A couple of years ago, Mom bought a ’95 STS (w/ N*). The first time I drove it I told her I wanted it when she was ready to get rid of it. WELL, yesterday, Dad bought Mom a 2004 CTS w/ ~20k miles. They called me up and told me they were getting the CTS and asked if I still wanted their ’95 STS. I said, ‘HECK, YEAH!!!.’ They just about gave it to me—said I deserved it… (blush, blush). So, Mom & Pop have an ’04 CTS, I now have a ’95 STS and my son has an 88 Coupe. They are all great cars.

I did a search and found this site and it looks like a really good one so I decided to join.

My ’95 STS has 153k miles. Mom has been running regular unleaded gas in it after a friend told her they ran reg in their N* with no problems—just don’t swap back and forth from reg to premium so the computer will adjust to the lower octane. This guy is knowledgeable and has built engines, so Mom took his word and started running reg. I agree that the computer will adjust to the reg, but am a little concerned about the long term effects of it. I think the premium will run cleaner and be less wear and tear on the engine and valve train—especially at high rpm’s. Please post your opinions and experiences regarding the use of regular unleaded and premium unleaded in a Northstar. I think want to swap and start using premium especially considering the high mileage on this engine. Thanks in advance.

 

octane, usually wont make any diffence except on your pocket book unless you race it. octane has to do with ping/detonation and that is about it. as far as i know, the computer adjusts the timing to fit he fuel beign used.

if the car has none of that, it is fine.
if it has, then you got other problems beside the octane.

you may wat to read this:

https://www.cadillacforum.com/m_7430/tm.htm





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i am sort of like u i believe that the premium gas has a little more kick to it and your car will get better performace.gas now days starts turn bad in few weeks it seems like to me i know my 4.9was gummed up so i bought some seafoam carb and fuel inj. cleaner and added to a tank of gas preked things up too. good fresh gas is the key............ buckshot601

 

I got a friend that swears his car goes better after he washes the car. perception is a nice feeling.

regarding the bolded phrase, that is news to me .. unless .. you are using grain fuel or a mix with grain fuel .. ethanol ...
then, the answer is a big yes .. they gum up fuel tanks and injectors in less than a few weeks. IMO, one shouldnt use that stuff unless you are going to use it up in about two weeks or less. a dirty little secret that no one wants to admit.









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I use premium and that is what the owners manual suggests. This is a fairly high compression engine at 10.3:1 and does require more octane to run properly and avoid detonation. If you plan on using all of the ponies and torque this engine was designed to deliver I think regular 87 octane will knock and potential cause damage. With all due respepct an older woman driving with 87 octane would never be a problem. I have on occassion had to run 89 octane because of premium gas shortages on trips to Northern Ontario and I noticed no ill effects.

 

Does it have to use high test?That is question that has been around since the late fifties when the first high compression engines appeared. It's mainly because high test costs more than regular.In the old days just jack the timing back and away you go on regular,a little loss in power gas was so cheap no one cared about economy.During Cadillac's low to no power days of the 80s,Cadillac responded with the 4.5pfi which came in thier most expensive cars eldos,sevilles,and the fwd fleetwoods.Their belief was if you can afford the car you can afford high test gas.From 1991 til 1999 all the fwd engines and the 94-96 LT1 fleetwoods required high test.The LT1 and early northstars had knock sensors to make up for substandard gas.On the 4.5 & 4.9 pfi just jack the timing back just like the old days.During the mid 90's Cadillac realized most cadillac owners weren't going to buy high test, but would hit the dealer with drivability concerns because of sub-standard fuel.For 2000 Cadillac redesigned the northstar to run on regular.With all that said engines designed for regular see no real benefit from high test other than it usually has more detergent to keep fuel injectors clean, but it can cause cold start concerns due to high flash point.Engines designed for high test achieve the best overall performance and economy when running on it. Along time ago I had cars that required high test high and someone told me it would run it would just fine on regular and I was wasting money on high test. I tried it not knowing any better and she responded by running terrible,after that I never second guessed the guy who built engine.I know high test is way too high but it is not that you weren't informed the first you looked at the dash and saw: PREMIUM FUEL REQUIRED

 

Octane is one of the most misunderstood aspects of a consumer product that has ever existed (today's computer characteristics rival it though). Octane is the measure of a fuel's resistance to autoignition due to temperature and pressure. High octane fuel is sold as "high performance". It has no more energy per lb/gal than regular. A spark ignition engine needsa minimum level of octane to operate as designed-that isthat thespark initiates the power stroke, not autoignition. Diesels use autoignitionto light the fire that produces heat for the power stroke. They measure their fuel's primary characteristic as cetane, the ease of auto-ignitiondue to temperature and pressure.

High octane is required for highcompression engines to enable the piston to compress the fuel air mixture to higher pressures/temperatures than in a low compression engine. By compressing the fuel/air mixture to higher pressures and temperatures, the engine can operate at a higherthermal efficiency-converting more heat energy to mechanical energy. This is why diesels get better fuel mileage-they are more thermally efficient due to the very high compression ratios they use. The use of high octane fuel in a low compression engine will do nothing for power since it cannot utilize the higher autoignitioncapability ofhigh octane fuel. All you are doing is emptying yourwallet faster.

That said, Cadillacs that say use"Premium Fuel Only" are high compression engines that need "High Octane" fuel to operate asdesigned. You can run Low Octane fuel inthe computer controlled engines and they will compensate by retarding the ignition timing, but this takes the engine off it's "designpoint". If you drive with an egg under your accelerator foot, the engine can live with this. If you want to use it like a normal car, you need to listen fordetonation. Detonationwill destroy the pistons fairly quickly. Most peoplecan't distinguishbetween mild
detonation(pinging)and severe detonation (pinging also). They also pass it off a occasional-it all adds up. It's the old pay me noworpay me lateridea.

chuck
89 Deville Concours
"Premium Fuel Only"

 

I agree with most of MECHANICAL MAN's post.DETONATION is the uncontrolled burn of the fuel mixture. The voilent explosion jars the internal of the engines. The pinging that most people hear in pre-ignition its a mild form of detonation most of the time harmless, but can damage outer portion of piston(s). There is a deeper toned detonation, that one can cause major engine failures. Not all Cads have spark retard systems, so do what you want. Just don't blame the car if you let it down.

 

IM IMPRESSED!!!!!!........THISTHREAD SHOULD BE SOME WHERE ELSE.....FAQ, MAYBE........ GOOD STUFF!! GOOD OPINIONS..... THANKS TO ALL
rickyd