Hi All,

Quick background, I inherited my father's 1975 Delta 88 Royale when he passed. It has sat in storage for a few years and I've gotten it back into running shape. However my google searches are coming up with different answers and I'm hoping you all can help. I remember my father would always put higher octane gas (I cant remember the level), in this car. It's getting to where it's able to drive again but I don't know what to put in and don't want to ruin the engine or cause further issues. My choices in Southern PA are 87, 89, and 92/93 depending on the station. I'm sorry if this has been answered previously but I have looked but I find conflicting answers. Thanks in Advance!!

 

87 or 89 will be fine, anything more is a waste of money. Octane level use is based on engine compression.

 

By 1975, all Oldsmobile engines ran on 87 [(R + M)/2] octane unleaded. Fill it up with regular at your local station and you will be fine.

 

Welcome SlotMachine,

87 or 89 will be fine as mentioned, 89 and fuel stabilizer may be wise unless driven regularly in case the octane drops from extended hibernation. Listen for pinging/detonation on any fuel you choose, if the timing and temp are ok then higher octane would help.

I stick to TopTier fuels for the claimed additive/detergent packages. Who knows may be a gimmick but it works for me.

Welcome again and good luck!!!

 

The lowest octane that doesn't knock should be what you need. If you can get it without ethanol so much the better.

 

As above, give it the octane level it needs to avoid detonation -- no more and definitely no less. If you're going to pay up, then get ethanol-free it it's available in your area.

 

I have a ‘74 Delta 88 Royale Convertible and been running it on 87 octane since I bought it in 1992 and it runs great.

 

Not that it makes a huge difference, but just something to be aware of: your '74 is not equipped with a catalytic converter as is the OP's '75.

Another thing is that 45 year old cars are rarely kept in the same state of tune as they left the factory, so their octane requirements may differ. Best to keep an open mind and if you hear any hint of pre-ignition, use a higher grade of gas.

 

Every Olds made since the 1971 model year was designed to run on regular grade low-lead (and later, no-lead) fuel. This is why CR dropped across the board in the 1971 model year. Unless the advance curve has been modified or the engine rebuilt with higher CR, using higher octane fuel is simply a waste of money.

 

Yes, but my point was that after 45 years, it's entirely possible that the advance curve has been modified or that the engine has been rebuilt to different specs or that there's even a whole other engine in there. All I'm suggesting is that it's unlikely the car is in the same state of tune as it left the factory, so if it detonates on regular, you better use something with higher octane. Don't insist on going with regular just because you're supposed to be able to use it.

 

I was agreeing with you...

MY point is that most people who use premium are wasting their money.

 

Oh. Well in that case, you're right.

 

I am not aware that octane rating impacts the catalytic converter since it is post-combustion but welcome further input. I believe octane rating impacts the ability of the engine to resist pre-ignition so pinging/knocking might warrant a higher grade. Both the ‘74 and ‘75 350ci 4BBL engines have the same 8.5:1 compression ratio hardly high performance. The ‘74 does have 180 HP (SAE net) — 10 more than the ‘75. And my ‘74 runs fine on Regular.

 

No, you're exactly right. I should have finished my thought which was that it doesn't make any difference unless you have access to leaded gas, which you could use in the '74 but not the '75.

Sorry about that.