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MG MGB Technical - 110 Octane

My friend just found a gas station that sells 110 octane racing fuel. I was thinking it would be fun to go fill up my '71 GT with a tank. It is a stock engine, 80,000 miles. Has anyone done this and had bad results, ie blown engine? If not, it should be quite a trip. Thanks.
Brooks Twist

I didn't think just putting in higher octane fuel in an older car did anything? My understanding was if your car is set up to run properly (i.e. no detonation) then putting in higher octane fuel by itself won't do anything. Wouldn't you need to adjust the timing (advancing it?) to take advantage of the better fuel?

Modern cars have automatic knock sensors so they can adjust the timing so it is always running near the limit depending on what octane fuel is in it.

I could have this all wrong though so hopefully someone who knows will explain :)

Simon Jansen

Don't bother. Higher octane does not mean more power. In fact it takes more energy to burn higher octane fuels than low octane. High octane is only necessary for engines with high compression, forced induction, or lots of carbon deposits wich cause knocking/pinging. The lower octane you can run without knock/ping the better. But if you want to waste your money go right ahead, you won't hurt your car or notice a difference.


About 15 years ago, just before a rebuild, I too put in very high test--114 leaded. Now maybe it was the power of suggestion but she seemed to run like a bat out of hell, especially low end torque! I know, others will confuse the issue with the facts, and I don't dispute them, but it was a blast. Perhaps the highest octane was making up for an otherwise tired engine..don't know.

Paul Hanley


Another vote for save your money and don't buy the 110. Unless you have a high performance engine that has been tuned to need that kind of octane, your engine won't show any appreciable increase in power.

The kind of cars that will benefit from this are high tuned modern sports cars with electronic fuel injection and are computer controlled. These cars have a knock sensor and basically will automatically advance the timing until knock is detected and then back off a bit. With higher octane fuel, the knocking doesn't occur and the engine controls can go to max power mode.

If you want to make permanent major high power modifications to your engine, including advancing the distributor, then you would want the 110 octane. To just run it through one time won't get the benefit that you think it will.

BTW. how did that MGB turn out that you were working on? I may have missed your final sets of photos and if you have them up on a site somewhere, I would appreciate a link.
Ron Kluwe

If you don't make any other changes the fuel will still be burning as it exits the exhaust valves. I well remember the tale an RAF friend told me of his mate who filled his Royal Enfield with aviation fuel for a w/e leave trip in the days of pool petrol. The thing flew but burnt out the exhaust valve. Rich.

Yep--Back in the good old days when military airplanes had pistons thrashing around in big round engines, a lot of guys were tempted to help themselves to a bit of 115/145 and soon found themselves with a ruined engine. Burnt valves were the main result--and they never did see any performance increase. In fact, running too much octane will actually decrease your performance, although, as Paul noted, you don't get any preignition, so you can really load it in the low rpm range. Don't waste your money on racing fuel unless you actually have a need for it.
R. L Carleen

An engine runs best when the fuel ignites at the proper time in the sequence. Higher octane takes longer to burn, so if hte car isn't tuned for it I would imagine a power loss. Properly built and tuned for it would probably gain horsepower but I doubt a stock engine tuned for 110 octane would yield worthwhile results. Certainly not $8USD per gallon worthwhile!
Steve Simmons

The people telling you there will be no power difference, and that the gas actually burns slower and puts out less power are correct.

You want to run the lowest octane you can without preignition.

I run 108/110 in my Twincam, but then it has 12:1.
Bill Spohn

I had a 64 Etype coupe once and it would run well on a mix of 2 gallons of 100 octane and the rest 93 octane. Wouldn't ping or diesel, but when I put straight 93 it would always ping and diesel.

I would reverse your formula: You want to advance your spark as far as you can without heavy pinging. With better -higher octane- gas you can advance your spark further and get higher engine output. The aircraft engines in world war 2 were all supercharged and produced effective combustion pressures under high power equivalent or in excess of 12:1 in a naturally aspirated engine.

The exception to the rule would be a low compression engine that won't ping even with poor gas. In those circumstances, advancing the spark or better gas won't do anything more. Buying better fuel for that engine is a waste of money. If you can move the distributor back and forth and it all seems to run ok and you aren't getting pinging, then you are running a low compression engine, or too hi an octane fuel, or both.

I find a pragmatic way to set the advance is to fill the tank with low octane fuel. Set the spark to produce a little "pinking" (UK) or pinging (US). Note the setting and then advance a little more for the premium fuel.

The early distributors have an Octane adjustment screw. You get a weak tank of fuel turn the knob back a few. You get some good gas then turn the knob the other way and advance the spark to hear the ping.
With a vacuum advance distributor the spark advances several degrees at light throttle. When you are cruising under light load, you should hear a little ping immediately when you hit the throttle. The vacuum advance takes a second or two to retard the timing as you punch the throttle and you get that transitional ping.

The old cars (early 20th century) had a lever at the steering wheel for manual adjustment of timing in real time. Modern cars have knock sensors that retard or advance the spark thousands of times per minute, depending on fuel, load, rpm, humidity, ambient temperature, etc. They don't eliminate ping, they just keep it on the edge of no apparent pinging and max horsepower and economy. A very light ping means your timing, fuel mixture etc. are just right.

I picked up a "universal" knock sensor kit at a flea market about a decade ago. I keep planning to try it on the MGB. I figure I'll advance the spark way out there and let the knock sensor retard the spark enough to inhibit the knock. It'll be the equivalent of computer spark timing.

Barry Parkinson

Its not just a matter of tuning a motor for 110oct. its a matter of having a motor that needs it and a stock B does not. The idea is always to have the engine develop max combustion pressure 12* after TDC to develop full power. If you keep advancing the spark to take advantage of its slow burn you would be producing reverse torque and there fore loosing power. A lot of people talk of spark advance as if it is gods gift for power but an engine will always develop more power if the engine can be built to burn the fuel faster without detonation. The normal ways to do this are turbulance from a good squish area, oxygen content and density of the mixture. I/m afraid I get a bad feeling when I here people using ping to set advance, although it is a good indication of over advance. MANY engines on a dyno will develop maximum power several degrees before ping so any advance in excess of this is not doing the engine any good. Denis

Barry, my 53 chevy has the octane adjustment knob on the side of the distrib as you described. Its an interesting toy to play with. Mine is now disabled and a pertronix point replacement is installed. That old 235 6cyl purrs.

This thread was discussed between 28/02/2005 and 02/03/2005

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