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I have a '69 L71 coupe which will not run without pinging on pump gas. Sunoco 94 won't do it and the octane boost stuff doesn't make much of a difference. A 50-50 mix of aviation 100 octane low lead and the 94 does the trick. I also have a 66 L79 which runs great on the 94. My questions are? Do people with stock L71s run ok on the 94? ( this one has been rebuilt and may not be truly 100% stock now.)And what was the recommended octane for the 69 in 1969 and how does it equate to todays numbers? Thanks for any help.
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Re: Octane
John,
I don't have the technical information to answer your questions but just wanted to point out some good discussion about lead and lead substitutes that is still posted on the discussion board. It's begins from Steve on Sunday May 30 (HELP! What do I do for a lead substitute)and has several reponses.
The response that reminded me of you is from Bill Clupper, Sunday May 30th and he makes mention to the octane rating, which is scaled somewhere around 4 points(?) less than what it used to be. That alone is something I really wasn't aware of. Sorry I couldn't be more helpful. Tom. -
Re: Octane
John,
I don't have the technical information to answer your questions but just wanted to point out some good discussion about lead and lead substitutes that is still posted on the discussion board. It's begins from Steve on Sunday May 30 (HELP! What do I do for a lead substitute)and has several reponses.
The response that reminded me of you is from Bill Clupper, Sunday May 30th and he makes mention to the octane rating, which is scaled somewhere around 4 points(?) less than what it used to be. That alone is something I really wasn't aware of. Sorry I couldn't be more helpful. Tom.Comment
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Re: Octane
John, "Lots" of things can contribute to your problem, but to answer your question re-the L-79 vs L-71, they are essentially the same compression ratio, with the L-71 having more camshaft duration, which should make it somewhat more tolerant of lower Octane, not less, all things being equal. However, since you state that the BB has been "rebuilt" you have more investigation to do to get your answers. Do you know exactly what was done to the engine? new pistons, if so what dome size?, was the block cut? were the heads cut? all of these things will go into determining the true compression ratio of your engine. An important thing to remember is that the advertised compression ratio on a bigblock (or any Chevrolet production engine for that matter) is just that, ADVERTISED. Few if any engines ever left the factory carrying the advertised number, generally they will run 1/2 to 3/4 point lower than advertised. If your car has been "blueprinted" to actually hit the max, you may have more of a problem on your hands than if it was just "rebuilt". One of the things often overlooked in following "pinging" conditions is the condition of the distributor. Often, well meaning "enthusiasts" will take it upon themselves to "recurve" the distributor, often bringing in much more timing than otherwise would be used at a particulay engine speed. The distributor should be checked on a Sun Machine or equivalent and returned to exactly the factory specs, including the vacuum advance. Bigblocks can be somewhat more touchy than smallblocks regarding detonation, but will usually run on pump hi-test if properly tuned. Make sure the timing is properly set to factory specs, and as a last resort, try different sources of high-test. I have a hard time on restarts with Sunoco 94, most of the guys around here use Amoco "Ultimate" with better results, so try some more. I don't know of any good way to check your compression ratio without pulling as head, so I've tried to provide some "workarounds". I hope they helpComment
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Re: Octane
John, "Lots" of things can contribute to your problem, but to answer your question re-the L-79 vs L-71, they are essentially the same compression ratio, with the L-71 having more camshaft duration, which should make it somewhat more tolerant of lower Octane, not less, all things being equal. However, since you state that the BB has been "rebuilt" you have more investigation to do to get your answers. Do you know exactly what was done to the engine? new pistons, if so what dome size?, was the block cut? were the heads cut? all of these things will go into determining the true compression ratio of your engine. An important thing to remember is that the advertised compression ratio on a bigblock (or any Chevrolet production engine for that matter) is just that, ADVERTISED. Few if any engines ever left the factory carrying the advertised number, generally they will run 1/2 to 3/4 point lower than advertised. If your car has been "blueprinted" to actually hit the max, you may have more of a problem on your hands than if it was just "rebuilt". One of the things often overlooked in following "pinging" conditions is the condition of the distributor. Often, well meaning "enthusiasts" will take it upon themselves to "recurve" the distributor, often bringing in much more timing than otherwise would be used at a particulay engine speed. The distributor should be checked on a Sun Machine or equivalent and returned to exactly the factory specs, including the vacuum advance. Bigblocks can be somewhat more touchy than smallblocks regarding detonation, but will usually run on pump hi-test if properly tuned. Make sure the timing is properly set to factory specs, and as a last resort, try different sources of high-test. I have a hard time on restarts with Sunoco 94, most of the guys around here use Amoco "Ultimate" with better results, so try some more. I don't know of any good way to check your compression ratio without pulling as head, so I've tried to provide some "workarounds". I hope they helpComment
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Re: Octane
John----
In the 60s and 70s I rarely saw a reference to the actual octane ratings which different engines required. Owner's manuals would simply indicate "regular" or "premium". In fact, in those days, the pumps did not display octane ratings, so even if you knew what octane your car required, you wouldn't know what the octane rating of the gasoline was that you were putting into it. The yellow and black octane labels didn't start happening until the late 70s.
One "hint" that we have of required octane ratings was the sticker placed on the console of 68-69 L-88 cars. This sticker said that the engine shouldn't be operated on gasoline having a rating lower than 103 Research Octane or 95 Motor Octane. The ratings on pumps today happens to be the average of the Research and Motor Octane ratings so the requirement for an L-88 today would be 99 by the R+M/2 system. Of course, an L-88 with its 12.5:1 compression ratio would have octane requirements dramatically greater than even an advertised 11.0:1 of your L-71. Consequently, I would say that 94 octane using today's system wouldn't be too far off the mark.
I can tell you this for certain though: in 1968 I factory-ordered a 1968 Chevelle Super Sport 396 with the L-78 engine option and M-22 trans. The L-78 has virtually the same internals, save for displacement, as your L-71. As a matter of fact, the 66-69 Chevelle L-78 engines were virtually identical to the 1965 Corvette L-78 engine. I used to run this engine on Chevron Custom Supreme("THE WHITE PUMP"), the highest octane pump gas that you could buy at that time(although as I said earlier, I never knew what the rating was). The engine ALWAYS "pinged", though. It just pinged less on Custom Supreme than on any other gas.In Appreciation of John HinckleyComment
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Re: Octane
John----
In the 60s and 70s I rarely saw a reference to the actual octane ratings which different engines required. Owner's manuals would simply indicate "regular" or "premium". In fact, in those days, the pumps did not display octane ratings, so even if you knew what octane your car required, you wouldn't know what the octane rating of the gasoline was that you were putting into it. The yellow and black octane labels didn't start happening until the late 70s.
One "hint" that we have of required octane ratings was the sticker placed on the console of 68-69 L-88 cars. This sticker said that the engine shouldn't be operated on gasoline having a rating lower than 103 Research Octane or 95 Motor Octane. The ratings on pumps today happens to be the average of the Research and Motor Octane ratings so the requirement for an L-88 today would be 99 by the R+M/2 system. Of course, an L-88 with its 12.5:1 compression ratio would have octane requirements dramatically greater than even an advertised 11.0:1 of your L-71. Consequently, I would say that 94 octane using today's system wouldn't be too far off the mark.
I can tell you this for certain though: in 1968 I factory-ordered a 1968 Chevelle Super Sport 396 with the L-78 engine option and M-22 trans. The L-78 has virtually the same internals, save for displacement, as your L-71. As a matter of fact, the 66-69 Chevelle L-78 engines were virtually identical to the 1965 Corvette L-78 engine. I used to run this engine on Chevron Custom Supreme("THE WHITE PUMP"), the highest octane pump gas that you could buy at that time(although as I said earlier, I never knew what the rating was). The engine ALWAYS "pinged", though. It just pinged less on Custom Supreme than on any other gas.In Appreciation of John HinckleyComment
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Re: Octane
Thanks guys for the replies. I've had the timing all over the place with no improvement. My conclusion is that the compression ratio of this particular engine is too high for pump gas and I will have to do surgery this winter. Thanks again to all of our consultants.Comment
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Re: Octane
Thanks guys for the replies. I've had the timing all over the place with no improvement. My conclusion is that the compression ratio of this particular engine is too high for pump gas and I will have to do surgery this winter. Thanks again to all of our consultants.Comment
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Re: Octane
Joe and Bill gave you SOLID advice/counsel. If you've played with retarding the timing and a 5-6 degree retard doesn't give you any relief, consider the advice to pull the distributor and have it checked on a Sun (or equal) tester. Short of that, you've used up the 'quick fixes'.
My '65 L-78 is bored 90 over and built stock (cam, crank, Etc.), I can get away with plain pump 91 here at altitude (7930 feet) due to O2 density reduction. At/near sea level, it needs full bore 94/95 octane pump and is borderline on ping. A 'dribble' of good boost additive KILLS the detonation issue. So, if you've got real noticeable/objectionable ping with all you've done, chances are your engine internals are pretty 'peaked'....Comment
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Re: Octane
Joe and Bill gave you SOLID advice/counsel. If you've played with retarding the timing and a 5-6 degree retard doesn't give you any relief, consider the advice to pull the distributor and have it checked on a Sun (or equal) tester. Short of that, you've used up the 'quick fixes'.
My '65 L-78 is bored 90 over and built stock (cam, crank, Etc.), I can get away with plain pump 91 here at altitude (7930 feet) due to O2 density reduction. At/near sea level, it needs full bore 94/95 octane pump and is borderline on ping. A 'dribble' of good boost additive KILLS the detonation issue. So, if you've got real noticeable/objectionable ping with all you've done, chances are your engine internals are pretty 'peaked'....Comment
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