Octane Requirements
03-27-2008, 01:31 PM
#16
"Motronic has a little known function that pulls back overall ignition timing as the intake air temps rise. This is the system's knock protection function."
That's totally misleading and has very little effect in reducing detonation,
as the key cause relates to head temp and engine LOAD. That's why
Porsche used/uses knock sensors on all later engines. As an example
the air temp could be moderate, but because of engine conditions (LOAD)
the head temp would very high resulting in a high probability of detonation.
This is VERY problematic in an air cooled vs a water cooled engine where
the head temps are more controlled.
That's totally misleading and has very little effect in reducing detonation,
as the key cause relates to head temp and engine LOAD. That's why
Porsche used/uses knock sensors on all later engines. As an example
the air temp could be moderate, but because of engine conditions (LOAD)
the head temp would very high resulting in a high probability of detonation.
This is VERY problematic in an air cooled vs a water cooled engine where
the head temps are more controlled.
"What this all means is that in general is the 87-89 Carreras can run on 87 octane fine."
A very ridiculous statement, which totally ignores Porsche's requirements, to promote the sale of "performance" chips and only the very naive or those who rationalize their purchase of such would believe.
A very ridiculous statement, which totally ignores Porsche's requirements, to promote the sale of "performance" chips and only the very naive or those who rationalize their purchase of such would believe.
What this all means is that in general is the 87-89 Carreras can run on 87 octane fine, but because of the +2.4 degree ignition spike which would show up at low ambient air temps, you should be using an octane of at least 89-90 octane in the instance you are running flat out for hours on the Autobahn right at 6000 rpm, on a cold German day, to maintain the amount of margin the factory intended. However with the real timing difference at the temperature extremes really more like 1.4 degrees, 89 octane (2 octane over the 84-86 requirement) produces the same margin.
Is there risk of predetonation running 87 octane on the 89-89 Carreras? Unless there is something wrong with the car, or perhaps you are in Death Valley at 138 degrees running flat out with an open exhaust, not too likely. That's also because the factory engineered another safety margin in the chip program by programming the car to run very rich at the upper rpms - by 4800 rpm, the air fuel ratios are in the 11s. The excess fuel keeps the combustion chamber cool, reduces the peak pressure of combustion, and burns slower, with the same effect as reducing ignition advance. This was done for two reasons. First as an insurance policy for the warranty department, and second, to keep the ceramic monoblocks of the catalytic converter cool under hours of flat out driving on the Autobahn at 150 mph. Analysis of the European/ROW programs with their freer exhaust and euro premufflers show that they are actually programmed leaner at the upper rpms than their U.S. counterparts because of their lack of cats.
All this does not mean I recommend everyone with their stock cars and stock chips go run 87 octane, as it is impossible to know the state of tune and operation of everyone's car. Using higher octane fuel than a car can run basically provides additional margin against predetonation as higher octane fuel has less propensity to self ignite. If your car has leaking intake manifold gaskets, clogged injectors, thick carbon buildup, a worn sensors and such, the higher octane gives you extra insurance, but the maintenance issues should really be addressed.
03-27-2008, 02:02 PM
#17
Race Car
Thread Starter
"Lower intake air temp = lower ambient temp = lower engine temp = lower head temp"
Wrong! Lower ambient temp doesn't ALWAYS equal (imply) lower head temp,
as was explained before. That's why there's a separate cylinder head temp
sensor. Besides, the intake air temp is mainly used to correct for air density
changes with temperature as it relates to the AFR.
Bottom line: The hyperbole from Pelican doesn't need to be PARAPHASED here on Rennlist.
No amount of "spin" can refute the technical facts as most who understand them will agree.
Wrong! Lower ambient temp doesn't ALWAYS equal (imply) lower head temp,
as was explained before. That's why there's a separate cylinder head temp
sensor. Besides, the intake air temp is mainly used to correct for air density
changes with temperature as it relates to the AFR.
Bottom line: The hyperbole from Pelican doesn't need to be PARAPHASED here on Rennlist.
No amount of "spin" can refute the technical facts as most who understand them will agree.
03-27-2008, 02:30 PM
#18
But YOU paraphrased, I provided a complete quote. Duh.
03-27-2008, 02:33 PM
#19
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There is no better teacher than experience. One fine spring day as I had my Harley with the fresh top-end on a ride with some other bikes through the Anza Borrego Desert, I decided to pass up the line of bikes and let them hear the music coming out of the straight drag pipes. At wide open open throttle somewhere a little north of 100 mph, I experienced a sudden loss of power (one of the guys said that he saw a big puff of smoke come out of the pipes) and I pulled to the shoulder. On dissassembly, I found a hole about the size of a quarter burned through the top of the front piston as if someone took a torch to it.
I always ran super unleaded in it and I used jets that were bigger than normal in the new S&S carb to prevent it from running too lean. The mistake that I made was running too much advance at the top of the rpm range for the 91 pump gas that I had available to me.
Given the potential for sudden and catastrophic engine failure and the likely effects of the extreme pressures resulting from auto-ignition of the air/fuel mixture on the rings and valve seats, it doesn't make any sense to me to run anything less than the highest octane available in these cars.
After I melted the Harley piston, I did some E-search into octane and, particularly, what I could do to "boost" the octane of the fuel that I was buying at the pump. There are some petroleum derivatives that are very resistant to auto ignition under pressure and that, when added to gasoline, will increase its octane rating. I found some anctedotes about old school drag racers using paint thinner with high concentrations of toulene or xylene for that purpose. The problem with "home brew" octane booster seems to be the effects that these highly caustic additives may have on both the internal components of the fuel delivery system and the emissions cleaning components of the exhaust system (e.g., the catalytic converter, oxygen sensor, etc.).
Bottom line, when in doubt, err on the side of too much octane for the Porsche. The only thing it will hurt is your wallet. Save the cheap gas for rental cars.
I always ran super unleaded in it and I used jets that were bigger than normal in the new S&S carb to prevent it from running too lean. The mistake that I made was running too much advance at the top of the rpm range for the 91 pump gas that I had available to me.
Given the potential for sudden and catastrophic engine failure and the likely effects of the extreme pressures resulting from auto-ignition of the air/fuel mixture on the rings and valve seats, it doesn't make any sense to me to run anything less than the highest octane available in these cars.
After I melted the Harley piston, I did some E-search into octane and, particularly, what I could do to "boost" the octane of the fuel that I was buying at the pump. There are some petroleum derivatives that are very resistant to auto ignition under pressure and that, when added to gasoline, will increase its octane rating. I found some anctedotes about old school drag racers using paint thinner with high concentrations of toulene or xylene for that purpose. The problem with "home brew" octane booster seems to be the effects that these highly caustic additives may have on both the internal components of the fuel delivery system and the emissions cleaning components of the exhaust system (e.g., the catalytic converter, oxygen sensor, etc.).
Bottom line, when in doubt, err on the side of too much octane for the Porsche. The only thing it will hurt is your wallet. Save the cheap gas for rental cars.
03-27-2008, 02:49 PM
#21
Race Car
Thread Starter
"But it''s also used to retard the ignition timing at higher temps."
Oh O.K. So Porsche really didn't need to use knock sensors to fully solve
the potential for detonation. They should've consulted those tech wizards
on Pelican Parts and just monitored air temp, right?
Bottom line: It has only a VERY minor effect and doesn't FULLY solve the problem
under instantaneous heavy engine loads as was exemplified by Hester. The AMOUNT
of retard on the 3.2 is NOT enough to protect the engine ALL load conditions.
The 964 DME retards up to 9 degrees.
Get it!!!!!!!!!!!!!!! Obviously not.
Bottom line: As posted on the other thread, when Porsche defines a specification
that you don't agree with either consult the Pelican Forum or ignore/redefine
the specification.
Oh O.K. So Porsche really didn't need to use knock sensors to fully solve
the potential for detonation. They should've consulted those tech wizards
on Pelican Parts and just monitored air temp, right?
Bottom line: It has only a VERY minor effect and doesn't FULLY solve the problem
under instantaneous heavy engine loads as was exemplified by Hester. The AMOUNT
of retard on the 3.2 is NOT enough to protect the engine ALL load conditions.
The 964 DME retards up to 9 degrees.
Get it!!!!!!!!!!!!!!! Obviously not.
Bottom line: As posted on the other thread, when Porsche defines a specification
that you don't agree with either consult the Pelican Forum or ignore/redefine
the specification.
Last edited by Lorenfb; 03-27-2008 at 03:13 PM.
03-27-2008, 02:51 PM
#22
Couldn't agree more Hester, and I only use 93 in my Carrera, with Steve Wong chip. But I do agree with Steve that for many owners, for most normal driving with an OEM chip, 87 is probably fine. And that's from a guy who has also holed a few pistons over the years (not in P-cars) 
03-27-2008, 02:58 PM
#23
You seem to have a huge emotional investment in this issue Loren. Lighten up.
03-27-2008, 03:10 PM
#25
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This place is a real hot bed for pissing contests isn't it?
How about this: Everyone put their recognized qualifications in their sig, like for ex. Master Mechanic, this way the laymen can recognize who's a professional and who's a loudmouth shade-tree mechanic.
How about this: Everyone put their recognized qualifications in their sig, like for ex. Master Mechanic, this way the laymen can recognize who's a professional and who's a loudmouth shade-tree mechanic.
03-27-2008, 03:19 PM
#26
Addict
Lorenfb there is a thread in OT that could use your help. It's called CRAP GASOLINE and it looks like it could use your help.
kick7ca, in my driveway as we speak are odometers totalling some 470,000 miles that I have driven using 91 octane. That puts me in the loudmouth shade-tree column.
oh and hester....he's a Lawyer who's profesor's estate gave him the car. That puts him in the redneck loudmouth can't trust a word he says grabbin' the free stuf shade-tree column.
kick7ca, in my driveway as we speak are odometers totalling some 470,000 miles that I have driven using 91 octane. That puts me in the loudmouth shade-tree column.
oh and hester....he's a Lawyer who's profesor's estate gave him the car. That puts him in the redneck loudmouth can't trust a word he says grabbin' the free stuf shade-tree column.
03-27-2008, 03:58 PM
#27
Rennlist Member
My knowledge of chips goes about as far as Ruffles or Fritos, neither of which require a certain octane, but either can, depending on what its consumed with, create gas.
Back in the early '80s we built an engine for my E/P car, ran the super-high octane gas from Willow Springs in it, and I thought that I had the mixture perfect (Solex carbs). Well, I couldn't hear the detonation, but the car was very fast, surprisingly so. Like Hester's Harley, on the 4th lap I burned two pistons - with a very conservative ignition timing setting. Back to the drawing board - we installed sensors at each cylinder, and gauges on the dash, so I could monitor their temps from the driver's seat in order to hopefully prevent future lean-mixture failures. Never had another mixture-induced failure, but looking back it would have been really nice to have had an ECU with a BBQ, no, a "regular" chip in it. The point is, there's more than one way to hurt an engine, and something really good cannot always protect against something that's really bad.
In closing, I haven't the foggiest idea if this has anything at all to do with this thread, but...
Back in the early '80s we built an engine for my E/P car, ran the super-high octane gas from Willow Springs in it, and I thought that I had the mixture perfect (Solex carbs). Well, I couldn't hear the detonation, but the car was very fast, surprisingly so. Like Hester's Harley, on the 4th lap I burned two pistons - with a very conservative ignition timing setting. Back to the drawing board - we installed sensors at each cylinder, and gauges on the dash, so I could monitor their temps from the driver's seat in order to hopefully prevent future lean-mixture failures. Never had another mixture-induced failure, but looking back it would have been really nice to have had an ECU with a BBQ, no, a "regular" chip in it. The point is, there's more than one way to hurt an engine, and something really good cannot always protect against something that's really bad.
In closing, I haven't the foggiest idea if this has anything at all to do with this thread, but...
Last edited by Peter Zimmermann; 03-27-2008 at 06:45 PM.
03-27-2008, 05:13 PM
#28
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Alright, this is getting out of hand...and there is no need to start calling me the five letter word that rhymes with "Sawyer," Mr. Bray (even if its true). I spent five years wrenching in a mom and pop foreign and domestic and every minute of as much of my 41 years that I can remember that I wasn't thinking about my other obsessions - 9ussy and money (listed in order of importance) - thinking about how stuff works. My opinions on this subject are based on those qualifications.
Loren is right - It is doubtful that Porsche's use of electronics to adjust the ignition timing and/or increase fuel delivery completely protects against the tendency of gasoline to auto-ignite in conditions of high compression and temperature. I'm sure that Dave Thomas and I could melt a hole in the top of just about any piston if we tried.
I also agree with Dave Thomas that, under normal driving conditions within the limits of the law, using 87 octane in your Porsche probably will not cause catastrophic engine failure. I suspect, however, that pre-ignition does cause incremental damage to the rings, valve seats, and rod bearings. Given the beaucoup bucks it takes to rebuild a flat six and the tendency of anyone driving a Porsche with any cajones to "put it through its paces" once in awhile, I wouldn't try to cut corners at the pump.
In my family, the Porsche, the Mercedes, and the Harley get premium. The Civic gets the cheap stuff (and 40 miles to the gallon last Sunday).
With that, my friends, I think you will all agree that this horse is officially dead.
Loren is right - It is doubtful that Porsche's use of electronics to adjust the ignition timing and/or increase fuel delivery completely protects against the tendency of gasoline to auto-ignite in conditions of high compression and temperature. I'm sure that Dave Thomas and I could melt a hole in the top of just about any piston if we tried.
I also agree with Dave Thomas that, under normal driving conditions within the limits of the law, using 87 octane in your Porsche probably will not cause catastrophic engine failure. I suspect, however, that pre-ignition does cause incremental damage to the rings, valve seats, and rod bearings. Given the beaucoup bucks it takes to rebuild a flat six and the tendency of anyone driving a Porsche with any cajones to "put it through its paces" once in awhile, I wouldn't try to cut corners at the pump.
In my family, the Porsche, the Mercedes, and the Harley get premium. The Civic gets the cheap stuff (and 40 miles to the gallon last Sunday).
With that, my friends, I think you will all agree that this horse is officially dead.
03-27-2008, 07:52 PM
#29
Addict
Hester, it was a joke. Come on now, it's me. Julian run, new years day, lunch, drive down to the 8. remember the Youtube? I'm sorry. I didn't intend to lump you in with the other guys.
EDIT: I totally agree with you on this and find you fully qualified. Now go to the beach.
EDIT: I totally agree with you on this and find you fully qualified. Now go to the beach.
!