Revisiting an old topic: Why use premium gas?

[BeauJangles]

I asked this question a couple of months ago in another thread. Today, I realized I wasn't satisfied with the explanation. Subbaculcha did a great job of explaining a good reason for using higher octane gas (higher compression). I understand how premium and regular gas are different.

So, if higher octane = higher compression, why does higher compression = higher performance? Why can't car companies build higher performance cars with lower compression ratios?

Below is my original post and the answer:

Originally posted by: Subbaculcha

Originally posted by: BlinderBomber
I'm going to ask a slightly tangential question, if that's alright.

Why do so many "performance" cars use premium gas? MS3, GTI, Civic SI, etc all use premium gas. As a counterexample my friend's Saab 9-3 2.0 liter is an incredibly fun fast car to drive and operates on regular gas.

It's probably a stupid question, but I've always wanted to know.

Not stupid, it's a great question.

Gasoline engines are designed to "burn" the air/fuel mixture, not explode. When people talk of "pinging" or "detonation", "knock", all terms meaning the fuel is exploding. This is undesireable as the engine can be damaged and fuel economy and power will suffer.

Higher performance cars will usually have a higher compression ratio (when the valves are closed and the piston is traveling upward -- the compression stroke), average cars are in the 8.5:1 or so range, performance cars 10.5:1 and higher. The higher the compression ratio the higher the risk for "knock".

Pump fuel has an octane rating, which measures the fuels resistance to detonation or knock. So the higher the number the more resistance to knock, hence you can run at a higher compression ratio, and get more power out of the engine.

Normal engines "suck" the air into the cylinder, a turbo or supercharger (forced induction), forces compressed air into the cylinder, -- detonation in this scenario can have serious consequences on the engine internals.

In a nutshell, higher octane (premium) allows a higher compression ratio without detonation.


Sidebar-- Then there's diesels. Diesel fuel, if it had an octane rating would be about 25. A diesel engine runs at very high compression ratio, 16:1 or so and higher. Diesels do not use a spark to ignite the fuel, the fuel ignites without aid under the extreme pressure. Diesels are noisy because they are designed to "knock", the fuel detonates.

Thanks guys!

 

[jagec]

Sure, you could built a low-compression engine that made as much power as a high-compression engine, but it would be bigger, heavier, and have more rotating mass to deal with--all of which are Bad Things for a performance car.

Higher compression ratio=higher performance out of the same displacement.

 

[BeauJangles]

Originally posted by: jagec
Sure, you could built a low-compression engine that made as much power as a high-compression engine, but it would be bigger, heavier, and have more rotating mass to deal with--all of which are Bad Things for a performance car.

Higher compression ratio=higher performance out of the same displacement.

That makes a great deal of sense.

I'm sorry for being so thick.

 

[DisgruntledVirus]

Originally posted by: BlinderBomber

Originally posted by: jagec
Sure, you could built a low-compression engine that made as much power as a high-compression engine, but it would be bigger, heavier, and have more rotating mass to deal with--all of which are Bad Things for a performance car.

Higher compression ratio=higher performance out of the same displacement.

That makes a great deal of sense.

I'm sorry for being so thick.[/q]

Everybody has their "Oh, now I see what you mean" moment, and you may have just needed it explained in a different way

 

[boomerang]

It's already been explained, but in different words;

Compression = Power
The more you compress the air, the more powerful the explosion (not the right word, but more easily understood.)

The reason you want higher octane fuel in a high compression motor is that higher octane fuel burns slower. Yes, slower. This greatly reduces the chance of pre-ignition (knock). Compressing air makes it hot. You want the air-fuel mixture to ignite from the spark provided by the spark plug, not because the air is hot enough to ignite it.

In the days of high compression motors and carburetors, the cylinders were filled with a mixture of air and fuel. High octane fuel was very important because the fuel was already in the cylinder and was being compressed along with the air. Nowadays, cars have fuel injectors. The fuel is sprayed in as the air is almost fully compressed. It's not as critical to have high octane fuel with a high compression motor and that's why you may see with some cars that high octane fuel is "recommended" not required. Also, computers in modern cars are able to control the spark timing and fuel delivery to further help prevent detonation.

So high octane fuel burns slower. What does this do when you run it in a low-compression motor? Well, there is a very good chance that it will actually reduce performance.

 

[alimoalem]

sorry BlinderBomber, but i have a question of my own

if you use lower octane fuel in a high performance car, would it ALWAYS be bad for the car or only at higher revs?

 

[Demo24]

well new cars now attempt to compensate for this when detected. Basically they have a little knock sensor which when it detects it retards the timing thus saving the engine. Some cars manage this better than others however. In general keeping it off load will usually be ok until the correct fuel can be added. Still it's not a good idea and in the long run you can do some damage to the engine.

 

[Dman877]

I've heard that just about any engine from the late 90's on can adequately compensate for lower octane fuels. The catch is that because some engines are designed around using 91+ octane, they are less efficient at lower octane levels. This decreases mileage, thus negating the monetary benefits of using lower octane gas.

I have a 2004 TSX and 91+ octane is recommended. However, the manual states that lower octane gasoline will work but it will reduce the engine's performance.

 

[marincounty]

Originally posted by: Dman877
I've heard that just about any engine from the late 90's on can adequately compensate for lower octane fuels. The catch is that because some engines are designed around using 91+ octane, they are less efficient at lower octane levels. This decreases mileage, thus negating the monetary benefits of using lower octane gas.

I have a 2004 TSX and 91+ octane is recommended. However, the manual states that lower octane gasoline will work but it will reduce the engine's performance.

Exactly. If you want all the power you paid for, use the proper octane. Extra octane
(anti-knock) does nothing for you, but won't hurt anything.
Too low an octane, you risk engine damage or just lower performance.

 

[Zenmervolt]

Originally posted by: boomerang
In the days of high compression motors and carburetors, the cylinders were filled with a mixture of air and fuel.

With the exception of a few very rare Gasoline Direct Injection engines, the cylinders of EFI cars are still filled with a mixture of fuel and air. The injectors sit in the intake tract just before the intake valve and inject fuel just an instant before the valve opens. The cylinder still sucks in a mix of fuel and air. Just exactly like it did with a carburetor.

The reason that modern cars have higher compression before higher octane is required is because computer aided design and combustion chamber modeling techniques have improved greatly, preventing the creation of hot spots within the cylinders and thereby reducing the likelihood of detonation/pre-ignition.

ZV

 

[Pacfanweb]

Higher compression=more power, because the fuel/air mixture is squeezed tighter, so you have basically a more powerful explosion, which means more force driving the piston down. More force pushing the piston IS more power.

There are ways to get more power without raising compression....better-flowing heads, intakes, headers, fuel injection, timing advance, etc.

But if you raise the compression, you'll get more power, but you'll need the higher octane to keep it from detonating/spark knocking/pinging.

Increasing the compression brings other factors into play that you can change: You can get better-flowing heads, larger cams, even bigger intakes/carbs/headers, etc, and so forth.

Generally, you don't just change one thing...it's always a combination of things.

You can run higher compression with aluminum heads than with cast iron heads that are otherwise identical....in fact, you have to in order to make the same power. The iron heads hold the heat in better, and all other things being the same, will make more power than aluminum heads. So you run a higher compression ratio with aluminum heads to make the same power, because they lose more heat and therefore more potential power. Yet they're lighter, and more easily modified, so are more desirable in race applications.

And this can go on and on and on....the possibilities are endless.

 

[alpineranger]

^ compression = ^ theoretical efficiency

one of the reasons why diesels get better fuel economy than gas engines

 

[jaedaliu]

Originally posted by: DEMO24
well new cars now attempt to compensate for this when detected. Basically they have a little knock sensor which when it detects it retards the timing thus saving the engine. Some cars manage this better than others however. In general keeping it off load will usually be ok until the correct fuel can be added. Still it's not a good idea and in the long run you can do some damage to the engine.

hopefully, your car will run fine even if it's on sub-optimal octane. The knock sensor will put your engine in a safe mode if it detects misfire. If the knock sensor goes bad (not uncommon) then the engine will be running in the safe mode at all times, no matter what type of gas you put inside the engine. (at least that's the way it works on my Nissan)

 

[alkemyst]

Originally posted by: marincounty

Originally posted by: Dman877
I've heard that just about any engine from the late 90's on can adequately compensate for lower octane fuels. The catch is that because some engines are designed around using 91+ octane, they are less efficient at lower octane levels. This decreases mileage, thus negating the monetary benefits of using lower octane gas.

I have a 2004 TSX and 91+ octane is recommended. However, the manual states that lower octane gasoline will work but it will reduce the engine's performance.

Exactly. If you want all the power you paid for, use the proper octane. Extra octane
(anti-knock) does nothing for you, but won't hurt anything.
Too low an octane, you risk engine damage or just lower performance.

Not necessarily true, too high an octane doesn't burn completely and can lead to fouling sensors and cats.