Home » Public Forums » GMCnet » Assumptions, Timing, Fuel...
| Assumptions, Timing, Fuel... [message #241296] |
Tue, 25 February 2014 08:37  |
corleyw
Messages: 130
Registered: June 2007
Location: Battle Ground, WA
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Assumptions:
1) Retarded timing causes greater heat
2) Retarded timing causes less power
3) GMCs (both 455 and 403) must retard the timing to run on todays 87 octane regular fuel.
Leaving the ethanol considerations out of this altogether, if you are running on regular 87 octane gasoline, you probably have your timing retarded from the stock factory setting. What would happen if you advanced it a couple degrees, and ran 91/92 octane ethanol fuel?
Would it a) run cooler, and b) have better performance? If both of those are true, then would you possibly get enough better mileage, due to the engine not working so hard, as to make up for the cost differential? (Another benefit would be cooler operation, a potential cost saving down the road.)
Keep in mind that premium fuel costs only .20 or .30 cents more than regular now days, and that is only about a 3-5% increase at todays prices. Personally, I'd be willing to pay a bit more for fuel if I thought it would help the coaches performance/engine life in any way. Just maybe, running premium is actually no more expensive when everything is considered???
Just wondering what the opinion of you "experts" is on this... (I do not expect any sort of consensus on this subject.)
Corley
'76 Glenbrook
29 other vehicles
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| Re: Assumptions, Timing, Fuel... [message #241308 is a reply to message #241296] |
Tue, 25 February 2014 10:06  |
Chris Tyler
Messages: 458
Registered: September 2013
Location: Odessa FL
Karma: 7
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In theory, 87 octane E10 would be the same as 87 octane straight gasoline. Ethonol has a better octane rating than even premium, so it is used to improve the octane value of different grades of gasoline, replacing other additives. And make no mistake, gasoline is always a blend of additives, a chemical stew.
The problem is that octane is resistance to knock [detonation] not power. It has a higher octane rating but less heat energy by weight. I have found a difference in milage of 5% under equal conditions comparing the two on my Corvette when it was stock. No difference in performance though. That was 93 octane.
To answer your question... Maybe. For every condition of RPM, load, throttle opening, inlet temp, air/fuel ratio and a dozen other factors, there is an optimum level of advance that will produce the most torque because it reuires just the right timing to produce the maximum cyl pressure at just the right time. 1970s vintage engines were hampered by emission requirments that resulted in retarded ignition and cam timing to meet the standards.
Now, with computer controlls all those factors can be considered and optimized electronicly. You can experiment with a lot of initial/ centrifigal/ vacuum timing curves to try to find the sweet spot but unless you know what you are doing its easy to get lost. If you invest in EGT, Wide band AFR gauges, Have a programable EFI and or ignition system or at least it makes the process much easier
But here is the rub...you will in all likelihood never recoup the invest ment in all that in terms of improved milage. So unless you are an inverterate tinkerer [like me], why bother? I probably will at some point as I have those things laying around.
The low tech/old school way: If you run more initial you have to limit the total timing. If you recurve the distributor with lighter springs and weights you will need premium fuel. Basicly, you power time the engine by advancing until there is no improvement in acceleration or it pings. When it does, back off because it is probably detnating before you can hear it. The vac advance has a major role in cruise milage, but again, too much can hurt.
Middle ground is an adjustable vac advance canister and a MSD ping control. A knock sensor helps too.
If you can, try to find an old school tuner with experiance. Most of them now can only tune electronicly... which I am just now beginning to learn.
Is it worth the chance? Thats up to you.
76 Glenbrook
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| Re: Assumptions, Timing, Fuel... [message #241325 is a reply to message #241296] |
Tue, 25 February 2014 12:20 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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| corleyw wrote on Tue, 25 February 2014 09:37 |
Assumptions:
1) Retarded timing causes greater heat
2) Retarded timing causes less power
3) GMCs (both 455 and 403) must retard the timing to run on todays 87 octane regular fuel.
Leaving the ethanol considerations out of this altogether, if you are running on regular 87 octane gasoline, you probably have your timing retarded from the stock factory setting. What would happen if you advanced it a couple degrees, and ran 91/92 octane ethanol fuel?
Would it a) run cooler, and b) have better performance? If both of those are true, then would you possibly get enough better mileage, due to the engine not working so hard, as to make up for the cost differential? (Another benefit would be cooler operation, a potential cost saving down the road.)
Keep in mind that premium fuel costs only .20 or .30 cents more than regular now days, and that is only about a 3-5% increase at todays prices. Personally, I'd be willing to pay a bit more for fuel if I thought it would help the coaches performance/engine life in any way. Just maybe, running premium is actually no more expensive when everything is considered???
Just wondering what the opinion of you "experts" is on this... (I do not expect any sort of consensus on this subject.)
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Corley,
Two of your opening assumptions are correct and one is not. The fact is that I run our coach's base timing advanced several degrees from specified. The thing that is incontrovertible is that the heat content of ethanol is only two-thirds of most gasoline*. This is clearly demonstrated if you can go between two fuels cleanly. In my case, with my coach, the fuel rate is increased almost exactly for the content of alcohol I measure when fueling. So, if I can take on fuel without alcohol, I will get fuel mileage that is 5~10% better than fuel without. Strangely, this is exactly what the results of laboratory tests over two decades ago predicted would happen. The other joke is that this was all done to lessen the US requirements for foreign oil imports and it has been a complete disaster in that regard, so what else could they do? ¿¿Double Down??
*Gasoline is not a chemical compound and not always the same thing. It is chemically described by a Carbon/Hydrogen ratio and is commercially a petroleum distillation between temperature between 90ºF and 400ºF.
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: Assumptions, Timing, Fuel... [message #241328 is a reply to message #241296] |
Tue, 25 February 2014 12:31 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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You made a lot of assumptions that are not exactly correct.
You are assuming that higher octane fuel has more energy and better performance. Actually increased octane fuel reduces the chance of pre-ignition (detonation, ping) in higher compression engines. That is it. If you increase compression of an engine then you can see increased power along with higher fuel consumption. If you do not increase the octane rating of he fuel a higher compression engine, the fuel will tend to explode in certain conditions. So higher Octane fuel is used to reduce that possibility.
If you increase octane beyond what is normally required for a given engine you can actually reduce the of power applied to the pistons because the fuel air mix burns slightly slower. The amount of fuel and air that you convert to mechanical power is directly related to where in the piston cycle the fuel is burned. So what you want is the maximum fuel/air burn at a point just AFTER after the piston reaches TDC. Since higher octane fuel burns slower we fire the spark plugs earlier to achieve maximum burn at that point just past TDC and beyond. You will gain nothing by using too high octane fuel in a lower compression engine like ours. If you want to replace the pistons with say 10.5 to 1 then you can achieve more power and will be forced to use higher octane fuel to prevent detonation.
Now fuel economy is another thing. Ethanol laced fuel while being higher in octane is also lower in energy (BTUs) per gallon and lower fuel economy will result when using it.
Our 455s and 403 engines are NOT retarded from the GM spec when using 87 octane fuel. My 455 is set for a static timing of 13 BTDC, 10 vacuum, and 18 centrifugal for a maximum of 41 BTDC. This is slightly advanced over the GM spec. I use 87 octane fuel exclusively except in areas above 4000 or 5000 feet where 86 or 85 is sold as regular. I also do my best NOT to buy craponol laced fuel (E10).
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
[Updated on: Wed, 26 February 2014 02:21] Report message to a moderator |
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| Re: Assumptions, Timing, Fuel... [message #241333 is a reply to message #241308] |
Tue, 25 February 2014 12:48 |
mikethebike
Messages: 331
Registered: January 2014
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Back in 1980 my wifes Ford Fiesta would not run on regular...and this was in Charleston, SC on flat land. Put 10% gasohol and no more sprark knock.....but mpg was down 5-10%. I kept records of every drop of gas that went into that car. Every time I used 10% mix, the milage went down. I've not seen the same results with the 2007 Accord?????
| Chris Tyler wrote on Tue, 25 February 2014 10:06 |
In theory, 87 octane E10 would be the same as 87 octane straight gasoline. Ethonol has a better octane rating than even premium, so it is used to improve the octane value of different grades of gasoline, replacing other additives. And make no mistake, gasoline is always a blend of additives, a chemical stew.
The problem is that octane is resistance to knock [detonation] not power. It has a higher octane rating but less heat energy by weight. I have found a difference in milage of 5% under equal conditions comparing the two on my Corvette when it was stock. No difference in performance though. That was 93 octane.
To answer your question... Maybe. For every condition of RPM, load, throttle opening, inlet temp, air/fuel ratio and a dozen other factors, there is an optimum level of advance that will produce the most torque because it reuires just the right timing to produce the maximum cyl pressure at just the right time. 1970s vintage engines were hampered by emission requirments that resulted in retarded ignition and cam timing to meet the standards.
Now, with computer controlls all those factors can be considered and optimized electronicly. You can experiment with a lot of initial/ centrifigal/ vacuum timing curves to try to find the sweet spot but unless you know what you are doing its easy to get lost. If you invest in EGT, Wide band AFR gauges, Have a programable EFI and or ignition system or at least it makes the process much easier
But here is the rub...you will in all likelihood never recoup the invest ment in all that in terms of improved milage. So unless you are an inverterate tinkerer [like me], why bother? I probably will at some point as I have those things laying around.
The low tech/old school way: If you run more initial you have to limit the total timing. If you recurve the distributor with lighter springs and weights you will need premium fuel. Basicly, you power time the engine by advancing until there is no improvement in acceleration or it pings. When it does, back off because it is probably detnating before you can hear it. The vac advance has a major role in cruise milage, but again, too much can hurt.
Middle ground is an adjustable vac advance canister and a MSD ping control. A knock sensor helps too.
If you can, try to find an old school tuner with experiance. Most of them now can only tune electronicly... which I am just now beginning to learn.
Is it worth the chance? Thats up to you.
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| Re: Assumptions, Timing, Fuel... [message #241350 is a reply to message #241333] |
Tue, 25 February 2014 14:54 |
tphipps
Messages: 3005
Registered: August 2004
Location: Spanish Fort, AL
Karma: 9
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The Accord's EFI is allowing for the difference. My experience, the minor difference in non-ethanol fuel price is more than offset by better performance and gas mileage. If ADM did not have politicians in their pocket, we would be now running straight gas.
Consider the Mazda Skyfire series of engines, and check how closely they are computer controlled. Wonder if they have a "limp home" condition.
We are stuck with the 10% stuff, hope the 15% is not forced on us. Interesting side note; Brazil has 100% alcohol fuel, made from Sugar Cane, for the entire country. With the drought, I wonder what will happen?
Tom, MS II
2012 Phoenix Cruiser model 2552
KA4CSG
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| Re: Assumptions, Timing, Fuel... [message #241354 is a reply to message #241296] |
Tue, 25 February 2014 15:39 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
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I have an MSD dash adjustable timing control on my coach. I have the base timing set in such a way as to have the ability to retard or advance the timing from the driver's seat. This gives me the ability to run optimum timing for different situations.
For example, on my annual sand dune trip, I tow my Jeep, on a trailer, from my home in Tucson (2,800 ft elevation) to Southeastern California (25' elevation). Near the western end of the trip, just before Yuma, AZ, there is a mountain pass to be crossed. It is not real high or long, but it is somewhat steep and requires full throttle and usually a downshift at some point. When I start to hear the slightest pre-ignition knock, I dial the timing back about eight degrees.
Conversely, when I am up in Northern AZ or NM, on I-40, I can advance the timing an additional 12 degrees if I think of it. Theoretically, that should give me a little better fuel efficiency.
I mostly installed the adjustable timing to save my engine under high loads at lower altitudes. Works for me. As I get older (I'm currently 58) and my hearing deteriorates further, I may (have to) invest in a knock sensor in order to get a jump on it.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: [GMCnet] Assumptions, Timing, Fuel... [message #241358 is a reply to message #241350] |
Tue, 25 February 2014 15:56 |
Jim Bounds
Messages: 842
Registered: January 2004
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I had an 81 Mazda 626 company car, drove it many miles. Was set up for ethanol fuel back then which I think was all ethanol. Ran really good, just had to have the carb built for it.
Jim Bounds
--------------------
On Tuesday, February 25, 2014 3:55 PM, Thomas Phipps <tph1pp5@yahoo.com> wrote:
The Accord's EFI is allowing for the difference. My experience, the minor difference in non-ethanol fuel price is more than offset by better performance and gas mileage. If ADM did not have politicians in their pocket, we would be now running straight gas.
Consider the Mazda Skyfire series of engines, and check how closely they are computer controlled. Wonder if they have a "limp home" condition.
We are stuck with the 10% stuff, hope the 15% is not forced on us. Interesting side note; Brazil has 100% alcohol fuel, made from Sugar Cane, for the entire country. With the drought, I wonder what will happen?
Tom, MS II
--
1975 GMC Avion
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| Re: [GMCnet] Assumptions, Timing, Fuel... [message #241381 is a reply to message #241333] |
Tue, 25 February 2014 18:24 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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Modern engine control systems will take advantage of whatever you put in them, within limits. Feed them good gas, they'll advance the timing and take advantage of better anti-knock fuel. Feed them 'rabbit gas' and they'll turn things down to compensate. They're doing this to keep emissions low and get the best mileage they can. As an ecercise with your new car, start with the cheeeep stuff and check the mileage. Step up one grade, and do it again. Advance till you gain nothing, then back up one. Quick and dirty way to figure out what grade of gas the thing does best with.
--johnny
'76 23' transmode norris
________________________________
From: mike foster <mafoster1@bellsouth.net>
To: gmclist@temp.gmcnet.org
Sent: Tuesday, February 25, 2014 1:48 PM
Subject: Re: [GMCnet] Assumptions, Timing, Fuel...
Back in 1980 my wifes Ford Fiesta would not run on regular...and this was in Charleston, SC on flat land. Put 10% gasohol and no more sprark knock.....but mpg was down 5-10%. I kept records of every drop of gas that went into that car. Every time I used 10% mix, the milage went down. I've not seen the same results with the 2007 Accord?????
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: Assumptions, Timing, Fuel... [message #241386 is a reply to message #241296] |
Tue, 25 February 2014 18:40 |
JohnL455
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So I would think the above threads answered your question. Basically we have noticed more response, less throttle required, cooler running (less heat rejection to coolant) and better MPG by running a little more advance than GM 70's stock setting that was to pass emissions. Oddly if you use less pounds of fuel you would produce less pounds of emissions. Running more retarded also required more throttle but lower cyl pressures were emissions friendly. Another extreme bad case is with premium/low compression is that if the flame event is not finished in time, the exhaust valves really suffer trying to survive
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Assumptions, Timing, Fuel... [message #241400 is a reply to message #241296] |
Tue, 25 February 2014 19:28 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
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Corley,
I think these two references will answer your question:
1) Maintenance Manual X-7525 / Section 6 Engine Electrical / Page 6Y-59 HIGH ENERGY IGNITION SYSTEM SPECIFICATIONS
2) http://www.gmcmhphotos.com/photos/random-photos/p32021-dick-paterson-s-distri.html
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Corley Wooldridge
Sent: Wednesday, February 26, 2014 1:38 AM
To: gmclist@temp.gmcnet.org
Subject: [GMCnet] Assumptions, Timing, Fuel...
Assumptions:
1) Retarded timing causes greater heat
2) Retarded timing causes less power
3) GMCs (both 455 and 403) must retard the timing to run on todays 87 octane regular fuel.
Leaving the ethanol considerations out of this altogether, if you are running on regular 87 octane gasoline, you probably have your
timing retarded from the stock factory setting. What would happen if you advanced it a couple degrees, and ran 91/92 octane ethanol
fuel?
Would it a) run cooler, and b) have better performance? If both of those are true, then would you possibly get enough better
mileage, due to the engine not working so hard, as to make up for the cost differential? (Another benefit would be cooler
operation, a potential cost saving down the road.)
Keep in mind that premium fuel costs only .20 or .30 cents more than regular now days, and that is only about a 3-5% increase at
todays prices. Personally, I'd be willing to pay a bit more for fuel if I thought it would help the coaches performance/engine life
in any way. Just maybe, running premium is actually no more expensive when everything is considered???
Just wondering what the opinion of you "experts" is on this... (I do not expect any sort of consensus on this subject.)
--
Corley
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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: Assumptions, Timing, Fuel... [message #241467 is a reply to message #241328] |
Wed, 26 February 2014 09:41 |
corleyw
Messages: 130
Registered: June 2007
Location: Battle Ground, WA
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| Ken Burton wrote on Tue, 25 February 2014 09:31 |
You made a lot of assumptions that are not exactly correct.
I made two assumptions that are givens, and one assumption that is true for my coach.
You are assuming that higher octane fuel has more energy and better performance. No I am not. I am assuming that advancing the timing will produce more power.
Actually increased octane fuel reduces the chance of pre-ignition (detonation, ping) in higher compression engines. That is it. If you increase compression of an engine then you can see increased power along with higher fuel consumption. If you do not increase the octane rating of he fuel a higher compression engine, the fuel will tend to explode in certain conditions. So higher Octane fuel is used to reduce that possibility.
If you increase octane beyond what is normally required for a given engine you can actually reduce the of power applied to the pistons because the fuel air mix burns slightly slower. The amount of fuel and air that you convert to mechanical power is directly related to where in the piston cycle the fuel is burned. So what you want is the maximum fuel/air burn at a point just AFTER after the piston reaches TDC. Since higher octane fuel burns slower we fire the spark plugs earlier to achieve maximum burn at that point just past TDC and beyond. You will gain nothing by using too high octane fuel in a lower compression engine like ours. Is it not true that if I advance the timing I will get better performance (to a point), but I will also get pre ignition, which can be counterred by high octane fuel?
If you want to replace the pistons with say 10.5 to 1 then you can achieve more power and will be forced to use higher octane fuel to prevent detonation.
Now fuel economy is another thing. Ethanol laced fuel while being higher in octane is also lower in energy (BTUs) per gallon and lower fuel economy will result when using it. I said not to consider ethanol, so I'll ignore this part.
Our 455s and 403 engines are NOT retarded from the GM spec when using 87 octane fuel. My '76 455 is retarded, by two degrees, in order to stop detonation. My 455 is set for a static timing of 13 BTDC, 10 vacuum, and 18 centrifugal for a maximum of 41 BTDC. This is slightly advanced over the GM spec. I use 87 octane fuel exclusively except in areas above 4000 or 5000 feet where 86 or 85 is sold as regular. I also do my best NOT to buy craponol laced fuel (E10).
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My comments above are in red. Engines vary a lot. Casting flashes, carbon deposits, hot spots, etc.. My dizzy has never been out of the hole, so it is a total unknown. I DO know I have to run slightly retarded on 87 fuels. If I could advance it a bit, it would seem I should run cooler, and get better performance. I can't do that without runing higher octane fuel. I know how it all works, I've been tuning engines since the early '50s. The question is, is it worth the extra in cost on MY coach? I know, you will say "get the distributor curves checked". If that is your answer, don't bother, it's already been said.
Corley
'76 Glenbrook
29 other vehicles
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| Re: Assumptions, Timing, Fuel... [message #241491 is a reply to message #241467] |
Wed, 26 February 2014 12:12 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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[quote title=corleyw wrote on Wed, 26 February 2014 09:41]| Ken Burton wrote on Tue, 25 February 2014 09:31 |
You made a lot of assumptions that are not exactly correct.
I made two assumptions that are givens, and one assumption that is true for my coach.
You are assuming that higher octane fuel has more energy and better performance. No I am not. I am assuming that advancing the timing will produce more power.
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That is a common misconception. Advancing timing only makes up for slower burning higher octane fuel. You can not have burning before TDC, I do not care what octane you are using. Power from burning can not be applied when the piston is still rising (BTDC). If you do apply burning power BTDC you will hear it ping. The only reason for using higher octane fuel is to prevent pre-ignition. Pre-ignition occurs when the compression ratio (and accompanying heat) is to high for the fuel being used.
If you are having to reduce your static timing, I would be looking for what else is causing your problem.
Some of the causes might be:
1. Slipped outer ring on the harmonic balancer. I have seen several of these on GMCs. Especially ones that got oil soaked from a leaking front seal.
2. The vacuum advance hooked to manifold vacuum instead of ported vacuum,
3. A vacuum advance sticking.
4. The centrifugal advance sticking
5. A weak, worn, or missing advance spring.
I would take a very good look at the advance weights, springs, and the advance weight pivots. These are a very common failure point on these distributors after all these years. While you are in there, if they look good, lube all of the weight and spring pivot points.
It might be time to pull that distributor and stick it on a distributor machine to see what you really have going on there.
Another possibility is to send your distributor in to Dick Paterson. He will replace those worn parts and set the vacuum and centrifugal advance to match your GMC MH requirements.
Severely retarded timing will cause engine overheating and possibly burned valves. Two degree retarded in NOT severely retarded.
Good luck with your quest.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Assumptions, Timing, Fuel... [message #241493 is a reply to message #241354] |
Wed, 26 February 2014 12:15 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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| Carl S. wrote on Tue, 25 February 2014 13:39 |
I have an MSD dash adjustable timing control on my coach. I have the base timing set in such a way as to have the ability to retard or advance the timing from the driver's seat. This gives me the ability to run optimum timing for different situations.
For example, on my annual sand dune trip, I tow my Jeep, on a trailer, from my home in Tucson (2,800 ft elevation) to Southeastern California (25' elevation). Near the western end of the trip, just before Yuma, AZ, there is a mountain pass to be crossed. It is not real high or long, but it is somewhat steep and requires full throttle and usually a downshift at some point. When I start to hear the slightest pre-ignition knock, I dial the timing back about eight degrees.
Conversely, when I am up in Northern AZ or NM, on I-40, I can advance the timing an additional 12 degrees if I think of it. Theoretically, that should give me a little better fuel efficiency.
I mostly installed the adjustable timing to save my engine under high loads at lower altitudes. Works for me. As I get older (I'm currently 58) and my hearing deteriorates further, I may (have to) invest in a knock sensor in order to get a jump on it.
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Did you see an improvement by advancing the total timing with the MSD ? I have one on my coach but I don't see any difference in performance so I just leave it at 8 or 10 degrees initial advance.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: [GMCnet] Assumptions, Timing, Fuel... [message #241496 is a reply to message #241488] |
Wed, 26 February 2014 12:19 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Probably more important from performance standpoint is to seek out fuel
that contains a lower level of Alcohol, than to select fuel based on
octane. Most fuels in today's market achieve their AKI from alcohol
blending, so this is more difficult to achieve. When we did the Cross
Canada rolling rally, all the coaches settled on PetroCanada 92 octane that
was a no alcohol fuel. We did not need the higher octane, What we were
after was the improved fuel mileage and no deterioration of the rubber
parts in these old coaches. As soon as we arrived in the States, we all
experienced vapor lock and similar problems on crapahol fuels. Your
experience may vary.
Jim Hupy
Salem Or
78 GMC Royale 403
On Feb 26, 2014 10:01 AM, "A." <markbb1@netzero.com> wrote:
>
>
> corleyw wrote on Wed, 26 February 2014 09:41
> > ...I DO know I have to run slightly retarded on 87 fuels. If I could
> advance it a bit, it would seem I should run cooler, and get better
> performance. I can't do that without runing higher octane fuel.
> > ...
> > The question is, is it worth the extra in cost on MY coach?...
> If you are asking if your cost per mile for fuel will drop if you advance
> your timing and burn higher grade fuel, my gut says "no".
>
> But the only way to know for sure is to try it.
>
> What most people here were telling you is that they DO NOT have to retard
> the timing on theirs to run modern 87 octane fuel. Several advance the
> timing and still do not experience pinging.
>
> My guess is that your carb runs leaner than theirs, since we seem to have
> beat the spark timing issue to death. The carb is the remaining variable.
> --
> '73 23' Sequoia For Sale
> '73 23' CanyonLands For Sale
> UA (Upper Alabama)
> CanyonLands most likely for a parts coach. Sequoia being restored to
> service.
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| Re: Assumptions, Timing, Fuel... [message #241503 is a reply to message #241491] |
Wed, 26 February 2014 12:50 |
corleyw
Messages: 130
Registered: June 2007
Location: Battle Ground, WA
Karma: 0
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[quote title=Ken Burton wrote on Wed, 26 February 2014 09:12]| corleyw wrote on Wed, 26 February 2014 09:41 |
| Ken Burton wrote on Tue, 25 February 2014 09:31 |
You made a lot of assumptions that are not exactly correct.
I made two assumptions that are givens, and one assumption that is true for my coach.
You are assuming that higher octane fuel has more energy and better performance. No I am not. I am assuming that advancing the timing will produce more power.
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That is a common misconception. Advancing timing only makes up for slower burning higher octane fuel. uh, what happens if you retarded your timimg, say 20 or 30 degrees? It will over heat, and it will be way down on power.You can not have burning before TDC, I do not care what octane you are using. Power from burning can not be applied when the piston is still rising (BTDC). If you do apply burning power BTDC you will hear it ping. The only reason for using higher octane fuel is to prevent pre-ignition. Pre-ignition occurs when the compression ratio (and accompanying heat) is to high for the fuel being used.
If you are having to reduce your static timing, It's been running with 2 degrees of static retard ever since I've owned it, about 20 years now, and about 50k miles.I would be looking for what else is causing your problem. As I indicated, there are a million things that could cause this. None of that matters to the question I asked.
Some of the causes might be:
1. Slipped outer ring on the harmonic balancer. I have seen several of these on GMCs. Especially ones that got oil soaked from a leaking front seal.
2. The vacuum advance hooked to manifold vacuum instead of ported vacuum,
3. A vacuum advance sticking.
4. The centrifugal advance sticking
5. A weak, worn, or missing advance spring.
I would take a very good look at the advance weights, springs, and the advance weight pivots. These are a very common failure point on these distributors after all these years. While you are in there, if they look good, lube all of the weight and spring pivot points.
It might be time to pull that distributor and stick it on a distributor machine to see what you really have going on there.
Another possibility is to send your distributor in to Dick Paterson. He will replace those worn parts and set the vacuum and centrifugal advance to match your GMC MH requirements.
Severely retarded timing will cause engine overheating and possibly burned valves. Two degree retarded in NOT severely retarded.
Good luck with your quest.
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From many years of experience, I can tell you that if an engine is retarded, it overheats more easily, and it will be down on power. When you approach ideal timing, a higher octane fuel lets you advance the timing further because higher octane fuel burns more slowly. It has been claimed that the slower burning in a slow turning engine like ours produces more power. The question is all about whether that is a true statement or not.
OK, so we don't exactly agree on some of that. Let's move on...
Now to introduce the ethanol subject, which is a totally different aspect, but related: Would ethanol fuel be easier to tolerate, with higher octane fuel, since higher octane fuel is less volitile? (Or is it?) Putting it a different way, if it is true that higher octane fuel burns slower, is it also less volitile and less likely to cause vapor locks?
Being slower and less volitile, does it also vapor lock at a higher temp?
That could be a real benifit, IF TRUE... (and that was the real reason for introducing this subject.)
Corley
'76 Glenbrook
29 other vehicles
[Updated on: Wed, 26 February 2014 12:53] Report message to a moderator |
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| Re: Assumptions, Timing, Fuel... [message #241523 is a reply to message #241493] |
Wed, 26 February 2014 15:01 |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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Senior Member |
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| roy1 wrote on Wed, 26 February 2014 11:15 |
| Carl S. wrote on Tue, 25 February 2014 13:39 |
I have an MSD dash adjustable timing control on my coach. I have the base timing set in such a way as to have the ability to retard or advance the timing from the driver's seat. This gives me the ability to run optimum timing for different situations.
For example, on my annual sand dune trip, I tow my Jeep, on a trailer, from my home in Tucson (2,800 ft elevation) to Southeastern California (25' elevation). Near the western end of the trip, just before Yuma, AZ, there is a mountain pass to be crossed. It is not real high or long, but it is somewhat steep and requires full throttle and usually a downshift at some point. When I start to hear the slightest pre-ignition knock, I dial the timing back about eight degrees.
Conversely, when I am up in Northern AZ or NM, on I-40, I can advance the timing an additional 12 degrees if I think of it. Theoretically, that should give me a little better fuel efficiency.
I mostly installed the adjustable timing to save my engine under high loads at lower altitudes. Works for me. As I get older (I'm currently 58) and my hearing deteriorates further, I may (have to) invest in a knock sensor in order to get a jump on it.
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Did you see an improvement by advancing the total timing with the MSD ? I have one on my coach but I don't see any difference in performance so I just leave it at 8 or 10 degrees initial advance.
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Not necessarily. The main reason I was interested in controlling timing from the dash was to be able to retard any detonation out of it. I don't know whether I get any improvement from more advance or not, but I like to think I do. I guess it would be hard to notice on a 15,000 pound (including the towd) vehicle anyway.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Assumptions, Timing, Fuel... [message #241543 is a reply to message #241296] |
Wed, 26 February 2014 17:18 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Another thing to check or timing may be erratic, inconsistent or misleading, is that even if the weights section and vacuum section looks lubed and A OK, the factory lube on the concentric shafts (breaker plate section) has long dried up and gone to grease heaven. This could cause pinging at some times and not others. Because of this you may not be able to set base timing to optimal. DickP points out the grease turned to cement in his presentation due to the age of these vehicles.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Assumptions, Timing, Fuel... [message #241546 is a reply to message #241328] |
Wed, 26 February 2014 17:35 |
mikethebike
Messages: 331
Registered: January 2014
Karma: 0
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| Ken Burton wrote on Tue, 25 February 2014 12:31 |
Increasing compression (actually increasing cylinder pressures)will not increase fuel consumption, quite the opposite. An increase in static C/R within the limits of available gasoline will increase the efficiency of an ICE. It the all that other stuff that guy do when the increase C/R that hurt MPG...you know, high lift, long duration big overlap cams, larger ports, valves, Dominator carbs, 4.56 ring & pinion.
One more thing...preignition and detonation are two entirely different things.
Actually increased octane fuel reduces the chance of pre-ignition (detonation, ping) in higher compression engines. That is it. If you increase compression of an engine then you can see increased power along with higher fuel consumption. If you do not increase the octane rating of he fuel a higher compression engine, the fuel will tend to explode in certain conditions. So higher Octane fuel is used to reduce that possibility.
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