Why sell do we still manufacture 87 octane gas?

First of all - there people outside of North America who will see 87/

91 octane and wonder why anyone would use 91 octane fuel. We typically use 91 octane (R+M)/2 ratings as unleaded premium.

Moving to higher compression engines isn't going to create some magic fuel economy gain. Honda uses regular unleaded for its hybrids - I would have thought they'd use premium if they thought it would help with fuel economy. Your examples aren't really any proof that premium unleaded results in better fuel economy.

I believe it's still created with different fractions. I doubt there have been any effective octane boosters (in small amounts) since lead was phased out. You'd also be surprised at how many cars can benefit from using premium - a Toyota Camry V6 is an example.

There's nothing particularly wrong with a lower compression engine that uses regular gas. It won't neccesarily perform that well, but most cars aren't driven all that hard.

So you are saying your 120 HP motor has 1/2 the displacment or the 240 HP one?

I'd always read that the lower octane has a higher heat value and therefore more "energy" per gallon.

Close. Its 2.3L versus 3.2L. So thats about 70%.

No it's not, it's stored in seperate tanks and is processed seperately at the refinery. I don't know who told you it's 'mixed at the pump', but they're dead wrong.

Cheers, - Gnarles (a former station manager for a a "Jet" gas station in W. St. Paul, MN)

There is no question that a higher compression engine converts fuel to work more efficiently than a lower compression motor. Thats just the nature of theromodynamics and combusting the fuel at a higher temperature. The higher the difference between the heat source and the heat sink, the greater the efficiency.

A seperate issue is the economics of whether or not you will get more miles per dollar of gas. This is controlled by the pricing that the largest gas companies decide on and is more an issue of man's law (or lack of) than natures law.

One of the secrets of Hondas success in getting great gas milage on 87 gas is being able to run a higher compression motor than its competitors using technology to control ignition timing and other techniques. Although they could have designed a higher efficiency engine that required 91 fuel and done even better, they designed the motor to be able to use cheap gas because they knew this is what economy minded people want to buy and know this is what oil companies want to sell (in greater quantities).

Nope, that was the old days. Now its all the same base gasoline before additives are used make different products. Thats great that many of the modern vehicles can get some benefit from advanced timing with the higher octane fuel, however, they would be even better if they had been designed from the start to use higher compression and got that benefit as well.

Then you put a smaller motor (but more efficient) in the car and call it an "economy car". It runs WOT on acceleration and sips "premium".

I'd say an engine that only makes 240 HP and REQUIRES 91 octane is the "crap motor." A garden-variety Chrysler 3.5L v6 produces over 250 horsepower on mid-grade (87) fuel, and it was putting out 214 horsepower on 87 back in 1993. Caddy Northstars put out over 300 HP on 87, and were also doing so back in 1993. Today, the Chrysler 5.7 Hemi puts out 350+ on 87 octane. So do lots of others, those examples just came to mind immediately. And there are many other common engines that put out around

200-220 horsepower on plain old 83 octane regular, nevermind 87. Any engine that REQUIRES 91 octane better be churning out 400+ HP.

Jay wrote:

The last 2 stations I worked at didn't have a 3rd tank for "Mid", and I'm still alive! How many years ago were you a a former manager?

I was under the impression that energy content was the primary factor in fuel economy - other than engine design. While I know higher compression engines yield higher output, I'd imagine that the fuel economy benefits are small, and as you say, outweighed by the increased prices for "premium" fuel. A hotter running engine will also have certain issues that will increase the price. Try explaining that to the buyer of an economy car.

As for efficiency, the equation is:

Theoretical Efficiency = 1-(ambient temp/engine temp).

I believe that for all intensive purposes, the ambient temp is between that of the engine block and outside air - I think.

Depends on the station. I once asked a driver with two tanks - which one was for mid-grade. He said there was a wall separating the mid- grade in one of his trailer tanks. I suppose others might mix on the fly.

There's a noticeable difference in the energy content per gallon between diesel and gasoline (diesel has a lot more energy per gallon). There's also a huge difference between gasoline and alcohol. But the difference between the energy density of premium and regular grades of gasoline (assuming that they both either have or do not have alcohol as a blend component) is NEGLIGIBLE. It may be measurable in a lab, but its not going to be measurable in terms of fuel economy, especially not when modern adaptive engines can milk more economy out of a higher octane fuel even if its a lower energy-density fuel.

Uh.... not quite. You need to look at the difference in temperature between the heat reservoir (the combustion chamber) and the heat sink, PLUS there are lots of other factors that add in. You're alluding to the theoretical Carnot efficiency that is the maxiumum possible for a heat engine that uses a cycle similar to the Diesel or Otto cycle. But that is just the theoretical maximum. Reality is always less...

"Jay" wrote in message news: snipped-for-privacy@posting.google.com... | y_p_w wrote in message news:

| >

| >No it's not, it's stored in seperate tanks and is processed seperately | >at the refinery. I don't know who told you it's 'mixed at the pump', but | >they're dead wrong. | >

| >Cheers, | > - Gnarles | >(a former station manager for a a "Jet" gas station in W. St. Paul, MN) | | The last 2 stations I worked at didn't have a 3rd tank for "Mid", and I'm | still alive! | How many years ago were you a a former manager?

Didn't Shell have a "dial-a-grade" pump several years ago. I think it had five or six grade choices. Surely they must have been mixing grades to do that.

||The 1st thing that happened in the early 70's with the oil embargo and new ||pollution standards was to reduce engine compression. The 360HP muscle cars ||went down to 200HP with the same engines.

Virtually all of that was a change in the way HP ratings were done. Advertised HP meant just that - what the manufacturer wanted the buyer to think, tempered by the possible negative effects on nsurance premiums should they get too far from reality. They were based on flywheel HP, plus a marketing fudge factor. About 1972 (?) they went to rear wheel HP, as measured on a chassis dyno.

All this is from a hazy recollection, so anyone with real facts feel free to correct me. Rex in Fort Worth

Lowering the compression ratio also decreases the NOx emissions.

High compression engines produce more NOx (at some point, you can even run the engine off of the NOx production, but emission controls for air diesels are an exercise for the diligent student).

It was Sunoco

Refinish King

You're forgetting about one REALLY important factor bud.................displacement. A 2.5 liter 4-cylinder that runs 91 octane and produces 240 HP is doing a hell of a lot better than a 5.7 liter,

8-cylinder hemi pumping 350 horsies on regular unleaded.

And where the hell can you buy 83 octane nowadays?

Doc

I've seen 87, 89 with ethanol, and 91 sold somewhere. Maybe this setup requires the third tank.

Actually the compression started dropping before the oil problems. The government started passing tighter emission control standars and the only way Detroit could meet them at the time was to drop compression. Fuel economy actually went down on the big engines like the Cadi 472/500. With new engine designs, fuel injection, and computer controls manufacturers have been able to run the compression back up again.

I often wonder what kind of power could be produced using todays technology to design another 500ci engine.

Steve B