Unleaded Plus vs. Unleaded Premium - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Lil bastard · 04-29-2008, 09:30 AM

So far as Calorific, BTU or Joules are concerned, generally speaking, lower octane fuel contains more energy than higher octane fuel.

This is because the additives used to boost octane contain less calories/volume than normal gasoline - they make the fuel more difficult to combust so that it can be better compressed and burn more thoroughly releasing more useful energy. Since these additives displace a % of normal gasoline when mixed for octane purposes, this gallon of octane boosted gas will contain less energy, somewhere on the order of 5-10%. This is also a problem with Ethanol - it contains 30% less energy/volume than regular gasoline.

Now, the issue becomes potential energy.

Some engines, due to better computer control, higher compression, etc. can release more useful energy from octane boosted fuels than normal gasoline, or perform better - create more HP or run more smoothly.

That is not to say they perform badly on regular gasoline, in fact, due to the wide variation in fuel quality around the world, most cars marketed internationally are setup to run just fine on a wide variance in octane. The Boxster is one of these.

Since there is no tetra-ethyl lead in any of these fuels, none of them will hurt the engine or emissions gear.

If mine were a daily driver, I'd use 90 Octane and only go to Super-premium for track or weekend spirited runs.

But, you hardly need to eek out that extra 0.1 sec. 0-60 when commuting.

Now, that said, there is often some variation between what is stated on the pump and the actual octane rating of any particular batch of fuel, in fact, it's usually slightly lower than stated due primarily to the volatility of the octane additives, which evaporate from the fuel as it sits in the Tankers or underground storage tanks. But, if you fill-up at the busiest name brand station you can find, this isn't usually a problem.

Try several grades of fuel, and use the one which gives you the best value for your money. Because of driving style, conditions, source of fuel, etc., there is no one answer to this question - you need to experiment on your own. You won't hurt the car at all. You may run a little rougher, or see reduced range and if so, just switch back. The savings may be small, but in this day of ever-rising fuel costs and uncertain immediate economic future, it pays to save wherever you can.

PSL-Boxster · 04-29-2008, 09:36 AM

thanks guys, i never thought of the octane<>gasmileage feud. i was mainly wondering what long term affects could come other then the occasional check engine light

blue2000s · 04-29-2008, 10:32 AM

Quote:

Originally Posted by Lil bastard

So far as Calorific, BTU or Joules are concerned, generally speaking, lower octane fuel contains more energy than higher octane fuel.

The key here is "generally speaking" because there are various mixtures used to boost fuel octane and they don't all decrease the energy content of the fuel.

Lil bastard · 04-29-2008, 02:39 PM

Quote:

Originally Posted by blue2000s

The key here is "generally speaking" because there are various mixtures used to boost fuel octane and they don't all decrease the energy content of the fuel.

Exactly... There are fuels which contain more energy and higher octane, but these are usually special concoctions costing much more and not generally available at the pump.

Benny986 · 04-30-2008, 12:26 AM

the higher octane is primarily to run more timing. lower your timing down, and run all the 87 octane you want. the car can automatically retard timing, but only so much.

but why would you want to retard timing when you own a porsche, to save a couple of dollars/fill-up?

on the flip side, if you run higher octane, like 100, etc. then bump the timing, and get more power : )