1985 + 928s on Low Octane Fuel?
09-03-2005, 08:56 PM
#31
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Originally Posted by Rich9928p
I was very careful to specify US 928s in my threads. The earlier Euro S motors had high compression and without knock sensors it is much more dodgy running lower octane fuel.
The 1980 - 1983 Euro S used the transistorized ignition with vacuum and mechanical spark advance, so the only compensation would be to manually change the base spark timing which affects the whole advance scheme.
However, from Euro and ROW 1984 - 1986 (EZF spark control) there is the jumper for selecting lower octane fuel. If a Euro owner was to plug in the jumper and go for lower octane fuel I'd keep my ears open and listen for knocking.
I would hope that gas companies post the MINIMUM octane for their fuels and that in reality they actually come in a little higher. Perhaps a renlister who works in the petrolium industry could speak to the accuracy of their fuel specs.
The 1980 - 1983 Euro S used the transistorized ignition with vacuum and mechanical spark advance, so the only compensation would be to manually change the base spark timing which affects the whole advance scheme.
However, from Euro and ROW 1984 - 1986 (EZF spark control) there is the jumper for selecting lower octane fuel. If a Euro owner was to plug in the jumper and go for lower octane fuel I'd keep my ears open and listen for knocking.
I would hope that gas companies post the MINIMUM octane for their fuels and that in reality they actually come in a little higher. Perhaps a renlister who works in the petrolium industry could speak to the accuracy of their fuel specs.
-I once put 100LL Avgas in this car...and it started to foul the plugs. 100LL is about 104 octane in the automotive nomenclature.
N!
09-03-2005, 09:07 PM
#32
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[QUOTE=Mark]"Our US regular @87 is equal to a RON of about 92!
I found this by googling... Octane rating
The most important characteristic of gasoline is its Research Octane Number (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation"
There's an RON/MON conversion equation posted on every US gas pump I've seen.
I found this by googling... Octane rating
The most important characteristic of gasoline is its Research Octane Number (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation"
There's an RON/MON conversion equation posted on every US gas pump I've seen.
09-03-2005, 09:21 PM
#33
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[QUOTE=Fogey1]
Porblem is, you cant solve R+M/2 equation when all you know is the result!
Our reg is R+M/2 = 87
No way to determine R without knowing what Z is...and vice versa.
(Hey, not bad!!! I haven't had an algebra class in about....ummm....a LONG time!)
Originally Posted by Mark
There's an RON/MON conversion equation posted on every US gas pump I've seen.
Our reg is R+M/2 = 87
No way to determine R without knowing what Z is...and vice versa.
(Hey, not bad!!! I haven't had an algebra class in about....ummm....a LONG time!)
09-03-2005, 09:30 PM
#34
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Originally Posted by Normy
Well, I used what I refer to as "road fuel" once in my S2, and I won't do it again. I was driving to south Florida, and put some cheap stuff in the tank. Listen- 87 octane DOES NOT work with 10.4:1 static compression! I had noticeable missing, and I could even hear the spark knock during acceleration! I had to drive the car very gently for the next 200 miles in order to burn off the gas. I use Chevron 93 octane, and that seems to be about right, octane wise.
-I once put 100LL Avgas in this car...and it started to foul the plugs. 100LL is about 104 octane in the automotive nomenclature.
N!
-I once put 100LL Avgas in this car...and it started to foul the plugs. 100LL is about 104 octane in the automotive nomenclature.
N!
09-05-2005, 10:53 AM
#35
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I summarized the information about 928 fuel requirements and the Australian "S4 RON 91 - 87 Octane coding plug" and created a nice PDF document. If you'd like a copy, drop me a line at info@electronikrepair.com and I'll send it.
09-05-2005, 12:17 PM
#36
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Dr. Bob provided this good information: You might want to remind folks in the smog
states like California that MTBE and ethanol are use extensively as octane
boosters. Unfortunately, the specific energy of those products is somewhere
around 15% of the gasoline displaced in your tank. With up to 20% of the
fuel composed of this diluent, the total energy available from the gallon is
reduced by about 15%. Many folks who track their mileage carefully have
seen this as a direct reduction in fuel economy. Lesser gradeds of fuel,
with less additives required, --may-- yield better performance and better
mileage if the actual octane rating is adeqwuate for the driveing and engine
conditions. On the other hand, with crude prices at record levels, gasoline
refiners would certainly be tempted to substitute MTBE and ethanol at
maximum allowable dosages into all grades of fuel. Ethanol is less than
$0.25/lb in small quantities, and the oil co's are certainly buying big
quantities for less. It may be that your "regular" grade has adequate
anti-knock capabilities to run in our cars.
Bottom line: Try a lesser grade of fuel in your car in it's normal driving
routine, especially if it's equipped with knock sensors ('87+ cars). Drive
with a lighter gas-saving foot, and you'll save double by stretching more
miles from a lower-cost fuel.
Bob
states like California that MTBE and ethanol are use extensively as octane
boosters. Unfortunately, the specific energy of those products is somewhere
around 15% of the gasoline displaced in your tank. With up to 20% of the
fuel composed of this diluent, the total energy available from the gallon is
reduced by about 15%. Many folks who track their mileage carefully have
seen this as a direct reduction in fuel economy. Lesser gradeds of fuel,
with less additives required, --may-- yield better performance and better
mileage if the actual octane rating is adeqwuate for the driveing and engine
conditions. On the other hand, with crude prices at record levels, gasoline
refiners would certainly be tempted to substitute MTBE and ethanol at
maximum allowable dosages into all grades of fuel. Ethanol is less than
$0.25/lb in small quantities, and the oil co's are certainly buying big
quantities for less. It may be that your "regular" grade has adequate
anti-knock capabilities to run in our cars.
Bottom line: Try a lesser grade of fuel in your car in it's normal driving
routine, especially if it's equipped with knock sensors ('87+ cars). Drive
with a lighter gas-saving foot, and you'll save double by stretching more
miles from a lower-cost fuel.
Bob
09-05-2005, 12:46 PM
#37
Looking the the factory manual of my 1985 US 32V it states on pg 107, "Fuel Recommendation-96 RON (92 CLC or AKI on U.S. fuel pumps) minimum. PREMIUM UNLEADED FUEL ONLY."
Their bolding on the last statement, not mine. So does that mean I can use lower than the 93 octane we have here in Fla without knocking & without changing the timing?
Their bolding on the last statement, not mine. So does that mean I can use lower than the 93 octane we have here in Fla without knocking & without changing the timing?
09-05-2005, 01:37 PM
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Originally Posted by 928SS
IIRC, 100LL will also trash/coat the cats....
09-05-2005, 03:25 PM
#39
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Correction for US '85 - 86, please read
Originally Posted by MGW-Fla
Looking the the factory manual of my 1985 US 32V it states on pg 107, "Fuel Recommendation-96 RON (92 CLC or AKI on U.S. fuel pumps) minimum. PREMIUM UNLEADED FUEL ONLY."
Their bolding on the last statement, not mine. So does that mean I can use lower than the 93 octane we have here in Fla without knocking & without changing the timing?
Their bolding on the last statement, not mine. So does that mean I can use lower than the 93 octane we have here in Fla without knocking & without changing the timing?
1985 - 86 RON 96 /MON 86 10.0 : 1 US = M28.43/44
I wasn't totally crazy, I read the wrong column, there is a M28.45/46 engine that is spec'd for RON 91/MON 82 that has 9.3 : 1 compression.
US '85 - '86 have slightly higher fuel quality requirements than the S4s (probably due to the lack of a knock sensor).
09-05-2005, 04:41 PM
#40
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I have used (and will continue to use) 87 octane , and in some instances when traveling across the USof A, 85 octane, for the past 20,000 miles in my '86.5.
I have never encountered pinging, misfiring or any other symptom of preignition or any other malady at al, all the way to redline in all but high gear (but close to it !!). Never. Not ever. Car runs like a clock. Smoothly. My understanding is the higher the octane, the more carbon is deposited on the tops of the pistons, and thus the more the compression ratio is increased = more octane is required. My engine runs this way in 110 deree heat, -10 degree ice and snow, and in pouring rain in Louisiana (thought I had mistakenly driven into the ocean and was in a huge endless wave !!). It keeps running smoothly and powerfully and evenly all day and night.
I will stay with lower octane thank you.
I have never encountered pinging, misfiring or any other symptom of preignition or any other malady at al, all the way to redline in all but high gear (but close to it !!). Never. Not ever. Car runs like a clock. Smoothly. My understanding is the higher the octane, the more carbon is deposited on the tops of the pistons, and thus the more the compression ratio is increased = more octane is required. My engine runs this way in 110 deree heat, -10 degree ice and snow, and in pouring rain in Louisiana (thought I had mistakenly driven into the ocean and was in a huge endless wave !!). It keeps running smoothly and powerfully and evenly all day and night.
I will stay with lower octane thank you.
09-06-2005, 10:27 AM
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Originally Posted by Ron_H
I have used (and will continue to use) 87 octane , and in some instances when traveling across the USof A, 85 octane, for the past 20,000 miles in my '86.5.
I will stay with lower octane thank you.
I will stay with lower octane thank you.
I'll be getting my first shark, an 86.5, in a couple of weeks. One of the 'perks' where I work is that they pay my gas, but the gas credit cards for all employees are programed to accept regular only. Hence regular gas is free, premium I pay.
I still run premium in my motorcycle though. I'll try both in the shark, but at least I'm less worried about it now.
Thanks for the info!
04-08-2008, 11:21 PM
#43
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Bump - gas is going to hit $4/gal this summer 
04-08-2008, 11:55 PM
#44
Ethanol was 3.20 this week. I am running it in the Honda. The 928 is designed for it. The 91 octane crap we get in the west is nearly 3.90. I think 70 cents a gallon is well worth the extra work.
07-13-2016, 02:35 AM
#45
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Octane loop wire for low grade fuel
On 1985-1986 Porsche 928 32valve U.S models there is a (green or brown not sure) wire called the octane loop on the fuse panel, when connected, it retards the timing for use of low octane fuel since these model year have no knock sensors. I've tried it once and noticed a loss of power and lower mpg, so you don't benefit from saving if mpg and power drops. Low octane fuels have less cleaning detergent causing carbon deposit. Same with my v6 VW Passat with knock sensor, less power with less mpg using low grade vs premium.