Arlie/All,
Steve is correct on the pistons. Same same on all 68 engines. I also agree with him on the effect on mileage. Minimal at best. Not worth the investment if increased mileage is your goal.
What is your goal? If more power is the goal and mileage a possible benefit, you may get where you want.
I have been dying to talk compression and octane. I am no chemist, but rely on yesterdays engines reaction to today’s gas. So here goes.
Today’s gas octane rating is a relative figure. The mixture and the additives (lack of lead) have no correlation to the gas in 1968. Lead was the primary anti-detonation (read slow down the burning) for engines with higher compression in the 60’s. The amount of lead was increased to raise the octane rating. In my mind today’s gas rating has no relation to old rating system. Today’s gas has no lead as we know. They are using other chemicals and compounds to reduce the combustion characteristics of “gas”. Does 87 octane gas react differently than 92 in today’s or yesterdays engines?? Yes. 92 will reduce ping in an engine that pings on 87. But I have found that 92 reacts very similarly to 99 or 101 from the days of old. My 3 cars, which 2 have iron heads, all run high compression. My 67 runs around 13 to 1. My 57 Wagon with the 428 Cobra Jet runs 10.7 to 1 or stock. My GT/CS runs 10 to 1 with aluminum heads. All run great on 92 octane with no booster our additives. Tuning is a major part of this, although they are not far away from stock timing.
Let’s talk about gas a bit more. Hot Rod ran an article about a year ago and used a 360 mopar engine with 10 to 1 hotted up to about 400 HP as a test mule. The ran 6 pulls on 87, 89, 92, and some race gas more like the old days with lead that were rated at 101 and I think 113. Each pull was at a different “total” timing. 38 degrees total is the industry standard for all naturally aspirated engines whether today or yesterday. Total timing is the addition of initial timing, plus vacuum advance, plus centrifugal timing. The total timing measure at above 2,500 RPM for the engine. Need a degree tape or a degreed damper to measure.
The results of these pulls demonstrated something I have felt would happen. Today’s gas ran fine at the 38 range and made good HP. The more they dropped the total to say 32 or 34 total on this engine the less power it made on today’s gas. The interesting thing that happened with the “old type” gas is that the engine actually made more power as they reduced the total timing. Down to 30 and below. The old gas was more “volatile” or burned quicker and liked less total advance. I will go way over my head next in my education….. One explosion or power stroke on yesterdays gas produced more “joules” (yes I said that) of energy or heat than a power stroke on today’s gas. The test mule made the most HP on the old gas!! I was not surprised.
Today’s gas burns slow and will run in high compression engines, but it does not make as much power. So you can tune your 4V 302 or 390 to run just fine on today’s. You need to adjust your engine properly. Now if you wish to add additives or run aviation gas or whatever you have to be consistent. If you tune it to run on the hot stuff, of course it will run differently on today’s gas.
Another thing that factors in is camshaft overlap. Believe it or not a hotter cam with more overlap actually bleeds off low speed compression and aids in elimination of mid-range pinging. If you measure compression of a stock engine and then install a cam with more overlap with no other changes, the compression measurement will be less across each cylinder. Would love to explain cam “overlap”.
Another area that can really help on a stock engine is the proper adjustment of the vacuum advance. The early vacuum advances have the ability to unscrew the end and use different shims and springs to adjust them. The later stamp steel models you can insert an Allen wrench in the tube where the vacuum nose plugs on and adjust it. I took an old one and cut in open to see how it works. This simple adjustment can tune away mid-range pings. I can go into that.
Make your choice on the fuel you will burn. Stay consistent and tune it to match. If you want the most power, get the old fuel. But you can make great power and run just fine on 92 with your old engine.
By the way, don’t get me started on hardened seats due to “unleaded” gas. Unless you are pulling a huge trailer or running a boat, hardened seats are unnecessary. Also, when you are putting so few miles on an engine, it will never factor in. Don’t tear into your engine just to put in hardened seats.
Not speaking for Paul with the miles on his car, but most of us put so few miles on the car. How many years will it take some of us to put 50k on our car? Forever….
Rob