octaine question

[jayjack]

ok my friend has a 2002 9-5 saab and it is sugested that he uses premium octaine. he being the person he is only puts regular octaine in it. i told him it probally wans't a good idea but he asked why and i wasn't really sure. i know how octaine is the measure of anti-knock so all i told him was that it would make him loose hp. i also realized he drives like a grandma abd RARELY puts the car above 3000 rpm,(which i find IMPOSSIBLE) so i thought he really wouldn't need all that octaine. what will happen if he continues like this, and what if he used regular gas and drove like a maniac(me)?

[curtis73]

Best case scenario is that its not doing any damage and he's enjoying cheaper fuel costs. Worst case scenario is that he's experiencing knock (which can be audible or not) and his bearings, pistons, pins, and crankshaft are fast on their way to a horrible and complete destruction. Seriously.

That car does require high octane. If he's not taking it over 3000 he MIGHT be ok, but I doubt it. I would use what they recommend.

[jayjack]

ok thx

[UncleBob]

most modern cars are very effective at sensing knocking and modifying the engine management to remove the knocking....but you will lose performance in the process, and probably mpg

If everything isn't working as it should, it might knock anyway and then things can go downhill quickly. But most setups are very smart and know when there's something wrong.

Most likely, there's no harm in running lower octane, but the recommendations from the manufacturer aren't just made up.

[jayjack]

alright one more question, if he took off the turbocharger, he probally wouldn't need the higher octaine, would he? and it would increase his mpg wouldn't it?

[curtis73]

That's REALLY not a good idea. He'll give up tons of performance for no real benefit.

If you do the math, running high octane will only account for $200 a year or so, but re-engineering one of the finest turbo engines on the planet is pretty... well, dumb. Saab spent years and millions of dollars engineering that engine, and randomly removing major components of the engine wouldn't be the wisest thing.

It will require rather major intake and exhaust plumbing, custom computer programming, sensor and wiring changes... a whole raft of crap just because he won't pony up a couple bills a year for good gas. He'll spend thousands removing the turbo, but high octane is a tiny price to pay for the package.

Tell him to buy a non-turbo and sell that fine piece of engineering to someone who'll appreciate it.

[Polygon]

Originally Posted by jayjack

alright one more question, if he took off the turbocharger, he probally wouldn't need the higher octaine, would he? and it would increase his mpg wouldn't it?

Yes, he could stop using high octane. However, he wouldn't really gain any fuel economy. It would probably get worse. Thanks to turbo lag you can manage to drive without using boost too much. As a result I can get about 25MPG in my Stealth and 30mpg in my GTC. If you took off the turbo you'd be left with a car with a low compression ratio and the engine would have to work harder and sue more fuel.

It would become less efficient.

Tell him to keep his foot out of it all the time or go buy a Civic.