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gas requirment for 1968 Eldorado 472 cu. inch


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Anyone on here have experience with the 1968 472 cu. inch engine? Found a great 68 eldo but concerned about todays fuels ruining the engine. I had one years ago...but high octain leaded fuel was available.

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There should be additives that you can buy to replace the lead and increase the octane rating, but I think that modern gasoline also has somewhat different burn characteristics, i.e., that it burns slightly slower. Today's engines are of course optimized for that, but in older engines it could mean degraded efficiency and also excessive heating of the exhaust valves and manifolds if the air-gas mixture has not finished burning when it is let out of the combustion chamber. I read about this a few years ago in Cadillac Club Sweden's member magazine, but I cannot verify that it is correct. As far as I understand, adjusting the ignition timing may only partially compensate. I think it should be possible to run the car without ruining the engine -- just throwing some thoughts out there, perhaps for someone with more insight to pick up on.

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The 1968 Eldorado used a 10.5:1 compression ratio, so I would definitely think twice about octane. Premium (91 octane or more) would be a must with an iron head engine with that compression.

Some old engines depended on lead deposits on the exhaust valves to keep heat transfer from the exhaust down, and clean exhaust valves would run too hot for maximum life. To me, this is a dumb move for a car that is designed for high speed cruising, because long high-speed drives tend to clean out deposits pretty thoroughly, and such a design weakness would mean reliability problems for cars driven on the highway a lot. I've never heard of a Cadillac with valve or cylinder head problems. I would be surprised if Cadillac had this problem with any engine they ever designed for use in Cadillacs. If anyone has ever had a problem with running a pre-unleaded-gas Cadillac on modern fuel, please come forward.

Alcohol is another problem. Nearly all gas you can get these days is 10% ethanol, which probably won't cause a problem, but watch out for 15% "flex fuel" gas and other products with more than 10% alcohol. It's not that they can't burn it, but some fuel system components like fuel lines, fuel pumps, internal carburetor parts, and fuel filters aren't compatible with alcohol and too much can result in damage; you might be able to identify and change over some or all of these components in a carbureted car. Also, make sure that the PCV system is in good working order so that blow-by doesn't let alcohol and water get into the oil and cause a sludge problem, even with 10% fuel mix.

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I agree with Kevin...

My understanding about running unleaded fuel in an engine designed for LEADED fuel was eventual damage to the valve seats.

I could very well be wrong about this (and probably am)... but this is what I remember from the distant past...

The lead formed slight deposits on the surface of the valve and the seat ... where they came into contact with each other.

The lead acted sort of like a cushion, to keep the valve and seat from banging against each other.

Without the lead cushion... valve seat damage would eventually occur and when the heads were pulled... specially hardened valve seats were then installed.

By using Premium Fuel and a modern lead substitute... there may no longer be a problem associated with running unleaded fuel.

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Those cylinder heads were made from high quality material, and unless you'll be towing a heavy trailer for hundreds of thousands of miles, valve seat recession from the use of unleaded fuel will not be a concern, assuming the engine is in good tune. I would not bother with a lead substitute, but premium fuel may be required unless someone has fiddled with the spark calibration. If the carburetor is all original, it may benefit from a mild rebuild to ensure all the soft bits are tolerant of 10% ethanol (try to find out which other fuel system components, as noted above, have been replaced over the years).


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Yes, I had one, but not recently. I agree with KevinW, use the highest octane fuel you can find, and if it pings back off the throttle a bit.

Exxon may still sell a 92 or 93 oct, but not sure.

Never underestimate the amount of a persons greed.

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