Capt. Mike
Moderator
Transfer from archives
8/7/99 (9:38 PM)
This has been hashed to death, but I fournd the following from a Bosch FI manual, so thought I'd pass on along with general comments since Bosch supplies VW's FI.
The VW is designed to run on "regular" gas. Anything more is wasted money and can cause running problems. Today, "regular" is generally 87 octane, Gov't R+M/2 method. That's about 90-91 RON.
From a Bosch FI manual (Pub #4232.21) discussing the changes in the mid-70's when even high performance cars dropped requirements to 87 octane:
Old tales claim high-test gas has better additive packages. That's no longer true in any major brand. FI systems are so touchy, no manufacturer wants the reputation of fouling expensive systems. Nor do they want to be pegged as a contributor to pollution by EPA.
There are circumstances when a slightly higher octane say 'middle' or 89 is prefered. Old engines may have sufficient carbon build-up to actually raise compression. This will cause a ping under load. If your engine is pinging, try a middle octane. If it cures the problem, you will probably want to consider a head job or overhaul. Not critical short term, but it indicates the engine has seen better days, contaminated with carbon, soot and other maladies, not just higher compression.
Some head jobs and overhauls may over-machine the heads and increase compression. This is not desireable, but leaves one with the choice of new heads or higher octane. A better mechanic also comes to mind.
Certain areas of the country sell winterized fuel, or have special blends that deviate from the normal 87 octane regular. In the Rocky Mountains, it's not unusual to find 'regular' at about 83-84 octane. VW's don't like this sub-grade fuel. Some brands, like Sunoco's old 'custom blend pump' would put their economy grades below 87. The only cure for this marketing or EPA numbers game is to READ THE NUMBERS on the pump. Every pump MUST be posted with the R+M/2 method octane. Get an 87, regardless of what quality level it's called.
Octane blends. If you get a tank of sub-87 by accident, or suspect so, you can put in some high test to balance it out. I once got a tank of 'regular' near Denver that turned out to be 83 octane. It didn't run worth diddley. Every time I could squeeze in 5 gallons, I put in high-test and by the 2nd or 3rd go-around, everything was back to normal.
Winterized fuel, and gasahol are oxygenated, theoretically giving cleaner burning. Unfortunately, they also tend to reduce power and mileage, reported to be by as much as 3%. Their octane is the same, but the energy contained is not. This drop in mileage is unrelated to the octane. With most fuels containing some ethanol these day, you will have little choice but to use it. VW's should have no problems below 15% and the 10% of most blends shouldn't cause any concern. They are required by law to have a sticker (not necessarily big and prominant) on the pump.
Leaded gas: Tetraethyl lead was an additive used to increase the effective octane of gas. It is now outlawed pretty much around the world (cancer suspect in fumes & spills) and has been superceded by new technology. It had the side-effect of being a lubricant for valves & valves seats. In any newer engine ('70s on), this was alleviated by the superior technology of hardnened valves & seats. Some believe that, in OLD engines, the lubricant replacement additives in the new unleaded fuels are not adequate and a lubricant additive should be used. The jury is out. I use one in a 60+ year old flathead engine that I know is all original, but not much else. Any COMPETENT overhaul in the last 25 years should have gone to hardened valves & seats, and thus make leaded gas unnecessary. The side-effects of fouling, exhaust corrossion, increased acidity, etc., overwhelm any benefits. Ruining a cat converter ought to convince you that additives aren't the best route -- to repeat, there are no magic cures in a bottle.
ADDITIVES: Generally, not recommended for gas engines, nor by VW. Good, name brand fuel and major independents have all the additives needed for clean burning and system maintenance.
I'll offer two additive exceptions: Before each major service, where I will be changing fuel filters (15K or 30K depending on model), I add a can of valve cleaner and a can of fuel injection system cleaner to the last tank fill before the service. My thoughts are these will help break free any varnish, carbon, etc., and it will be trapped by the filter and discarded. A good FI cleaning at that interval has kept all of my vehicles going over 100K with never an injector failure. (My Porsche just turned 40 years with the original injectors.) If your system needs additives beyond these 'maintenance shots' you should address the cause or reevaluate your fuel source.
8/7/99 (9:38 PM)
This has been hashed to death, but I fournd the following from a Bosch FI manual, so thought I'd pass on along with general comments since Bosch supplies VW's FI.
The VW is designed to run on "regular" gas. Anything more is wasted money and can cause running problems. Today, "regular" is generally 87 octane, Gov't R+M/2 method. That's about 90-91 RON.
From a Bosch FI manual (Pub #4232.21) discussing the changes in the mid-70's when even high performance cars dropped requirements to 87 octane:
They then go on to say it will cause CO levels to be high and difficult to adjust. In the catalytic converter and oxygen sensor era, this also has the effect of causing the FI system to try to compensate for the excess CO levels by leaning the engine beyond specifications. Too lean a mixture can do serious engine damage. It also strains the catalytic converter's capacity.
Old tales claim high-test gas has better additive packages. That's no longer true in any major brand. FI systems are so touchy, no manufacturer wants the reputation of fouling expensive systems. Nor do they want to be pegged as a contributor to pollution by EPA.
There are circumstances when a slightly higher octane say 'middle' or 89 is prefered. Old engines may have sufficient carbon build-up to actually raise compression. This will cause a ping under load. If your engine is pinging, try a middle octane. If it cures the problem, you will probably want to consider a head job or overhaul. Not critical short term, but it indicates the engine has seen better days, contaminated with carbon, soot and other maladies, not just higher compression.
Some head jobs and overhauls may over-machine the heads and increase compression. This is not desireable, but leaves one with the choice of new heads or higher octane. A better mechanic also comes to mind.
Certain areas of the country sell winterized fuel, or have special blends that deviate from the normal 87 octane regular. In the Rocky Mountains, it's not unusual to find 'regular' at about 83-84 octane. VW's don't like this sub-grade fuel. Some brands, like Sunoco's old 'custom blend pump' would put their economy grades below 87. The only cure for this marketing or EPA numbers game is to READ THE NUMBERS on the pump. Every pump MUST be posted with the R+M/2 method octane. Get an 87, regardless of what quality level it's called.
Octane blends. If you get a tank of sub-87 by accident, or suspect so, you can put in some high test to balance it out. I once got a tank of 'regular' near Denver that turned out to be 83 octane. It didn't run worth diddley. Every time I could squeeze in 5 gallons, I put in high-test and by the 2nd or 3rd go-around, everything was back to normal.
Winterized fuel, and gasahol are oxygenated, theoretically giving cleaner burning. Unfortunately, they also tend to reduce power and mileage, reported to be by as much as 3%. Their octane is the same, but the energy contained is not. This drop in mileage is unrelated to the octane. With most fuels containing some ethanol these day, you will have little choice but to use it. VW's should have no problems below 15% and the 10% of most blends shouldn't cause any concern. They are required by law to have a sticker (not necessarily big and prominant) on the pump.
Leaded gas: Tetraethyl lead was an additive used to increase the effective octane of gas. It is now outlawed pretty much around the world (cancer suspect in fumes & spills) and has been superceded by new technology. It had the side-effect of being a lubricant for valves & valves seats. In any newer engine ('70s on), this was alleviated by the superior technology of hardnened valves & seats. Some believe that, in OLD engines, the lubricant replacement additives in the new unleaded fuels are not adequate and a lubricant additive should be used. The jury is out. I use one in a 60+ year old flathead engine that I know is all original, but not much else. Any COMPETENT overhaul in the last 25 years should have gone to hardened valves & seats, and thus make leaded gas unnecessary. The side-effects of fouling, exhaust corrossion, increased acidity, etc., overwhelm any benefits. Ruining a cat converter ought to convince you that additives aren't the best route -- to repeat, there are no magic cures in a bottle.
ADDITIVES: Generally, not recommended for gas engines, nor by VW. Good, name brand fuel and major independents have all the additives needed for clean burning and system maintenance.
I'll offer two additive exceptions: Before each major service, where I will be changing fuel filters (15K or 30K depending on model), I add a can of valve cleaner and a can of fuel injection system cleaner to the last tank fill before the service. My thoughts are these will help break free any varnish, carbon, etc., and it will be trapped by the filter and discarded. A good FI cleaning at that interval has kept all of my vehicles going over 100K with never an injector failure. (My Porsche just turned 40 years with the original injectors.) If your system needs additives beyond these 'maintenance shots' you should address the cause or reevaluate your fuel source.
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Fuel stabilizers: