bump the timing ahead on degree and do this again. And Again. And Again. Until it is clear that the peak has been passed over. Then we look back at the data for the drop in torque and split that curve and mark that MBT (Minimum for Best Torque). We now have one point at one RPM and Throttle position. Then we increase the load that the absorber (the brake part of a dynomometer) is putting on our test engine until it is one or two hundred RPM lower. We would repeat these loops. When you get to as low an RPM as is practical, then you start over again at a different throttle position and do it all again.
Keep repeating the above until you have filled a small telephone book with data. Are you bored yet??? This is called engine mapping. And in a modern development test cell, the system has recorded all that data along with any applicable data about exhaust emissions. The cells I built last could do this without operator intervention. (Even unattended if not a BU issue.)
Even long before the current era, this map still had to be created early in an engine family’s life so the production distributor curve and power enrichment points could be set.
Though I had little respect for the man’s technical capability, Smokey Yunick once said that most carburetor problems are cause by ignition issues. Yes, they are that related, but until the introduction of the ECU with knock sensing, there was little that could be done but run with “safe” timing.
**** Safe Timing **** (Finally)
Here we get back to the “safe” timing issue. Yes, all the production engines are set with a base calibration (timing, advance curve and fuel delivery) that proved safe for all but about 1% of the engines produced (they just hope grama gets that 1% so the warranty doesn’t eat then alive). As I remarked I a prior post, there can be significant scatter in an engine family. Much less these days than were accepted in the past. Before GM’s Central Foundry was retooled in the mid 80s, that scatter was about 10° of timing and about 20 HP on the small block line. Ford and Chrysler were a little tighter, but not to any great degree.
How big was this? An acquaintance used to run box stock drag cars and win all the time. This was before bracket racing. He liked chevy, so had an order in for a 8V auto mid size (he would chose a body style that year on weight) and he would get called when ever one came in so he could drive it. About once a year he would pick one. Then he would break it in and take racing and keep on winning until he sold it. Then put another order in.
Toward the end of my time in engine development, everyone was pushing to go the mathematical modeling to try to shorten the mapping cycle, last I heard, it still was only good at reducing the number of stops to make loops that had to be run.
Only in recent years have I run my own vehicles with the “out of the box” calibration. They can almost always be improved on.
What can you (a typical citizen- GMC owner) do about this? Short of going to EFI with EBL and a knock sensor, not much. The easiest thing to do is play with the base timing. Adjusting the centrifugal and vacuum advance takes tools you don’t have. I don’t even know how much Dick Patterson modifies the advance curve, but I am sure that this is based on his significant experience as he does not have the time or facility to run and MBT-LBT on every engine.
Now, if it were in a boat, you could take it out on the bay and open the throttle to cruise, then go down and nudge the distributor ahead until there was no gain and then retard until there was a little loss, then advance to recover the loss. This is what I have done for years. Many marine engines don’t even have timing marks. (This is the actual procedure in the manual of all the Universal engines since the late thirties.) Fortunately, in a GMC we can come pretty close to this. One can, and I have done this, but it requires a good stretch of road and a patient driver.
*** Timing by vacuum ***
As to setting the timing by vacuum, this presumes that the advance curves are correct as all you can do is set for best idle by juggling the base timing with the throttle stop screw. If the vehicle has a manifold (not ported) vacuum advance, then that usually has to be disconnected. But, again, all this will get you is a good base timing. The standard method is to just tweek the distributor to a best idle speed. The big problem here is that if that best speed gets near the mechanical advance threshold, you may be setting a timing that is not actually at the base and that will be reflected all the way up the curve, this may require adjusting the throttle stop. In short, it takes knowing the engine. Even with a best idle timing, you may have to back it out a few degrees or an inch or so of vacuum to get to a timing that actually works with what we now have available for fuel. There just is no substitute for experience - even with all the nifty electronical gismos that are available. Listen to your engine, it will complain when it is not happy and you would do well to listen.
Well, I’m tired of writing about this and I do have other things to do. But if you have questions, I will try to answer them in words normal humans (not just engineers) can understand.
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
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| Re: [GMCnet] Timing [message #193624 is a reply to message #193547] |
Fri, 21 December 2012 18:02 |
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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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Matt,
Thanks for the detailed response; I'm going to have to read this a number of times to comprehend it completely, however, after the
first read I reckon the operative sentence is:
"I won't go into the complete detail simply because it is nothing that can be done outside of a completely equipped and well
instrumented test facility."
This reinforces my plan to choose an EFI system of some kind for the Patterson Stage II engine I bought off the burned up coach in
Canada and have it dyno tuned before I install it in Double Trouble.
Regards,
Rob M.
-----Original Message-----
From: Matt Colie
Are you two trying to gang up on me??
I don't mind explaining the process in detail, but I won't go into the complete detail simply because it is nothing that can be done
outside of a completely equipped and well instrumented test facility. Start by controlling the intake air quality to 68°F+/-3 and
RH 50%+/-5 the coolant in at Spec (typically 160°F) and a fuel temperature also at 68°F. We have (for years) had cell readouts that
return actual timing and I like to do a "pull down", so I would have (or the tech if BU) start at the engine's max rev and set WOT.
Then we keep incrementing the timing forward one degree at a time, and when it is set, then do a fuel loop. That means
incrementally increasing the fuel flow until the torque “flattens out. (Going past that serves only to carbon foul the
engine.) We would look for the “knee” in the curve and mark that one LBT (Lean for Best Torque) at that timing. Bump
the timing ahead on degree and do this again. And Again. And Again. Until it is clear that the peak has been passed over. Then
we look back at the data for the drop in torque and split that curve and mark that MBT (Minimum for Best Torque). We now have one
point at one RPM and Throttle position. Then we increase the load that the absorber (the brake part of a dynomometer) is putting on
our test engine until it is one or two hundred RPM lower. We would repeat these loops. When you get to as low an RPM as is
practical, then you start over again at a different throttle position and do it all again.
Keep repeating the above until you have filled a small telephone book with data. Are you bored yet??? This is called engine
mapping. And in a modern development test cell, the system has recorded all that data along with any applicable data about exhaust
emissions. The cells I built last could do this without operator intervention. (Even unattended if not a BU issue.)
Even long before the current era, this map still had to be created early in an engine family’s life so the production
distributor curve and power enrichment points could be set.
Though I had little respect for the man’s technical capability, Smokey Yunick once said that most carburetor problems are
cause by ignition issues. Yes, they are that related, but until the introduction of the ECU with knock sensing, there was little
that could be done but run with “safe” timing.
**** Safe Timing **** (Finally)
Here we get back to the “safe” timing issue. Yes, all the production engines are set with a base calibration (timing,
advance curve and fuel delivery) that proved safe for all but about 1% of the engines produced (they just hope grama gets that 1% so
the warranty doesn’t eat then alive). As I remarked I a prior post, there can be significant scatter in an engine family.
Much less these days than were accepted in the past. Before GM’s Central Foundry was retooled in the mid 80s, that scatter
was about 10° of timing and about 20 HP on the small block line. Ford and Chrysler were a little tighter, but not to any great
degree.
How big was this? An acquaintance used to run box stock drag cars and win all the time. This was before bracket racing. He liked
chevy, so had an order in for a 8V auto mid size (he would chose a body style that year on weight) and he would get called when ever
one came in so he could drive it. About once a year he would pick one. Then he would break it in and take racing and keep on
winning until he sold it. Then put another order in.
Toward the end of my time in engine development, everyone was pushing to go the mathematical modeling to try to shorten the mapping
cycle, last I heard, it still was only good at reducing the number of stops to make loops that had to be run.
Only in recent years have I run my own vehicles with the “out of the box” calibration. They can almost always be
improved on.
What can you (a typical citizen- GMC owner) do about this? Short of going to EFI with EBL and a knock sensor, not much. The
easiest thing to do is play with the base timing. Adjusting the centrifugal and vacuum advance takes tools you don’t have. I
don’t even know how much Dick Patterson modifies the advance curve, but I am sure that this is based on his significant
experience as he does not have the time or facility to run and MBT-LBT on every engine.
Now, if it were in a boat, you could take it out on the bay and open the throttle to cruise, then go down and nudge the distributor
ahead until there was no gain and then retard until there was a little loss, then advance to recover the loss. This is what I have
done for years. Many marine engines don’t even have timing marks. (This is the actual procedure in the manual of all the
Universal engines since the late thirties.) Fortunately, in a GMC we can come pretty close to this. One can, and I have done
this, but it requires a good stretch of road and a patient driver.
*** Timing by vacuum ***
As to setting the timing by vacuum, this presumes that the advance curves are correct as all you can do is set for best idle by
juggling the base timing with the throttle stop screw. If the vehicle has a manifold (not ported) vacuum advance, then that usually
has to be disconnected. But, again, all this will get you is a good base timing. The standard method is to just tweek the
distributor to a best idle speed. The big problem here is that if that best speed gets near the mechanical advance threshold, you
may be setting a timing that is not actually at the base and that will be reflected all the way up the curve, this may require
adjusting the throttle stop. In short, it takes knowing the engine. Even with a best idle timing, you may have to back it out a
few degrees or an inch or so of vacuum to get to a timing that actually works with what we now have available for fuel. There just
is no substitute for experience - even with all the nifty electronical gismos that are available. Listen to your engine, it will
complain when it is not happy and you would do well to listen.
Well, I’m tired of writing about this and I do have other things to do. But if you have questions, I will try to answer them
in words normal humans (not just engineers) can understand.
Matt
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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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