[1968_CONVERTIBLE_GOAT]

Okay i heard the stories about reg. octane such as 87 making your car "ping" and make werid noises...but has this actually happen to anyone who uses reg. fuel?.. i've been using 91 since i've had my car n some other goat owner said to use a lower octane because back in the 60's..they used lower octane...any comments???..or experience...?

[1968_CONVERTIBLE_GOAT]

Okay i heard the stories about reg. octane such as 87 making your car "ping" and make werid noises...but has this actually happen to anyone who uses reg. fuel?.. i've been using 91 since i've had my car n some other goat owner said to use a lower octane because back in the 60's..they used lower octane...any comments???..or experience...?

[Young One79TA]

The reason they probably used lower octane in the 60's was because it was leaded gas which helped boost the octane and prevent detonation. Todays gas is unleaded.

[goatnut4909]

from what i understand the fuel in the 60's octane was way higher w they also ran leaded gas corret me if i'm wrong but royal bobcat gto's had arond 11-1 so the octane was proble 95 or higher the higher the octane the slower the burn so you can run more advance hench more power I dont run anything but 91 or higher cheap insurance in my opion

[judge_jury_executioner_69]

i have a 428 with 6x-4 heads ( 8-8.5:1 cr ) and its pickey about gas. i can run 87 octane but it hesitates and dont want to go when i romp on it. 89 is a little better but not as good as 91

[78 GHOST]

Octane is a term describing the particular fuel's resistance to detonation.

If your car isn't running well on 87, it's because you're suffering inaudible detonation, and the car is struggling to accellerate despite this tremendous problem. Upping the octane removes this detonation, permitting the engine to achieve it's healthy level of power given the fuels available.

Octane is only helpful to the point where you're no longer experiencing detonation at the optimum timing setting. If you're NOT experiencing any detonation, your octane is sufficient and you will see ZERO benefit switching to a higher octane. In other words, higher octane fuels DO NOT contain more BTU's of energy than lower octane fuels, as is commonly perceived by the public.

However, if you build up a high C.R. engine that develops a lot of dynamic compression, you may have to set the ignition timing way retarded to prevent detonation.. thus robbing you of huge power. In addition, retarded timing makes more heat than properly set advanced timing, resulting in overheating problems.. etc.

If you build an engine with iron heads and 9.5:1 C.R., todays pump premium should be adequate to permit you to run the ideal timing without detonation. Aluminum heads require about 10.5:1 for peak pump gas power, due to aluminum shedding heat better than iron.

Ideally, you would design your setup predicated upon the fuels available, and work backward from there to setup your C.R., cam, etc.

Lead was a popular additive since the late 30's to raise the octane of a given fuel. This push was driven by the need to develop more speed from the early "pursuit" aircraft in the Army Air Corps.

Today, we use different more environmentally friendly additives to achieve the same thing.

[Trev]

Correct me if I am wrong Matt, but I thought that lead was used primarily as a valve seat lubricant, and had nothing to do with octane levels. That is why engines designed to run on unleaded fuel require hardened valve seats to make them live without the lubricating effect.

[78 GHOST]

Octane Linky

The valve seat lubricant thing is a side benefit to the primary goal, raise the octane. Even still, 10 different engine builders will give you 10 different answers as to whether or not that ever made a difference.

[george kujanski]

Nope, Trev, I think Matt is correct. The Tetraethel lead additive was used to raise octane ratings, especially since the in the late 50's and early sixties compression ratios were going up; raising the CR raises the torque output, everything else being equal. raising the CR, tho, raises the tendency to ping, the TEL additive raises the octane rating to reduce that.

The valve lube, I believe was a side benefit; once the TEL went away, the mfrs started hardening the exhaust seats to compensate.

Raisng the CR is a great way to get more torque output, unfortunately, we're limited by the gas we can buy.

Diesels have more output torque everything else being equal, because they run CR, of, say, 18 or 20 to 1. Diesel also has a higher energy content, a win-win situation.

George

[68GTOConv]

Someone else said it correctly - if your car runs fine (Ping free all the way across the RPM range) on the gas you are running, then you don't need more octane.

By running better gas, you could dial in a more aggressive timing scheme which will result in more power. When using puny gas, you could take a little timing out. It only takes about 5 minutes to change the springs on the mechanical advance of most distributors. For those of you running vacuum advance, it's even easier.

Oops, forgot to add >> Using MSD with a Timing device plugged into the pill slot, you can change it on the fly inside the car. I do this sometimes if I want to run 104 for "street nights", and then I want to run 91 for nice leisurely trips to work.

Mike M

[carbking]

There is no postive benefit in using a higher octane fuel than is needed to prevent detonation; and, in fact, there may be a negative effect, as many refiners use ethanol (less energy per gallon) to boost octane.

If you have ping or detonation, use the higher octane; else, use the lower octane and spend your money on other items.

Jon.

[68GTOConv]

Carb, I agree with what you are saying, but there is a level of performance above "normal" that can be had with higher octane fuel. I'll give you an example using my Formula.

With 91 octane (which it runs fine), I have to limit my timing to 17 degrees WOT. If I go more aggressive, I get knock. The car makes 462 horsepower at the wheels in that config.

With 104 octane (which I use for racing), I can use the PC to set my timing to a much more aggressive 24 degrees WOT. The same car will now make 492 horsepower at the wheels.

So the "wherever it doesn't ping" analogy is accurate. It is fine for application like my stock Monte SS 3.8

But to think you can't make more power with greater octane would be missing a BIG part of the performance picture.

You are also right in that running more octane than what is needed for a particular set-up actually hurts performance. Slower burn rates come into play. The trick is matching your combo to the right gas.

Mike M

[Bob68]

I had an old gas station sign that said
Regular 95+ octane
Premium 100+ ocane

Yes octane used to be higher.

Just look in your old owners manuals and it will recomend nothing less than 100 octane, sometimes 105 for the high performance models

[carbking]

68GTOConv - my comments were assuming everything else held constant. If you tune to take advantage of the extra octane, additional power is certainly possible. This being the street forum, I assumed (possibly incorrectly) that most readers would not be doing pc tuning on their engines.

Jon.

[george kujanski]

Bob: I used to have a OHC SPrint, 10.5:1 CR, it needed 100 octane. Would ping like mad using the regular of the day, 94! octane.

Bought it at $.399 a gallon, too.

George

[GOAT8U2]

Back in the day there were tons of HIGHER octane fuel, all leaded. My Dad still tells me stories of using 104 oct leaded with his 283.

Lead will add both octane points and valve train lubrication. I use a 32oz. bottle of lead to raise a 10 gallon tank of 93 to 102-103 octane. If interested I can let you know what it is and where to get it.

Lower octane fuel has more BTU's and will make more power up until your compression ratio is beyond the octane ratings detonation control.

There is no such fuel as a 115 unleaded, to date there is a 108 oct available in unleaded through VP Racing Fuels and everything else above that has lead in it.

[68GTOConv]

Jon, understood. You probably get blank stares like I do when I tell people that a car running perfectly fine on 91/93 octane will actually make less power on 100+, unless they are tuning for it.

It runs contrary to that old popular bench racing phrase "I went and put some high test in her, and it roasted the tires all the way to 100 miles per hour ..."

Mike M

[GOAT8U2]

On the chart it shows Turbo Blue Unleaded as 100 octane. Sounds like a nice marketing ploy as Turbo Blue (Extreme) is 116 octane, leaded.

What are you paying for it?

By the way George, I live about 10 minutes from you. What car do you have, maybe we have seen each other on the road.

[george kujanski]

GOAT8U2: You probably haven't seen it unless you looked in my garage, LOL!

It's currently got the windshield out for cowl repair and dash painting so it hasn't been on the street for awhile.

Email me and we can xchange addresses & maybe get together sometime.

Live long and prosper.