Howdy all, I have a 73, early SU's with air horns and ITG filters. I have started putting lower octane gas into my tank and have not really noticed a performance drop. Hills are fine, temperature and predetonation are fine. My question is: How important is it to have a lead substitute for our Z's without the hardened valve seats? It drives me crazy. The day CA gas went to that stupid MTBE crap, my gas mileage went from the mid 20's (22-26) to the low 20's high teens (16-21). Is that just the gas, or am I missing something else? If anyone has any info, shoot it my way!

If I'm not mistaken, I don't think you can buy any "real" lead (tetrahydrol?) additives anymore. Most of the octane boosters sold just create deposits on your valves.

You car should be fine on premium unleaded. If you're really worried about it, you can buy a barrel of racing fuel from VP Racing and add 5 gallons of it to each tank of pump gas.

Thanks for the info. Anyone else out there have anything, just let me know. Thanks...

My understanding is that if you don't have hardened valve seats or guides and you don't use leaded gas or a lead substitue your valve seats and guides would wear dramatically faster! I admit that I am not a chemistry expert or a fuel sciences expert but I don't see why running preium fuel would be any better than running lower fuel grades for preserving these parts. In my opinion if you are using premium gas or race gas and don't need it your are wasting money. Also, if you don't have hardened valve seats or hardened valve guides you should use leaded gas or a substitue or you will be wasting larger chuncks of money soon.

I wouldn't be surprised to learn that the modern lead substitues are of poor quality but it is probably still cheap insurance. When I had my engine rebuilt over 10 years ago that valve seats were severly worn, I always thought it was due to not using the lead substite. I had hardened valves and guides put in at that time.

No clue about the drop in gas mileage! Have you tuned up the car and adjusted the valves recently?

Corrections and comments welcome!

Thanks for the comments Royce. I am installing an electronic ignition (crane cams) this weekend and tuning. New spark plugs and setting the timing. That will be about all. I know that the California MTBE has a lot to do with all vehicles getting fewer mpg. Any other insight is welcome too. Thanks:)