# What octane do I need to be at?



## advag (Jan 17, 2012)

I have a 1970 Judge Ram Air III and wanted to know what fuel I should be running. In my 1970 LS6 Chevelle I run a blend of 55/45 or 50/50 of 110 Sunoco racing fuel with 91 unleaded premium. This mix brings me in at around 99-100 octane with the lead needed. Thanks for the help.


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## GTO JUDGE (May 14, 2005)

I run Sunoco 93 oct and add 1oz of Lucas Ethanol Treatment per every 5 gallons. The car screams.


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## geeteeohguy (Feb 2, 2008)

If your Judge is stock, around 96-100 octane will be fine.


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## advag (Jan 17, 2012)

It is stock. I will shoot for a blend of 110 and 91 to end up around the upper 90's.


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## skurfan (Sep 19, 2012)

I can't find any more than 91 here. And the octane booster is pretty much useless.


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## kilkm68 (May 5, 2011)

I was wondering the same thing. My motor is only 9.5 compression. Runs well on 92 octane. Would there be any performance gain at the track if I do a 
50/50 mix with 100 octane that's available at a few stations here?


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## geeteeohguy (Feb 2, 2008)

Klikm, no. Actually, a performance decrease. If it doesn't ping with 92, you are fine. Higher octane fuel burns slower, which helps with HC engines, but is a detriment to power on lower compression applications. Skurf, the one octane booster that actually works is TEL 130, sometimes called Octane Supreme 130. It's real Tetraethyl Lead, so it actually works, and works well. Not cheap, and not meant for street use, either.


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## skurfan (Sep 19, 2012)

Thanks geetee...! I am going to try it.


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## skurfan (Sep 19, 2012)

Just ordered a case from Wild Bill's Corvette for $120 plus $12 shipping. The only one I saw on eBay was a quart for $20.95 plus $16 shipping. Thanks again for thi tip. Now to get the mix right.


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## skurfan (Sep 19, 2012)

Just got my Octane booster case today. Not sure what my compression ratio is. Pretty much a stock rebuilt 400 1967. Any clues on how much to add? I figure if I have 10:1 and go 1oz. To the gallon I should be close to 95. Does that sound about right?


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## geeteeohguy (Feb 2, 2008)

You could try. My '65 seems to like about 2 oz per gallon. It's got 10.75+ compression, though. Your stock '67 is probably 10.25-10.5 to one.....If it pings with one ounce/gallon, add more. The true test is the hot weather/summer season. That's when you really need it. My 9.3:1 '67 GTO will run fine on 89 octane most of the year, but when June hits, even 91 octane won't quite cut it.


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## BearGFR (Aug 25, 2008)

kilkm68 said:


> I was wondering the same thing. My motor is only 9.5 compression. Runs well on 92 octane. Would there be any performance gain at the track if I do a
> 50/50 mix with 100 octane that's available at a few stations here?


Zero, none, zilch. That's one of the popular misconceptions about octane rating, that it somehow relates to how much power the fuel can make. Octane rating relates to a fuel's ability to "resist" detonation and that's all. A higher octane fuel doesn't contain any more "potential energy" than a lower octane fuel. In fact, because it does tend to burn a little slower, it might even make _less_ power than a lower octane fuel because the piston is going to be a little further down the cylinder by the time the fuel finishes burning (and therefore the burning fuel is heating a larger volume of gasses, and since Boyles Law says pressure is inversely proportional to volume, cylinder pressure is going to be lower).

Bear


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## geeteeohguy (Feb 2, 2008)

Bear is correct. Run only enough octane to prevent detonation. Too much= less power. Lower octane fuel has more heat energy than higher octane due to it's quicker burn rate.


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## 67GTO4SPD (Oct 4, 2012)

BearGFR said:


> Zero, none, zilch. That's one of the popular misconceptions about octane rating, that it somehow relates to how much power the fuel can make. Octane rating relates to a fuel's ability to "resist" detonation and that's all. A higher octane fuel doesn't contain any more "potential energy" than a lower octane fuel. In fact, because it does tend to burn a little slower, it might even make _less_ power than a lower octane fuel because the piston is going to be a little further down the cylinder by the time the fuel finishes burning (and therefore the burning fuel is heating a larger volume of gasses, and since Boyles Law says pressure is inversely proportional to volume, cylinder pressure is going to be lower).
> 
> Bear


I agree with what you are saying, but that's assuming his car running is optimum tune with 93 octane. I have had several engines in the 9.5 : 1 range that would show some improvement at the track with a blend of pump gas and race fuel (like 75% pump/25% race) because you could run more total timing. Its not as simple as just saying "it doesn't ping with 93, so it must be good". A lot of Pontiac engines have been lunched over the years due to inaudible detonation, especially if they been driven hard. I've had a few that I had to keep the timing down on the street on pump gas, but I could run a little more timing at the track with a blend. The older closed chamber heads are bad about it, even if you have the static compression ratio at what is considered to be a streetable ratio. 

My answer to that question would be get a log book and to take it to the track and experiment. I had engine combinations that run best at 32 degrees total timing and some that run their best as high as 38. 

If the engine is running its best combination with 93 octane, then I'd agree that higher octane would not help.


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## 67GTO4SPD (Oct 4, 2012)

I'll add that in my experience, a 9.5 : 1 Pontiac engine behaves more a like 10 : 1 Chevy small block with iron heads on pump gas in terms of being prone to detonation. I have ran many 10 : 1 SBC's and I consider it the max limit with iron heads, but 9.5 : 1 is pushing it with an iron headed Pontiac on pump gas.


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## geeteeohguy (Feb 2, 2008)

67GTO4SPD, very well said. I agree 100% and my experience has been the same as yours with Poncho V8's. And I'll add that sometimes it'll detonate and you can't hear it doing it. Happened to me in the '80's and I busted the lands out of 4 or 5 pistons in my 428. I dropped the compression ratio of my '67 GTO to about 9.3:1 five years ago. It turned out great. Was able to run 87-89 octane in it with no ping with the stock advance curve/total timing setting. Until last summer. On a 6-700 mile trip, in 105-110 temps, it started pinging going up long grades under load....even on 91 octane. It wasn't overheating, but _everything_ was hot, especially the intake air. On a Pontiac, you can "get away" with less octane in the winter than in the summer, IMO. Good post.


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## 67GTO4SPD (Oct 4, 2012)

One of my friends snapped the crank in the 455 in his '70 GTO to due to long term inaudible detonation. He was running around 10 : 1 and the engine wasn't pinging, so in his mind it wasn't detonationing. He told me several times that he had run over 10 : 1 on Chevy engines, and was convinced it wouldn't be different on a Pontiac. The rod bearings in that engine looked like you had taken a hammer and beat the piston down on the crank. It was a very mild engine, and he believed he "needed" the compression for it perform well, even though he rarely drove it hard. 
Luckily for him the block (and the #64 heads) survived. 


Out curiousity, how did you go about lowering the compression in '67 GTO? If you swapped pistons, which ones are yo running? Are you still running the 670 heads, or did you change heads to lower the compression?


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## BearGFR (Aug 25, 2008)

:agree Very much on the topic of inaudible detonation. Just because you can't hear it doesn't mean it ain't happening. (Reminds me of an old Bill Cosby or maybe George Carlin joke -- "It's the only male mosquito who buzzes, but only the female who bites. So if you wake up and hear buzzing, don't worry and go back to sleep. However, if you wake up and hear _nothing_...")

Until you know for sure your engine is safe, it's a good idea to pull the spark plugs periodically and examine them with a magnifying glass. if you can see little shiny metallic specs, you've got trouble. Those shiny specs are most likely flecks of aluminum that are flaking off the tops of your pistons due to detonation - even though you're not hearing it.

Bear


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## geeteeohguy (Feb 2, 2008)

67, I lowered the compression by installing a set of 1970 455 heads, #15's, because the #64's were much harder to find and much more expensive. I upgraded the heads to screw in studs. They cc'd at 87 before the clean-up and machine work. I didn't cc them after they were done, but it didn't look like they milled them...just a clean and straightness check. I left the pistons alone (forged Sealed Power flat tops) because I had rebuilt the engine 70,000 miles prior, and everything checked out. If I were to do a fresh build, or when this one gets tired, I would go with dished pistons and re-install the original 670 heads, or a set of #12 heads I have.


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## 67GTO4SPD (Oct 4, 2012)

BearGFR said:


> :agree Very much on the topic of inaudible detonation. Just because you can't hear it doesn't mean it ain't happening. (Reminds me of an old Bill Cosby or maybe George Carlin joke -- "It's the only male mosquito who buzzes, but only the female who bites. So if you wake up and hear buzzing, don't worry and go back to sleep. However, if you wake up and hear _nothing_...")
> 
> Until you know for sure your engine is safe, it's a good idea to pull the spark plugs periodically and examine them with a magnifying glass. if you can see little shiny metallic specs, you've got trouble. Those shiny specs are most likely flecks of aluminum that are flaking off the tops of your pistons due to detonation - even though you're not hearing it.
> 
> Bear


Exactly. The sharp edges left from the machining of the combustion chambers certainly doesn't help with detonation on a Pontiac. I've had success with smoothing them out and polishing the chamber to an 80 grit sandpaper finish to help with detonation. It is tedious and time consuming to get all the chambers to cc the same, but its worth it, IMO. 






geeteeohguy said:


> 67, I lowered the compression by installing a set of 1970 455 heads, #15's, because the #64's were much harder to find and much more expensive. I upgraded the heads to screw in studs. They cc'd at 87 before the clean-up and machine work. I didn't cc them after they were done, but it didn't look like they milled them...just a clean and straightness check. I left the pistons alone (forged Sealed Power flat tops) because I had rebuilt the engine 70,000 miles prior, and everything checked out. If I were to do a fresh build, or when this one gets tired, I would go with dished pistons and re-install the original 670 heads, or a set of #12 heads I have.


I am in same boat right now. I have a pair of ported #16's with the smoothed out combustion chambers and Ferrea valves, but the need freshening up and new springs, a set of stock, rebuilt ready to go #62's and I have the #670's that need springs, a valve job, and probably/possibly a few guides. I tend to want to keep the original 670's, but it is awfully tempying to just go with the ready to go #62's. OTOH, those ported #16's were awesome on my old '69 Firebird , and I've wanted to use them again for years. Either way, I'll have to run dished pistons with all of them. I have also thought about taking the 400 with the #670's out and putting it in the shop and just building the '76 455 that I have in the shop with idished pistons and the ported #16's. Whatever I do, I'm going to run the stock ( probably ported) intake, Q jet, and the H.O. manifolds and to keep it stock appearing. Decisions, decisions! -lol


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