Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

66 thunderbird - what additive is best for the gas

I currently drive my 66 thunderbird to and from work (50 miles round trip) four days a week. the ethanol free gas I us is only rated 89 octane and the owner’s manual recommends 98 octane (390 cubic engine). The question is should I be more concerned with using an octane booster or a lead substitute to help maintain the valves/cylinders/pistons? Or do I need to use both

That 98 octane is on a different scale, I think. Is your engine knocking? If not, there’s not an octane problem. You might have a valve seat problem eventually because of the absence of lead, but the fix for that once there’s a problem is the same as it is now - pull the heads and have valve seat inserts installed. So I’d wait on that.

The ethanol free gas prevents the fuel system components from being destroyed.

The lead substitute prevents the valve seats from wearing.

This wouldn’t normally be a problem with a vehicle of this vintage. Because most aren’t driven on a daily basis.

But in your situation, you want to use a lead substitute.

Either that, or have hardened valve seats installed in the heads.


1 Like

Yes the hardened valves and seats would be good, If it were me I would replace fuel lines and pump diaphram and carb needles with ethanol resistant parts also. Also seafom or techron or heet in the cold months to fight condensation of water into the gas.

Back in the 50w and 60s you could buy pump gasoline with over 100 octane. I remember Golden Esso with 102 octane and lead free Amoco premium 101 octane.

I don’t remember when we changed but now the number on the pump is supposed to be an average of research and motor methods of measuring octane.

If it isn’t pinging on accelerations or going uphill, the octane you are using is ok. Do some research on whether your engine was manufactured w/ hardened valve seats or not. If not, use a lead substitute additive.


My 67 Galaxie 500 with the 390 didn’t have hardened valve seats.


1 Like

There’s a book titled “Fix Your Ford” which I believe says when the switch to hardened seats was made. It might not have occurred at the same time for all engines, models, or for all plants. My guess is a 66 Thunderbird/390 doesn’t have hardened seats.

texasas is on the right track.

Right now you’re doing the best things for your car; running it regularly, using ethanol free gas if you can get it and enjoying it so if it ain’t broke, don’t fix it.

Back when the lead was first removed, there was a whole lot of tumult about “valve guide wear due to lack of lubrication” and everybody and their brother was selling some kind of a lead replacement and octane booster concoction.

My experience has been that by the time the valves are starting to show some wear, there’s other parts that will need attention too so you may as well keep your money in your pocket for an eventual rebuild.

thank you so much for your feedback it is very much appreciated

It’s not the valve guides that wear.

It’s the valve SEATS that get pounded out from the lack of lubrication due to the lack of lead.



thank you for that - I will look into that book

Many years ago, I worked on several old pre 1970 Ford trucks that ran on propane and never dealt with a problem with worn valve seats caused by lead free, virtually dry fuel. In fact I have never seen any damage that seemed to be the result eliminating the lead from the gasoline used in old engines including many old Continental flat head engines used in portable welding generators and air pumps. Apparently there was a serious problem with some applications in the early 70s but they slipped right by me thankfully.

1 Like

Thanks for the correction