Packard Forum Index » Post-War (1946-54)

What grade/octane does everyone use?

I don't think any of the straight eights need premium from the perspective of compression ratio/knock. Possibly the additives offer some significant advantage, though I have never seen it.

Senior V8s generally can profit from the higher octane stuff although our club has several V8 owners that insist on the cheapest regular in a fifty mile radius. Most are partly deaf, which I do not regard as a coincidence.

Premium is said to hold up better in storage, as in for the winter.

I use hi-test, the 56 374 engine has 10:1 compression ratio.

I don't think any of the straight eights need premium from the perspective of compression ratio/knock

I'd make one exception to Pat's statement above; the 54 "359" engine at 8.7/1 generally wants premium or at least mid-range gasoline, and even more so if the cylinder head has been replaced with a 327 or 288 head.

I agree completely. Clearly I have spent too much time with the regular only group....

I run the the alcohol free premium from the local BP station.

I use the high octane, especialy summerin the PAckard V8 and Indian (low compression engine too). Prefer SHell but often use BP and alot of Marathon. Of course next year or 5 years from now i mite find that some other brand runs better. The alcohol free is always "sold out" around here. Everything else i use the cheapest.

According to the petroleum engineers handbook inaudible detonation results in accelerated motor wear. So I would refer to the owners manual and use at least what they recommend. Modern car knock sensors can monitor spark and bearing knock below threshold and the computer control retard timing accordingly.

The rule is your octane should look like your compression ratio. In other words if your compression ratio is below 8.7:1 use 87 octane regular. If your compression is 9.2:1 or higher, use high octane 92. If your compression ratio is 10:1 you can't get 100 octane anymore except at an airport so you may need octane boost.

Naturally this is just a guide, not a hard and fast rule. As others pointed out, flathead engines with high compression engines need more octane than OHV engines with the same compression. This only applies to the last year or 2 of flathead Packards, the earlier models had compression of under 8:1 and do fine on regular. Some engines need more octane than others, even with the same compression ratio.

You have to use your own judgement. Try different brands of gas and see which one your car likes best. If it starts easier, runs smoother and has more pep on one brand, that is the one to use.

There is NO advantage to using high octane gas if you do not have a high compression motor. A low compression motor will actually start easier, run better and cooler, and make more power on low octane fuel because that is what it is designed for. Some very low compression engines from the twenties and thirties, actually run better if you add some diesel or kerosene to the gas.

For 1951 Packard was recommending high octane for the engines with Ultramatic and regular for manual transmission cars. By 1954 they recommended premium for everything; the 288, 327, 359 and the V-8.

How did they make these recommendations? They drilled and tapped knock sensors into the block, took the car out on the test track, and recorded the knock readings on special meters inside the car. Or they did it on a dyno. Or both.

The reason modern cars generally run great on regular is because a computer controlled engine retards the timing to accommodate the grade of gasoline. And the owner's manuals will sometimes state that optimum performance will be achieved with higher octane, but that regular is acceptable.

Now those old gasolines used tetraethyl lead which was a very effective, and cheap, anti knock compound. I don't know what the octane ratings were in those days, but I'm sure they can be found. However, the octane number methods are subject to change.

Suffice it to say the Packard owners manual is the appropriate source for deciding what grade of gasoline to buy. Notwithstanding that the cars burn so much of it.

I wouldn't tell somebody to disregard that recommendation.