Took my 71 on a long trip, around 1000 miles... the car performed really well. Had a couple minor problems that got worked out thanks to the “on road service” from the forum... Anyway I was feeling really good about how peppy and smooth it was running so I felt like taking her out for a ride yesterday... In Spain our unleaded has is 95 or 98 octane, I always use the 95. The 71 was the first year of lower compression so it could run on no lead gas and I was told that the 95 octane was equivalent to high test in the states. Well yesterday I thought I’d fill up with 98 to see if I get some extra zip... Well soon after the car starts running like sh_t, not smooth and lacking power. Sort of felt like one cylinder wasn’t firing... I pulled over to see if a vacuum line got disconnected or something but everything looked find. I drove back to my garage and parked it. Needless to say I’m very worried. Could it have been the higher octane gas or is something else going on?

 

I'm in Australia. We to have the same octane rating as you in spain. We have standard fuel for the eco **** box family cars. 91. and 95 mid grade and 98. The 98 would be like 91 in the States. I lived in the states until 12 years ago.
a low compression engine does not need nor will it benefit from 98 octane fuel. However, it won't cause the issues you speak of.
sorry, perhaps bad fuel??? More likely something else. Sorry.

 

Most likely contaminated fuel (water?).

 

OP, if the car indeed started running poorly, soon after filling the car with gas, then I think it's a sure bet that you got a bad tank of gas (water content, probably). I would get some "dry gas", if you can, or some fuel treatment like STP's if dry gas isn't available where you are, and give the car a good dose of it. The higher octane gas will give you more "zip", but only if you adjust the ignition timing, to take advantage of the increased octane's ability to prevent "pinging".

 

Perhaps with 98 being more expensive than 95 fewer people will buy it - and perhaps a reason for water contamination at the pumps. Presumably Spain, Europe (and Auzzie) use Ron octane rating (rather than USA's Mon rating?) - wonder if the addition of 5% Ethanol (soon to be 10%) has a bearing on fuel/water contamination being as Ethanol, Methanol, Alcohol etc are hygroscopic (water absorbing) - whereas water in your tank with 100% gasoline falls to the bottom of the tank and usually only creates issues (other than a rusted-out tank) when the fuel level is low. Water contaminated at the filling station is gonna be more difficult to ascertain if actually held in suspension within the gasoline.

 

Seems to me more like a timing issue due to the higher octain....? I got a couple carburetor sputters sort of like a back fire. I may just go for a long ride and use up the tank of gas and then fill it up with my regular 95 octane...

 

Higher octane serves one purpose, ping resistance. Otherwise lower octane fuel contains more energy than the higher octane fuels as long as were not talking about oxygenated fuels.
So feeding high octane to an engine that doesn't need it will result in less power, not more. It's all dependent on cylinder pressures/temperatures. High enough and you need the detonation prevention of high octane. Otherwise you run the risk of destroying the engine due to detonation.

Your problem, I would agree with the other posters that it's fuel contamination NOT octane related.

 

Sometimes the gas pumps in Spain are easily confused with diesel fuel, or could have been mixed in a storage tank by mistake.
Ed

 

Added 4 gallons of Shell Vpower Diesel (in error) to the 4 gallons of Super Unleaded already in the tank - like a WW2 destroyer laying down a smoke screen. It wouldn't have even run if not for the MSD ignition......

 

Add me to those who are telling you that its not the octane rating of the fuel. If the fuel is of good quality you won't notice any difference going to higher octane fuel. If its the fuel causing your problem, you got a bad batch. And if that's the case, the problem will go away as you use up the bad gas and replace it with good.

 

If you have any deposits in your cylinders they could be responsible for the poor running on higher test gasoline. A lower compression engine can make lots of carbon and other materials on the piston crown. If it gets too much it will start to glow and cause pre-ignition or detonation.

Sometimes I put additives in my gasoline to get rid of the deposits in the cylinders. There are plenty of products to do this on the market. I tend to use B&G 44K fuel system cleaner as it cleans everything well.

I used to build air cooled VW engines for people and in my years I remember the trick we used when a VW beetle would require premium to stay quiet. Even (8-1 compression ratio) VW's can have detonation believe it or not.... The solution was to pour a small quantity of cold water down the carburetor while the engine is hot. You only add enough water that the engine stays running, don't dump a pint down the throat of the carburetor. While adding the water we rev the engine to keep it running and the thermal re-action in the cylinders was enough to blow large quantities of carbon out the exhaust system. It worked and was a cheap way to stop a VW from pinging.

The other way would be to put the best quality gasoline you can get and "drive it like you stole it". If it is still pinging or detonating just retard the timing a bit as well if needed.

 

Dave, do you know if or when your timing chain and gears were replaced?
Dave in Tampa

 

Guys, you're getting WAY off base on this. OP said the problem began right after he filled up the car with gas......

Step 1 of basic analytic troubleshooting, a course I took back in 1972. When a problem arises, question number 1 is, "what changed". In the case of the OP's Corvette the gas "changed". Therefore start troubleshooting there. Since he's in Europe, I doubt there are Advance Auto Parts, Autozone, NAPA, or Pep Boys store all over the place, but I would hope there is something equivalent that he can go to, and buy 2-3 bottles of gas treatment....

 

The September, 2020 issue of Hot Rod magazine discusses this in their article "What is Octane Creep? p. 56. The article mentions that the octane requirement of a new engine can increase as much as 4 numbers in the first 10,000 miles, perhaps as much as 6 numbers by 20,000 miles. This is due to carbon build up in the cylinders. This is particularly a problem for the newer computer/ping sensor equipped engines, because you won't realize this is happening. These engines, when they sense a ping, they automatically reduce ignition timing.......and hence engine performance. As a driver you have no awareness this is happening. The article recommends fuel additives that reduce carbon build up, such as GM's TopEngine Cleaner (PN 88861803) or ACDelco X66P.. I've found Chevron Techron works for me.

 

Sounds like water in the gas...I have had this a few times over the years. I would buy a $2 bottle of dry gas from any gas station or auto store that carries it and pour it in the tank... this sould be the first thing done at this point before wasting time guessing about less likely or more expensive causes. a low compression SBC should run fine on any unleaded 87 or above (US method)fuel available at the gas pump. anything available at a regular gas station isnt going to to be rocket fuel or require the timing to be adjusted just to run smooth.

 

I don't buy it. Maybe for engines that never warm up fully or maybe direct injection engines and build up on the valves which could possibly lead to hot spots, but generally speaking I don't believe it's a real thing.
I've torn down many a modern engine and found no appreciable carbon build up that would cause a need for a higher octane rated fuel over time. Engines with 200,000+ miles.
If this were true then take an engine that requires 91 octane right out of the box and over time, if this "octane creep" were a real problem you would not be able to increase the octane in states like California and so timing would be rolling back progressively, further and further retarded until it got to it's limits then uncontrollable detonation would set in.

 

Horse ****.

 

My '71 runs fine on 91 /93 octane fuels . I'm not a chemist but the older fuels .. ie. Sunoco 260 ,Super Shell , Mobil 260 ...... were rated above octane in proformance numbers like AVGAS in airplanes ....... 115/145 [purple] ...105/130 [green] ......

 

In the days before all these fuel injected cars with their catalytic converters our cars actually used a bit of oil where as todays engines are much built to tighter tolerances and burn Very little compared to the older days.
When was the last time you had to add a quart every 1000 miles (Fords One quart for every 500 miles) in your GM?

Today's engines frequently go all the way to the next oil change without using much oil at all. This would have been fairly rare back in the mid sixties. When the engine starts burning oil it will start making deposits on the piston crown or in the cylinder head. That is when you have problems. When you get burned oil in the exhaust it will damage the Oxygen sensor.

I read the same article in the Hot Rod, If some of you think it is bologna that is fine by me. I have seen "Octane Creep" in action many times during my years. The first time I saw a VW engine that was pinging like crazy it blew my mind but I found the cause on top piston crowns. That is why a fellow VW mechanic showed me the "Water down the carburetor" trick and the first couple times I did it the carbon blowing out the exhaust was mind blowing.

 

Any engine that is poorly jetted or running rich for any reason is going to develop carbon deposits. Those deposits can cause hot spots or even a slight (very slight) increase in CR due to less space in the combustion chamber. But to say this is common in today's engines I would still say is overreaching.
The problem may actually be caused and/or exacerbated by using high octane fuel that cannot be fully burned by a low compression engine that does not have the timing sufficiently advanced and therefore cannot completely burn the fuel creating a condition in which many deposits may be left behind.
High octane fuel burns slower than low octane fuel. That in of itself would require a greater lead time in the ignition timing to allow for the fuel burn to be complete for a given CR and combustion chamber design, before the exhaust cycle comes around. Higher CR will make the fuel burn faster and hotter.
So combine low CR with relatively retarded ignition lead and your going to create a condition that leads to incomplete fuel burn, cooler combustion chambers, more deposits making hot spots more likely. Leading you to burn higher octane fuel to bandaid the condition, and at the same time making it worse.
In the carbureted era it would be far more common since carburetors cannot fully adjust to changing atmospheric conditions. Fuel injection can adjust. Fuel enrichment still occurs when the engine is cold though, so if the engine does not fully warm up (as far as the computer is concerned or in reality) that condition can persist creating a long term problem.

One fable that has persisted for as long as I've been aware is that high octane fuel makes more power. I suspect that those VW folks who had a problem with their VW's probably believed that and ran high octane where it was not needed eventually creating their own problems. Also Many believed in the carb era and even believe it today, that the more fuel the more power, again creating their own problem by running too rich.
The VW engines that were air cooled of course are a bit of a different creature compared to a water cooled engine and tended to have uneven cooling and hot spots compared to it's water cooled counterpart. I grew up working on VW type 1 engines. Didn't hop them up or anything myself just working on them since that's what we had as a kid. Didn't seem to have any issues with detonation and used the cheapest low octane fuel of the time.
Did have friends though who did believe that richer and higher octane was the secret to high performance. LOL, high performance being a relative term.

This is irrespective of oil consumption. Any engine burning oil is going to have a long term problem.