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Varies greatly from car to car Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 22 Feb 2005 07:18:02
In Reply to: what kind of gas???, kcox, Mon, 21 Feb 2005 17:51:37 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Octane is just the rating of how resistant the fuel is to knock (pre-ignition). Knock is caused by a lot of things - compression, boost levels, timing, hot spots in the engine, spark plug types, driving style, phase of the moon.
In general, higher boost cars require higher octane - an Aero runs at higher boost, which can require higher octane. But that doesn't mean that a LPT 185hp car doesn't need higher octane.
Turbo engines are low compression, but that's to compensate for boost. If you cram a lot of compressed air into a cylinder (boost), that effectively increases the compression ratio. The 'base' compression ratio - cylinder geometry - is reduced so that you don't blow the head off during boost.
But it all comes down to need. If the engine starts to knock, the Trionic will first dial back the ignition timing in that cylinder. If that doesn't do it, it messes with fuel, and finally, cuts back on boost. This all happens much faster than you can tell.
As engines age, they change. Some get higher compression - carbon buildup, variations in timing, etc. Some get lower - leaky valves. Some develop hot spots in the cylinders, so are more prone to knock.
So, what octane should you use? The highest octane you need to reach full performance, and NO HIGHER. Higher octane than you need is just wasting money. It provides not advantage - it doesn't 'burn hotter', or provide more power - in fact, higher octane fuel contains slighly less energy than lower octane fuel, but the difference is miniscule.
If you're getting full performance on 87, then run 87. If it runs better - more boost, better throttle respone on 91, then run 91, and so on. If there is no improvement between 91 and 93, run 91; 93 is wasting money.
I can tell you to run 93, and be pretty sure your car will be hitting full performance there. It's easy - it's not my money. And anyone that can tell you precisely what octane your car needs without having examined the engine pretty carefully should be out picking lottery numbers.
If you're happy with the performance at 87, then that's pretty good. If you want, try a full tank of 89, and see if you get better performance. If so, try 91, until you don't see an improvement. There is also a lot of variation with season - fuel mixtures change in winter vs. summer; and in summer, when the air is warmer, engines are more prone to knock. So you might see a difference between the seasons, if your engine is 'on the edge' between needing a higher grade.
I don't have a 9-5, but I'm running a 9000 CSE. I get full performance on 87 octane, and am very happy about that. Absolutely no reason to run at higher octane
posted by 192.249....
, Tue, 22 Feb 2005 07:18:02 <-- Viewing This Message
, Mon, 21 Feb 2005 18:07:54
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