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  1. #1
    Petty Officer 1st Class
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    Default Leaded vs Unleaded Fuel

    Just noticed a sticker on the side of Mercruiser 165 inline 6 that says to use leaded fuel. Anyone experience any issues with using unleaded fuel in these old engines?

    Randy

  2. #2
    Fleet Admiral HT32BSX115's Avatar
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    Default Re: Leaded vs Unleaded Fuel

    Howdy,


    It's just not a problem. Run it on regular unleaded.(unless you use AVGAS, you don't have a choice anyway)




    Regards,


    Rick
    73 de Rick
    ----------------------------------------------------------------------------------
    I use xxx oil and have never had a problem........
    1987 FourWinns 211 Liberator, 7.4L- Bravo III installed by ME,
    Formerly OMG 460 King Kobra powered
    1947 Stinson 108-2 Station Wagon
    , Franklin 6A-335B. Hartzell CS 2-blade prop
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  3. #3
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    Default Re: Leaded vs Unleaded Fuel

    With a lot of hours of use on unleaded gas the exhaust seat will recede and the valves will sink deeper into the head.
    If you have the head off anytime, I would put hardened exhaust valve seats in the head.

  4. #4
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    Default Re: Leaded vs Unleaded Fuel

    My Father had a 1974 Merc 120 with the "Use Leaded fuel only" sticker. We used no lead for many years without issue. I believe somewhere in the 1972-74 years they started to install factory hardened valve seats, but I don't recall the exact year.

    What year is the in-line 6?
    1989 Four Winns 170 Cobra 3.0

  5. #5
    Rear Admiral Fishermark's Avatar
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    Default Re: Leaded vs Unleaded Fuel

    Well, you're not going to find leaded gas, and additives are a waste of money. Run good old regular unleaded. If and when you ever have issues, (and you probably will not), you can address the valve seat issue then. And remember this - leaded fuel hasn't been available in decades. Sounds like the boat has made it fine this far.

  6. #6
    Fleet Admiral HT32BSX115's Avatar
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    Default Re: Leaded vs Unleaded Fuel

    Quote Originally Posted by coheej View Post
    With a lot of hours of use on unleaded gas the exhaust seat will recede and the valves will sink deeper into the head.
    If you have the head off anytime, I would put hardened exhaust valve seats in the head.

    How many hours will it take?

    My previous boat, a 1966 Mark Twain/150 Mercruiser that Dad bought new, didn't get leaded gas for about 25 years!! I sold it in 2005.

    Engines that run on propane or natural gas seem to run for EVER!

    I think there may be "much ado" about unleaded gas...... It's right up there with E10 and the Oil-Snakes.......
    73 de Rick
    ----------------------------------------------------------------------------------
    I use xxx oil and have never had a problem........
    1987 FourWinns 211 Liberator, 7.4L- Bravo III installed by ME,
    Formerly OMG 460 King Kobra powered
    1947 Stinson 108-2 Station Wagon
    , Franklin 6A-335B. Hartzell CS 2-blade prop
    -
    All PM's cheerfully answered! Please ask tech questions in the appropriate forum so everyone's included

  7. #7
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    Default Re: Leaded vs Unleaded Fuel

    Quote Originally Posted by HT32BSX115 View Post
    How many hours will it take?

    My previous boat, a 1966 Mark Twain/150 Mercruiser that Dad bought new, didn't get leaded gas for about 25 years!! I sold it in 2005.

    Engines that run on propane or natural gas seem to run for EVER!

    I think there may be "much ado" about unleaded gas...... It's right up there with E10 and the Oil-Snakes.......
    That's something we won't know for sure, will we?
    We just have to do what works for us.
    I've taken old heads (60's automobile) apart that the seats we 1/8" deeper than stock. How many miles (hours), I didn't ask. I tell my customers if they're going to drive it more than 10000 miles a year and the heads are already off; I would put new exhaust seats in them.

  8. #8
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    Cool Re: Leaded vs Unleaded Fuel

    Also, the old "leaded gas" engines did not have SST valves in them.
    One of the leads' effects was cooling. Thus, mild steel stem valves were used.
    Take the lead out, and you lose the cooling effect that lead brings. Thus why SST valves are used in 'unleaded gas" engines. SST does not conduct heat very well. Hence it's use in exhaust heat shields.

    Also, the lead made the gas much more stable under compression. That's why the old muscle cars could run a 12:1 compression without detonation. That lead was a huge advantage in an engine. But, it causes cancer and such and is bad for the envirionment.

    Progress, I guess.

    1988 Chris Craft Scorpion
    4.3L V6 Mercruiser
    Alpha 1 Drive

  9. #9
    Fleet Admiral HT32BSX115's Avatar
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    Default Re: Leaded vs Unleaded Fuel

    That's why the old muscle cars could run a 12:1 compression without detonation.
    Um.....how many pleasure boats out there have 12:1 compression?

    In fact, how many over the road cars have 12:1 compression? Or better yet, how many cars (other than on a drag-strip) have 12:1 compression?


    I've taken old heads (60's automobile) apart that the seats we 1/8" deeper than stock. How many miles (hours), I didn't ask. I tell my customers if they're going to drive it more than 10000 miles a year and the heads are already off; I would put new exhaust seats in them.
    I'm not saying one shouldn't install hard seats in an engine when it's apart...... (I had hard seats put in my Ford Y-block when I rebuilt it)

    I'm just saying it's not worth doing it to boat engine (unless you're going to do a complete rebuild)

    It's not worth tearing the engine down or even removing the head if it's running fine.

    It's also not worth worrying about it. When it comes time to rebuild the engine or repair a head that DOES have a valve problem......it's absolutely worth it to put hard seats in it. (I don't think ANYONE does soft seats any more........)


    Leaded gas is pretty much gone and when they finally get a replacement for 100LL Avgas, the ONE company that still makes tetraethyl lead will vanish.

    this thread probably needs to be moved to Engine Frequently Asked Questions (FAQ)
    73 de Rick
    ----------------------------------------------------------------------------------
    I use xxx oil and have never had a problem........
    1987 FourWinns 211 Liberator, 7.4L- Bravo III installed by ME,
    Formerly OMG 460 King Kobra powered
    1947 Stinson 108-2 Station Wagon
    , Franklin 6A-335B. Hartzell CS 2-blade prop
    -
    All PM's cheerfully answered! Please ask tech questions in the appropriate forum so everyone's included

  10. #10
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    Default Re: Leaded vs Unleaded Fuel

    valve cooling is an engine design and valve face and seat contact design, has nothing to do with fuel.
    tetraethyl lead was a very good octane booster and it would cushion and lubricate the valve seats and faces.
    however, it was determined to be bad juju.
    not many stock production run engines ever ran 12/1 compression ratios.
    however 10.5/1 was pretty common in the sixties to the early 70's.
    would I sweat it or add a snake oil? nope.
    run it until if and when it has issues, fix it and keep running it.
    no tech questions by PM, they wont get answered.
    you have to be trusted by the people that you lie to .

  11. #11
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    Exclamation Re: Leaded vs Unleaded Fuel

    Quote Originally Posted by HT32BSX115 View Post
    Um.....how many pleasure boats out there have 12:1 compression?

    In fact, how many over the road cars have 12:1 compression? Or better yet, how many cars (other than on a drag-strip) have 12:1 compression?



    I'm not saying one shouldn't install hard seats in an engine when it's apart...... (I had hard seats put in my Ford Y-block when I rebuilt it)

    I'm just saying it's not worth doing it to boat engine (unless you're going to do a complete rebuild)

    It's not worth tearing the engine down or even removing the head if it's running fine.

    It's also not worth worrying about it. When it comes time to rebuild the engine or repair a head that DOES have a valve problem......it's absolutely worth it to put hard seats in it. (I don't think ANYONE does soft seats any more........)


    Leaded gas is pretty much gone and when they finally get a replacement for 100LL Avgas, the ONE company that still makes tetraethyl lead will vanish.

    this thread probably needs to be moved to Engine Frequently Asked Questions (FAQ)
    What cars or boats run 12:1 is not the point. The point is, with the old leaded gas YOU COULD run 12:1 and gain much more power from this set-up. It's a fact that leaded gas creates more power through an engine because of it's cooling effect and extra stabitlity making higher compression possible. Why do you think engineers placed lead in the gas in the first place? For fun? No, for perfomance gains.

    But whatever. It's neither here nor there. It's in the past. On a side note, Hi-test leaded was 100 octane all day for real.

    1988 Chris Craft Scorpion
    4.3L V6 Mercruiser
    Alpha 1 Drive

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