will high octane fuel help carbureted engines?

Question

I have a 1986 Civic hachback with a 1.3liter carbureted engine that hesitates at hwy speeds and while running through the gears (4spd). I decided to run 93 octane instead of the usual 87 and performance appeared to improve. So now i'm kind of curious if there's any proof of this.

Answer 1

This is possible. Premium fuel (91 octane or higher) sometimes has more additives designed to keep fuel injectors clean. These additives may be cleaning and/or demoisturizing your carburetor. Test this by going back to regular after a couple of tanks of premium. If the problem has disappeared, you have found the answer.

If the hesitation - defined as the engine stumbling then coming back to life - is caused by fuel system problems, I would like to know if: the fuel filter has been changed and if the hesitation is caused by too much or too little fuel. The answer to this should tell you where to look. Carburetors, even Honda carburetors, always have been far more unreliable than fuel injection.

Answer 2

Octane has nothing to do with performance in an otherwise unmodified car -- with one partial exception, which I'll get to in a bit. First, let's talk about what octane does.

Octane keeps your car's gas from igniting until the spark plug sets it off. That's all it does -- there is no more energy, performance, or power in 93 octane than in 87 octane gas. And it has nothing to do with carburetors or fuel injection; octane's only effect is inside the cyinder, when the piston is compressing the air and fuel before the spark plug fires.

Under some circumstances, low-octane gas can ignite before the plug fires. This is called pre-ignition, and can be very bad in some vehicles (particularly those with turbochargers or superchargers). Even if it doesn't cause damage, pre-ignition does not produce as much power as combustion at the correct time, so you are wasting fuel and your car will run rough.

High-octane gas is associated with performance because high-performance engines often introduce additional heat or pressure to the air-fuel mix, therefore they require higher octane fuel. But it's not the fuel that has the power, it's the engine -- it just requires high octane fuel to prevent pre-ignition. This, again, is a factor of the engine's internal design (and often the presence of a supercharger or turbocharger) -- not, strictly speaking, whether it's got a carburetor or fuel injection.

So what did you notice -- a real improvement or just an imaginary improvement in performance? Your car MIGHT be running more smoothly with high-octane gas, and you MIGHT be getting more power with 93 than with 87 -- but not because there's more "power" in the gas. It's more accurate to say that the low-octane gas, in your car's engine, is producing LESS power than your engine was designed to make. Here's why.

Older cars, such as your '86 Honda, often develop carbon deposits inside the engine, like the burned-on bits at the bottom of a skillet. These can retain heat in the cylinder, like embers on a log in the fireplace, or increase the pressure inside the cylinders and raise the temperature. If there are enough of these deposits, the increased temperature in the engine can cause pre-ignition -- that is, make the air-fuel mixture ignite before the spark plug goes off. You may either hear this as "knocking" or "pinging" (a light rattle from the engine compartment), or you may feel it as hesitation or a "miss" during acceleration.

So most likely, your car has these carbon deposits which are causing pre-ignition under the conditions you describe. Using the 93-octane gas resists the pre-ignition, so your car runs more smoothly than it did with the 87. It's not that there's more "performance" in the 93, it's that it is correcting a fault in your engine.

You may be able to get some improvement by using the newer generation of detergent gasolines or gasoline additives; Chevron's Techron brand of fuel system cleaner is a genuinely useful product, though many fuel additives do nothing but transfer money from your pocket to the manufacturer. And while Chevron includes Techron in their pump gas, you can buy it in concentrated form at auto parts stores. You might try running a bottle of Techron through each of your next two tanks of gas and see if that makes a difference; it did on an old car I used to own.

And finally, your ignition timing may be off. If your timing is too far advanced, the spark plug fires too early in the piston's cycle which can lead to the same performance problems as pre-ignition -- unless you compensate for it by using higher octane fuel to keep the gas from igniting till the plug fires. If you haven't checked your car's timing lately (or had it checked), that could be a useful next step as well.

Answer 3

Carbureted Engines

Answer 4

yes it will help. the higher the octane the cleaner it burns. more of it is also burning then passing through and out the pipes. it will also keep your engine cleaner and you will not get as much carbon deposits as you would with lower octanes. another good idea to do so it runs better is to change your plugs, wires, battery terminals, and distributor cap. then it will have a much better connection. you can also replace your fuel filter.