Reader Question: Leaded Premium and High Performance?

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Here’s the latest reader question, along with my reply!

Gary asks: Thank you for kindly answering my prior inquiry about switching off all electric stuff when you turn off a modern car. I’d learned to shut everything off when I turned the key to Off (manual transmission set in reverse). Maybe my late father was concerned about battery drain. From you, I understand how it works now. Please let me impose with another inquiry:

A person I know, who is in his 70s, commented recently that in respect of muscle cars, approximately 1970 and before, that we, today, can never really experience those cars as they were meant to be driven with high octane leaded gasoline. He also offered car-folks of his generation agree. Memory is subjective, and despite miles of movie film from those days, nothing could convey a difference of leaded vs. unleaded. But then again, Sunoco 107 was the designed diet of those cars and I fed it to my 70 Duster. Were they ever so quicker with lead? Or quieter? I add here that I’ve seen young people at car shows laugh when a car with a ’60s Hemi is started because, to them, it’s ridiculously loud! Or was it all about compression ratio in pre-computer days, so that modern gasoline with an octane boost provides the same drive? What do you think? Is this a distinction with a difference, or just folklore? Or some of both?

My reply: Lead was added as an octane booster (and lubricant) but withdrawn for environmental reasons, resulting in a wholesale lowering of compression ratios circa 1970, to accommodate the new unleaded (and lower octane) gas. This gas was very hard on engines designed to run on leaded – and which needed the high-octane. In those days, there were no knock sensors and cars didn’t have computer-controlled engines that can adjust for lower (or higher) octane by adjusting ignition/cam timing, turbo boost and so on.

The older high-compression engines would suffer “pinging” – pre-ignition – if fed gas without a sufficiently high octane ratio; this could cause physical damage to the engine and would absolutely hurt the engine’s power/performance.

Many owners of older cars with high CR engines would buy aviation gas or race gas in drums; or add octane booster (and lead) to bring the fuel up to spec. Or retard their engine’s ignition timing – and try not to load the engine too much.

Higher-octane unleaded began to become available again in the ’80s and through to today. The way octane is measured today makes it seem as though today’s “hi test” (usually 92-93 octane) is much lower than the 100-plus octane fuel that was available in the ’60s, but it’s analogous to the way horsepower is measure today vs. back in the ’60s.

Back then, engine horsepower was rated according to the SAE “gross” standard – without accessories and (usually) with headers and a performance tune – as opposed to the current SAe “net” standard, which rates the output of an engine in full production trim, with accessories and exhaust (including mufflers) installed, etc. An engine that was rated as making 335 SAE “gross” hp in 1970 would only rate 290 (or thereabouts) in 1972. Lowering compression and cam aggressiveness also gimped the output, of course. But you see the point.

Today’s high-octane unleaded is enough for today’s engines, many of which run extremely high compression ratios (and boost). It is common for a passenger car engine – not a high-performance engine – to have a CR of 10.0:1 or higher; some go up to 13.0:1 – and it’s routine for turbo-boosted cars to run 26-plus pounds of boost.

So, an old muscle car with its original engine should run just as hard as it did back in the day on today’s gas s it did on yesterday’s gas. The main issue with those older engines is the lack of lubrication from the lead; of course, by now – 50 years down the road – most have been rebuilt with hardened seats and so on to handle low-lead gas!

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  1. I don’t know I’d agree lead boosted octane. What it did was prevent premature ejac…..uh…..detonation and lubricated the engine’s cylinders and valves. 107 octane fuel had no more lead per liter than lesser octane, it was simply higher octane containing lead. The other thing it did was to reduce the temperature in the cylinder since a 12-1 C.R. would run ok on 103 but ran better with 107 due to better combustion. Of course the higher the octane number, the less power gasoline had. That was back in the days when 6-8 C.R. was common on engines not meant for high performance.

    Everyone was still running regular valve seats when lead was removed and if you didn’t use some sort of octane booster you’d quickly burn the valves and seats. Fuel was extremely expensive for me back then since I’d use the highest octane I could get and then dose it up with Moroso octane booster, a really dangerous cocktail of all sorts of petroleum based chemicals. It even came with a cancer warning sticker on the can……eventually.


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