View Full Version : Silly question, but does my car require 93 octane?

10-19-07, 12:45 PM
I'm wading through the myriad of owner's info, but haven't found the answer yet.

Its a 2005 4.6 V8 STS.


10-19-07, 01:59 PM
Page 5-5 in your owners manual discusses fuels. You can download a pdf copy of your owners manual at: https://www.mygmlink.com/pdf/go2content/manual/US/en/cadillac/2k05sts.pdf.

Much easier to find info in manuals using search feature of your pdf reader.

Benjamin Simon
10-19-07, 02:21 PM
No. I run 87 fine all day long if you want a quick answer.
I think I notice a power increase with 93, but it is not really enough to notice.

Benjamin Simon
10-19-07, 02:29 PM
No. I run 87 fine all day long if you want a quick answer.
I think I notice a power increase with 93, but it is not really enough to notice.

Curious George
10-19-07, 04:22 PM
No. My '06 Northstar runs just fine on regular. As I have oft written in these boards, the only place I have seen a repeatable and measurable performance difference between 87 and 91 octane is at the Infinion (nee Sears Point) Raceway Wednesday night drag races: 87 octane fuel yields a 14.7-second et (elapsed time) in the standing start 1/4 mile where 91 octane yields a 14.5-second et, i.e. 0.2 second quicker. 100 octane Sunoco unleaded racing fuel, available at the track for about $6/gallon, is even better. The following advice is offered at no extra charge. It is extracted from a valuable list of hints and advice, somewhere on this site, that I cannot find as I write this. Perhaps a moderator can provide the link.... Occasional Full-Throttle Acceleration Is Good For Your Engine (check this discussion for references): (back to the top) There are many advantages to occasional full throttle accelerations with a Northstar and any engine. [In some circles this is referred to as an "Italian Tune-up". -CG] It keeps the carbon cleaned out of the combustion chamber. This is maybe a little more important with the Northstar than some other engines due to the tight squish volumes between the piston and the cylinder head. It's designed this way to promote good in-cylinder mixture motion (good combustion) but it has the down side of providing a ready place for carbon build-up to touch the piston - causing noise. Ever heard of the Northstar "cold carbon rap" problem?? Simply put you'll hear a rythmic, piston slap-like noise when the engine is cold. Very prominent and very annoying. Cause: excessive carbon build up causing the the piston to contact the carbon on the head - causing it to rock in the bore and "slap" Much more evident when the engine is cold and the pistons haven't expanded to full diameter yet. Simplest and easiest "fix" for this: A few good WOT (wide open throttle) accelerations to clear the carbon out. That is all it takes to eliminate the problem and prevent it from re-occurring. Occasional WOT accelerations also help seat the rings to the ring lands and exercise the rings and keep them mobile and from becoming stuck in carbon in the ring lands. At high RPM and WOT the rings move around on the piston - they actually rotate on the piston and will polish away any carbon and seat themselves to the sides of the ring grooves. This is especially important on the 2000 and later Northstars which had hard anodized top ring lands on the pistons. Very hard and wear resistant - also harder to break-in and seat the rings to the sides of the ring-lands to promote the best possible seal. Many oil consumption complaints on the 2000 and later engines are related, to some extent, with the rings never seating to the sides of the ring-grooves due to lack of load as the engine was babied around forever. Even engines with rings stuck in the ring-grooves due to carbon build up can eventually be freed up with enough high RPM operation. WOTs warm up the engine thoroughly and clean out the exhaust due to temperature in the exhaust and high flow rates blasting particles, rust and such out of the system. Frequent WOT operation will not hurt the engine or the transmission. They're designed for that. The healthiest engines that I have seen at high miles are always the ones that are run the hardest. Rings are free on the pistons and sealing; no carbon buildup. The exercise that I think works best for many things is to select manual 2nd gear on an isolated stretch of expressway. This takes the transmission shifting out of the question if you are worried about hurting it. Start at 55 MPH or so and go to WOT in 2nd gear and hold it until the RPM reaches near the normal shift point - i.e. 6500 for an L37 and 6000 for an LD8. Hold the throttle wide open until the engine reaches, say, 6200 for an STS and then just let completely off the throttle. Leave the transmission in 2nd so that the engine brakes the car and creates some pretty heavy over-run conditions at high vacuum levels. Let it slow until it is about 55 or so and then go to WOT again and repeat. This exercise really loads the rings, allows variable RPM operation at WOT for several seconds continuously, creates heavy over-run which tends to unload the rings and make them move and thus exercise them in the ring grooves and it will blow-out carbon and the exhaust - all without creating a spectical of yourself and attracting the attention of cops. You can do it on most any freeway and stay within the 70-75 MPH range allowable. Once a week like this will keep the engine cleaned out and healthy and is DEFINITELY recommended for the Northstar in particular. The Northstar engine was designed/developed/validated to be run hard. It was expected that people would use the performance of the engine - which few people seem to do. The biggest single problem that many issues stem from is lack of use at full throttle by the owners. It just doesn't like to be babied around. The rings are low-tension by design for good high RPM operating characteristics and low friction/good power. They work best if "used" and kept free. In every conversation with owners I have had, once the owner started doing the WOTs and using the power of the engine they report no more carbon rap, better oil economy, no "smoke" when they do light it up (keep the exhaust cleaned out. If you notice a "cloud" at WOT then you are not doing enough WOTs...) etc... A bit of judicious use of the other end of the throttle travel is a GOOD thing...

10-20-07, 01:05 PM
i used 91 as instructed in the manual and dealer as well, but i had my throttle valve body clogged and had to take it to dealer.then i used 95 and it is just fine and no problems indicated.

10-20-07, 01:23 PM
The difference comes not from the gasoline base stocks but from the additives introduced by the various bulk suppliers. "Top Tier" gasoline contains additives sufficient to keep your fuel system clean over and above the level of additives required by federal regulation. Differences in octane rating don't mean a thing in that regard. Regular gasoline from a "Top Tier" producer may keep your engine cleaner than premium gasoline from Joe Scooterberry.

10-22-07, 08:59 AM
Thanks George - great information! When I was younger, my dad bought an 88 Thunderbird with the 5.0 V8. He would do a WOT like you've described, and he told me it was to "clean out the engine". I've always assumed he just liked to get on it pretty hard when mom wasn't in the car and that was his way of justifying it! Of course, when they handed the car down to me at 16, I did just as dad had said to do!

So in my previous car, I had always bought Wal-mart's Murphy Oil gasoline because I would get slightly higher fuel mileage on it. I assume this isn't a good fuel for the STS, being that it probably isn't top tier. So would a Chevron or BP fuel be a better choice?


10-22-07, 10:26 AM
Do what your manual tells you to do. You are not smarter than GM.