Home » Public Forums » GMCnet » [GMCnet] Higher octane, weaker engine?
| [GMCnet] Higher octane, weaker engine? [message #96111] |
Mon, 16 August 2010 16:27  |
980
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Registered: July 2010
Location: United States
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I got a piece of advice about different octane fuels to only bother with
higher octane when you want to run a tank of Chevron every once in a
while through the coach because the higher octane will have the most
"Techron" additive/detergent.
So I filled it up with Chevron-91 and it doesn't seem to run as well as
the usual Costco-87 I normally run. The PO also regularly ran Costco
(warehouse store) gas through it as well. Is my slight loss of power
due to just the gas, or something between the timing and the different
octane or should I start looking see if I have some other problem?
It's not a dramatic loss but I can feel the difference.
Desmond
--
1977 GMC ex-Palm Beach, 26-3
Treasure Island, CA
KC6VHG, KAG0675 "980"
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| Re: [GMCnet] Higher octane, weaker engine? [message #96114 is a reply to message #96111] |
Mon, 16 August 2010 16:42  |
Harry
Messages: 1888
Registered: October 2007
Location: Victoria, BC CANADA
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Octane numbers are highly over rated.
Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where octane is raised by blending in ethanol, energy content per volume is reduced.
[Updated on: Mon, 16 August 2010 16:43] Report message to a moderator |
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| Re: [GMCnet] Higher octane, weaker engine? [message #96116 is a reply to message #96115] |
Mon, 16 August 2010 17:02 |
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hnielsen2
Messages: 1434
Registered: February 2004
Location: Alpine CA
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You can also pickup a four pack of Techron at Costco.
I add that to our F I GMC and F I cars.
Howard
Alpine CA
----- Original Message -----
From: "Emery Stora" <emerystora@mac.com>
To: <gmclist@temp.gmcnet.org>
Sent: Monday, August 16, 2010 2:52 PM
Subject: Re: [GMCnet] Higher octane, weaker engine?
>
> On Aug 16, 2010, at 3:27 PM, Desmond's GMC wrote:
>
>> I got a piece of advice about different octane fuels to only bother with
>> higher octane when you want to run a tank of Chevron every once in a
>> while through the coach because the higher octane will have the most
>> "Techron" additive/detergent.
>>
>> So I filled it up with Chevron-91 and it doesn't seem to run as well as
>> the usual Costco-87 I normally run. The PO also regularly ran Costco
>> (warehouse store) gas through it as well. Is my slight loss of power
>> due to just the gas, or something between the timing and the different
>> octane or should I start looking see if I have some other problem?
>>
>
> Desmond
>
> You really don't have to run higher octane at all. With the amount of
> ethanol in the fuels now it will give as much detergent action as any
> added detergent in Techron premium fuel.
>
> Most of the time detergents in fuel are really for the fuel injectors in
> modern engines anyway.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
>
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| Re: [GMCnet] Higher octane, weaker engine? [message #96126 is a reply to message #96117] |
Mon, 16 August 2010 17:49 |
LarryInSanDiego
Messages: 336
Registered: September 2006
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There's a false and largely unjustifiable romance with "premium" gas. I forget the numbers, but 91 definitely has less energy content (BTUs) than 87. If it doesn't ping on 87 you'll lose power, mileage, and money running 91.
FWIW, John Meaney, an efi whiz who drives a 1300+ hp turbo'ed Vette on the street, does his tuning on 87, NOT high octane race gas. Of course, one thing is you must have a very high percentage squish/quench peanut shaped combustion chamber design, the type found on modern race heads (forget about oem or Edelbrocks, they don't even come close).
Larry Engelbrecht
San Diego, CA
'73 26' ex-Glacier
TZE063V100319 03/07/73
[Updated on: Mon, 16 August 2010 17:54] Report message to a moderator |
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| Re: [GMCnet] Higher octane, weaker engine? [message #96131 is a reply to message #96117] |
Mon, 16 August 2010 18:17 |
Steven Ferguson
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Registered: May 2006
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Desmond,
The higher octane actually retards flame travel across the top of the
piston. Higher octane could very well lower performance in an engine
designed for low octane fuel. Stick with the 87.
On Mon, Aug 16, 2010 at 3:02 PM, Desmond's GMC <980@opg.org> wrote:
> But would it be why my power isn't that great anymore? It will be
> interesting to see what happens after I've burnt it off and go back to
> my cheap gas again.
>
> DC
>> Desmond
>>
>> You really don't have to run higher octane at all. With the amount of ethanol in the fuels now it will give as much detergent action as any added detergent in Techron premium fuel.
>>
>> Most of the time detergents in fuel are really for the fuel injectors in modern engines anyway.
>>
>> Emery Stora
>> 77 Kingsley
>> Santa Fe, NM
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> List Information and Subscription Options:
>> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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>
>
> --
> 1977 GMC ex-Palm Beach, 26-3
> Treasure Island, CA
> KC6VHG, KAG0675 "980"
>
> _______________________________________________
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--
Steve Ferguson
'76 EII
Sierra Vista, AZ
Urethane bushing source
www.bdub.net/ferguson/
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| Re: [GMCnet] Higher octane, weaker engine? [message #96135 is a reply to message #96131] |
Mon, 16 August 2010 18:36 |
Steve Stolley
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Registered: April 2008
Location: Ivins, Utah
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For what it's worth, I was told that Costco's regular typically does not contain ethanol unless the state you're in requires it. Costco puts the ethanol sticker on the regular pumps just to cover their rears. Perhaps what you're seeing is the power difference between regular without ethanol versus premium with ethanol.
Steve Stolley
76 Glenbrook
Ivins, Utah
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| Re: [GMCnet] Higher octane, weaker engine? [message #96153 is a reply to message #96147] |
Mon, 16 August 2010 21:28 |
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just advance your static timing to about 16* BTDC, and make sure your advance is working correctly, you will make the most from that 91 then. I can run 10 to 12* on 87 with a pretty heavy vac advance and total timing of about 45* but sometimes I get slight det on a grade, so if im going somewhere hilly I back the timing down a few degrees or run some 91 to take care of it...
| 980 wrote on Mon, 16 August 2010 20:45 |
I'm actually excited about burning off this $150 worth of gas and
putting the cheap stuff in. I was just trying to wash some detergent
through it once and see if it made any difference.
I'm in CA, so I'm sure everything has ethanol. The Costco gas ran
pretty good. It will be interesting to see if that comes back.
DC
> For what it's worth, I was told that Costco's regular typically does not contain ethanol unless the state you're in requires it. Costco puts the ethanol sticker on the regular pumps just to cover their rears. Perhaps what you're seeing is the power difference between regular without ethanol versus premium with ethanol.
>
--
1977 GMC ex-Palm Beach, 26-3
Treasure Island, CA
KC6VHG, KAG0675 "980"
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73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] Higher octane, weaker engine? [message #96179 is a reply to message #96111] |
Tue, 17 August 2010 07:43 |
Gary Casey
Messages: 448
Registered: September 2009
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I'm with Steve on this one. Over the years I've heard stories about the effects
of different fuels. One is that premium fuels have less energy (heating value)
because they have more aromatic (closed chain) molecules and are less dense
(fewer pounds per gallon). This story is true, but the difference is pretty
small - a couple percent. Another is that the flame speed is lower. I've never
seen proof of this, and I doubt, if there is a difference, that it is enough to
measure. Sure, higher octane fuel resists auto-ignition, but that doesn't mean
it resists being lit from a flame front. It takes maybe 30 degrees of crank
rotation to complete combustion and if it took even 5% longer to burn that would
mean that the combustion would be retarded less than 1 degree on average.
Enough to notice? No way. Could the premium fuel contain more ethanol than a
regular grade? If allowed or if subsidized adding ethanol is a great way to
improve octane, so I'll bet it is likely that at the same pump premium will have
more ethanol. Only problem is that ethanol contains only 60% as much energy as
gasoline. Big, big difference. since the carburetor can't tell the difference
an engine will definitely be down on power when burning any significant amount
of ethanol. In theory the difference is about half the concentration - 10%
ethanol will drop the power 5%. YRMV (your results may vary)
Only one itsy-bitsy problem - All Q-jet applications that I know of were set up
to run significantly richer than best power (LBT - Lean Best Torque) at full
throttle. This is because the fuel distribution of the air valve secondaries
was so dismal. So a little leaning will only reduce power in some cylinders,
not all, and some cylinders will make more power when leaned. Somewhere in the
deep recesses of my memory I recall that the average mixture was typically 12,
more than 10% rich of LBT which is about 13.5 for gasoline. Some cylinders were
probably running 11 and some 13.5 or so. Q-jet engines would always make black
smoke at full throttle.
Gary Casey
'73 23 in Colorado, running rich with a Q-jet
For what it's worth, I was told that Costco's regular typically does not contain
ethanol unless the state you're in requires it. Costco puts the ethanol sticker
on the regular pumps just to cover their rears. Perhaps what you're seeing is
the power difference between regular without ethanol versus premium with
ethanol.
--
Steve Stolley
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| Re: [GMCnet] Higher octane, weaker engine? [message #96184 is a reply to message #96179] |
Tue, 17 August 2010 08:38 |
LarryInSanDiego
Messages: 336
Registered: September 2006
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| Gary Casey wrote on Tue, 17 August 2010 05:43 |
...Q-jet engines would always make black smoke at full throttle...
|
Maybe with a sunk float or bad needle. No Q-Jet I've ever completed would ever smoke black, not even at RBT, let alone LBT. And I've tuned them for both on dual and single planes. Watch a (successful) NHRA Q-Jet equipped Super Stock launch and go down the track from behind the bleach box. Nothing. You can check it out on YouTube.
The choke, air valve spring, float level, fuel pressure, something would have to be really off whack to get that rich. Any of the grayish stuff coming out at WOT was likely deposits. AFR would have to be single digit to smoke black, somewhere around 8:1. The only black stuff I'd EVER see behind a properly Q-Jetted vehicle were tire marks.
Add edit: Unless maybe someone left out that plate under the air valve? And I'm not even sure that would do it. Some apps had a booster venturi-looking thing on one side to actually richen the lean plane a tad IIRC.
Check your float. Those plastic foamy things easily become fuel absorbent. Perhaps ethanol aggravates that, I'm not sure. But I DO know it can be hard to tell unless you do have a new float and compare the weight side by side. A new one will seem surprisingly light if the present one is "sunk". Unfortunately, new floats don't come in carb kits. I have to go to an AC Delco dealer to get mine. Kragen and the like don't stock those. Maybe NAPA has them.
Larry Engelbrecht
San Diego, CA
'73 26' ex-Glacier
TZE063V100319 03/07/73
[Updated on: Tue, 17 August 2010 09:36] Report message to a moderator |
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| Re: [GMCnet] Higher octane, weaker engine? [message #96194 is a reply to message #96111] |
Tue, 17 August 2010 09:55 |
Gary Casey
Messages: 448
Registered: September 2009
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You might be right, Larry, but every Production example I saw back in the 60's
and 70's while working at the GM Tech Center would smoke at WOT. Granted, you
usually couldn't see it from inside the car, but could when following. Yes, the
A/F would have to be close to 10 in order to smoke, but I don't think it has to
be 8 - at 8 I think it would be close to a flame-out. Some Chevy's were built
with Holley equal-bore carburetors and those were able to be set up leaner at
WOT. We would joke about being able to tell which carb was in the car by the
black smoke at WOT. There were many, many tricks on how to set up the air
valves, but I don't know of any that would match the Holley equal-bore carb for
WOT A/F distribution. Now, light load drivability was another thing entirely.
A good Q-jet would come off idle silky smooth, but the Holley was, shall we
say, a challenge.
All this is from memories from way back when. But they say only the short-term
memory is affected, right?
Take care,
Gary casey
Gary Casey wrote on Tue, 17 August 2010 05:43
> ...Q-jet engines would always make black
> smoke at full throttle...
Maybe with a sunk float or bad needle. No Q-Jet I've ever completed would ever
smoke black, not even at RBT, let alone LBT. Any of the grayish stuff coming out
at WOT was likely deposits. AFR would have to be single digit to smoke black,
somewhere around 8:1.
--
Larry Engelbrecht
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| Re: [GMCnet] Higher octane, weaker engine? [message #96354 is a reply to message #96117] |
Wed, 18 August 2010 19:22 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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""But would it be why my power isn't that great anymore? It will be
interesting to see what happens after I've burnt it off and go back to
my cheap gas again.
""
Higher octane slows the flame front thereby decreasing the tendency to ping. Unless you have a knck sensor, it has the same effect as retarding the timing--hence reduced performance.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Higher octane, weaker engine? [message #96376 is a reply to message #96111] |
Wed, 18 August 2010 22:37 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
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Qjets RICH from the factory???? My experience was LEAN from the factory. My 70 GS350 would pop thrugh the carb at WOT when new. I had to put in thinner tipped secondary rods to fix it. What years were you refering to, the early like 65 when they first came out? Maybe the first couple years but by 68 they had the AC Autothermac preheated aircleaner and were tuned to the lean side of the controled air temp thourough most driving conditions. I don't think GM would have passed emissions with visible black rich conditions. If the secondary air valve is stuck shut that may happen but that's the only time I've found.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Higher octane, weaker engine? [message #96379 is a reply to message #96376] |
Wed, 18 August 2010 23:13 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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| JohnL455 wrote on Wed, 18 August 2010 21:37 |
Qjets RICH from the factory???? My experience was LEAN from the factory. My 70 GS350 would pop thrugh the carb at WOT when new. I had to put in thinner tipped secondary rods to fix it. What years were you refering to, the early like 65 when they first came out? Maybe the first couple years but by 68 they had the AC Autothermac preheated aircleaner and were tuned to the lean side of the controled air temp thourough most driving conditions. I don't think GM would have passed emissions with visible black rich conditions. If the secondary air valve is stuck shut that may happen but that's the only time I've found.
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You got it!!
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Higher octane, weaker engine? [message #96385 is a reply to message #96379] |
Thu, 19 August 2010 00:36 |
LarryInSanDiego
Messages: 336
Registered: September 2006
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WOT is not part of the FTP (Federal Test Procedure) drive cycle that mfrs have to have their vehicles pass. Therefore, technically, it could smoke black at WOT. Not that you'd want that of course.
We all know that high combustion temps and pressures cause high NOx emissions. Consider that today's higher performance vehicles definitely have to have those types of cylinder conditions to generate the levels of power that were considered legally unattainable 35 years ago.
The heated O2 sensors and really good cats can't possibly keep up at WOT. They don't have to. The hardest part is keeping emissions at bay during the cold start cycle. Since WOT isn't in the drive cycle, it's not measured, therefore, emission output, whatever they are, is irrelevant.
Larry Engelbrecht
San Diego, CA
'73 26' ex-Glacier
TZE063V100319 03/07/73
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| Re: [GMCnet] Higher octane, weaker engine? [message #96396 is a reply to message #96111] |
Thu, 19 August 2010 08:16 |
Gary Casey
Messages: 448
Registered: September 2009
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What Larry said, that's what I was trying to say. The first emission
requirements were met without catalyst and there were no NOX requirements or
grams/mile standards, only percent HC and CO. Therefore, the way to meet the
standard was to run lean (low CO) and retard the timing (higher EGT's reduce
HC). The Q-jet was excellent in enabling lean operation at part throttle on the
primaries because the small bores and 3-stage venturis provided a highly
atomized and consistent fuel delivery. Since the emissions requirement didn't
address full throttle operation full throttle just didn't matter. And the
attitude at GM was (sadly for us car enthusiasts) that no one used full throttle
anyway(probably true). The Air Valve Secondary (AVS) Q-jet was a good design
considering the conditions of the time. The secondary throttle had to be very
large to compensate for the very small primary. A standard venturi secondary
would probably have impossibly poor fuel control when opening while the AVS
could control fuel flow reasonably well while opening. What it couldn't do is
provide a nicely atomized central fuel "fog" at all throttle openings.
Solution? Just pour in more fuel - a little extra only marginally reduced power
of some cylinders. A pragmatic solution of the time. I never, having driven
perhaps hundreds of new Q-jet equipped cars experienced a case of lean secondary
operation. The choke vacuum break is critical to good secondary operation and
some people would drill out the orifice thinking it would improve secondary
response (I did and it would), but going too far would create a secondary tip-in
stumble. Or if done too far would create what we called a stip-in tumble :-).
The net result is that in all the applications I was aware of traces(or more) of
black smoke could be observed at full throttle. It had no impact on emission
test results. Oh, and the air valve is always "stuck shut" until the choke is
fully open - going full throttle then was a non-event.
Maybe I'm a little sensitive since I was one - there was, and still seems to
be - a sentiment that the automotive OEM employees were a bunch of incompetant
bafoons that could never get it right. In my experience there were a lot of
intelligent, hard working engineers that did as well as conditions allowed.
Gary Casey
From John:
Qjets RICH from the factory???? My experience was LEAN from the factory. My 70
GS350 would pop thrugh the carb at WOT when new. I had to put in thinner tipped
secondary rods to fix it. What years were you refering to, the early like 65
when they first came out? Maybe the first couple years but by 68 they had the
AC Autothermac preheated aircleaner and were tuned to the lean side of the
controled air temp thourough most driving conditions. I don't think GM would
have passed emissions with visible black rich conditions. If the secondary air
valve is stuck shut that may happen but that's the only time I've found.
--
John Lebetski
Then from Larry:
WOT is not part of the FTP (Federal Test Procedure) drive cycle that mfrs have
to have their vehicles pass. Therefore, technically, it could smoke black at
WOT. Not that you'd want that of course.
We all know that high combustion temps and pressures cause high NOx emissions.
Consider that today's higher performance vehicles definitely have to have those
types of cylinder conditions to generate the levels of power that were
considered legally unattainable 35 years ago.
The heated O2 sensors and really good cats can't possibly keep up at WOT. They
don't have to. The hardest part is keeping emissions at bay during the cold
start cycle. Since WOT isn't in the drive cycle, it's not measured, therefore,
emission output, whatever they are, is irrelevant.
--
Larry Engelbrecht
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